African horse sickness
African horse sickness, also known as equine plague, pestis equorum, African horse sickness (AHS) or equine plague, is a peracute to acute seasonal viral disease of solipeds transmitted by bloodsucking insects. There is no animal-to-animal transmission, but the disease can be transmitted indirectly through used syringes. The pathogen, an orbivirus of the family Reoviridae, is ingested by the bite of mosquitoes (gnats), in mosquitoes of the genus Culicoides the virus multiplies and is further transmitted with the next sucking act. Other mosquito species can spread the virus mechanically. Nine serotypes have been differentiated so far. Strains with different virulence occur under natural conditions.
The African horse sickness virus is labile to low pH-values and is rapidly destroyed by temperatures of 50-60 °C, by sunlight and by putrefaction.
In general, African horse sickness is characterised by fever, oedematous swelling of the head, pulmonary oedema, haemorrhage in internal organs and exudation into the body cavities. This is caused by viral replication in the endothelial cells and consequent increased permeability of the capillary and vascular walls. Four courses of African horse sickness are distinguished:
- Peracute pulmonary course: incubation period 2-5 days, high fever (40-41 °C), increased respiratory rate, distended nostrils, muscle tremors, staggering, dyspnoea, sweating and coughing. 95 % lethality, death usually occurs within a few hours after the first clinical manifestations.
- Subacute cardiac course: fever for 3-6 days and edema of the head, neck, chest, and limbs. Foamy discharge from the nostrils, petechiae on the conjunctiva, muscle weakness, colic symptoms. Lethality up to 50
- Acute mixed course: a mixture of symptoms of the two forms mentioned, subclinical cardiac form followed by acute dyspnoea. Most frequent form, death by heart failure after approx. 1 week in 70-80 % of cases.
- Abortive course ("horse sickness fever"): mild course, incubation period 5-14 days, intermittent mild fever for 5-8 days. Complete recovery, occurrence mostly in less susceptible animals (donkeys) or vaccinated animals.
The suspected diagnosis, especially in the mosquito season, is made clinically by frequent peracute deaths of equidae and the presence of typical oedema.
The diagnosis is confirmed by PCR diagnostics, virus isolation or serology, especially by means of the elisa technique. Commercially available elisa test kits (VP7 protein) allow reliable detection of antibodies from serum of horses from 8-12 days after infection. With competitive formats, testing of sera from other equids (donkey, zebra, etc.) is also possible.
Virus isolation as well as antigen detection by real-time PCR is possible from blood during the fever phase, after which spleen, lungs and lymph nodes are suitable as sample material.
Differential diagnosis: equine arteritis, trypanosomiasis, spirochetoses, piroplasmoses, babesioses, infectious anaemia, anthrax, various poisonings.
Last updated: 13.01.2022