Veterinary · Para-clinical · also MSVE VPP-311
The lesions of specific infectious diseases and poisonings, how to perform a post-mortem and dispatch samples, and the basics of clinical pathology.
In the exams: ICAR AICE-JRF core group 06; ICAR AIEEA-PG 13 (Pathology); ECFVG / NEB CPE necropsy section
Special pathology is the study of the lesions that particular infectious agents and poisons produce, and necropsy is the systematic post-mortem examination that finds them. Clinical pathology is the laboratory half of the same work: blood, cytology and urine read in the living animal. This subject assumes you already know general pathology (cell injury, inflammation, neoplasia) and systemic pathology (lesions organ by organ). Here the question is practical: this animal died or is ill, so how do I open it safely, which lesions point to which disease, which samples do I take, how do I keep them fit for the laboratory, and what do the laboratory numbers mean?
In practice a necropsy is often the cheapest and fastest diagnostic test a herd, flock or kennel can buy. A single well-done examination with correct samples can confirm a notifiable disease, settle an insurance or legal dispute, or stop an outbreak of poisoning. A badly done one can spread anthrax, destroy evidence, or send a laboratory tissue so autolysed that nothing can be read. The same logic governs clinical pathology: a result is only as good as the sample, the anticoagulant and the time it spent on the bench.
Exams everywhere test this subject in three ways. Indian postgraduate and research-fellowship papers (ICAR AIEEA-PG pathology, AICE-JRF core group) ask one-line recall of pathognomonic lesions, inclusion bodies, stains and preservatives. Licensing exams (NAVLE, BCSE, the PAVE and ECFVG clinical routes, the RCVS and Australasian exams) ask applied questions: interpret a haemogram, classify an effusion, choose a sample. The ECFVG and NEB clinical proficiency examinations include a hands-on necropsy section. Where law or practice differs by country, such as notifiable-disease lists, carcass disposal and the rules for legal post-mortems, this manual says so; always follow your national veterinary authority.
A necropsy begins with the history: species, age, sex, number sick and dead in the group, clinical signs, duration, treatment, feed and water changes, and time of death. The history decides both the risk and the samples. The first rule is that a carcass suspected of anthrax is not opened. Sudden death in cattle, sheep or wild herbivores, with dark unclotted blood from the nostrils, anus or vulva, incomplete rigor mortis and rapid bloating, must be treated as anthrax until a blood smear proves otherwise. Opening the carcass exposes Bacillus anthracis to air, where it forms spores that survive in soil for decades. The correct sample is a thin smear of blood taken from a superficial vein (ear or tail) or by needle, stained with polychrome methylene blue to show square-ended bacilli with a pink capsule (the M'Fadyean reaction); a swab may be sent for culture or PCR. Other zoonoses shape the method: rabies (never open the skull outside a suitable facility; send the head or brain), bovine spongiform encephalopathy and other prion diseases (brainstem at the obex), glanders, tuberculosis, brucellosis, psittacosis in birds and Q fever at parturition. Protective equipment is gloves (with cut-resistant gloves under latex where possible), apron or gown, boots, eye protection, and a mask or respirator when aerosol-generating zoonoses are suspected. The site must be cleanable and the carcass disposed of by the route national rules permit: rendering, incineration, or deep burial with lime.
Whatever the species, the order is the same: identification (tag, microchip, brand, tattoo, description), external examination (body condition, natural orifices, skin, feet, udder, wounds, parasites, post-mortem changes), then reflection of the skin and inspection of subcutis, superficial lymph nodes and muscles, then the abdominal cavity in situ, the thoracic cavity in situ, removal and examination of organs, and finally the head, brain, spinal cord, joints and bone marrow as the case requires. Look before you cut: note fluid in cavities, displacement, adhesions and the position of organs before anything is moved, because a torsion or displacement cannot be diagnosed once the organs have been lifted out. Pathology texts name four techniques. Virchow's method removes organs one by one. Rokitansky's method dissects organs largely in situ. Letulle's method removes the thoracic, cervical, abdominal and pelvic organs en masse as one block. Ghon's method removes them in functional blocks (thoracic and neck, coeliac organs, intestines, urogenital). Veterinary necropsies usually combine en bloc removal of the pluck (tongue, larynx, trachea, oesophagus, lungs and heart) with organ-by-organ examination of the abdomen. Describe each lesion by location, number, distribution, size, shape, colour, consistency and contour before you interpret it, and photograph it with a scale and case number.
Ruminants are placed in left lateral recumbency, right side up, so the rumen lies underneath and the abomasum, omasum, liver and intestines are accessible from the right. The rumen contents are examined for smell, pH, consistency and foreign bodies, and the reticulum for penetrating wire in traumatic reticuloperitonitis. The horse is usually placed in right lateral recumbency, left side up, which exposes the large colon, caecum base and spleen; colic cases demand careful in-situ inspection of intestinal position before anything is cut. Pigs are opened in dorsal recumbency. Dogs and cats are opened in dorsal recumbency, or in left lateral recumbency, with the limbs reflected so the carcass stays stable. Birds are wetted with disinfectant to control feathers and dust, placed on their back, and the skin and keel reflected; the bursa of Fabricius, thymus, caecal tonsils, proventriculus and sciatic nerves are examined as routine because many flock diseases are diagnosed there. Small laboratory animals and neonates can be opened whole and fixed whole after the cavities have been opened. In all species the brain is removed by sawing the skull, and the spinal cord by removing the dorsal laminae. Aborted fetuses are examined with the placenta, which is often the most informative tissue.
After death the body cools (algor mortis), blood settles by gravity into the dependent parts (livor mortis or hypostatic congestion), muscles stiffen (rigor mortis) as ATP is exhausted and actin-myosin cross-bridges lock, and then relax again as proteolysis proceeds. Autolysis is self-digestion by the cell's own enzymes; putrefaction is decomposition by bacteria, mostly gut clostridia and coliforms, which produce gas and hydrogen sulphide. The speed of all these changes depends on ambient temperature, body size, fleece or fat cover, fever before death and the rumen's fermentation, so large, well-covered ruminants in hot weather decompose fastest. Recognised post-mortem artefacts include hypostatic congestion of the down-side lung and kidney, haemoglobin imbibition (diffuse red staining of the intima of vessels and tissues as red cells lyse), bile imbibition (yellow-green staining of tissues next to the gall bladder), pseudomelanosis (green-black discoloration, usually of intestine and abdominal organs, from iron sulphide formed when hydrogen sulphide reacts with haemoglobin iron), gas bubbles in the liver and other organs (foamy or 'Swiss cheese' liver), sloughing of rumen and intestinal mucosa, softening of the kidney medulla and the brain, corneal clouding, and post-mortem bloat with prolapse of the rectum or vagina. Post-mortem clots are moist, shiny, elastic, unattached to the vessel wall and often separated into a dark red lower part and a yellow upper part ('currant jelly' and 'chicken fat' clots). A thrombus, by contrast, is dull, dry, friable, attached to the wall and may show laminations (lines of Zahn).
The key test is a vital reaction: haemorrhage, inflammation, oedema, fibrin or repair show that the tissue was alive when the change occurred. A rupture of the stomach, rumen, diaphragm or intestine that happened after death has clean edges without haemorrhage or fibrin; one that caused death has haemorrhage at the margins and contents mixed with blood or fibrin in the cavity. In bloat of ruminants the 'bloat line' is a useful sign: the cervical oesophagus is congested and haemorrhagic while the thoracic oesophagus at the thoracic inlet is pale, because the distended rumen compressed venous return before death; post-mortem gas does not produce this sharp demarcation. Congestion that follows gravity is hypostasis; congestion that ignores gravity is real. Some diseases are diagnosed only in fresh carcasses because autolysis mimics or hides their lesions: ovine enterotoxaemia, where the 'pulpy kidney' is itself largely autolytic change accelerated by the disease, so glucosuria and brain lesions are more reliable; and enteric diseases generally, since intestinal villi autolyse within hours. The practical rule is that a carcass examined late must be interpreted cautiously and the report should state the degree of autolysis.
A medico-legal or vetero-legal necropsy is done when the result may be used in court, an insurance claim, a welfare prosecution or a dispute between buyer and seller. It differs from a diagnostic necropsy in its process, not its anatomy. The request should be written and come from the proper authority (police, court or insurer, depending on the country). The animal must be identified beyond doubt and the identification recorded, including ear tags, which insurers usually require to be kept. Everything is documented as it is found: photographs with scale and case number, contemporaneous notes, and a diagram of wounds. Samples are collected in clean containers, sealed, labelled and signed, and a chain of custody record shows every person who held them. Common questions are the cause and manner of death (disease, accident, poisoning, deliberate injury), the time since death, whether wounds were inflicted before or after death, and the species of origin of meat. Time since death is estimated only as a range from body temperature, rigor, livor, decomposition, stomach contents and insect colonisation, and the range widens with time. Wounds are described by type (incised, lacerated, punctured, contused, gunshot, bite), and entry gunshot wounds are usually smaller with inverted margins, exit wounds larger and everted. Lightning stroke leaves singed hair, sometimes branching burn marks and a carcass near a tree, fence or water. Suspected poisoning requires frozen stomach or rumen contents, liver, kidney, urine, blood and samples of feed and water, kept without preservative. Species of meat is identified by immunological or DNA (PCR) methods. The veterinarian writes the report as an expert witness, keeping observation separate from opinion; the relevant evidence law differs by country.
The sample must match the question. For histopathology, take slices of lesion together with adjacent normal tissue, no thicker than about 1 cm (thinner is better, around 0.5 cm), and place them in 10% neutral buffered formalin at about ten times the tissue volume. '10% formalin' means one part commercial formalin (a 37 to 40% formaldehyde solution) in nine parts water, so it contains roughly 4% formaldehyde. Formalin penetrates slowly, so thick pieces autolyse in the centre. Brain can be fixed whole or halved; for many neurological cases one half goes fresh or frozen and one half into formalin. For bacteriology, collect aseptically, before antibiotic treatment where possible, in sterile containers or swabs in transport medium, and keep chilled, not frozen, for delivery within a day or two. Intestinal loops are tied off at both ends. For virology, tissues are chilled if they arrive within about 48 hours or frozen (ideally very low temperature) for longer delays, in viral transport medium or, where a cold chain is not available, 50% buffered glycerol saline, which preserves many viruses but is not suitable for bacterial culture of most agents. For toxicology, take large samples of liver, kidney, stomach or rumen contents, urine, blood and suspect feed or water, in separate clean glass or plastic containers without preservative, and freeze them, except whole blood, which is refrigerated. For parasitology, fresh faeces are chilled; blood smears are air-dried and fixed in methanol. For serology, clotted blood or separated serum is sent chilled; a paired sample taken two to four weeks apart that shows a four-fold rise in titre indicates active infection. For molecular tests, fresh or frozen tissue is best, and formalin-fixed tissue gives degraded but often usable DNA.
Diagnostic specimens travel under the UN model regulations for dangerous goods, which national postal and courier rules follow. The basic principle is triple packaging: a leak-proof primary container, a leak-proof secondary container with enough absorbent material to take up all the liquid, and a rigid outer package. Most diagnostic specimens are shipped as 'Biological Substance, Category B' under UN 3373. Material known or strongly suspected to contain an agent in the list of Category A infectious substances, such as foot-and-mouth disease virus cultures or high-consequence zoonotic agents, is shipped as UN 2814 (affecting humans) or UN 2900 (affecting animals only), by trained shippers with special packaging and documents. Samples of a suspected notifiable disease normally go only to the laboratory designated by the national veterinary authority, often after official notification. Each container is labelled with owner, animal, tissue and date in waterproof ink. The submission form carries the history, clinical signs, necropsy findings, tentative diagnosis, treatment given and the tests requested; a laboratory cannot choose the right test without it. Formalin must not travel in the same compartment as fresh tissues for culture, because its fumes kill microbes. Samples should arrive early in the week, not on a Friday or before a holiday.
A routine paraffin section passes through a fixed sequence. Fixation, usually in neutral buffered formalin, cross-links proteins so that the tissue stops autolysing and keeps its structure. The fixed tissue is trimmed into cassettes, then dehydrated through graded alcohols up to absolute alcohol, because paraffin and water do not mix. It is then cleared in an agent such as xylene, which is miscible with both alcohol and paraffin and makes the tissue translucent. Infiltration in molten paraffin wax and embedding in a block follow. Sections about 4 to 5 micrometres thick are cut on a microtome, floated on a warm water bath, picked up on glass slides, dewaxed, rehydrated and stained. Haematoxylin and eosin (H&E) is the routine stain: haematoxylin, with a mordant, stains nuclei and other basophilic (acidic) structures blue-purple, and eosin stains cytoplasm, collagen and other eosinophilic structures pink. The section is then dehydrated again, cleared and covered with a coverslip in mounting medium. Frozen sections cut on a cryostat skip dehydration and wax, so they keep lipids and many enzymes; they are used for fat stains and for rapid diagnosis during surgery. Bone and teeth must first be decalcified, with acids such as formic or nitric acid (fast but damaging to staining and nucleic acids if overdone) or with EDTA (slow but gentle, better for immunohistochemistry and PCR). Special fixatives exist: glutaraldehyde for electron microscopy, Bouin's and Davidson's solutions for eye and testis, and Carnoy's fluid where glycogen or nucleic acids matter.
Special stains answer specific questions that H&E cannot. Ziehl-Neelsen stains acid-fast organisms red: mycobacteria (tuberculosis, paratuberculosis), Nocardia (partially acid-fast), and the acid-fast intranuclear inclusions of lead poisoning. Gram stains separate bacteria in tissue. Periodic acid-Schiff (PAS) stains glycogen, mucins, basement membranes and fungal walls magenta, and diastase digestion removes glycogen to confirm it. Grocott's methenamine silver stains fungi and Pneumocystis black. Warthin-Starry silver shows spirochaetes such as Leptospira and small curved bacteria such as Lawsonia intracellularis. Congo red stains amyloid, which shows apple-green birefringence in polarised light. Perls' Prussian blue shows haemosiderin (ferric iron) blue. Von Kossa stains calcium salts black, and alizarin red stains calcium red. Masson's trichrome stains collagen blue or green and muscle red, to measure fibrosis. Oil red O and Sudan stains show neutral fat, but only on frozen sections, because processing dissolves lipid. Luxol fast blue stains myelin. Toluidine blue shows mast cell granules metachromatically. Immunohistochemistry uses labelled antibodies to locate a specific antigen in fixed tissue, for example rabies virus, prion protein, or a tumour marker; in-situ hybridisation locates nucleic acid; PCR can be run on fixed, embedded tissue.
Several artefacts are common in veterinary material. Autolysis produces loss of nuclear detail, detachment of epithelium and diffuse eosinophilia, and in the gut it begins at the villous tips within hours. Formalin pigment (acid formaldehyde haematin) is a brown-black, birefringent granular deposit in blood-rich tissue fixed in unbuffered formalin; it can be mistaken for haemosiderin or parasite pigment, and it is avoided by using buffered formalin. Freezing before fixation creates ice-crystal holes. Crushing with forceps squeezes nuclei into dark streaks, especially in lymphoid tissue. Delayed fixation and thick slices produce a well-fixed rim with an autolysed centre. Bacterial overgrowth after death produces colonies without any inflammatory reaction around them, which distinguishes them from true infection. Knife chatter, folds and bubbles are section-cutting faults. Contamination by 'floaters', fragments of another case picked up from the water bath, can put tumour cells or organisms on the wrong slide, so any finding that does not fit the case should be checked on a new section. Poor dehydration or clearing leaves soft, crumbling blocks that cut badly. Recognising artefacts matters because a pathologist who mistakes autolysis for necrosis, or formalin pigment for haemosiderin, reaches a wrong diagnosis on a well-stained slide.
Anthrax (Bacillus anthracis) causes peracute septicaemia in ruminants: sudden death, dark tarry blood that does not clot, bleeding from natural orifices, incomplete rigor, rapid bloating and, if the carcass has been opened by mistake, a greatly enlarged soft dark spleen. Pigs and dogs, more resistant, show oedematous swelling of the throat and haemorrhagic lymphadenitis. Haemorrhagic septicaemia (Pasteurella multocida, serotype B:2 in Asia and E:2 in Africa) affects cattle and especially buffalo, with hot painful oedema of the throat, dewlap and brisket, petechiae on serosae and a short fatal course. Blackquarter or black leg (Clostridium chauvoei) strikes young, well-grown cattle, typically between about six months and two years: a heavy muscle mass is swollen, crepitant, dark red to black, dry and spongy in the centre with gas, and smells of rancid butter. Malignant oedema (C. septicum and other clostridia) follows wounds and produces gelatinous oedema with less gas. Enterotoxaemia of sheep and goats (C. perfringens type D, epsilon toxin) follows sudden access to rich feed; lesions include excess pericardial fluid with a fibrin clot, petechiae, glucosuria, rapid autolysis of the kidney ('pulpy kidney') and, in subacute cases in lambs, focal symmetrical encephalomalacia. C. novyi type B causes black disease (infectious necrotic hepatitis) where migrating liver flukes create necrotic foci, and C. haemolyticum causes bacillary haemoglobinuria, with a large anaemic infarct in the liver, haemolysis and red urine. Braxy (C. septicum) is a haemorrhagic abomasitis of sheep in cold weather. Tetanus and botulism produce no specific gross lesions; diagnosis rests on signs, toxin detection and exclusion.
Bovine tuberculosis (Mycobacterium bovis) produces granulomas (tubercles) with caseous, often calcified, yellow-white centres in the lungs and the bronchial, mediastinal, retropharyngeal and mesenteric lymph nodes. Histologically, a caseous core is surrounded by epithelioid macrophages, Langhans-type multinucleated giant cells, lymphocytes and a fibrous capsule; acid-fast bacilli may be scarce in cattle. Nodules on the pleura and peritoneum are called 'pearl disease'. Johne's disease (M. avium subsp. paratuberculosis) thickens and corrugates the ileal and caecal mucosa into transverse folds without ulceration, with corded, thickened lymphatics and enlarged mesenteric nodes; sheets of epithelioid macrophages full of acid-fast bacilli fill the lamina propria. Caseous lymphadenitis of sheep and goats (Corynebacterium pseudotuberculosis) produces abscesses with concentric 'onion-ring' laminations in lymph nodes and lungs. Actinomycosis ('lumpy jaw', Actinomyces bovis) is a rarefying pyogranulomatous osteomyelitis of the mandible or maxilla; actinobacillosis ('wooden tongue', Actinobacillus lignieresii) affects soft tissue. Both contain club-shaped radiating 'sulphur granules' (Splendore-Hoeppli material). Glanders (Burkholderia mallei) in equids forms nodules and ulcers on the nasal septum that heal as star-shaped scars, pulmonary nodules and cutaneous 'farcy buds' along lymphatics; it is a notifiable zoonosis in most countries.
Contagious bovine pleuropneumonia (Mycoplasma mycoides subsp. mycoides) gives a fibrinous pleuropneumonia in which distended interlobular septa give a 'marbled' cut surface, and chronic cases leave encapsulated necrotic sequestra. Contagious caprine pleuropneumonia (M. capricolum subsp. capripneumoniae) is a fibrinous pleuropneumonia of goats. Pneumonic mannheimiosis ('shipping fever', Mannheimia haemolytica) is a cranioventral fibrinous bronchopneumonia with coagulative necrosis bordered by streaming 'oat cells'. Brucellosis causes placentitis with a thickened, leathery intercotyledonary placenta and necrotic cotyledons, orchitis and epididymitis in males, and carpal hygroma in cattle. Leptospirosis causes haemolysis with red urine and jaundice in calves, abortion, and interstitial nephritis with white foci in the kidney. Listeriosis (Listeria monocytogenes) in ruminants causes brainstem microabscesses with perivascular cuffs, as well as abortion and septicaemia. Swine erysipelas (Erysipelothrix rhusiopathiae) produces raised rhomboid 'diamond' skin lesions, vegetative valvular endocarditis and chronic arthritis. Salmonellosis produces fibrinonecrotic enterocolitis and, in pigs, button ulcers and small necrotic foci in the liver ('paratyphoid nodules'). Oedema disease of weaned pigs (Shiga toxin-producing Escherichia coli) causes oedema of the eyelids, forehead, gastric submucosa and mesocolon. Strangles (Streptococcus equi subsp. equi) produces abscesses in the submandibular and retropharyngeal lymph nodes; spread to other organs is called 'bastard strangles', and purpura haemorrhagica is an immune-complex vasculitis that may follow.
Foot-and-mouth disease (an aphthovirus) produces vesicles and erosions on the tongue, dental pad, gums, interdigital skin, coronary band and teats; in young calves, lambs and piglets it can kill through myocarditis, seen as pale grey-yellow streaks in the myocardium ('tiger heart'). Vesicular lesions in pigs cannot be told apart by eye from swine vesicular disease or vesicular stomatitis, so laboratory confirmation is essential. Peste des petits ruminants (a morbillivirus) in goats and sheep causes necrotic stomatitis, erosions along the tract, congestion along the longitudinal folds of the colon and rectum ('zebra stripes'), and bronchointerstitial pneumonia with multinucleated syncytial cells carrying eosinophilic intracytoplasmic and intranuclear inclusions. Rinderpest caused similar lesions in cattle and was declared eradicated worldwide in 2011. Bovine viral diarrhoea virus causes mucosal disease in persistently infected cattle, with erosions and ulcers from mouth to colon and necrosis over Peyer's patches, and infection of the fetus in mid-gestation can cause cerebellar hypoplasia and ocular defects. Malignant catarrhal fever (ovine herpesvirus 2 from sheep, alcelaphine herpesvirus 1 from wildebeest) produces bilateral corneal opacity, erosions, lymph node enlargement and a lymphocytic vasculitis in many organs. Infectious bovine rhinotracheitis (bovine herpesvirus 1) causes necrotising rhinotracheitis, pustular vulvovaginitis, and abortion with small necrotic foci in the fetal liver. Bluetongue (an orbivirus spread by Culicoides) causes facial oedema, a cyanotic tongue in some sheep, coronitis and haemorrhage in the wall of the pulmonary artery at its base. Sheep pox, goat pox and lumpy skin disease (capripoxviruses) produce skin nodules that may involve the full skin thickness, with similar nodules in the respiratory and digestive tracts. Orf (contagious ecthyma, a parapoxvirus) causes proliferative scabs on the lips of lambs and kids and is zoonotic.
Classical swine fever (a pestivirus) produces petechiae in the kidney cortex ('turkey-egg kidney'), infarcts at the margin of the spleen, peripheral haemorrhage in lymph nodes and, in chronic cases, button ulcers in the colon. African swine fever (an asfivirus) produces similar haemorrhages, but the acute form typically has a greatly enlarged, dark, friable spleen and lymph nodes that look like blood clots; only the laboratory separates the two. Equine infectious anaemia (a lentivirus) causes haemolytic anaemia, splenomegaly and haemosiderin in macrophages. African horse sickness produces a pulmonary form with massive lung oedema and froth, and a cardiac form with oedema of the supraorbital fossae and head. Canine distemper (a morbillivirus) causes pneumonia, enteritis, hyperkeratosis of the nose and footpads ('hard pad'), enamel hypoplasia in survivors, and demyelinating encephalomyelitis, with eosinophilic intracytoplasmic and intranuclear inclusions in epithelium of the bladder, bronchi and stomach and in glial cells. Canine parvovirus 2 destroys the intestinal crypt epithelium, leaving dilated crypts and collapsed villi, and depletes lymphoid tissue; infection of neonates can cause myocarditis. Infectious canine hepatitis (canine adenovirus 1) produces hepatic necrosis, oedema of the gall bladder wall, and large intranuclear inclusions in hepatocytes and endothelium; corneal oedema ('blue eye') can follow recovery. Feline infectious peritonitis, a disease produced by a mutated feline coronavirus, causes a pyogranulomatous vasculitis; the effusive form fills cavities with viscous, straw-coloured, protein-rich fluid, and the dry form produces granulomas in kidneys, eyes and brain.
Rabies (a lyssavirus) usually leaves no specific gross lesion. Histologically there is a non-suppurative encephalomyelitis with perivascular lymphocytic cuffs and glial nodules, and eosinophilic intracytoplasmic Negri bodies in neurons, most easily found in the hippocampus in carnivores and in the cerebellar Purkinje cells in ruminants. Absence of Negri bodies does not exclude rabies; the fluorescent antibody test on fresh brain is the standard test, with immunohistochemistry and PCR as alternatives. Transmissible spongiform encephalopathies (scrapie, bovine spongiform encephalopathy, chronic wasting disease) produce no gross lesions and no inflammation; the microscopic lesion is vacuolation of the neuropil and neuronal cell bodies in the grey matter of the brainstem, with the obex as the key site, confirmed by detection of abnormal prion protein. Pseudorabies (Aujeszky's disease, suid herpesvirus 1) causes intense pruritus and death in cattle, dogs and cats, and non-suppurative encephalitis in piglets. Other viral encephalitides with non-suppurative lesions include West Nile and other arboviral infections of horses, bovine herpesvirus 5 in calves, and the 'old dog encephalitis' form of distemper. In all of them the pathologist describes the distribution (grey or white matter, brainstem or cortex) and the cell types, and relies on immunohistochemistry, PCR or virus isolation for the agent, because gross examination of the brain is usually normal.
Most poisonings leave few specific gross lesions, so the pathologist's job is to recognise the few that do, exclude infection and send the right samples. Lead poisoning, common in cattle that lick old batteries, paint or engine oil, may leave the offending material in the reticulum; microscopically there are acid-fast intranuclear inclusions in renal tubular epithelium, and cerebrocortical laminar necrosis in some cases. Blood smears may show basophilic stippling. Chronic copper poisoning, especially in sheep, accumulates copper in the liver until a stress releases it; a haemolytic crisis follows with jaundice, a swollen yellow-orange liver, port-wine urine, and dark 'gunmetal' kidneys stained by haemoglobin. Inorganic arsenic causes severe gastroenteritis with a red, velvety abomasal or gastric mucosa. Chronic fluorosis mottles and wears the teeth and causes bony exostoses. Nitrate and nitrite poisoning turns haemoglobin into methaemoglobin, giving chocolate-brown blood. Cyanide blocks cellular use of oxygen, so venous blood stays bright cherry red, and the rumen may smell of bitter almonds (not everyone can smell it). Oxalate poisoning from plants such as Halogeton, and ethylene glycol poisoning in dogs and cats, deposit birefringent calcium oxalate crystals in renal tubules; ethylene glycol also produces the monohydrate crystals seen in urine. Anticoagulant rodenticides cause haemorrhage into body cavities, joints and tissues without a traumatic source. Zinc phosphide releases phosphine, and the stomach contents may have a garlic or fish-like odour; pulmonary oedema is common.
Pyrrolizidine alkaloids (from Senecio, Crotalaria, Heliotropium and others) are activated in the liver and cause chronic liver disease: megalocytosis (giant hepatocytes with large nuclei), fibrosis and bile duct proliferation, with hepatogenous photosensitisation and hepatic encephalopathy. Aflatoxins (Aspergillus flavus and A. parasiticus) cause acute centrilobular necrosis and haemorrhage in high doses, and in chronic exposure megalocytosis, bile duct hyperplasia and fibrosis; young animals, ducklings and turkey poults are especially sensitive, and aflatoxin is a recognised liver carcinogen. Lantana camara, a weed across the tropics, causes intrahepatic cholestasis with jaundice, a distended gall bladder, photosensitisation and rumen stasis. Bracken fern (Pteridium) in cattle causes an acute syndrome of bone marrow aplasia with haemorrhage, and on long exposure enzootic haematuria with bladder tumours; in horses and pigs its thiaminase causes thiamine deficiency. Ionophore coccidiostats such as monensin cause necrosis of cardiac and skeletal muscle; horses are far more sensitive than cattle or poultry. Gossypol from cottonseed damages the heart of pigs and pre-ruminant calves. Ergot alkaloids (Claviceps purpurea) cause vasoconstriction and dry gangrene of the extremities. Fumonisins (Fusarium) cause leukoencephalomalacia in horses and pulmonary oedema in pigs. Zearalenone causes swelling of the vulva and vaginal prolapse in gilts. Ochratoxin causes nephropathy in pigs and poultry.
Organophosphate and carbamate insecticides, strychnine, urea and many plant neurotoxins kill without characteristic lesions. Suspicion comes from the history (access, several animals at once, a new batch of feed), the clinical signs and the exclusion of infection; proof comes from the laboratory. For organophosphates and carbamates, cholinesterase activity is measured in chilled whole blood or in one frozen half of the brain, and the pesticide itself is sought in stomach or rumen contents. Urea poisoning is supported by an ammonia smell and high pH of rumen contents, which should be frozen at once because ammonia is lost. In every suspected poisoning, sample the source as well as the animal, keep samples separate, do not add preservative, and if legal action is possible, follow the chain-of-custody steps of a vetero-legal necropsy. The pathology report should list what was found, what was excluded (for example, infectious causes excluded by culture and histology), and what analyses are pending, so that a negative laboratory result is not mistaken for proof that no poison was involved. Many laboratories screen only for the toxins requested, so the history and suspicion must be written on the submission form.
The complete blood count needs blood in EDTA, mixed gently and analysed or smeared within a few hours; an underfilled tube has too much EDTA, which shrinks cells and lowers the packed cell volume. Heparin suits many birds and reptiles, whose blood can haemolyse in EDTA (notably chelonians and some birds), though it stains smears poorly. Sodium citrate at one part to nine parts blood is used for coagulation tests, and fluoride-oxalate for glucose. The red cell indices are calculated: mean cell volume (MCV, fL) = PCV (%) × 10 ÷ RBC (×10^12/L); mean cell haemoglobin (MCH, pg) = Hb (g/dL) × 10 ÷ RBC (×10^12/L); mean cell haemoglobin concentration (MCHC, g/dL) = Hb (g/dL) × 100 ÷ PCV (%). As a check, Hb in g/dL is roughly one-third of the PCV in mammals. Anaemia is classed as regenerative or non-regenerative. Regeneration is shown by reticulocytes (counted with new methylene blue), polychromasia, anisocytosis and often a raised MCV with lowered MCHC. It takes about three to five days after acute blood loss or haemolysis for a reticulocyte response to appear, so an early anaemia can look non-regenerative. The corrected reticulocyte percentage adjusts for anaemia: observed % × patient PCV ÷ normal PCV. Horses do not release reticulocytes into the blood even when regenerating, so bone marrow or serial PCV and MCV are used. Ruminants release reticulocytes only in marked anaemia, and cats have aggregate and punctate reticulocytes, of which the aggregate count reflects current regeneration. Blood loss anaemia of long duration becomes iron deficient, with microcytic, hypochromic cells. Non-regenerative anaemia follows chronic inflammation, chronic kidney disease (lack of erythropoietin) and marrow disease. Spherocytes and autoagglutination suggest immune-mediated haemolytic anaemia in dogs; Heinz bodies and eccentrocytes indicate oxidative damage (onions, garlic, paracetamol in cats, red maple in horses); basophilic stippling suggests lead poisoning or strong regeneration in ruminants.
Absolute counts, not percentages, are interpreted: absolute count = total WBC × percentage ÷ 100. Nucleated red cells are counted as white cells by many methods, so the corrected WBC = counted WBC × 100 ÷ (100 + nRBC per 100 WBC). Species differ: dogs, cats and horses normally have more neutrophils than lymphocytes, whereas cattle, sheep and goats normally have more lymphocytes, and cattle often respond to acute inflammation with neutropenia and a left shift. A stress (corticosteroid) leukogram is mature neutrophilia, lymphopenia, eosinopenia and, in dogs, monocytosis. A physiological (adrenaline) leukogram, seen in excited cats and horses, shows neutrophilia with lymphocytosis. A left shift is an increase in immature (band) neutrophils; it is 'degenerative' when bands outnumber segmented neutrophils or the total neutrophil count is normal or low, which signals overwhelming demand and a worse prognosis. Toxic change (cytoplasmic basophilia, foamy vacuoles, Döhle bodies) reflects rapid marrow production. Eosinophilia suggests parasitism or allergy. Platelets clump easily in cats, producing false thrombocytopenia, so always check the smear. Anticoagulant rodenticides prolong prothrombin time first, because factor VII has the shortest half-life. Plasma fibrinogen, measured by heat precipitation, is a useful inflammation marker in cattle and horses. Stained thin smears show blood parasites: Babesia as piroplasms inside red cells, Anaplasma marginale as dots at the red cell margin, Theileria piroplasms in red cells and schizonts ('Koch's blue bodies') in lymphocytes and lymph node aspirates, Trypanosoma evansi free in plasma (concentrate by buffy coat), Ehrlichia canis morulae in monocytes and Anaplasma phagocytophilum in neutrophils, and Hepatozoon gamonts in neutrophils.
Cytology examines individual cells rather than tissue architecture, and it is fast, cheap and usually safe. Fine needle aspiration uses a fine needle (commonly 22 to 25 gauge) moved through a mass several times, with or without suction, and the material is expelled onto a slide and spread gently; heavy pressure ruptures cells. Impression smears are made by blotting a cut surface or ulcer and touching it to the slide. Scrapings suit firm lesions that exfoliate poorly. Fluids (effusions, cerebrospinal fluid, synovial fluid, washes) are examined for cell count, total protein and a smear, concentrated by centrifugation or cytocentrifuge when cell numbers are low. Air-dried smears are stained with a Romanowsky-type stain such as Wright's, Giemsa or a rapid aqueous variant; the rapid stains sometimes fail to stain mast cell granules, which can make a mast cell tumour look like another round cell tumour. Slides for cytology must be kept away from formalin fumes, which spoil staining. The first decision on any smear is whether it is inflammatory, hyperplastic or neoplastic, and whether the sample is adequate at all.
Inflammation is described by its predominant cell. Neutrophilic inflammation with degenerate neutrophils (swollen, pale nuclei) suggests bacterial infection, especially when bacteria are seen inside the cells. Macrophage-rich (granulomatous or pyogranulomatous) inflammation suggests mycobacteria, fungi, foreign bodies or feline infectious peritonitis. Eosinophilic inflammation suggests parasites, allergy or some tumours such as mast cell tumour. Neoplastic cells are grouped into three types. Epithelial cells exfoliate well, in cohesive clusters or sheets, and have distinct borders. Mesenchymal cells exfoliate poorly, come singly or in loose aggregates, and are spindle or stellate with tails of cytoplasm. Round (discrete) cells exfoliate well as individual round cells; this group includes lymphoma, mast cell tumour (purple cytoplasmic granules), histiocytoma (common in young dogs, often regresses), plasma cell tumour and transmissible venereal tumour (round cells with clear punctate cytoplasmic vacuoles, spread by coitus in dogs). Nuclear criteria of malignancy include anisokaryosis, a high nucleus-to-cytoplasm ratio, coarse chromatin, large or multiple or angular nucleoli, abnormal mitoses, nuclear moulding and multinucleation. Several criteria should be present before calling a population malignant, and inflammation can make benign cells look atypical. Histopathology remains the standard for grading and margins. In lymph node aspirates, a monotonous population in which most cells are large lymphocytes supports lymphoma, whereas a mixture of small lymphocytes with some large cells and plasma cells suggests a reactive node.
Effusions are classified by total protein and total nucleated cell count into transudates (low protein, low cells), modified transudates (intermediate) and exudates (high protein, high cells); the cut-off values differ between textbooks and laboratories, and many clinical pathologists now classify by mechanism instead. A septic exudate contains degenerate neutrophils with intracellular bacteria and needs surgical and antimicrobial treatment. The effusion of feline infectious peritonitis is straw-coloured, viscous and high in protein with a low albumin to globulin ratio, yet has relatively few cells, mostly non-degenerate neutrophils and macrophages. Chylous effusion is milky and has a triglyceride concentration higher than that of serum. Haemorrhagic effusion from a ruptured splenic haemangiosarcoma contains erythrophagocytosis and no platelets. Vaginal cytology in the bitch tracks the cycle: superficial, cornified cells rise to nearly all of the epithelial cells during oestrus, and a sudden return of intermediate and parabasal cells with neutrophils marks the onset of dioestrus. Ear cytology shows Malassezia as small peanut-shaped yeasts. Bone marrow cytology reports the myeloid to erythroid (M:E) ratio: a high ratio with a normal cell mass suggests myeloid hyperplasia or erythroid hypoplasia, and a low ratio suggests erythroid hyperplasia.
Urine can be collected by free catch (midstream), catheterisation, or cystocentesis. Cystocentesis, a needle passed through the abdominal wall into the bladder, avoids contamination from the urethra and genital tract and is the method of choice for bacterial culture in dogs and cats; it may add a few red cells. Urine should be examined within about an hour of collection or refrigerated; on standing, bacteria multiply, pH rises, casts and cells break down, and cooling can precipitate crystals that were not present in the animal, so a refrigerated sample should be warmed to room temperature and crystals interpreted with care. Colour and clarity are noted: horse urine is normally cloudy and mucoid because of calcium carbonate crystals and mucus, and rabbit urine is often cloudy and may be red or orange from plant pigments that can be mistaken for blood. Urine specific gravity is measured with a refractometer; the dipstick pad for specific gravity is unreliable in animals. Glomerular filtrate has a specific gravity of about 1.008 to 1.012, so urine in this range is isosthenuric, below it hyposthenuric (actively diluted), and above it concentrated. Commonly used thresholds for adequate concentrating ability are above about 1.030 in dogs, 1.035 in cats and 1.025 in horses and cattle, though figures vary slightly between texts. An azotaemic animal with poorly concentrated urine suggests renal azotaemia, whereas azotaemia with concentrated urine suggests a pre-renal cause.
Urine pH is usually acidic in carnivores and alkaline in herbivores. Cattle with abomasal displacement or obstruction may pass acidic urine despite a metabolic alkalosis ('paradoxical aciduria'), because the kidney conserves sodium and potassium at the expense of hydrogen ions. The protein pad mainly detects albumin and gives false positives in highly alkaline urine, so protein is quantified by the urine protein to creatinine (UPC) ratio, interpreted only when the sediment is inactive. Under the International Renal Interest Society (IRIS) scheme, a UPC below 0.2 is non-proteinuric in dogs and cats, and proteinuria is above 0.5 in dogs and above 0.4 in cats, with the values between being borderline. Glucose appears in urine when blood glucose exceeds the renal threshold (diabetes mellitus, stress hyperglycaemia in cats) or when tubules fail to reabsorb it (proximal tubular disease, and the glucosuria of ovine enterotoxaemia). Ketone pads using nitroprusside detect acetoacetate and acetone but not beta-hydroxybutyrate, the main ketone in bovine ketosis, so they can underestimate ketosis. Small amounts of bilirubin are normal in concentrated dog urine, especially in males, but any bilirubinuria in a cat is abnormal. A positive blood pad has three causes: haematuria (red cells visible in the sediment, supernatant clears on spinning), haemoglobinuria (pink plasma, from haemolysis) and myoglobinuria (clear plasma, from muscle damage). The leucocyte pad misses many cases in dogs and gives false positives in cats, and the nitrite pad is unreliable in animals; sediment and culture are used instead.
Sediment from a standard volume of centrifuged urine is examined unstained or stained. Casts are moulds of the renal tubules made of mucoprotein: a few hyaline or granular casts may be normal, but many granular, cellular or epithelial casts indicate active tubular injury, and red or white cell casts locate bleeding or inflammation in the kidney itself. Casts dissolve in dilute or alkaline urine. Struvite (magnesium ammonium phosphate) crystals are colourless prisms like coffin lids and form in alkaline urine. Calcium oxalate dihydrate crystals are small squares with an 'envelope' cross; calcium oxalate monohydrate crystals are picket-fence or dumbbell shaped and, in the right history, strongly suggest ethylene glycol poisoning. Ammonium biurate crystals ('thorn apples') suggest liver dysfunction such as a portosystemic shunt, and uric acid or urate crystals are common in Dalmatian dogs, which have a breed defect in urate handling. Hexagonal cystine crystals indicate inherited cystinuria. Bilirubin crystals are yellow needles. Bacteria seen in sediment should be confirmed by culture of a cystocentesis sample, since contaminants and debris look alike.
Laboratory rodents are kept free of specified pathogens because even subclinical infection alters research results. Health status is checked by regular monitoring, often of sentinel animals exposed to soiled bedding, following schemes such as the FELASA recommendations in Europe. Important mouse infections include mouse hepatitis virus (a coronavirus causing enteritis in infant mice and hepatitis or encephalitis in others), Sendai virus (a paramyxovirus causing pneumonia), ectromelia or mousepox (an orthopoxvirus causing foot swelling, gangrene of extremities and necrosis of liver and spleen with intracytoplasmic inclusions), and pinworms (Syphacia and Aspiculuris). Murine respiratory mycoplasmosis (Mycoplasma pulmonis) causes rhinitis, otitis media and chronic bronchopneumonia with bronchiectasis and abscesses, most severely in rats. Rats also suffer sialodacryoadenitis, a coronavirus infection that swells the cervical salivary glands and inflames the eyes. Red 'bloody' tears in rats (chromodacryorrhoea) are porphyrin pigment from the Harderian gland and are a sign of stress or illness, not bleeding. Tyzzer's disease (Clostridium piliforme) affects mice, rats, gerbils, hamsters, rabbits and foals, causing necrotic foci in the liver, intestine and heart, with bundles of long filamentous bacilli in hepatocytes at the edge of the lesions, shown by silver, Giemsa or PAS stains. Hamsters develop proliferative ileitis ('wet tail') from Lawsonia intracellularis. Rodents can carry zoonoses such as lymphocytic choriomeningitis virus, hantaviruses and the rat-bite fever agent Streptobacillus moniliformis.
Pasteurella multocida in rabbits causes 'snuffles' (rhinitis), otitis media with head tilt, pneumonia, abscesses and genital infections. Head tilt is also caused by Encephalitozoon cuniculi, a microsporidian that produces granulomatous encephalitis, interstitial nephritis and lens rupture with uveitis. Rabbit haemorrhagic disease (a lagovirus) causes acute hepatic necrosis, disseminated intravascular coagulation and sudden death. Myxomatosis (a leporipoxvirus spread by fleas and mosquitoes) causes gelatinous skin swellings around the head and genitals. Hepatic coccidiosis (Eimeria stiedae) produces white nodules and dilated, hyperplastic bile ducts full of oocysts, and intestinal Eimeria species cause diarrhoea in young rabbits. Guinea pigs, like primates, cannot make vitamin C because they lack the enzyme L-gulonolactone oxidase; deficiency (scurvy) causes haemorrhage around joints and gums, enlarged costochondral junctions and poor wound healing. Guinea pigs and hamsters are very sensitive to antibiotics with a mainly Gram-positive spectrum (for example penicillins, lincosamides and macrolides given by mouth), which disturb the gut flora and allow a fatal Clostridium difficile or clostridial typhlocolitis. Guinea pigs also get cervical lymphadenitis ('lumps') from Streptococcus zooepidemicus and pneumonia from Bordetella bronchiseptica. Macaques carry Macacine herpesvirus 1 (B virus), which is mild in monkeys but can cause fatal encephalitis in people bitten or scratched; tuberculosis and measles are major threats to primate colonies.
Wildlife pathology serves conservation, public health and livestock disease control at once. Carcasses are often found late and far from a laboratory, so the finder records location (GPS), date, habitat and number of dead, photographs the scene, and handles every carcass as a possible zoonotic source: anthrax, rabies, highly pathogenic avian influenza, tuberculosis, brucellosis, and bat-borne henipaviruses. Sampling and export of tissues from protected species may need national and CITES permits. Many wild species are reservoirs of livestock infections. Bovine tuberculosis is maintained in badgers in the United Kingdom and Ireland, brushtail possums in New Zealand, African buffalo in parts of southern Africa and white-tailed deer in parts of North America. African buffalo carry the SAT serotypes of foot-and-mouth disease virus. Rabies is maintained by foxes, raccoons, skunks, mongooses and bats in different regions. Brucellosis persists in bison and elk in the Greater Yellowstone area. Flying foxes carry Hendra virus (spread to horses in Australia) and Nipah virus (spread through pigs to people in Malaysia). Kyasanur Forest disease, a tick-borne flavivirus of South India, kills monkeys and infects forest workers.
Capture myopathy is a stress and exertion syndrome after chase, capture or transport: pale, streaked, necrotic skeletal and heart muscle, myoglobinuria and myoglobinuric nephrosis; prevention depends on short pursuits and careful handling. Chronic wasting disease is a prion disease of deer, elk and moose spreading in North America and found in Scandinavia, with spongiform change in the brain like other prion diseases. Canine distemper killed many lions in the Serengeti in the 1990s and threatens other carnivores. Elephant endotheliotropic herpesvirus causes rapidly fatal haemorrhagic disease in young Asian elephants, and elephants can carry Mycobacterium tuberculosis acquired from people. Tasmanian devil facial tumour disease is a transmissible cancer passed by bites. Chytridiomycosis (Batrachochytrium dendrobatidis) infects amphibian skin and has driven declines worldwide; white-nose syndrome (Pseudogymnoascus destructans) kills hibernating bats in North America. Avian botulism type C kills waterfowl in warm, stagnant water. Lead poisoning from ingested shot and bullet fragments harms waterfowl and scavenging raptors. On the Indian subcontinent, vulture populations collapsed after birds fed on carcasses of cattle treated with the anti-inflammatory drug diclofenac, which causes kidney failure and visceral gout in vultures; veterinary diclofenac was then banned in India, Pakistan and Nepal. Sarcoptic mange causes severe crusting and death in foxes, wombats and other wildlife. Anthrax outbreaks recur in wild herbivores in parts of Africa and Asia, and carcasses must again be left unopened.
Learn one necropsy routine and use it every time, in the same order, so nothing is missed. Practise describing lesions in plain terms before naming a disease. Know by heart which container and temperature each sample needs, the red cell index formulas, and the difference between a post-mortem clot and a thrombus, because these are asked in every exam and are needed in the first week of practice. Before any field post-mortem, rehearse the anthrax rule: sudden death with unclotted blood means a smear, not a knife.
In practice this subject decides what happens after a sudden death on a farm. You decide whether to open the carcass, which organs to sample, whether to notify the authorities, and how to package samples so they arrive usable. In the clinic you read haemograms, urine and fine needle aspirates daily: a stress leukogram versus an inflammatory one, whether azotaemia is renal, whether an effusion is septic. Knowing the limits of in-house cytology tells you when to send tissue for histopathology.
A senior clinician or state veterinary officer uses necropsy to manage outbreaks and disputes: confirming a notifiable disease before movement controls, investigating a poisoning with the feed supplier present, and writing vetero-legal and insurance post-mortem reports that will stand up in court. The senior role also sets standards for juniors, such as a submission form every laboratory can act on, safe carcass disposal, and deciding which cases justify a full laboratory work-up and which can be closed on gross findings.
For a researcher the subject supplies the tools of pathology research: well-fixed tissue, controlled sampling, special stains, immunohistochemistry and molecular methods on fixed tissue, and health monitoring of laboratory animal colonies so that infection does not confound experiments. It also supports wildlife and One Health surveillance, where carcass findings provide the first evidence of a new disease, a spill-over from wildlife, or an environmental toxin, as in the vulture and diclofenac story. Research questions in the field include better markers of time since death, validated molecular tests on archived fixed tissue, and standardised necropsy and sampling protocols for multi-centre studies.
Avian diseases are covered in the separate avian pathology subject and organ-by-organ lesions in systemic pathology, so they appear here only in passing. Species reference intervals for haematology and urinalysis are not given, because they differ by laboratory, method, breed and age; use your laboratory's own. Effusion cut-off values are described but not fixed, since textbooks disagree. Fish, reptile and exotic pet clinical pathology, clinical biochemistry (enzymes, liver and kidney function tests) and the evidence-law sections of particular countries are not covered yet. Dangerous-goods rules for shipping are summarised in principle only; check the current national and carrier rules before sending samples.
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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