Veterinary · Para-clinical · also Diagnostic Parasitology
The laboratory and field methods that find parasites in faeces, blood and skin, the immune response to parasites, parasitic zoonoses and integrated control. Advanced and postgraduate level.
In the exams: ICAR AICE-JRF subject paper (Parasitology); ICAR AIEEA-PG 13
Clinical, diagnostic and immuno-parasitology is the part of parasitology that answers the practitioner's questions: is this animal or this herd carrying parasites, which ones, how many, do they matter, will the drug work, and is anyone else at risk? It sits on top of the descriptive subjects (helminthology, protozoology, entomology and acarology), which tell you what the parasites are and how their life cycles run. This subject is about finding them, measuring them, understanding the host's immune response to them, and using that knowledge to control them without breeding drug resistance or exposing people to zoonoses.
The work is mostly done with cheap tools: a balance, a measuring cylinder, a flotation solution, a McMaster slide, a microscope, Giemsa stain, a scalpel blade and some mineral oil. Around that core sit serological tests (ELISA, immunofluorescence, card agglutination), antigen tests on faeces and blood, and DNA methods. A clinician who understands what each test detects, and therefore when it will be falsely negative or falsely positive, makes better decisions than one who owns more equipment.
The subject is examined as an advanced or postgraduate topic. In India it forms part of the Parasitology paper of ICAR AIEEA (PG) Code 13 and of core group 06 in ICAR AICE-JRF/SRF, which ask for technique details, immune mechanisms, vaccines and zoonoses. Licensing examinations worldwide (NAVLE, RCVS, AVE, SAVC, the Philippine and Chinese boards) test the same material as clinical judgement: which sample, which test, what the result means, and what to tell the owner. Disease chapters on individual helminthoses and blood protozoa are in the preventive medicine, helminthology and protozoology manuals; this manual refers to them only where the diagnostic or immunological point needs it.
A faecal examination is only as good as the sample. Collect faeces from the rectum, or pick up a freshly voided pat or pile you saw passed, so you know which animal it came from and that it has not been contaminated by soil nematodes or fly larvae. Fill the container and exclude air, label it with the animal, date and time, and keep it cool (about 4 °C in a refrigerator). Do not freeze it: freezing distorts eggs and kills larvae. Warm storage lets strongyle-type eggs embryonate and hatch within a day or two, so a sample left in a hot vehicle can give a falsely low count and confuse egg identification.
For a flock or herd question, sample several animals (commonly around ten of the same management group) rather than one. Individual samples can be counted separately, or equal weights can be mixed into a composite sample; a composite gives a good estimate of the group mean at lower cost but loses information on spread between animals. Parasite burdens are overdispersed: most worms are carried by a minority of hosts, so a few animals produce most of the eggs, and a small sample can easily miss them.
Some parasites are shed intermittently (Giardia cysts, tapeworm segments) or have long prepatent periods, so a single negative sample does not rule them out. For Giardia, testing samples collected over several days increases sensitivity.
The McMaster technique is a quantitative flotation method that reports eggs (or coccidial oocysts) per gram of faeces (EPG or OPG). A known weight of faeces is mixed thoroughly with a known volume of flotation solution, the suspension is strained through a sieve or gauze to remove coarse debris, and while it is still being mixed a sample is drawn into the McMaster slide. Each chamber of the standard slide has a ruled grid of 1 cm by 1 cm and a depth of 0.15 cm, so the volume under one grid is 0.15 mL. After a minute or two the eggs float up against the upper glass, and those inside the grid lines are counted under low power.
The general formula is: EPG = eggs counted × (total volume of suspension ÷ volume examined) ÷ grams of faeces. The total volume is the volume of fluid plus, approximately, the volume of faeces (taking 1 g as roughly 1 mL). With 4 g of faeces in 56 mL of solution (about 60 mL in total) and one chamber read (0.15 mL), each egg represents 60 ÷ 0.15 ÷ 4 = 100 EPG. With 2 g in 28 mL (30 mL) and both chambers read (0.30 mL), each egg represents 30 ÷ 0.30 ÷ 2 = 50 EPG. With 3 g in 42 mL (45 mL) and both chambers read, each egg is again 50 EPG.
The multiplication factor is also the analytical sensitivity: with a factor of 50, one egg seen means 50 EPG, and zero eggs seen means fewer than 50 EPG, not zero. Counting both chambers, or using devices that read a larger volume, lowers the detection limit. Some modern flotation devices read two 1 mL chambers from a 1 in 10 dilution, which gives a factor of 5 EPG and is useful when post-treatment counts are low, as in resistance testing.
Egg counts are an index of the egg-laying adult worm population, not a direct measure of worm burden. The relationship is weakened by several facts. Fecundity varies widely between species: Haemonchus contortus females lay thousands of eggs a day, whereas Trichostrongylus and Teladorsagia lay far fewer, so a modest count in a Haemonchus-dominated flock can mean a damaging burden while the same count of Trichostrongylus may be harmless. Immune hosts suppress worm fecundity. Prepatent infections, arrested (hypobiotic) larvae and larval stages produce no eggs at all, so disease from larvae can occur with a zero count; acute fasciolosis, larval cyathostominosis in horses and type II ostertagiosis are the classic examples.
Counts are therefore most useful at group level and in sequence: to decide whether a group needs treatment, to monitor pasture contamination, and to test whether a drug worked. Strongyle-type eggs of most trichostrongyles and strongyles cannot be told apart reliably by eye, so the count is usually reported as 'strongyle-type' with Nematodirus, Strongyloides, Trichuris and others counted separately. Generic identification needs larval culture or a DNA method. Interpretive thresholds for 'low', 'moderate' and 'high' counts differ by host species, parasite mix, region and laboratory; use local guidance rather than a universal cut-off.
Flotation works because most helminth eggs and protozoan cysts are lighter than a dense salt or sugar solution, so they rise to the surface while heavier debris sinks. The key property is specific gravity (SG). Common solutions are saturated sodium chloride (SG about 1.18–1.20), zinc sulphate at about 33% (SG about 1.18–1.20), magnesium sulphate, sodium nitrate, and Sheather's sugar solution (SG about 1.27). A solution of higher SG floats more kinds of egg, including some heavier ones, but also floats more debris and can distort delicate stages by osmosis. Zinc sulphate is preferred for Giardia cysts because it distorts them less than sugar or concentrated salt; sugar solution is widely used for Cryptosporidium oocysts and for heavier nematode eggs. Check the SG of each batch with a hydrometer, because evaporation and temperature change it.
In simple (passive) flotation the faecal suspension is placed in a tube or vial filled to a slight positive meniscus, a coverslip is laid on top and left for about 10–20 minutes, then lifted onto a slide. Centrifugal flotation spins the tube, with the coverslip either applied after spinning or during it depending on the rotor, and is consistently more sensitive than passive flotation, especially for small numbers of eggs, Giardia cysts and heavier eggs such as Trichuris.
Trematode eggs (Fasciola, paramphistomes, Dicrocoelium is the partial exception, and schistosomes) are heavy, and the operculate eggs of Fasciola and paramphistomes float poorly or collapse in ordinary flotation solutions. They are recovered by sedimentation: faeces are washed through a coarse sieve with water, the filtrate is allowed to settle or is passed through finer sieves, the supernatant is poured off repeatedly until the sediment is clean, and the sediment is examined, often after adding a dye such as methylene blue that stains debris but leaves fluke eggs their natural colour. Fasciola eggs are yellowish-brown and operculate; paramphistome eggs are similar in shape but clear to pale grey-green, which allows them to be told apart in the same sediment.
Fluke egg output is low and intermittent because eggs are stored in the gall bladder, so a negative sedimentation does not exclude chronic fasciolosis and is useless in acute disease, when immature flukes are migrating through the liver and no eggs are produced. Coproantigen and antibody tests (Chapter 6) fill that gap.
The Baermann technique recovers live nematode larvae from faeces or other material by their own activity. Faeces wrapped in gauze are suspended in lukewarm water in a funnel closed by a clamped rubber tube (or in a glass with the faeces resting in a sieve); larvae migrate out, sink, and collect at the bottom, where the first few millilitres are drawn off after several hours or overnight and examined. It is the standard test for lungworm first-stage larvae (Dictyocaulus in cattle, sheep and horses; Aelurostrongylus abstrusus in cats; Angiostrongylus vasorum and Crenosoma vulpis in dogs) and for Strongyloides larvae. Fresh faeces must be used so that larvae of gastrointestinal strongyles have not had time to hatch and confuse the result. Larvae per gram can be calculated by dividing the larvae recovered by the weight of faeces used.
Coproculture (larval culture) is used because strongyle-type eggs look alike. Faeces are mixed with a little vermiculite or charcoal, kept moist and warm (around 25 °C) for about a week, and the third-stage larvae (L3) are recovered by a Baermann apparatus and identified by features such as total length, the length of the sheath tail and the number and shape of intestinal cells. The proportions of genera present guide treatment choice and are essential when interpreting a resistance test.
Identification starts with size, shape, shell and contents, read against a reference atlas at the same magnification. Strongyle-type eggs are oval, thin-shelled and contain a morula of cells. Nematodirus eggs are much larger with a few large cells, which matters because N. battus causes disease in lambs. Strongyloides eggs are small and already contain a larva. Trichuris eggs are lemon- or barrel-shaped with a plug at each pole; Capillaria (Eucoleus) eggs are similar but the plugs are often asymmetrical and the shell sculptured. Ascarid eggs have a thick shell: Toxocara canis has a pitted surface, and Ascaris suum and Parascaris a thick mammillated coat. Moniezia eggs are triangular or square and contain a hexacanth embryo in a pear-shaped (pyriform) apparatus. Dipylidium eggs are passed in packets, usually seen when a segment is teased apart. Taeniid eggs are round with a thick, radially striated embryophore around a six-hooked embryo; the eggs of Taenia and Echinococcus cannot be told apart by microscopy, which has public-health consequences.
Coccidial oocysts (Eimeria, Cystoisospora) are counted as OPG on the McMaster slide. Species identification of Eimeria needs sporulation and measurement, and matters because pathogenicity differs widely between species: many oocysts of a mild species may be less important than fewer of a highly pathogenic one. Giardia cysts are oval with up to four nuclei and axonemes and are best seen in zinc sulphate centrifugal flotation, sometimes with iodine. Cryptosporidium oocysts are tiny (about 4–6 µm) and are easily overlooked in flotation; a faecal smear stained by the modified Ziehl–Neelsen method shows them as red to pink round bodies on a blue-green background, and antigen tests or PCR are alternatives.
Not everything that looks like a parasite is a parasite of the animal sampled. Pseudoparasites are objects that resemble parasite stages: pollen grains, plant cells and hairs, fungal spores, yeasts, grain mites and their eggs from stored feed, and air bubbles or oil droplets. Spurious (pass-through) parasitism is the passage of genuine parasite stages that do not infect the host: a dog that eats rabbit or ruminant faeces may pass Eimeria oocysts or strongyle eggs of those hosts, and a dog that eats raw offal may pass eggs of liver-dwelling parasites of the prey. The clue is a parasite that does not belong to that host, often in a dog known to be coprophagic; repeating the test after the dog has been prevented from scavenging resolves it.
Free-living nematodes and their larvae appear when faeces have touched soil or were left on the ground, and hatched strongyle larvae appear in old warm samples; both can be mistaken for lungworm larvae on a Baermann. Fly larvae and mites colonise faeces left in the open.
A positive faecal test establishes infection; it does not establish that the parasite caused the illness. Many healthy adult ruminants carry low strongyle burdens and coccidial oocysts, and many healthy calves pass a few Eimeria oocysts. Attribution needs consistency between the parasite, the signs and the numbers: anaemia, bottle jaw and a high strongyle count in summer lambs point to Haemonchus; profuse diarrhoea in weaned lambs with a heavy Nematodirus count in spring points to nematodirosis; bloody diarrhoea and high counts of a pathogenic Eimeria species in housed calves support coccidiosis. Supporting tests include packed cell volume and total protein (blood-feeding worms cause anaemia and hypoproteinaemia), plasma pepsinogen (raised with abomasal damage from Ostertagia), the FAMACHA eye score, and, at herd level, necropsy worm counts, which remain the gold standard.
Conversely, a negative faecal test does not clear an animal with consistent signs, because the disease may be caused by larval or prepatent stages, by a parasite shed intermittently, or by one not detected by the method chosen. In those situations the next step is a different test (antigen, antibody, PCR, Baermann, sedimentation), a repeat sample, or a post-mortem examination of a casualty.
Blood smears are the cheapest and fastest test for blood protozoa and rickettsiae in a sick animal, and remain the first test in most of the world. A thin smear is made by spreading a small drop of blood with a second slide at about 30–45° so that it ends in a feathered edge where cells lie in a single layer. It is air-dried, fixed in methanol and stained with Giemsa (or a rapid Romanowsky-type stain). Parasites are looked for in the monolayer and at the feathered edge, under oil immersion. A thick smear is a larger drop spread over a small area, dried and not fixed, so that the red cells lyse in the aqueous stain; it concentrates parasites and is more sensitive, but morphology is poorer, so thick smears screen and thin smears identify.
For Babesia bovis, which sequesters in capillaries, blood from a capillary bed (ear tip or tail tip) gives more parasites than jugular blood; at post-mortem a squash smear of cerebral cortex shows capillaries packed with parasitised red cells. In the carrier state parasitaemia is usually far too low for smears to detect, so smears confirm acute clinical disease but cannot certify freedom from infection.
Babesia bigemina are large, pear-shaped piroplasms, typically paired at an acute angle. B. bovis are small, often paired at an obtuse angle and lie centrally in the red cell. Babesia canis (sensu lato) are large and B. gibsoni small in dogs. Theileria piroplasms in red cells are small, rod-, comma- or ring-shaped; in theileriosis the schizonts (Koch's blue bodies) are found in lymphocytes in Giemsa-stained smears of a lymph node aspirate, which is the key diagnostic sample in early disease. Anaplasma marginale appears as dense, dark, round inclusions without surrounding cytoplasm at the margin of the red cell, A. centrale nearer the centre. Hepatozoon canis gamonts are seen inside neutrophils. Mycoplasma haemofelis and related haemotropic mycoplasmas sit on the red-cell surface as small cocci, rods or rings and must be distinguished from stain precipitate and Howell–Jolly bodies.
Trypanosomes are extracellular and motile. A fresh wet blood film under a coverslip shows their movement among the red cells. Sensitivity improves with concentration: in the microhaematocrit centrifugation technique blood is spun in a capillary tube and the junction of the buffy coat and plasma is examined directly for motile trypanosomes; in the dark-ground buffy coat method the capillary is cut and the buffy coat examined under dark-ground or phase contrast.
Microfilariae of Dirofilaria immitis circulate in the blood of many infected dogs. A direct wet drop shows motile microfilariae but is insensitive. The modified Knott's test concentrates them: blood is mixed with 2% formalin, which lyses red cells and fixes the larvae, centrifuged, and the sediment is stained with methylene blue and examined. It also allows identification: D. immitis microfilariae are longer, with a tapered head and a straight tail, whereas Acanthocheilonema (Dipetalonema) reconditum, a harmless subcutaneous filarid transmitted by fleas, has a blunt head and a hooked tail. Many infected dogs, especially those on macrocyclic lactone preventives, are amicrofilaraemic (occult infection), which is why antigen testing (Chapter 6) is the screening test of choice.
Haematology supports the parasitological diagnosis: a regenerative anaemia with haemoglobinuria fits babesiosis; anaemia with icterus and no haemoglobinuria fits anaplasmosis; lymphadenopathy, fever and later pancytopenia fit theileriosis; eosinophilia suggests tissue-migrating helminths or allergy to ectoparasites but is not specific. Packed cell volume measured before and after treatment is one of the most useful single numbers in haemoparasitic disease.
A skin scraping is a sample of the epidermis, hair and debris examined for mites. Choose fresh, untreated lesions at the edge of the affected area rather than old crusted or excoriated centres. Clip hair if needed, apply a drop of mineral oil (liquid paraffin) to the blade and the skin, and scrape in the direction of hair growth. The material is transferred to a slide with more oil and a coverslip and scanned at low power. Alternatively 10% potassium hydroxide clears keratin and hair, which helps when debris is heavy, but it kills mites so motility cannot be seen.
Deep scrapings are taken for mites that live in the epidermis or hair follicles. For Sarcoptes scabiei, which burrows in the stratum corneum, scrape several sites (ear margins, elbows, hocks in dogs) until capillary oozing appears. Sarcoptes is notoriously hard to find; repeated negative scrapings do not rule out canine scabies, and a positive pinnal–pedal scratch reflex, the distribution of lesions, serology where available and a response to a treatment trial are often used together. For Demodex, which lives deep in hair follicles, squeeze the skin before and during scraping to express follicular contents; finding several adult mites, immature stages or eggs is significant, whereas a single adult in a normal dog may not be. Trichograms (hair plucks) and tape strips are useful in sites that are hard to scrape, such as the face and feet.
Superficial scrapings, skin surface collections and tape tests serve mites that live on the surface: Psoroptes and Chorioptes in ruminants and horses, Cheyletiella ('walking dandruff') in dogs, cats and rabbits, Demodex gatoi in cats, and poultry mites. Notoedres cati in cats is usually easy to find.
Ear mites (Otodectes cynotis) are found by examining ear canal exudate in mineral oil; in heavy infestations mites are seen moving in the canal through an otoscope. Acetate tape pressed onto the coat picks up Cheyletiella, lice and eggs. Combing with a fine-toothed flea comb, onto white paper, recovers fleas and flea dirt; the 'wet paper test' confirms flea faeces because they dissolve into a reddish-brown halo of digested blood. Ticks are identified to genus from the mouthparts, scutum and festoons, which matters because tick genus predicts the pathogens transmitted.
In farm animals, skin scrapings from the edge of lesions diagnose psoroptic mange (sheep scab) and chorioptic mange. For sheep scab, an ELISA for antibodies to Psoroptes ovis is used in some countries as a flock test, detecting infestation early, before obvious lesions, and in flocks where clinical signs are masked. Antibody ELISAs for Sarcoptes are also available for dogs and for pig herds.
Ectoparasite diagnosis is often a probability judgement rather than a single definitive test. Fleas may be absent on examination in a cat with flea allergy dermatitis because the cat grooms them away; the distribution of lesions, flea dirt in the environment and response to strict flea control carry the diagnosis. In canine scabies, a dog with intense pruritus of the ear margins and elbows that scratches when the ear margin is rubbed is likely to be infested even if scrapings are negative, and in-contact people with itchy papules strengthen the suspicion because Sarcoptes from dogs causes a transient zoonotic dermatitis.
Record the number and stages of mites found and the sites sampled; repeat scrapings from the same sites are how the response to treatment is monitored, and in canine demodicosis treatment is usually continued until repeated scrapings are negative, as current guidance advises. Treatment choices and doses depend on the product licensed in each country; confirm against the product label and your national formulary.
Immunodiagnostic tests detect either the host's antibodies against the parasite or the parasite's own antigens. Antibody tests show exposure: they become positive some weeks after infection and may stay positive for months or years after the parasite has gone, so a positive antibody result does not prove current infection, and a negative result early in infection does not exclude it. Antigen tests detect molecules released by living parasites (circulating antigen in blood, coproantigen in faeces) and so indicate current infection, and fall after successful treatment. DNA tests (PCR and its relatives) detect the parasite's genetic material; they are sensitive and can identify species, genotypes and drug-resistance mutations, but they detect DNA from dead as well as live organisms and from environmental contamination.
The enzyme-linked immunosorbent assay (ELISA) is the commonest format. In an indirect ELISA, parasite antigen is fixed to a plate, test serum is added, and bound antibody is detected with an enzyme-labelled anti-species antibody and a colour-producing substrate; results are usually expressed as optical density relative to positive and negative controls. A sandwich (capture) ELISA uses an antibody on the plate to capture parasite antigen, which is how coproantigen and circulating antigen tests work. A competitive ELISA measures the ability of test serum to block a labelled monoclonal antibody; because it does not need a species-specific conjugate it suits many host species, and it is used for equine piroplasmosis. ELISAs on bulk tank milk give a cheap herd-level index of exposure to Ostertagia ostertagi and Fasciola hepatica in dairy herds.
The indirect fluorescent antibody test (IFAT) uses parasites fixed on a slide; antibody in serial dilutions of serum binds them and is shown with a fluorescent anti-species conjugate. It gives a titre, is widely used for Babesia, Theileria, Toxoplasma, Neospora and Leishmania, but needs a fluorescence microscope and subjective reading. Agglutination tests are simple: the card agglutination test for trypanosomosis (CATT/T. evansi) uses fixed, stained trypanosomes expressing a common variant surface antigen and is read by eye on a card in minutes, making it a field screening test for surra. The complement fixation test, older and more laborious, has long been used for dourine (Trypanosoma equiperdum) in international trade. Rapid immunochromatographic (lateral flow) tests now exist for several infections, including recombinant-antigen tests for canine leishmaniosis. For any test used for trade, check the current WOAH Terrestrial Manual, which lists recommended methods disease by disease.
Canine heartworm antigen tests detect a glycoprotein released mainly by adult female Dirofilaria immitis. They are highly specific but can be negative when the burden is very low, when only male worms are present, or when the infection is younger than about five to seven months; in some dogs antigen is bound in immune complexes, and heat treatment of serum before testing can release it.
Coproantigen ELISAs detect Echinococcus antigen in dog faeces and are used in surveillance and control programmes, with copro-PCR to identify the species; they replaced the old practice of arecoline purgation. Fasciola coproantigen tests become positive before eggs appear and fall within weeks of effective treatment, which makes them useful for checking whether flukicide treatment has worked. Giardia antigen tests (ELISA and lateral flow) are more sensitive than a single flotation.
Sensitivity is the proportion of truly infected animals that test positive; specificity is the proportion of truly uninfected animals that test negative. Neither changes with prevalence, but predictive values do. The positive predictive value (PPV) is the probability that a test-positive animal is infected, and it falls as prevalence falls: screening a low-prevalence population with an imperfect test yields many false positives. For example, with sensitivity 90%, specificity 95% and prevalence 10%, in 1,000 animals there are 90 true positives and 45 false positives, so PPV = 90 ÷ 135 = 67%. The negative predictive value (NPV) is high in the same population (855 ÷ 865, about 99%).
Two common strategies exploit this. Serial testing (retest positives with a second, more specific test) raises specificity and PPV, at some cost to sensitivity. Parallel testing (call an animal positive if either test is positive) raises sensitivity, useful when missing an infected animal is costly, as when importing animals into a free area. Apparent prevalence from a survey can be corrected for test error with the Rogan–Gladen formula: true prevalence = (apparent prevalence + specificity − 1) ÷ (sensitivity + specificity − 1). Cross-reactions between related parasites (Fasciola and other flukes, Babesia species, Taenia and Echinococcus) lower specificity, and vaccination can produce antibodies indistinguishable from infection.
Helminths are too large to be phagocytosed and most live in the gut lumen, tissues or blood vessels. The host's characteristic response is a type 2 (Th2) response. Epithelial damage releases alarmins that activate innate lymphoid cells and, with dendritic cells, drive CD4 T helper 2 cells. These secrete interleukin-4 (IL-4) and IL-13, which switch B cells to IgE (and, in ruminants, IgA and IgG1), increase mucus production by goblet cells and increase smooth muscle contractility and epithelial turnover; IL-5 drives eosinophil production; IL-9 and IL-3 expand mucosal mast cells. The combined effect in the gut is often summarised as 'weep and sweep': fluid secretion, mucus and peristalsis dislodge worms. IgE bound to mast cells triggers degranulation when worm antigen cross-links it, and eosinophils can damage larvae by antibody-dependent cellular cytotoxicity.
In sheep the response to gastrointestinal nematodes develops with age and repeated exposure. Immunity first limits worm growth and fecundity (shorter worms laying fewer eggs, partly linked to local IgA), then prevents establishment of incoming larvae, and finally expels adult worms. The self-cure phenomenon, described in sheep with Haemonchus contortus, is the expulsion of an existing burden when the sheep meets a new challenge of larvae; it is an immediate hypersensitivity reaction in the abomasal mucosa. Immunity relaxes around lambing and early lactation, so ewes shed more eggs (the periparturient rise), which contaminates pasture just as susceptible lambs begin grazing.
Intracellular protozoa are controlled mainly by type 1 (Th1) responses. Interferon-gamma from CD4 Th1 cells and NK cells activates macrophages to kill intracellular Toxoplasma and Leishmania through reactive oxygen and nitrogen species, and CD8 cytotoxic T cells kill infected cells. In Theileria parva infection, protection depends on MHC class I-restricted CD8 T cells that kill the schizont-transformed lymphocytes; antibody plays little part. In Leishmania, mouse models showed that a Th1 response leads to healing and a Th2 response to progressive disease; in dogs, high antibody titres accompany clinical disease rather than protection, and immune complexes cause glomerulonephritis, uveitis and polyarthritis.
Antibody and complement matter more against extracellular stages: trypanosomes in the blood are lysed and opsonised by antibody against their surface coat, and merozoites of Babesia are neutralised before they enter red cells. Immunity to Eimeria is strongly species-specific and depends largely on T cells in the intestinal mucosa; birds recovered from one species remain susceptible to others, which is why live coccidiosis vaccines must contain every important species.
Premunition (concomitant immunity) describes resistance to reinfection that depends on persistence of the existing infection, as with Babesia and Anaplasma in endemically infected cattle. Young calves are relatively resistant to clinical babesiosis and anaplasmosis (inverse age resistance) and, when exposed while young under protection of colostral antibody, become immune carriers. A herd in which almost all calves are infected early is in endemic stability, with little clinical disease; reducing tick challenge too far can leave a susceptible adult population and cause outbreaks.
Parasites persist for months or years in immunocompetent hosts because they evade or manipulate immunity. African trypanosomes have a dense coat of a single variant surface glycoprotein (VSG) and switch, from a large gene repertoire, to a new VSG; each wave of parasitaemia is cleared by antibody against the current coat, and a new variant then grows out, giving the characteristic undulating parasitaemia and making a conventional vaccine impractical. Intracellular protozoa hide inside host cells, sometimes inside a parasitophorous vacuole that avoids fusion with lysosomes (Toxoplasma) or inside macrophages themselves (Leishmania). Tissue cysts of Toxoplasma and Sarcocystis, and the hydatid cyst with its acellular laminated layer, shield parasites from effector cells.
Adult schistosomes acquire host molecules, including blood group antigens and MHC products, on their tegument, so that they are disguised as self, and concomitant immunity allows adults to persist while incoming larvae are killed. Many helminths secrete molecules that modulate immunity: they induce regulatory T cells and IL-10, dampen inflammation and divert responses, which is also why chronic helminth infection can reduce responses to unrelated vaccines.
Much parasitic disease is immune-mediated. Flea allergy dermatitis in dogs and cats involves immediate (type I, IgE-mediated) and delayed (type IV) reactions to flea salivary antigens. Insect bite hypersensitivity ('sweet itch') in horses is an allergic reaction mainly to Culicoides saliva. Immune complexes cause the renal and ocular lesions of canine leishmaniosis; much of the anaemia of babesiosis and trypanosomosis involves antibody-coated red cells removed by macrophages.
Hosts differ genetically in their response. Resistance is the ability to limit parasite establishment, development or fecundity; resilience (tolerance) is the ability to stay productive despite infection. Some sheep breeds (for example Red Maasai, St Croix and other hair sheep, Gulf Coast Native) resist Haemonchus better than many wool breeds; Bos indicus cattle carry fewer ticks than Bos taurus; N'Dama and other West African taurine cattle are trypanotolerant. Within breeds, faecal egg count is moderately heritable and is used to select rams in breeding programmes, a long-term control tool discussed in Chapter 11.
Compared with viral and bacterial vaccines, few vaccines against parasites have reached the market. Parasites are complex multicellular or eukaryotic organisms with many life stages, each with different antigens; many evade immunity (Chapter 7); natural infection often does not produce sterile immunity, so there is no simple correlate of protection to copy; and the protective responses (for example Th2 responses at mucosal surfaces, or CD8 T cells against Theileria) are hard to induce with killed or subunit antigens.
Two antigen strategies are recognised. Natural (exposed, conventional) antigens are those the host meets during infection, such as surface or secreted proteins of larvae. Hidden (concealed) antigens are molecules the host's immune system does not normally encounter, such as proteins on the gut lining of blood-feeding worms and ticks. Antibody induced by vaccination is ingested with the blood meal and damages the parasite's gut.
The first commercial vaccine against a helminth was the irradiated larval vaccine against the cattle lungworm Dictyocaulus viviparus, introduced in the UK in 1959. Third-stage larvae attenuated by irradiation are given orally in two doses before calves first go to grass; the larvae migrate but do not mature, stimulating immunity. Vaccinated calves must still be exposed to some field challenge to maintain immunity.
Several live vaccines protect against tick-borne protozoa. Attenuated cell-culture vaccines against tropical theileriosis use schizont-infected lymphoid cell lines of Theileria annulata, attenuated by prolonged culture; they are produced and used in India and several countries of North Africa, the Middle East and Asia. Against East Coast fever (T. parva), the infection-and-treatment method gives cattle live sporozoites (a mixture of strains) together with a long-acting tetracycline that restrains the infection while immunity develops; it produces carrier animals and needs a liquid-nitrogen cold chain. Live attenuated Babesia bovis and B. bigemina vaccines, often combined with Anaplasma centrale, are produced in splenectomised calves in Australia, South Africa, Argentina and elsewhere; they are best given to young cattle, which react less severely, and carry risks of transmitting other blood-borne infections.
Live coccidiosis vaccines for chickens contain oocysts of the main Eimeria species, either wild-type strains or attenuated 'precocious' lines selected for a shortened life cycle and lower pathogenicity. They are given to young chicks, by spray, water or feed, and immunity is built up as vaccine oocysts recycle in the litter, so litter management matters. A live vaccine containing an incomplete strain of Toxoplasma gondii (the S48 strain, which does not form tissue cysts or oocysts) is used in some countries to protect ewes against toxoplasma abortion; it is given before mating, and handlers must avoid self-injection because the organism is live.
The first recombinant vaccine against a parasite was the 45W antigen vaccine against Taenia ovis cysticercosis in sheep, developed in New Zealand and Australia. Its success led to the EG95 recombinant vaccine against Echinococcus granulosus in sheep, which prevents hydatid cyst development in the intermediate host and has been used in control programmes in China and South America alongside dog deworming. The Bm86 vaccine against the cattle tick Rhipicephalus (Boophilus) microplus is based on a hidden antigen from the tick gut; it reduces the number, weight and egg output of engorging ticks, lowering pasture infestation over generations rather than protecting the individual animal immediately, and its efficacy varies between tick populations.
A vaccine made from native hidden gut antigens (H11 and H-gal-GP) purified from adult Haemonchus contortus was first registered in Australia in the mid-2010s; it reduces egg output and anaemia but must be given repeatedly through the risk season because natural infection does not boost it. Vaccines against canine leishmaniosis are licensed in Europe and Brazil; they reduce the risk of progressing to clinical disease but do not prevent infection, and may complicate serological diagnosis, so they are used alongside sandfly repellents. A killed vaccine against Neospora caninum abortion in cattle was marketed in some countries and later withdrawn because protection was inconsistent.
A parasitic zoonosis is a parasitic infection naturally transmitted between vertebrate animals and people. Grouping them by route of transmission helps prevention. Food-borne zoonoses come from meat, fish or vegetables: Taenia saginata (beef) and T. solium (pork) tapeworms, Trichinella spp. (pork, wild boar, horse, bear and other game), Toxoplasma gondii (tissue cysts in undercooked meat, and oocysts on unwashed produce), Sarcocystis spp. (raw beef and pork), Fasciola hepatica and F. gigantica (metacercariae on watercress and other aquatic plants), Anisakis and other fish-borne nematodes, and fish-borne liver flukes. Environmental and faecal–oral zoonoses come from soil, water or animal contact: Toxocara canis and T. cati, Echinococcus granulosus and E. multilocularis, Cryptosporidium parvum and zoonotic Giardia, and hookworm larvae that penetrate skin. Vector-borne zoonoses include Leishmania infantum (sandflies, with the dog as main reservoir), zoonotic Babesia species (ticks) and Dirofilaria (mosquitoes). Direct-contact zoonoses include Sarcoptes and Cheyletiella mites from pets.
Toxocara is the commonest zoonotic helminth of pets in most countries. Eggs passed by puppies, kittens and foxes need some weeks in the environment before they are infective, so the risk is from contaminated soil, sandpits and hands rather than fresh faeces. Ingested larvae migrate in human tissues, causing visceral larva migrans, ocular larva migrans (which can lead to blindness), neurotoxocarosis or covert infection. Hookworm larvae (Ancylostoma braziliense and others) penetrating human skin wander in the epidermis, causing cutaneous larva migrans.
For Taenia solium, people are the definitive host and pigs the intermediate host; people who swallow eggs, from a tapeworm carrier or contaminated food, develop cysticercosis, and cysts in the brain (neurocysticercosis) are a leading preventable cause of epilepsy in endemic regions. Control combines meat inspection, treatment of human carriers, pig confinement, sanitation and, in some programmes, pig vaccination and treatment. Cystic echinococcosis (hydatid disease) follows ingestion of Echinococcus granulosus eggs from dogs that have eaten offal containing hydatid cysts; alveolar echinococcosis from E. multilocularis, maintained mainly between foxes and rodents, behaves like an invasive tumour in the human liver. Because taeniid eggs cannot be told apart microscopically, any dog passing taeniid eggs in an endemic area must be treated and handled as a potential Echinococcus carrier. Regular praziquantel treatment of dogs, denying dogs access to raw offal, and safe disposal of condemned organs break the cycle; several countries require dogs to receive tapeworm treatment before entry to keep E. multilocularis out.
Trichinella larvae encyst in muscle and infect people who eat raw or undercooked meat. Meat inspection in many countries uses artificial digestion of pooled muscle samples from predilection sites such as the diaphragm pillars, which is more sensitive than compression (trichinoscopy). Some Trichinella species in wildlife are freeze-resistant, so freezing does not make game meat safe. Toxoplasma gondii infection in pregnant women can cause abortion or congenital disease; most human infections come from undercooked meat or oocyst-contaminated produce and soil. Cats shed oocysts usually only once, for a short period after first infection, and oocysts take one to several days to become infective, so daily removal of litter, wearing gloves for gardening and washing vegetables reduce risk; a seropositive cat is less likely to shed than a recently infected seronegative one.
Veterinarians prevent zoonoses at several points: deworming and parasite control in pets and livestock, advice to owners (especially pregnant women, children and immunocompromised people), meat inspection and condemnation, surveillance and reporting, and participation in One Health programmes with medical and environmental colleagues.
Cryptosporidium parvum outbreaks among veterinary students, farm visitors and staff are regularly linked to diarrhoeic calves and lambs; hygiene at petting farms and in calf rearing protects people. Of the Giardia duodenalis assemblages, A and B infect people as well as animals, whereas others are largely host-adapted (C and D in dogs, F in cats, E in hoofed livestock), so genotyping helps judge zoonotic risk. Several parasitic zoonoses (for example echinococcosis, trichinellosis and porcine cysticercosis) are listed by WOAH, and national notification rules vary by country.
Anthelmintic resistance is the heritable ability of a parasite population to survive a drug dose that would normally kill it. It is widespread in gastrointestinal nematodes of sheep and goats worldwide, increasingly reported in cattle, and common in horse cyathostomins (to benzimidazoles) and Parascaris (to macrocyclic lactones). The faecal egg count reduction test (FECRT) is the standard field test and can be used for any drug class. Animals with adequate egg counts are allocated to groups, egg counts are measured, a group is treated at the correct dose for its weight, and counts are repeated after treatment. The interval matters, because drugs differ in how quickly egg output stops; the 2006 WAAVP revision suggested about 8–10 days for benzimidazoles, 3–7 days for levamisole and 14–17 days for macrocyclic lactones, and the 2023 WAAVP guideline revises these and other details, so follow the current document.
The percentage reduction can be calculated in two ways, and the method must be stated. Using an untreated control group: FECR% = 100 × (1 − T2 ÷ C2), where T2 and C2 are the arithmetic mean counts of the treated and control groups after treatment. Using the same animals before and after treatment: FECR% = 100 × (1 − T2 ÷ T1). For example, if a treated group falls from a mean of 1,000 to 40 EPG, the reduction is 96%; if post-treatment means are 120 EPG treated and 800 EPG control, the reduction is 85%. Under the 1992 WAAVP criteria, resistance was declared when the reduction was below 95% and the lower 95% confidence limit was below 90%. The 2023 guideline sets the number of animals and eggs to be counted and uses expected efficacy thresholds that differ by host species and drug, to give statistically sound conclusions; it is the reference for current field decisions.
A meaningful FECRT needs enough animals (commonly 10–15 per group as a minimum in older guidance, with sample size now driven by egg numbers counted), pre-treatment counts high enough to measure a reduction, accurate weighing and correct dosing, and a sensitive counting method. Underdosing, for example dosing to the average rather than the heaviest animal, is the commonest reason a 'failure' is not resistance. Larval culture before and after treatment shows which genera survived; resistance is usually genus-specific (for example Haemonchus surviving a drug that removed Trichostrongylus), and a bulk result can hide this.
In horses the egg reappearance period (ERP), the time after treatment until strongyle eggs reappear in faeces, is also monitored; a shortened ERP for macrocyclic lactones in cyathostomins is an early sign of reduced efficacy.
In vitro tests expose eggs or larvae to a range of drug concentrations. The egg hatch test detects benzimidazole resistance, because benzimidazoles prevent embryonated eggs from hatching. The larval development test measures the ability of larvae to develop from eggs to L3 in the presence of several drug classes. Larval migration and motility tests are used for levamisole and macrocyclic lactones. These tests need fresh eggs and laboratory skill and do not correlate perfectly with the field test.
Benzimidazole resistance has a well-understood molecular basis: single nucleotide changes in the isotype-1 beta-tubulin gene, most commonly at codon 200 (phenylalanine to tyrosine, F200Y), and also at codons 167 (F167Y) and 198 (E198A). PCR and sequencing-based assays detect these alleles and estimate their frequency in a worm population. The molecular basis of resistance to levamisole and macrocyclic lactones is more complex and not yet captured in a routine test.
For ticks, acaricide resistance is tested by exposing larvae (larval packet test, recommended by FAO) or engorged females (adult immersion test) to a range of acaricide concentrations and comparing survival with a susceptible reference strain.
Integrated parasite control combines drugs with grazing management, nutrition, host genetics, biological control, vaccines and monitoring, so that parasites are kept below the level that harms health and production while drug efficacy is preserved. It rests on knowing the local epidemiology: when infective larvae build up on pasture, which classes of stock are susceptible, when hypobiosis occurs, and which parasites dominate. For many years control relied on frequent whole-flock treatment, and in some systems on 'treat and move' to clean pasture; these practices select strongly for resistance because the next generation on the pasture comes almost entirely from worms that survived the drug.
The concept of refugia underpins modern control: refugia are the part of the parasite population not exposed to treatment, such as larvae on pasture, worms in untreated animals and arrested stages unaffected by the drug. Worms in refugia carry susceptible genes and dilute resistant survivors. Strategies that keep refugia larger slow resistance: leaving some animals untreated, avoiding treatment just before a move to clean pasture, and avoiding treatment when few larvae are on pasture (for example in drought).
Targeted treatment (TT) means treating a whole group at the right time based on evidence, such as egg counts or risk forecasts. Targeted selective treatment (TST) means treating only the individuals that need it, leaving the rest as refugia. For Haemonchus in sheep and goats, the FAMACHA system compares the colour of the lower eyelid mucosa with a five-point chart from red (1) to almost white (5); animals scoring 4 or 5, and often 3, are treated, because pale mucous membranes reflect anaemia. It only applies to blood-sucking worms. The Five Point Check extends the idea to other parasites: eye (anaemia), back (body condition), tail (dagging and diarrhoea), jaw (submandibular oedema) and nose (nasal discharge from Oestrus ovis). Other TST indicators include growth rate below target, body condition and milk yield.
In horses, routine interval dosing has been replaced in many countries by egg-count-based treatment: adults with low counts are left untreated, high shedders are treated, and young horses, which are more susceptible to ascarids and small strongyles, are managed more intensively. In dogs and cats, risk-based guidance recommends treatment frequency according to lifestyle and zoonotic risk, from monthly treatment for high-risk animals to less frequent treatment or faecal monitoring for low-risk ones.
Grazing management reduces exposure: alternating or co-grazing sheep with cattle (most gastrointestinal nematodes are host-specific, though Haemonchus contortus and Trichostrongylus axei cross between them), using hay or silage aftermaths and new leys as low-contamination pasture for susceptible young stock, avoiding wet fluke habitats or fencing them off, and avoiding overstocking. Good nutrition, especially metabolisable protein, improves both resistance and resilience and reduces the periparturient rise. Spores of the nematophagous fungus Duddingtonia flagrans fed to animals pass in the dung and trap developing larvae, reducing pasture contamination. Selection of rams with low estimated breeding values for egg count builds genetic resistance over years.
When drugs are used, use them well: dose to the weight of the heaviest animal in a group or weigh individually, use the correct route, check efficacy by egg count after treatment, use combination products of unrelated classes where licensed, and quarantine-treat purchased animals with effective drugs, hold them off pasture until eggs have passed (commonly 24–48 hours), then turn them onto pasture already carrying the resident worm population so survivors are diluted. Doses and withdrawal periods vary by product and country; confirm against the product label and your national formulary. Tick control likewise integrates acaricide use with endemic stability, tick-resistant breeds, pasture management and, where available, the Bm86 vaccine; mosquito and sandfly control with repellents protects dogs from heartworm and Leishmania.
An anaemic, weak lamb with submandibular oedema in late summer, from a flock with high egg counts, fits haemonchosis; the work-up is FAMACHA scoring of the group, packed cell volume and total protein in a few animals, a group egg count with larval culture, and necropsy of any casualty with an abomasal worm count. The control decision then depends on whether the drug class still works, so a FECRT is planned for the next treatment. Sudden deaths in sheep in autumn on wet pasture, with no fluke eggs in faeces, suggest acute fasciolosis; necropsy shows a swollen, haemorrhagic liver with tracts of immature flukes, and coproantigen or antibody tests and the timing of treatment with a drug active against immature flukes follow.
Diarrhoea in young calves in the first weeks of life commonly involves Cryptosporidium (modified Ziehl–Neelsen smear, antigen test or PCR), with coccidiosis a little later (OPG with species identification) and rotavirus, coronavirus and bacteria in the differential list. Coughing in first-season grazing calves in late summer suggests lungworm; Baermann on fresh faeces, and if needed bronchoalveolar lavage for larvae and eosinophils, confirm it. In a young horse with weight loss and acute diarrhoea in late winter or spring, larval cyathostominosis is likely even with a low egg count, because the disease is caused by mass emergence of encysted larvae; hypoalbuminaemia supports the diagnosis.
A new puppy with a pot belly and diarrhoea is assumed to carry Toxocara, because infection crosses the placenta and through milk, and disease can appear before eggs are passed; deworming begins early under national guidance, and owners receive zoonotic advice. A dog with chronic intermittent diarrhoea and a negative flotation should have a Giardia antigen test or zinc sulphate centrifugal flotations on samples from several days. A pruritic dog with crusted ear margins and elbows is investigated with multiple deep scrapings, the pinnal–pedal reflex and, where available, serology; if scrapings are negative and suspicion remains, a treatment trial with an appropriate licensed acaricide is reasonable.
For heartworm in endemic areas, dogs are screened annually with an antigen test combined with a microfilaria test (modified Knott's), and a positive antigen test in a low-risk dog is confirmed with a second test of a different format before treatment. A coughing dog from an area where Angiostrongylus vasorum occurs, with bleeding tendencies, is tested by Baermann and, where available, by a blood antigen test. A febrile, anaemic dog with haemoglobinuria after a tick bite in an endemic area needs a thin smear and PCR for Babesia; a dog from the Mediterranean or South America with weight loss, skin lesions, lymphadenopathy and proteinuria needs Leishmania serology (quantitative IFAT or ELISA), cytology of lymph node or bone marrow, and PCR.
Learn to do the core tests with your own hands: weigh and mix a McMaster count and calculate EPG from first principles, set up a sedimentation and a Baermann, make and stain a thin blood smear, do a deep and a superficial skin scraping, and run a modified Knott's test. For each test, write down what it detects, its detection limit and when it will be falsely negative. Learn the egg and blood-parasite morphology from an atlas until you can name the common ones on sight, and learn the Th2 and Th1 pathways well enough to explain self-cure, the periparturient rise and premunition. These are the questions in PG and PhD entrance papers and in practical exams.
Use parasitology to make decisions, not just diagnoses. In sheep and goat flocks, set up FAMACHA or Five Point Check training for the farmer, run group egg counts before treating, and do a FECRT on each drug class every few years. In horses, move clients to egg-count-based treatment. In small-animal practice, match deworming frequency to risk, screen for heartworm where it occurs, and give clear zoonotic advice to families with young children or pregnant women. Know which tests your laboratory runs, their turnaround and cost, and when to send samples for PCR or serology instead of repeating flotations.
At this stage you design and audit control programmes for many farms, a cooperative, a state laboratory or a corporate practice group. Decisions include which diagnostic tests a regional laboratory should offer and how to quality-assure them, how to monitor anthelmintic and acaricide resistance across districts, when to recommend live vaccines such as Theileria annulata or Babesia vaccines and how to manage their cold chain and reactions, and how to integrate dog deworming, meat inspection and public education in hydatid or cysticercosis control with public health colleagues. You also mentor younger vets in interpreting borderline results.
Research questions at this level include discovering and validating new antigens for vaccines and diagnostics, understanding the genetic basis of anthelmintic resistance beyond benzimidazoles, developing molecular tools that quantify species composition and resistance alleles from faeces, evaluating diagnostic tests without a gold standard (latent class models), modelling refugia-based strategies and climate effects on parasite epidemiology, and measuring the efficacy and cost-effectiveness of vaccines and selective breeding. Grant and policy work involves One Health programmes against zoonoses and advising regulators on resistance guidelines and test standards for trade.
Drug doses, withdrawal periods and treatment protocols are deliberately not given; they vary by product, species and country and belong to pharmacology and the disease manuals. The current WAAVP (2023) FECRT thresholds and sample-size rules are described in principle only and must be taken from the published guideline. Egg and oocyst dimensions and identification keys for L3 larvae and Eimeria species are summarised, not reproduced, and no images are included yet. Diagnostic performance figures (sensitivity, specificity) of named commercial kits are not given because they differ between kits, populations and studies. Test lists for international trade change; the WOAH Terrestrial Manual is the authority. Molecular methods are described in general terms; protocols, primers and sequencing pipelines are out of scope. Individual parasitic diseases are covered in the helminthology, protozoology, entomology and preventive-medicine manuals.
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.
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