Veterinary · Para-clinical · also MSVE VMC-321
Virus structure and replication, the virus families behind the major animal plagues and pet diseases, and how viral infections are diagnosed and vaccinated against.
In the exams: ICAR AICE-JRF core group 06; ICAR AIEEA-PG 13 (Microbiology); China infectious diseases 30
Systematic veterinary virology is the study of the viruses that cause disease in animals, arranged by family: what each virus is made of, how it enters and multiplies in a cell, how it spreads between animals, what disease it causes, how the laboratory proves it is present, and how vaccines and control programmes keep it out.
The subject matters because viruses cause most of the great animal plagues. Rinderpest killed cattle across three continents until a vaccine and a global campaign eradicated it; it was declared eradicated in 2011, only the second disease after human smallpox. Foot-and-mouth disease, peste des petits ruminants, African swine fever, highly pathogenic avian influenza and lumpy skin disease now shape trade in animals and animal products between countries. Rabies still kills tens of thousands of people each year, almost all bitten by dogs. In companion animal practice, parvovirus, distemper and the feline retroviruses are daily work. Most of these diseases are listed by WOAH (the World Organisation for Animal Health, formerly OIE), which means countries must report them and trade rules depend on their status.
The subject is examined everywhere. In India it is part of the ICAR postgraduate entrance (Microbiology within Veterinary Science) and core group 06 of the ICAR PhD entrance; the Chinese national veterinary examination gives 30 marks to infectious diseases in its preventive paper; licensing exams such as NAVLE, the RCVS statutory examination, the Australasian and South African registration exams test the same diseases as clinical cases. Questions ask for the family and genome of a virus, its distinguishing lesion, the confirmatory test, and the vaccine strain or type.
This manual follows the curriculum units: structure and replication first, then the major viruses one by one, then diagnostics and vaccine technology. Where practice or law differs by country, it says so. It gives no product names and few doses; always follow the product label and your national authority.
A virus is an obligate intracellular parasite that carries its genetic information as either DNA or RNA, never both, and cannot make energy or protein on its own. The complete infectious particle is the virion. At its core is the genome, which may be single- or double-stranded, linear or circular, and in one piece or in segments. The genome is wrapped in a protein coat, the capsid, built from many copies of a few proteins; the structural units seen by electron microscopy are called capsomers. Genome plus capsid is the nucleocapsid. Some viruses stop there and are called naked or non-enveloped (parvoviruses, picornaviruses, orbiviruses, birnaviruses). Others acquire a lipid envelope from a host cell membrane as they bud; viral glycoproteins stud this envelope as spikes (paramyxoviruses, orthomyxoviruses, rhabdoviruses, herpesviruses, retroviruses). Capsids follow two main designs. In helical symmetry the protein units wind around the genome as a rod or flexible coil, as in the nucleocapsids of paramyxoviruses, orthomyxoviruses and rhabdoviruses. In icosahedral symmetry the capsid is a closed shell with 20 triangular faces and 12 vertices, the most economical way to build a strong container from identical units; parvoviruses, picornaviruses, reoviruses, herpesvirus capsids and adenoviruses are icosahedral. Poxviruses are called complex because their brick-shaped virion fits neither pattern. Size ranges from about 20-25 nm for parvoviruses and circoviruses to 200-400 nm for poxviruses, which are just visible by light microscopy after special staining.
Enveloped viruses lose infectivity when the lipid envelope is dissolved, so they are sensitive to ether, chloroform, detergents and drying. Most enveloped viruses spread by close contact, aerosol, bites or vectors rather than by long survival in the environment, though there are exceptions: African swine fever virus, a large enveloped DNA virus, survives for months in chilled or cured pork. Non-enveloped viruses resist lipid solvents and many disinfectants and may persist for months in the environment; canine parvovirus and infectious bursal disease virus are notorious for contaminating kennels and poultry houses for long periods, and need oxidising agents, aldehydes or sodium hypochlorite at adequate strength and contact time. Acid stability separates viruses within the same family. Within the picornaviruses, enteroviruses survive stomach acid, whereas the aphthoviruses, including foot-and-mouth disease virus, are destroyed below about pH 6.
Viruses are classified by the International Committee on Taxonomy of Viruses (ICTV) into orders, families (ending -viridae), subfamilies (-virinae), genera (-virus) and species. Since 2021 ICTV has been moving species names to a two-part (binomial) form, so textbooks and papers use several names for the same virus; the names of the diseases and the viruses in daily use have not changed. The Baltimore scheme groups viruses by how the genome gives rise to messenger RNA (mRNA). Class I is double-stranded DNA (herpesviruses, poxviruses, adenoviruses, asfarviruses). Class II is single-stranded DNA (parvoviruses, circoviruses). Class III is double-stranded RNA (orbiviruses such as bluetongue virus, birnaviruses such as infectious bursal disease virus). Class IV is positive-sense single-stranded RNA, whose genome can act directly as mRNA (picornaviruses, flaviviruses including pestiviruses, coronaviruses, caliciviruses, arteriviruses). Class V is negative-sense single-stranded RNA, which must first be copied into mRNA by a polymerase carried in the virion (paramyxoviruses, rhabdoviruses, orthomyxoviruses). Class VI is single-stranded RNA viruses that replicate through a DNA intermediate using reverse transcriptase (retroviruses such as feline leukaemia virus and equine infectious anaemia virus). Class VII is double-stranded DNA viruses that also use reverse transcription (hepadnaviruses).
Attachment is the binding of a viral surface protein to a receptor on the host cell, and the distribution of receptors largely decides which species, tissues and cells a virus can infect (its tropism). Examples used in this manual: foot-and-mouth disease virus binds integrins through an arginine-glycine-aspartic acid (RGD) motif on capsid protein VP1; influenza haemagglutinin binds sialic acid; morbilliviruses such as rinderpest, peste des petits ruminants and canine distemper viruses use SLAM (CD150) on immune cells and nectin-4 on epithelial cells; canine parvovirus binds the transferrin receptor. Penetration follows, either by fusion of the viral envelope with the plasma membrane at neutral pH (paramyxoviruses, which carry a separate fusion protein) or by endocytosis, after which a pH drop in the endosome triggers fusion (influenza) or release of the capsid contents. Uncoating frees the genome. The virus then makes its proteins and copies its genome. Positive-sense RNA genomes are translated at once, often into one long polyprotein that viral proteases cut into pieces. Negative-sense RNA viruses first transcribe mRNA with the polymerase they carried in. DNA viruses usually use the host nucleus and its enzymes, with the important exceptions of the poxviruses and African swine fever virus, which replicate in the cytoplasm and therefore carry their own transcription machinery. Most RNA viruses replicate in the cytoplasm; influenza viruses are the classic exception, transcribing in the nucleus and taking the capped ends of host mRNAs (cap snatching). Retroviruses convert their RNA into DNA and integrate it into the host chromosome. Assembly and maturation follow, and virions leave either by lysis of the cell (typical of non-enveloped viruses) or by budding through a membrane, which gives them their envelope.
RNA polymerases lack proofreading, so RNA viruses make roughly one error per genome copy and exist as a cloud of related variants, called a quasispecies. This is why foot-and-mouth disease virus has seven serotypes and many strains within each, why vaccine strains must be matched to field viruses, and why escape from antibodies occurs. Viruses with segmented genomes, such as influenza (8 segments), bluetongue virus (10 segments) and infectious bursal disease virus (2 segments), can exchange whole segments when two strains infect the same cell. This reassortment produces sudden large changes; for influenza A the sudden acquisition of a new haemagglutinin or neuraminidase gene is called antigenic shift, whereas the gradual accumulation of point mutations in these proteins is antigenic drift.
Pathogenesis is the sequence of events from entry to disease. A virus enters through the respiratory tract, alimentary tract, skin (bites, vectors, wounds), conjunctiva or genital tract, multiplies locally, and may spread by viraemia (free in plasma or carried in leucocytes), through lymphatics or along nerves. Rabies virus travels inside axons, which protects it from antibodies. Target cells then determine the disease: parvoviruses need cells in the S phase of division, so they damage intestinal crypts, bone marrow and, in very young pups, the dividing heart muscle; infectious bursal disease virus destroys immature B lymphocytes in the bursa; Marek's disease virus transforms T lymphocytes. Persistence takes several forms. Latency, typical of herpesviruses, is a silent state in which the genome is kept in nerve or lymphoid cells and reactivates under stress. Chronic infection with continuous shedding occurs with some pestiviruses and retroviruses, and lifelong carriage of equine infectious anaemia virus is why every positive horse is a risk to others. Immune tolerance after infection in the foetus produces persistently infected animals, as in bovine viral diarrhoea and in the late-onset form of classical swine fever. Immunosuppression is itself a major outcome: infectious bursal disease, Marek's disease, canine distemper and feline immunodeficiency virus infections all make animals prone to other infections and blunt the response to vaccines.
Foot-and-mouth disease (FMD) virus was the first animal virus recognised, by Loeffler and Frosch in 1898. It belongs to the family Picornaviridae, genus Aphthovirus. The virion is small (about 25-30 nm), non-enveloped and icosahedral, with a single-stranded positive-sense RNA genome of about 8 kb. The capsid is made of four structural proteins, VP1 to VP4; VP1 carries the main antigenic site, a flexible loop (the G-H loop) containing the RGD motif by which the virus binds integrins on epithelial cells. The genome also codes for non-structural proteins, among them the polymerase 3D and the proteins 3A, 3B and 3C. There are seven serotypes, O, A, C, SAT1, SAT2, SAT3 and Asia1, and infection or vaccination with one gives little or no protection against another. Serotype O is the most widespread in the world. Serotype C has not been reported for about two decades. The SAT types are largely confined to sub-Saharan Africa, where African buffalo are their natural maintenance hosts. The virus is inactivated below about pH 6 and above about pH 9, which is why the acid formed in muscle during rigor mortis inactivates it in matured carcass meat, though not in lymph nodes, bone marrow or offal, where the pH stays higher.
All cloven-hoofed animals are susceptible: cattle, buffalo, sheep, goats, pigs, deer and camelids to a variable extent (dromedaries are relatively resistant). Horses are not susceptible. The disease spreads by direct contact, aerosols, contaminated feed (especially swill containing meat products), people, vehicles and fomites, and through milk and semen. Pigs excrete very large amounts of airborne virus and are called amplifying hosts; cattle are very sensitive to airborne infection and are called indicator hosts; in sheep and goats the disease is often mild and easily missed, so they can spread it unnoticed. After recovery, many ruminants carry virus in the pharynx for more than 28 days, which defines a carrier; African buffalo may carry virus for years. Pigs do not become carriers. Clinically, cattle show fever, drooling of ropy saliva, smacking of lips, and vesicles on the tongue, dental pad, interdigital space, coronary band and teats. Vesicles rupture within a day or two, leaving raw erosions; lameness and a sharp fall in milk yield follow. Adult mortality is low, but young animals may die of myocarditis, seen at post-mortem as grey-white streaks in the heart muscle, the so-called tiger heart.
FMD cannot be distinguished clinically from other vesicular diseases (swine vesicular disease, vesicular stomatitis, vesicular exanthema in pigs), so laboratory confirmation is required. Epithelium from fresh or recently ruptured vesicles, placed in transport medium at a pH of about 7.2-7.6, is the best sample; oesophageal-pharyngeal fluid collected with a probang cup is used to find carriers. Antigen ELISA serotypes the virus, RT-PCR detects the genome, and virus is isolated in sensitive cell cultures. Antibodies against structural proteins, measured by virus neutralisation or ELISA, are raised by both infection and vaccination. Antibodies against the non-structural 3ABC protein are raised by infection but not by vaccination with purified inactivated vaccines, and the NSP ELISA is therefore the standard DIVA test (differentiating infected from vaccinated animals). FMD is a WOAH-listed disease for which WOAH officially recognises country and zone status: free without vaccination, free with vaccination, or not free. Countries that are free control outbreaks by movement restrictions, culling of infected and in-contact animals and sometimes emergency vaccination. Endemic countries rely on mass vaccination with inactivated vaccines. These are made by growing the virus in cell culture, inactivating it with binary ethyleneimine (an agent that does not damage the capsid antigen), concentrating and purifying it, and formulating it with an aluminium hydroxide-saponin or oil adjuvant. Vaccine strains must match the serotypes and strains in circulation, and immunity is short, so a first dose is followed by a booster and then regular revaccination, commonly every six months; the exact schedule depends on the product and national programme.
Rinderpest, or cattle plague, was caused by a morbillivirus in the family Paramyxoviridae. It infected cattle, buffalo, yaks and many wild ruminants, and in naive populations killed most animals it infected. The virus is enveloped, with a negative-sense single-stranded RNA genome, a haemagglutinin (H) protein for attachment and a fusion (F) protein, and it destroys lymphoid tissue and the epithelium of the alimentary tract. The classic signs were fever, erosive stomatitis, profuse diarrhoea and dehydration, often summarised as the three Ds: discharges, diarrhoea and death. At post-mortem, congested ridges in the large intestine gave a striped pattern called zebra striping. Eradication rested on three tools. The first was a safe, cheap, highly effective vaccine: the tissue culture rinderpest vaccine developed by Walter Plowright and colleagues in Kenya by serial passage of a virulent strain in bovine kidney cells, later made thermostable so that it could be used far from refrigerators. The second was diagnostic: competitive ELISA and PCR to measure immunity and detect virus. The third was organisation: the Global Rinderpest Eradication Programme, launched by FAO in 1994, which moved from mass vaccination to targeted vaccination, then to stopping vaccination and proving freedom by surveillance, including community-based participatory surveillance by animal health workers. The last confirmed case was recorded in Kenya in 2001. WOAH and FAO declared global freedom in 2011.
Peste des petits ruminants (PPR) is caused by PPR virus, a close relative of rinderpest virus in the genus Morbillivirus (ICTV now calls the species small ruminant morbillivirus). It was first described in Cote d'Ivoire in 1942 and in India in 1987, and has since spread across Africa, the Middle East and Asia, with incursions into Europe. There is one serotype, but four genetic lineages (I to IV) defined by sequencing part of the N or F gene; lineage IV dominates in Asia. Goats usually suffer more severely than sheep; cattle and pigs can be infected without disease, and camels and some wild small ruminants can be affected. Spread is by close contact, mainly through aerosols and discharges, so movement and mixing at markets drive outbreaks. The disease resembles rinderpest in small ruminants: sudden high fever, serous then mucopurulent discharges from eyes and nose that crust and block the nostrils, necrotic erosions in the mouth with a foul smell, profuse diarrhoea, and a secondary bronchopneumonia, a pattern sometimes called the stomatitis-pneumoenteritis complex. Post-mortem shows erosions along the alimentary tract, zebra striping in the colon and rectum, and pneumonia; histology shows syncytia with eosinophilic inclusion bodies. Diagnosis uses ocular and nasal swabs, mouth scabs and, at post-mortem, lymph nodes and lung. RT-PCR is the main test; immunocapture ELISA and lateral flow tests detect antigen; competitive ELISA against the H or N protein detects antibodies. Live attenuated homologous vaccines, first the Nigeria 75/1 strain and in India strains such as Sungri 96, give strong immunity lasting at least three years from a single dose; heterologous rinderpest vaccine was used before, but its use ended with rinderpest eradication. PPR is WOAH-listed, has an official status recognition procedure, and is the target of the FAO-WOAH Global Control and Eradication Strategy launched in 2015, which aims at global eradication by 2030.
The genus Capripoxvirus in the family Poxviridae (subfamily Chordopoxvirinae) contains three viruses: sheeppox virus, goatpox virus and lumpy skin disease virus. Like other poxviruses they are large, brick-shaped, enveloped DNA viruses with a double-stranded genome of about 150 kb, and they replicate entirely in the cytoplasm, forming eosinophilic intracytoplasmic inclusion bodies. The three viruses are closely related genetically and serologically, so infection with one gives cross-protection against the others. This is why sheeppox and goatpox strains have been used as heterologous vaccines for cattle in some countries, although homologous lumpy skin disease vaccines (based on the Neethling strain) generally protect cattle better.
Lumpy skin disease (LSD) of cattle and water buffalo was first described in Zambia in 1929 and was confined to Africa for decades. It then spread to the Middle East, south-eastern Europe (2015-2016, where it was brought under control by mass vaccination), and from 2019 into South and East Asia, including Bangladesh, India and China; India experienced a large epidemic in 2022. Transmission is mainly mechanical by blood-feeding arthropods: stable flies, mosquitoes and hard ticks have all been shown to carry the virus, which explains its seasonality and its spread over distances. Direct contact plays a smaller role. Virus is also present in semen. Signs are fever, enlarged superficial lymph nodes, and firm, round skin nodules of about 1-5 cm that may cover the whole body, including the muzzle, udder and genitalia. Nodules involve the full thickness of the skin and may slough to leave deep sores (sit-fasts). Nodules also occur in the mouth, respiratory tract and alimentary tract. Oedema of the limbs and brisket, lameness, pneumonia, abortion, temporary infertility in bulls, a sharp and prolonged fall in milk yield and permanent hide damage make the economic losses large. Diagnosis is by PCR on skin nodules, scabs or blood; PCR assays exist that distinguish vaccine from field strains, which matters when vaccinated animals develop reactions. Virus neutralisation and ELISA detect antibodies. Control relies on vaccination of the whole cattle population with live attenuated vaccine, movement control, vector control and, in free countries, culling of infected animals. LSD, sheep pox and goat pox are all WOAH-listed.
Rabies is caused by rabies virus, the type species of the genus Lyssavirus in the family Rhabdoviridae. Other lyssaviruses, found mainly in bats, cause the same disease. The virion is bullet-shaped, about 75 by 180 nm, enveloped, with a negative-sense single-stranded RNA genome of about 12 kb coding for five proteins: nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G) and the large polymerase (L). The glycoprotein forms the surface spikes, binds receptors on neurons and is the only antigen that induces virus-neutralising antibodies, so it is the basis of all vaccines. Virus in saliva enters through a bite wound, may multiply for a time in muscle, and then enters peripheral nerve endings. It travels to the central nervous system inside axons by retrograde transport, hidden from antibodies. The incubation period is therefore variable, from days to many months, and is shorter for bites on the face and in richly innervated sites. Once in the brain the virus spreads widely and then travels outward along nerves to the salivary glands and other organs, so an animal sheds virus in saliva from shortly before signs appear. Clinical rabies is almost always fatal. In the furious form, animals become restless, aggressive and bite at objects; in the paralytic or dumb form, paralysis of the throat and jaw causes drooling and a dropped jaw, and cattle may bellow and strain. Both lead to paralysis, coma and death within days. The characteristic lesion is a non-suppurative encephalomyelitis, sometimes with Negri bodies, eosinophilic intracytoplasmic inclusions in neurons (typically in the hippocampus in carnivores and in cerebellar Purkinje cells in herbivores), but their absence does not rule out rabies.
Dogs are the source of nearly all human rabies deaths, which WHO and partners estimate at around 59,000 a year, most of them in Asia and Africa. Rabies is a WOAH-listed disease and a notifiable zoonosis in most countries. The global Zero by 30 plan of WHO, WOAH and FAO aims to end human deaths from dog-mediated rabies by 2030, chiefly through mass dog vaccination (coverage of about 70% of dogs is the usual target) and access to post-exposure prophylaxis for people. Diagnosis is made on brain tissue after death. The direct fluorescent antibody test on brain smears (brainstem, cerebellum and hippocampus) is the reference test recommended by WOAH and WHO; the direct rapid immunohistochemical test and RT-PCR are alternatives, and virus isolation in neuroblastoma cell culture has replaced mouse inoculation in most laboratories. No ante-mortem test on an animal can rule out rabies. Antibody tests (the fluorescent antibody virus neutralisation test and the rapid fluorescent focus inhibition test) are used to confirm vaccination, for example for pet travel, where many countries require at least 0.5 IU per mL. A healthy dog, cat or ferret that bites a person is usually confined and observed for ten days; if it remains healthy, it was not shedding virus at the time of the bite. Veterinary vaccines are inactivated cell-culture vaccines for dogs, cats and livestock, and oral bait vaccines for wildlife (modified live strains and a vaccinia virus recombinant expressing the rabies glycoprotein), which eliminated fox rabies from much of western Europe.
Bluetongue virus (BTV) belongs to the genus Orbivirus, historically in the family Reoviridae (now placed by ICTV in the family Sedoreoviridae). The virion is non-enveloped, about 70-80 nm, with a double-layered capsid and a genome of ten segments of double-stranded RNA. The inner core protein VP7 is common to all strains and is the target of group-specific antibody tests; the outer capsid protein VP2 determines the serotype and is the main target of neutralising antibodies. Twenty-four classical serotypes are recognised, and further atypical serotypes have been described since 2008, some of which spread by direct contact rather than by vectors. Immunity is largely serotype-specific. Because the genome is segmented, strains reassort, including with live vaccine strains. Bluetongue is not contagious in the usual sense. It is transmitted by biting midges of the genus Culicoides, whose distribution and season decide where and when disease occurs; Culicoides imicola is the main vector in Africa and southern Europe, while other species carried the virus in the northern European epidemic caused by serotype 8 from 2006 and the serotype 3 epidemic that began in 2023. Cattle usually show few signs but remain viraemic for weeks, often up to about 60 days, and act as reservoirs; the WOAH Terrestrial Code uses 60 days as the infective period. Transplacental infection occurs with some strains and can cause brain malformations (hydranencephaly) in calves and lambs. Bluetongue is WOAH-listed and is not a zoonosis.
Sheep, especially fine-wool and European breeds, suffer most. The virus damages the endothelium of small blood vessels, causing fever, hyperaemia and oedema of the lips, muzzle, face and ears, excess salivation, erosions and ulcers in the mouth, a swollen and sometimes cyanotic tongue (which is rare but gave the disease its name), coronitis with lameness, muscle degeneration and stiffness, pneumonia and a later break in the wool. Haemorrhage at the base of the pulmonary artery is a characteristic post-mortem finding. Group-specific RT-PCR (usually real-time) detects any serotype, and serotype-specific PCR targeting the VP2 gene identifies the serotype; virus isolation uses embryonated chicken eggs inoculated intravenously and cell lines derived from Culicoides or mammals. The competitive ELISA against VP7 detects antibodies to all serotypes, whereas the serum neutralisation test is serotype-specific. Control combines vaccination with movement restrictions, vector control, housing at dusk and dawn, and surveillance with sentinel animals and vector trapping. Live attenuated vaccines, long used in southern Africa, give strong immunity but can cause viraemia, reassort with field strains, cross the placenta and be transmitted by midges; inactivated vaccines, used widely in Europe, are safer but need boosters. Any vaccine protects only against the serotypes it contains, so vaccine choice follows serotype identification.
Avian influenza viruses are influenza A viruses (family Orthomyxoviridae, genus Alphainfluenzavirus). They are enveloped, pleomorphic viruses with eight segments of negative-sense RNA, replicating in the nucleus. The two surface glycoproteins name the subtype: haemagglutinin (HA), which binds sialic acid receptors and causes red cells to clump, and neuraminidase (NA), which cleaves sialic acid to release new virions. Sixteen HA and nine NA subtypes circulate in birds (two further subtypes have been found in bats). Wild aquatic birds, especially ducks, geese and shorebirds, are the natural reservoir and usually carry the viruses without disease. Pathogenicity is defined by a laboratory test and a molecular feature. A highly pathogenic avian influenza (HPAI) virus is one with an intravenous pathogenicity index greater than 1.2 in six-week-old chickens, or that kills at least 75% of four- to eight-week-old chickens inoculated intravenously; H5 and H7 viruses with multiple basic amino acids at the HA cleavage site are also treated as HPAI. That cleavage site lets the HA be activated by proteases present in many tissues, so the virus spreads systemically. Only H5 and H7 viruses have naturally become highly pathogenic, usually after low pathogenic H5 or H7 viruses circulate in poultry, which is why low pathogenic H5 and H7 infections in poultry are also closely watched. HPAI causes sudden death, cyanosis and oedema of comb and wattles, haemorrhages on the shanks, drops in egg production and mortality approaching 100% in chickens and turkeys. The H5N1 lineage that emerged in geese in Guangdong in 1996 has spread across the world and evolved into many clades; clade 2.3.4.4b has caused a panzootic since about 2020 with mass deaths in wild birds and marine mammals, infections of carnivores, and from 2024 infections of dairy cattle in the United States. HPAI is WOAH-listed. Control in poultry relies on biosecurity, surveillance, stamping out, movement control and, in some countries, vaccination with inactivated or vectored vaccines, combined with surveillance that can distinguish vaccinated from infected flocks.
Newcastle disease (ND), also called Ranikhet disease in India, is caused by virulent strains of avian paramyxovirus type 1, now named avian orthoavulavirus 1 (family Paramyxoviridae). The virus is enveloped, with non-segmented negative-sense RNA and two surface glycoproteins: haemagglutinin-neuraminidase (HN) and fusion (F). There is one serotype. Virulence depends on the cleavage site of the F0 precursor protein: virulent strains have several basic amino acids (arginine or lysine) at positions 113-116 and phenylalanine at 117, allowing cleavage by ubiquitous proteases; avirulent strains need trypsin-like enzymes found only in the respiratory and intestinal tracts. Strains are grouped into pathotypes: velogenic (viscerotropic, with haemorrhagic intestinal lesions, or neurotropic), mesogenic, lentogenic (mild respiratory) and asymptomatic enteric. WOAH defines ND as infection of poultry with a virus that has an intracerebral pathogenicity index (ICPI) in day-old chicks of 0.7 or more, or the virulent cleavage site motif. Velogenic ND causes depression, greenish diarrhoea, respiratory distress, twisted neck and paralysis, a fall in egg production with thin-shelled eggs, and high mortality. Post-mortem lesions include haemorrhages at the tips of the proventricular glands, necrotic haemorrhagic foci in the intestine and caecal tonsils, and tracheal congestion. ND is WOAH-listed and can cause conjunctivitis in people. Diagnosis is by RT-PCR with sequencing of the F cleavage site, virus isolation in embryonated eggs inoculated into the allantoic cavity followed by haemagglutination and haemagglutination inhibition (HI) tests, and HI serology to monitor flock immunity. Vaccines include live lentogenic strains (Hitchner B1, LaSota, and thermostable strains such as V4 and I-2 for village poultry) given by eye drop, drinking water or spray; live mesogenic strains used in some countries only as boosters in older birds; inactivated oil-emulsion vaccines; and vectored vaccines.
Infectious bursal disease (IBD), also called Gumboro disease after the town in Delaware where it was first recognised in 1962, is caused by a birnavirus (family Birnaviridae, genus Avibirnavirus). The virion is non-enveloped, icosahedral and about 60 nm, with a genome of two segments of double-stranded RNA (segments A and B). Segment A codes the capsid protein VP2, the main protective antigen, whose hypervariable region determines antigenicity and much of virulence. Serotype 1 causes disease in chickens; serotype 2, isolated from turkeys, does not. Within serotype 1 there are classical strains, antigenic variant strains (important in North America) and very virulent strains (vvIBDV), which emerged in Europe in the late 1980s and spread widely. The virus targets actively dividing IgM-bearing B lymphocytes in the bursa of Fabricius. Clinical disease occurs mainly at three to six weeks of age, when the bursa is at its largest: sudden depression, ruffled feathers, whitish watery diarrhoea, vent pecking and death over about a week, with mortality depending on strain. At post-mortem the bursa is first swollen, oedematous and yellowish with a gelatinous coat, sometimes haemorrhagic, and then shrinks to atrophy within about a week; haemorrhages in the thigh and breast muscles are common. Infection before about three weeks of age, in chicks with little or no maternal antibody, often causes no signs but lasting immunosuppression, poor response to other vaccines and more bacterial and viral disease. IBD is WOAH-listed. Diagnosis rests on lesions, antigen detection and RT-PCR with sequencing of the VP2 hypervariable region; ELISA measures antibodies. Control depends on biosecurity, vaccinating breeder hens with inactivated vaccines so that chicks receive high uniform maternal antibody, and vaccinating progeny with live vaccines (mild, intermediate or intermediate-plus, the last able to break through higher antibody but causing some bursal damage). Timing is critical because maternal antibody neutralises live vaccine, so vaccination age is often predicted from flock antibody titres and the antibody half-life (the Deventer approach). Immune-complex vaccines and herpesvirus of turkeys (HVT) vectored vaccines expressing VP2 can be given in ovo or at hatch and avoid the timing problem.
Marek's disease, first described by Jozsef Marek in 1907, is a lymphoproliferative disease of chickens caused by gallid alphaherpesvirus 2, also called Marek's disease virus serotype 1, a member of the genus Mardivirus. Serotype 2 (a non-oncogenic chicken virus) and serotype 3 (herpesvirus of turkeys, HVT) are related but do not cause tumours. The virus is strongly cell-associated in lymphocytes; fully infectious enveloped virus is produced only in the feather follicle epithelium, and chickens become infected by inhaling contaminated dander, which remains infectious in poultry house dust for months. Infection begins with a lytic phase in B cells, then activated T cells become latently infected, and some CD4 T cells are transformed into lymphoma cells; the viral Meq protein is the key oncogene. The disease has several forms. In the classical neural form, peripheral nerves such as the sciatic nerve and brachial plexus become enlarged, greyish and lose their cross-striations, causing paralysis, with the typical posture of one leg forward and one back. In the acute form, lymphomas appear in visceral organs, skin and muscle. The ocular form causes grey eye with an irregular pupil, and there is also a cutaneous form and a transient paralysis. Marek's disease must be distinguished from lymphoid leukosis, caused by avian leukosis virus (a retrovirus), which produces B-cell tumours arising in the bursa in birds usually older than about 16 weeks, without nerve lesions. Vaccination at hatch by subcutaneous injection, or in ovo at 18 days of incubation, is universal in commercial flocks. The first vaccine was HVT, the first vaccine against a cancer; bivalent HVT with serotype 2 vaccines act synergistically, and the attenuated serotype 1 strain CVI988 (Rispens) gives the strongest protection. Cell-associated vaccines are stored in liquid nitrogen. Vaccines prevent tumours and death but not infection or shedding of field virus, which is one reason virulence has increased over time.
African swine fever (ASF) virus is the only member of the family Asfarviridae (genus Asfivirus). It is a large, complex, enveloped virus with a double-stranded DNA genome of about 170-190 kb, replicating mainly in the cytoplasm of monocytes and macrophages, and it is the only known DNA arbovirus. Genotypes are defined by sequencing the gene for the major capsid protein p72; more than twenty have been described, mostly in Africa. The virus is remarkably stable: it survives for months in chilled and frozen meat, in some cured pork products, and in contaminated pens and vehicles, which is why swill feeding and the movement of pork products spread it over long distances. ASF was first described in Kenya in 1921. In Africa it is maintained in a sylvatic cycle between warthogs and soft ticks of the genus Ornithodoros living in their burrows; warthogs are infected without disease. From there it spills into domestic pigs, where it spreads by contact, pork products and fomites. Genotype II entered Georgia in 2007, spread through the Caucasus and Russia into the European Union, where wild boar maintain it in a wild boar-habitat cycle, and in 2018 reached China, then much of Asia, including India from 2020, and the Dominican Republic and Haiti in 2021. It is WOAH-listed, is not a zoonosis, and is the most important threat to pig production worldwide. Acute ASF causes high fever, loss of appetite, reddening or cyanosis of the skin of the ears, belly and legs, abortion and death within about a week, with mortality near 100% for virulent strains. The spleen is greatly enlarged, dark and friable, lymph nodes (especially gastrohepatic and renal) are enlarged and haemorrhagic, resembling blood clots, and there are petechiae on kidneys and serosae. Clinically ASF cannot be told from classical swine fever, so laboratory tests are needed: PCR on blood, spleen, lymph nodes or tonsil; antigen ELISA; antibody ELISA confirmed by immunoblot or indirect immunoperoxidase test in survivors; and virus isolation in pig macrophage cultures, where the haemadsorption test shows red cells adhering to infected cells. The virus does not induce effective neutralising antibodies, which is one reason conventional inactivated vaccines failed. Live attenuated vaccines made by deleting virulence genes have been licensed in Vietnam, but concerns about safety and reversion remain; control elsewhere still depends on biosecurity, stamping out, movement control, bans on swill feeding and management of wild boar.
Classical swine fever (CSF), also called hog cholera, is caused by a pestivirus (family Flaviviridae, genus Pestivirus), a small enveloped virus with a positive-sense RNA genome. It is closely related to bovine viral diarrhoea virus and border disease virus, which can also infect pigs and cause cross-reacting antibodies, so positive results are confirmed by comparative neutralisation tests. The envelope glycoprotein E2 is the main target of neutralising antibodies; the glycoprotein Erns is used as a marker in DIVA testing. Acute CSF causes high fever, huddling, conjunctivitis, constipation then diarrhoea, a staggering gait, convulsions and purplish discoloration of the skin, with high mortality in young pigs. Post-mortem lesions include haemorrhagic, marbled lymph nodes, petechiae in the kidney (turkey-egg kidney), infarcts along the edge of the spleen, which are considered characteristic, and in chronic cases button ulcers in the caecum and colon. Infection of pregnant sows with low-virulence strains causes abortion, mummification, congenital tremor and piglets born persistently infected, which shed virus and die later (late-onset CSF), and such sows are called carrier sows. Diagnosis uses PCR, antigen detection by immunofluorescence on tissues, virus isolation in pig kidney cells detected by immunostaining (the virus usually causes no cytopathic effect), and antibody ELISA with neutralisation tests. CSF is WOAH-listed with official status recognition. Free countries use stamping out and do not vaccinate. Endemic countries, including India, use live attenuated vaccines, notably the lapinised C-strain (Chinese strain, attenuated by passage in rabbits), which is safe and gives rapid, solid immunity. E2 subunit vaccines and live chimeric marker vaccines allow DIVA testing with an Erns antibody ELISA.
Canine parvovirus type 2 (CPV-2) belongs to the family Parvoviridae, genus Protoparvovirus, in the same species as feline panleukopenia virus, from which it emerged as a new pathogen of dogs in 1978. It is small (about 25 nm), non-enveloped, with a linear single-stranded DNA genome, and it binds the transferrin receptor. Within a few years CPV-2 was replaced by antigenic variants, now called 2a, 2b and 2c, which differ mainly at amino acid 426 of the capsid protein VP2; the variants also infect cats. The virus survives for months or longer in the environment, resists most disinfectants except oxidising agents, aldehydes and hypochlorite, and spreads by faeces and fomites. Because parvoviruses need cells in the S phase, CPV destroys the crypt epithelium of the small intestine, the bone marrow and lymphoid tissue. Unvaccinated pups from about six weeks to six months are most at risk, particularly as maternal antibody wanes. Signs are depression, vomiting, fever and foul-smelling, often bloody diarrhoea with severe leukopenia and dehydration; sepsis from gut bacteria is a common cause of death. Myocarditis occurs in pups infected in the first weeks of life from non-immune dams, but is now rare. Faecal antigen ELISA is the usual clinic test (it may be transiently positive after modified live vaccination), and PCR is more sensitive.
Canine distemper virus (CDV) is a morbillivirus (family Paramyxoviridae) with a very wide host range among carnivores: dogs, foxes, wolves, ferrets and other mustelids (in which it is almost always fatal), raccoons, seals, and large cats, as in the lion deaths in the Serengeti in 1994. It spreads by aerosol, infects lymphoid tissue first and causes immunosuppression, then spreads to epithelium and the central nervous system. Signs include fever, conjunctivitis, oculonasal discharge, cough, pneumonia, vomiting and diarrhoea, thickening of the footpads and nose (hard pad), enamel hypoplasia in dogs infected while their teeth develop, and neurological signs such as myoclonus, seizures and paralysis, which may appear weeks after the systemic phase or in dogs that never had it. Eosinophilic inclusions occur in the cytoplasm and nuclei of epithelial cells and glia. RT-PCR on conjunctival swabs, blood, urine or cerebrospinal fluid is the main test. Modified live and recombinant vectored vaccines give strong immunity. The WSAVA vaccination guidelines treat distemper, canine adenovirus and parvovirus (plus rabies where it is endemic) as core vaccines for every dog, given every two to four weeks from 6-8 weeks until 16 weeks of age or later, with a booster at 26 weeks to one year and then usually no more often than every three years, depending on the product and local risk. Neither CPV nor CDV infection is a WOAH-listed disease. Cats carry two important retroviruses. Feline leukaemia virus (FeLV, genus Gammaretrovirus) spreads by saliva through close contact and causes anaemia, immunosuppression and lymphoma; infection may be progressive, regressive, abortive or focal. Clinic tests detect the p27 capsid antigen in blood, and PCR detects provirus. Feline immunodeficiency virus (FIV, genus Lentivirus) spreads mainly by bites, so free-roaming entire tomcats are most at risk; it causes a slow immunodeficiency, and tests detect antibody, which may be maternal in kittens under six months or vaccine-induced where an FIV vaccine has been used.
Equine influenza is caused by influenza A virus subtype H3N8; the H7N7 subtype has not been isolated from horses for decades. Current strains belong to the Florida sublineage, clades 1 and 2, and a WOAH expert panel reviews strains each year and recommends which should be in vaccines. The disease spreads fast by aerosol after an incubation of one to three days, causing fever, a harsh dry cough, nasal discharge and depression; secondary bacterial pneumonia is the main complication. Diagnosis is by RT-PCR or antigen tests on nasopharyngeal swabs, or haemagglutination inhibition on paired sera. Inactivated, vectored and live attenuated vaccines are used, and racing authorities in many countries set compulsory booster intervals. Equine influenza is WOAH-listed. Equine infectious anaemia (EIA), or swamp fever, is caused by a lentivirus in the family Retroviridae. Infection is lifelong. The virus is carried mechanically on the mouthparts of large biting flies, especially horse flies and deer flies, and by contaminated needles, instruments and blood products. Acute disease brings fever and thrombocytopenia; chronic disease brings recurrent cycles of fever, anaemia, oedema and weight loss as antigenic variants emerge; many horses become inapparent carriers that still transmit infection. The agar gel immunodiffusion test (Coggins test) for antibodies to the core protein p26 is the classic test, and ELISAs are used for screening with positives confirmed by immunodiffusion or immunoblot. There is no internationally used vaccine or treatment. Positive horses are destroyed or kept permanently isolated from other horses, according to national law. EIA is WOAH-listed and a negative test is required for movement and export in many countries.
A good diagnosis starts with the right sample, taken at the right stage and kept in the right condition. Virus is usually most plentiful early in disease, so samples should come from acutely ill or freshly dead animals, from the tissue the virus targets: vesicle epithelium for FMD, brain for rabies, bursa for IBD, blood for bluetongue and ASF, ocular and nasal swabs for PPR, tracheal and cloacal swabs for avian influenza and Newcastle disease. Swabs go into viral transport medium, samples are kept chilled (not frozen, unless they will be held for days, when freezing at minus 70 degrees is preferred), and packaging and shipping of suspected high-consequence pathogens follows national and international transport rules. Virus isolation grows the virus in a living system. Cell cultures are primary cells, diploid cell strains or continuous cell lines; growth is shown by cytopathic effect (rounding and detachment, syncytia typical of paramyxoviruses and herpesviruses, inclusion bodies), by haemadsorption (as with ASF virus in macrophages), by immunostaining for non-cytopathic viruses such as CSF virus, or by interference. Embryonated chicken eggs are inoculated by different routes: the allantoic cavity at 9-11 days for influenza and Newcastle disease viruses, the chorioallantoic membrane for poxviruses (producing pocks), the yolk sac, the amniotic cavity, or intravenously for bluetongue virus. Antigen detection is faster. Fluorescent antibody tests on tissue smears or sections (as for rabies), immunohistochemistry on fixed tissues, antigen-capture ELISA (as for FMD serotyping), and lateral flow devices used at the pen side all use antibodies to find viral proteins.
The polymerase chain reaction (PCR) amplifies a target sequence of DNA; for RNA viruses the RNA is first copied into complementary DNA by reverse transcriptase (RT-PCR). Real-time PCR measures product as it forms using fluorescent probes; the cycle threshold (Ct) is the cycle at which fluorescence crosses a set level, so a lower Ct means more target. With 100% efficiency the target doubles each cycle, and a ten-fold difference in starting amount corresponds to about 3.3 cycles. Isothermal methods such as loop-mediated isothermal amplification (LAMP) need only a heating block and suit field use. Sequencing of key genes identifies serotypes, lineages, virulence markers (the F cleavage site of Newcastle disease virus, the HA cleavage site of influenza) and outbreak links; whole-genome and metagenomic sequencing are now standard in reference laboratories.
Antibody tests show exposure or vaccination. The virus neutralisation test is the most specific and serotype-specific but needs live virus and cell culture. Haemagglutination inhibition measures antibodies that block viral haemagglutination; it is the standard test for Newcastle disease and avian influenza subtypes and uses four haemagglutinating units of antigen per test. Agar gel immunodiffusion is cheap and simple (Coggins test for EIA, group test for influenza A). ELISAs are indirect (detecting antibody bound to antigen on the plate with an anti-species conjugate) or competitive (serum antibody blocks a labelled monoclonal antibody, which makes the test usable in any species, as in the bluetongue VP7 and PPR ELISAs). Because antibodies persist, a single positive titre shows past exposure, not current disease; current infection is shown by a four-fold or greater rise in titre between paired acute and convalescent sera taken two to four weeks apart, or by IgM. Every test has a sensitivity (the proportion of infected animals that test positive) and a specificity (the proportion of uninfected animals that test negative). The predictive value of a positive result depends also on prevalence: when a disease is rare, even a good test gives many false positives, which is why screening tests are followed by a confirmatory test with high specificity. DIVA tests, such as the FMD non-structural protein ELISA and the CSF Erns ELISA, depend on vaccines that lack the protein detected.
Live attenuated vaccines contain virus weakened by serial passage in cell culture, eggs or an unnatural host (the C-strain of CSF virus in rabbits, rinderpest vaccine in bovine kidney cells), by selection of cold-adapted or naturally mild strains (lentogenic Newcastle disease strains), or, more recently, by deleting virulence genes. They multiply in the host, stimulate antibody and cell-mediated immunity including mucosal immunity, often protect after one dose, and are cheap; but they may revert to virulence, spread to unvaccinated animals, cause disease in immunosuppressed or pregnant animals, contain contaminating viruses, and need a cold chain. Inactivated (killed) vaccines contain whole virus inactivated by formaldehyde, beta-propiolactone or binary ethyleneimine. They are safe and stable, can be combined, and suit exotic diseases such as FMD, where live virus cannot be used; but they need adjuvants, usually two doses and regular boosters, and mainly stimulate antibody. Adjuvants include aluminium salts, saponin and water-in-oil or oil-in-water emulsions. Subunit vaccines contain only the protective protein, produced by recombinant expression (for example the CSF E2 protein made in insect cells with a baculovirus) or assembled into virus-like particles, which mimic the virion without genome. Vectored vaccines insert the gene for a protective antigen into a harmless carrier virus: poxviruses such as fowlpox, canarypox and vaccinia (the wildlife rabies bait vaccine), herpesvirus of turkeys expressing IBD VP2, Newcastle disease or avian influenza genes, and adenoviruses. They give live-vaccine immunity without the pathogen, and many allow DIVA testing. Reverse genetics, the recovery of infectious RNA viruses from cloned DNA, allows rational attenuation, marker deletions and fast updating of vaccine strains. A marker (DIVA) vaccine lacks an antigen, or carries one, so that infected animals can be told from vaccinated animals by a companion test.
A vaccine works only if it reaches the animal potent and on time. Most live vaccines must be kept at 2-8 degrees Celsius from factory to animal, protected from light and used soon after reconstitution; Marek's cell-associated vaccines are kept in liquid nitrogen. Thermostable vaccines were decisive for rinderpest eradication and matter for village poultry vaccination against Newcastle disease. Maternal antibody passed through colostrum or egg yolk protects the young but also neutralises live vaccine, creating a window of susceptibility when antibody is too low to protect but high enough to block vaccination; this is why puppies receive a series of doses until at least 16 weeks and why IBD vaccination is timed from flock titres. At population level, vaccination aims to reduce transmission below the level needed to sustain spread. If each infectious animal infects R0 others in a fully susceptible population, transmission falls when the immune proportion exceeds 1 minus 1/R0; a vaccine that protects only a fraction of animals needs correspondingly higher coverage. Vaccine efficacy in a trial is the proportional reduction in attack rate among vaccinated animals compared with unvaccinated ones. Vaccination is always combined with other tools: surveillance, movement control, quarantine, biosecurity, vector control and stamping out. For WOAH-listed diseases, the choice between vaccination and stamping out affects trade, because free status without vaccination is valued most.
Learn each virus as a set: family and genome, target cell, how it spreads, the one lesion that points to it, the confirmatory test and the vaccine type. Build this grid for the sixteen units and revise from it. Practise working out properties from structure, for example why an enveloped virus is easy to disinfect and a parvovirus is not, or why a segmented virus can reassort. In practicals, learn to inoculate eggs by the allantoic and CAM routes, read an HA and HI plate, and collect and pack a swab properly, because examiners ask for these steps.
Five years into practice, virology is about recognising and acting. A vesicular lesion in cattle, sudden deaths in a pig herd or high mortality in a poultry flock must trigger a report to the veterinary authority and correct sampling before anything else. In companion practice you decide when to run a parvovirus antigen test or PCR, how to manage an infected litter and disinfect the ward, how to test and counsel on FeLV and FIV, and how to handle a dog that has bitten someone. You set vaccination schedules around maternal antibody and local risk, and you explain to farmers why vaccines fail when the cold chain breaks or the timing is wrong.
With twenty-five years' experience, the work shifts to systems: running district or national vaccination campaigns for FMD, PPR, LSD or rabies, designing serosurveillance with suitable tests and sample sizes, judging whether a DIVA strategy is feasible, organising outbreak response and compensation, and advising on trade and zoning under WOAH standards. Senior veterinarians decide between stamping out and vaccination, weigh cost against trade status, audit vaccine quality and cold chains, and train field staff and community animal health workers. They also watch for emerging threats, such as new influenza clades in mammals, and link animal and public health services under One Health.
A researcher with a PhD and long field experience works on the unsolved problems: an effective and safe ASF vaccine, thermostable and DIVA-compatible vaccines for PPR and FMD, vaccine strain matching for rapidly evolving viruses, the evolution of virulence under leaky vaccines as seen with Marek's disease, and the ecology of spillover from wildlife reservoirs. They use reverse genetics, structural biology, genomic epidemiology and transmission modelling, and they judge whether laboratory findings will hold in the field. They also advise reference laboratories and international bodies, and must handle dangerous pathogens and dual-use research under strict biosafety and biosecurity rules.
The manual covers the sixteen curriculum units and a short account of FeLV and FIV. It does not give full accounts of many other viruses of veterinary importance: bovine viral diarrhoea, infectious bovine rhinotracheitis and other herpesviruses (apart from Marek's), coronaviruses (infectious bronchitis, porcine epidemic diarrhoea, feline infectious peritonitis), porcine reproductive and respiratory syndrome, porcine circovirus, African horse sickness, sheep and goat viruses such as orf and maedi-visna, canine adenovirus and parainfluenza, fowl pox, egg drop syndrome and chicken infectious anaemia, and prions. It does not state whether Marek's disease is on the current WOAH list, because the writer could not confirm it. ICTV species names are changing to a binomial form and are given only where the writer was certain. Vaccine schedules are given in outline only; they vary by country, product and label, and doses are deliberately not given. Epidemic dates and country status change quickly and should be checked against WOAH WAHIS and the national authority.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Numerical answers were computed before the options were written, and the working is shown in each explanation. Values that differ by country, product or national programme are flagged as such rather than fixed to one figure.
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