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Diseases caused by viruses

African horse sickness (AHS)

AHS is a highly fatal insect-borne febrile viral disease of equidae characterized by edema of the subcutaneous tissue and lungs, haemorrhage and serous fluid in the body cavities. At room temperature, the virus is persistent for a few months in urine, dried blood, faeces and serum. It is resistant to boiling up to 15 minutes, and to common disinfectants. It is destroyed by sun light.

Transmission : AHS is transmitted by various Culicoides spp. and several species of mosquitoes. Mechanical transmission by biting flies is also possible. Dogs get infected by eating infected horse meat.

Antemortem findings :

  1. Incubation 3 – 5 days

Cardiac form

  1. Mortality up to 50 %
  2. Fever up to 40°C
  3. Conjunctivitis
  4. Non pitting-edema in the supraorbital fossa (above the eye, Fig. 179) is the most characteristic sign.
  5. Subcutaneous edema of the head, neck, brisket, thorax and ventral abdomen

Pulmonary form

  1. Sudden onset and increased temperature (41°C)
  2. Respiratory distress and associated coughing
  3. Frothy exudate from the nares for several hours prior to death
  4. Mortality is over 90 %

In some animals both forms may occur.

Mild form “horse sickness fever”

  1. Fever in duration of a few days
  2. Slightly laboured breathing
  3. Loss of appetite

Postmortem findings :

  1. Petechial haemorrhage on the ventral surface of the tongue
  2. Gelatinous edema in the periorbital tissue, neck muscles and ligamentum nuchae (Fig. 180)
  3. Intermuscular and subcutaneous edema and haemorrhage
  4. Trachea and bronchi filled with froth
  5. Pleural exudate and pulmonary edema (Fig. 181)
  6. Inflammation of the heart and haemorrhage on the pericardium
  7. Excessive pericardial fluid
  8. Congestion of the fundic portion of the stomach
  9. Haemorrhage of the intestinal serosa
  10. Haemorrhagic kidneys

Judgement : Carcass of an animal affected with acute AHF showing generalized clinical signs and postmortem lesions is condemned. The carcass of recovered and reactor animals is approved for limited distribution.

Differential diagnosis : Colics, anthrax, equine rhinopneumonitis. equine infectious anaemia, equine viral arteritis, equine piroplasmosis, equine influenza

fig 179

Fig. 179: AHS. This horse is near death. The animal is depressed and showing edema of supraorbital fossa, neck and chest.

fig 180

Fig. 180: AHS. Subcutaneous and intermuscular edema in the neck. This may be the only lesion in some cases of AHS.

fig 181

Fig. 181: AHS. Thoracic lesion. Pleural exudate and pulmonary edema are noted.

Equine infectious anaemia

Equine infectious anaemia is an acute, subacute, chronic and latent disease of solipeds caused by a virus. It is manifested with intermittent fever, depression, weakness, edema, anaemia and icterus.

Transmission : Close continued contact with susceptible animals and biting insects mainly Tabanidae and mosquitoes. Contaminated surgical instruments, needles, contaminated feed, bedding and intrauterine infection. Horse may die of anaemia during early viraemia or may recover and have recurrent episodes of viraemia.

Antemortem findings :

Acute form

  1. Fever up to 41°C; it may rise and fall rapidly
  2. Sudden onset of disease lasting from 3 days to 3 weeks
  3. Depression and weakness
  4. Jaundice
  5. Edema of the ventral abdomen, legs and prepuce
  6. Serosanguineous nasal discharge
  7. Abortion in pregnant mares
  8. Rapid dehydration
  9. Droopy ears and half closed eyes
  10. Dyspnea at terminal stages

Subacute form

  1. An acute onset of disease and subsequent recovery
  2. Swollen icteric conjunctivae
  3. Exertion may cause an increase in temperature and pulse, sweating and incoordination.
  4. Death caused by exhaustion

Chronic form

  1. Anaemia, weakness
  2. Icteric conjunctivae and mucous membranes
  3. Diarrhoea
  4. Recurrence of disease in a one to three month period

Latent form

  1. No symptoms observed although the animal is infected
  2. May become active. An acute attack may be caused by hard work, poor diet and parasitic infections.

Postmortem findings :

Acute case

  1. Subcutaneous edema
  2. Jaundice
  3. Subserosal haemorrhage

Chronic case

  1. Emaciation and anaemia
  2. Hydrothorax and ascites
  3. Enlarged spleen and liver (Fig. 182) with swelling of the edges
  4. Superficial haemorrhages in the organs
  5. Enlarged, edematous and haemorrhagic lymph nodes
  6. Replacement of bone marrow fat with dark red hemopoietic tissue (Fig. 183)

Judgement : Carcass of an animal affected with the clinical disease of EIA is condemned. Reactor animals may have the carcass approved for limited distribution if no systemic lesions are noted on postmortem examination.

Differential diagnosis : Emaciation, other acute septicemias, anthrax, piroplasmosis, glanders, tuberculosis, virus encephalomyelitis, purpura haemorrhagica, babesiosis, leptospirosis, parasitic infections (strongylosis and fascioliasis) and purulent infections causing anaemia

fig 182

Fig. 182: Equine infectious anaemia. Enlarged grey red liver showing lobular pattern and haemorrhage under the capsule.

fig 183

Fig. 183: Equine infectious anaemia. Replacement of bone marrow fat with dark red hemopoietic tissue (erythroid hyperplasia).

Viral encephalomyelitis of horses

Viral encephalomyelitis of horses is characterized by disturbed consciousness, motor irritation and commonly high mortality. The disease has been found in cattle and sheep as well as in humans.

Transmission : The disease is mostly spread from birds through insects to horses and humans. Mosquitoes of the Culex, Aedes and Mansonia genera are vectors of this disease. Wild birds are reservoirs of infection.

Antemortem findings :

  1. Incubation 1 – 3 weeks
  2. Fever
  3. Depression and anorexia

Nervous signs

  1. Hypersensitivity to sounds and touch, fascial muscle twitch and walking blindly into objects or in circles
  2. Paralysis, collapse and death between 2 – 4 days of the onset of symptoms
  3. Mouth and eyes may be closed.

Postmortem findings :

  1. Gross lesions are not usually found.
  2. Visceral haemorrhages
  3. Lesions in the lungs and rarely other organs
  4. Histological findings include the lesions in the brain showing perivascular accumulation of leucocytes and damage to neurons

Judgement : Carcass and viscera of the animal showing clinical signs of this disease are condemned. The carcass of reactor animal may have a limited distribution through specially licensed and closely supervised commercial channels. The brain and medulla must be condemned.

Differential diagnosis : Plant poisoning, botulism, equine infectious anaemia and the dumb form of rabies.

Diseases caused by bacteria

Contagious equine metritis (CEM)

CEM is a contagious, acute venereal disease of horses and other equidae caused by Haemophilus equigenitalis.

Transmission : Venereal, contaminated fomites, personnel examining infected animals and rarely stud handlers.

Antemortem findings :

  1. Incubation 2 – 10 days after breeding
  2. Morbidity up to 100 % from direct contact with an infected stallion
  3. Mortality is none
  4. Inflammation of the vagina (vaginitis) and copious mucopurulent vaginal discharge (Fig. 184)
  5. Hind legs of a mare soiled with exudate
  6. No systemic disturbance in affected mares

Postmortem findings :

  1. Inflammation of the vagina, cervix and uterus
  2. Congestive inflammation of the cervix
  3. Mucopurulent exudate in the uterus (Fig. 185) and vagina

Judgement : Carcass of an animal affected with CME is approved. If acute inflammation of the vagina and uterus is associated with septicemia, the carcass is condemned.

Differential diagnosis : Klebsiella spp. infection, Pseudomonas spp. infection, chronic Staphylococcus infection

fig 184

Fig. 184: CEM. White, stringy mucous exudate dripping from the vagina of a mare.

fig 185

Fig. 185: CEM. Accumulation of the mucopurulent exudate in the uterus.


Tetanus is an acute fatal disease of horses caused by Clostridium tetani. It is manifested by spasmodic contraction of the voluntary muscles and increased sensitivity to stimuli. Sheep and cattle and rarely pigs are also susceptible.

Transmission : Most frequent transmission of agent in horses is caused by nail wounds. In sheep, the agent may enter after castration and docking; in cows it may enter during puerperal infection, dehorning or castration. In swine, tetanus is mostly seen as a result of wound infection, castration or umbilical infection in new born animals. Clostridium tetani is found in the soil and more commonly in horse manure. It can also be demonstrated in the intestine of healthy horses. Clostridium tetani forms spores which are extremely resistant and may remain viable for years if protected from light and heat. They can however, be destroyed by boiling water. Digestive juices have no effect on spores. Clostridium tetani produces toxins which are responsible for the clinical picture of tetanus. Neuromuscular activity favours migration of tetanus toxins through peripheral nerves which reach the lumbar and cervical region of the cord and the brain stem. In this ascending form of the disease, tetanus develops first in the limbs, followed by the muscles of the trunk. Descending tetanus is observed in horses and humans. Toxins circulate in the blood and lymph and cause tetanus in the muscles of the forelimbs, upper trunk and hind limbs. The first symptoms are the protrusion of the nictitating membrane, and the involvement of facial and jaw muscles leading to lock jaw.

Antemortem findings :

  1. Incubation 4 – 14 days to up to 4 months
  2. Increased breathing and heart rate
  3. Congestion of mucous membranes
  4. Stiffness of the masseter muscles and stiff stilted gait
  5. Difficulties in mastication of food, hence the common term “lockjaw”
  6. Erect ears and prolapse of the third eyelid
  7. Animal observed in “sawback stance”
  8. Tetany and convulsions
  9. Death. At the end of the fatal attack the temperature rises to 42.2°C – 43.3°C. Sheep, goats and swine fall to the ground.

Postmortem findings : No significant postmortem lesions are present.

Judgement : Carcass of an animal affected with tetanus is condemned. The musculature is usually grey yellow in colour and the carcass is inadequately bled.

Differential diagnosis: Strychnine poisoning, hypocalcemia (eclampsia) of mares, cerebrospinal meningitis, lactation tetany of cattle, enzootic muscular dystrophy, enterotoxaemia of lambs, polioencephalomalacia


Glanders is a bacterial disease of horses and other solipeds characterized by lesions in lymph glands, lymph vessels, respiratory tract and skin. It is caused by Actinobacillus (Malleomyces) mallei.

Transmission : Ingestion of food and drinking of water contaminated with secretions and excretions of sick animals. Wound infection and the respiratory route in acute glanders, contaminated needles, grooming equipment, urine, nasal discharges, purulent skin lesions are also associated with the transmission of this disease.

Antemortem findings:

Acute form

  1. High fever
  2. Breathing difficulties and coughing
  3. Snoring sound if lesions are in the larynx
  4. Ulcers in the nasal mucosa. Star shaped scars upon healing of ulcers
  5. Nodules on the skin, abdomen and lower limbs
  6. Death due to septicemia

Chronic form

  1. Low mortality and high rate of recovered animals
  2. Intermittent fever and coughing
  3. Unthriftiness and loss of weight
  4. Unilateral rhinitis and yellowish-green or bloody nasal discharge
  5. Heavy exudate on the skin surface
  6. Enlarged submaxillary lymph nodes, abscess formation and abscess rupture.
  7. Granulomatous nodules along the lymphatics under the skin, especially on the legs.
  8. Swollen pipe like lymphatics (Farcy)
  9. Enlarged rear legs

Postmortem findings :

  1. Inflammation of the lymph nodes. The nodes are enlarged, fibrotic and abscessed
  2. Ulcers in the nasal mucosa, larynx and trachea.
  3. Nodules in lungs scattered throughout the lung tissue. These nodules have greyish centres.
  4. Nodules on the skin and in the subcutis
  5. Necrosis in the internal organs and testicles

Judgement : Carcass of an animal affected with glanders is condemned. The animal should not be admitted to the abattoir.

Differential diagnosis : Epizootic lymphangitis, ulcerative lymphangitis, strangles, dourine and melioidosis

Strangles (Distemper)

Strangles is a contagious disease of equines characterized by inflammation of the upper respiratory tract and purulent lesions in the regional lymph nodes. It occurs in stressed young horses 1 – 5 years old. It is caused by Streptococcus equi.

Transmission : Source of infection is nasal discharge from infected animals and contaminated food and water. Infection is spread by ingestion or via respiratory route by inhalation of droplets. It may spread for at least 4 weeks after the initial attack due to organism developing resistance to diverse environmental conditions. The spread of infection is also caused by parasites and infected animals during copulation. Infection of the udder of the mare may occur from an infected foal.

Antemortem findings :

  1. Incubation 4 – 8 days
  2. Temperature 39.5°C – 40.5°C
  3. A soft, moist and constant cough
  4. Severe pharyngitis and laryngitis
  5. “Hot” painful abscesses in submaxillary, pharyngeal and parotid lymph nodes and lymph vessels (Fig. 186)
  6. Necrosis of skin and eruption of abscesses
  7. Edema of lower limbs (swollen limbs are 3 – 4 normal size)
  8. Empyema of the guttural pouch complications
  9. Spread of lesions to lungs causing acute pneumonia
  10. Purulent inflammation of the brain followed by excitement, neck rigidity and terminal paralysis
  11. Pericarditis
  12. Lameness and difficult breathing

Atypical form of strangles is manifested by subclinical infection and mild disease.

Postmortem findings :

  1. Abscesses in the internal organs including the liver, spleen, lungs etc.
  2. Abscesses on the pleura and peritoneum
  3. Abscesses in the mesenteric lymph nodes

“Bastard strangles” denotes multiple abscessation in the vital organs and generalized systemic infection.

fig 186

Fig. 186: Strangles (Distemper). Hot painful abscesses of the lymph nodes of the throat.

Judgement : Carcass of an animal affected with strangles is condemned if the animal shows signs of acute infectious disease accompanied with fever and systemic generalized lesions. Atypical, mild form of the disease has a favourable judgement.

Differential diagnosis : Epizootic lymphangitis, ulcerative lymphangitis, dourine, melioidosis, equine viral rhinopneumonitis, equine influenza and equine viral enteritis

Parasitic diseases

Diseases caused by protozoa

Trypanosomiasis (Dourine, Mal du coit)

Contagious trypanosomiasis in horses is manifested by edematous swelling and inflammation of the genitalia, cutaneous lesions and paralysis. This disease is caused by flagellated protozoan called Trypanosoma equiperdum.

Transmission : Transmission by coitus and rarely by bloodsucking flies (Tabanidae and Stomoxys)

Antemortem findings :

  1. Incubation: 1 week to several months
  2. Low recurrent fever
  3. Loss of condition and anaemia
  4. Rough hair coat
  5. Acquired loss in cutaneous pigmentation (vitiligo) noted as white discrete patches of various shapes and sizes in the skin and external genitalia.
  6. Transitory urticarial plaques which do not ulcerate, appear on the mucosa and skin, particularly on the flanks.
  7. Mucopurulent urethral or vaginal discharge
  8. Frequent urination and increased sexual desire
  9. Edema of the scrotum, prepuce and penis in stallions and the udder edema in mares
  10. Edema of the ventral abdomen
  11. Depigmented genital mucosa and rarely ulceration of vaginal mucosa
  12. Nervous signs are manifested by incoordination, irregular muscular contractions, facial paralysis and complete paralysis of the body.
  13. Clinical signs may be absent in chronic disease.

Postmortem findings :

No specific lesions on postmortem

  1. Edema of genitalia
  2. Emaciation, anaemia and characteristic depigmentation in the skin and external genitalia
  3. Edematous fluid in the pleural, pericardial and peritoneal cavities

Judgement : Carcass of an animal showing chronic lesions of trypanosomiasis without systemic involvement and the carcass of recovered animals is approved. Horse carcass affected with the disease is condemned if clinical signs are accompanied with emaciation and edema or anaemia.

Differential diagnosis : Equine infectious anaemia

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