Equine Infectious Anaemia (Swamp Fever)

Equine Infectious Anaemia (Swamp Fever) is a chronic persistent infectious viral disease of equines characterized by, intermittent fever, anaemia, emaciation, edema and generalized lymphoproliferative changes in the body.

The anaemia may be for few days or may persist for a longer period and this is considered to be immunologically induced problem.

Etiology

• Equine Infectious Anaemia (Swamp Fever) is caused by Equine infectious anemia virus (EIAV) of genus Retrovirus, a member of subfamily Lentivirinae, family Retroviridae.
• The EIAV shares antigenic cross reactivity with human and feline immunodeficiency virus.
• The virus has one major group specific antigen p26.
• There is considerable antigenic drift in the surface glycoprotein (gp45 and gp90) responsible for emergence of novel antigenic strain is associated with the relapsing febrile reaction is characteristic of the disease.

Epidemiology

• The disease is present in all continents except antartica.
• The prevalence of infection depends on the density of equine population, proportion of carrier animal and vector population.
• The outbreak of EIAV is rare in developed countries.
• The morbidity rate varies with the strain of virus infected and inoculums delivered by insect
• Host affected: horse, donkey and mule.
• Source of infection: persistently infected or clinically affected horse.

Transmission

• Transfer of contaminated blood or blood products by biting insects mechanically occurs in summer season (under hot and humid condition).
• Through contaminated surgical instruments or needles.
• Use of common needle when injecting group of horse.
• Intrauterine infection can occur.
• It can also transmitted through semen of infected stallion.

Pathogenesis

Clinical manifestation

• Incubation period is 2-4 weeks.
• It occur as acute, subacute or chronic form.

Acute form

• Anorexia, depression and profound weakness and ataxia.
• Intermittent fever.
• Jaundice, edema of the ventral abdomen, propuce and legs.
• Petechial haemorrhage under the tongue and conjunctiva.
• Increase in heart rate and intensity of heart sound.
• Respiratory signs are not marked, but  may be thin serosanguineous nasal discharge.
• Considerable enlargement of spleen can be detected per rectum.
• Pregnant mares may abort.
• Many animals show temporary recovery 3 day-3 weeks of illness.
• Others progressively weak, recumbent and die after 10- 14 days of illness.
• Relapse occur in temporarily recovered animals.
• Relapse continue to occur  at interval of 2 weeks and cease about a year.

Chronic form

• Appetite is good, may show allotriphagia.
• Affected animals are recovered completely but relapse occur in the later years.

Necropsy Finding

• Considerable enlargement of spleen, liver and local lymph node.
• Reddened bone marrow.
• Glomerulonephiritis.

Sample collection

• Live animals-  buffy coat and paired sera sample.
• Dead animal- spleen, liver, bone marrow and perihepatic lymph node.

Diagnosis

• Based on clinical signs and lesion.
• Haematology and biochemical changes include anemia, thrombocytopenia, presence of sideroleukocyte, Hyper gamma globulinemia, increased bilirubin level and decrease in serum iron concentration.
•  AGID (Coggin’s test), competitive ELISA used to detect antibodies against group specific p26 core antigen of EIAV after 45 days of infection.
• Western blot test useful to detect antibodies against surface glycoprotein gp45 and gp90 (relapse occur with this antigen) after 60 days of infection.
• Experimental transmission of the disease to susceptible horse.

Differential diagnosis

• Purpura haemorrhagica
• Babesiosis
• Equine viral enteritis
• Equine granulocytic ehrlichiosis
• Leptospirosis
• Autoimmune haemolytic anemia
• Idiopathic thrombocytopenia
• Parasitism

Chronic disease

• Internal abscessation.
• Chronic hepatitis, neoplasia.
• Streptococcal infection- cough and respiratory distress are common.

Treatment

• No specific treatment available.
• Supportive therapy include blood transfusion, haematinic drugs may used to facilitate clinical recovery.

Prevention and control

• Killed whole virus vaccine is available and safe to use.
• Subunit vaccine enhance the disease occurrence.
• Lifelong quarantine of infected animals.
• Quarantine and testing of new stock.
• Compulsory testing of imported horse.
• Restriction of introduction of infected horse into the clean area.
• Maintenance of strict hygiene during  blood collection and vaccination.
• Draining of marshy area and control of biting insects.
• Disinfection of tattoo equipment and surgical instruments by immersion of less corrosive phenolic compound for 10 min.
• Cleaning and disinfection of premises with sodium hypochlorite and iodine compound.