Babesiosis - Horse

Also known as: Equine Piroplasmosis and Biliary fever

Description

Equine babesiosis is a tick-borne protozoal infection of donkeys, mules, horses and zebra characterised by acute haemolytic anaemia. The disease is caused by the intraerythrocytic protozoa Theileria equi (formerly Babesia equi) and Babesia caballi and is transmitted primarly by ixodid ticks. Equine babesiosis is endemic in most tropical and sub-tropical regions of the world where tick vectors are present. It is associated with high economic losses relating to treatment costs, loss of performance, abortion and death.

Aetiology and Pathogenesis

B. caballi and T. equi are transmitted by ticks, which become infected when they ingest parasites in the blood of infected horses. Approximately 15 species of ticks in the genera Dermacentor, Hyalomma and Rhipicephalus can be vectors for these organisms. T. equi can also be transmitted by Boophilus microplus. Infected adult ticks host several cycles of replication of Babesia spp and final maturation of the parasite occurs when the tick attaches to a new host. For this reason, the tick must remain attached to the host for a period of time before infection can occur; this may require up to a few days. Equine babesiosis may also be transmitted via contaminated surgical instruments, blood products or syringes. Currently, there is no evidence suggesting transmission by other insects.

Following recovery from infection, horses may become lifelong carriers of the disease and represent a potential reservoir of parasites.

Clinical signs

Clinical signs are generally dependent on the causative parasite. Infection with B. caballi tends to result in a milder form of disease that is often clincally inapparent whereas T. equi results in a more serious disease. The clincal presentation of the disease is variable and dependent on the immunological status of the affected animal. In rare cases, the presentation may be peracute with animals found dead within 24-48 hours of the onset of clinical signs. More often cases occur acutely with clinical signs including depression, pyrexia, inappetance, icterus, haemoglobinuria, conjunctival petechial haemorrhages amd anaemia. Small and dry faeces are often reported. Other common clinical signs include tachypnoea, ataxia and oedema of the head.

The subacute form of the disease has a similar but less severe presentation, characterised by intermittent fever, anorexia, weight loss, signs of mild colic and oedema of the distal limbs. Chronic infections are typically accompanied by varying clinical presentations including mild inappetance, weakness, weight loss and mild anaemia.

Pathology

At post mortem, the acutely affected horse is usually emaciated, icteric and anaemic. Gross lesions include hepatomegaly and splenomegaly, pericardial effusion and a pale, flabby heart. Petechial haemorrhages may be present on the kidneys with oedema of the renal capsule a common feature.

References

• Sellon, D. C., Long, M. T. (2007) Equine Infectious Diseases Elsevier Health Sciences