A major outbreak of African horse sickness (AHS) in Thailand earlier this year was a timely reminder for animal health officials, veterinarians, and members of the horse industry of the vulnerability of equine populations to this disease.
Veterinarian ready to vaccinate horse while doing field work
African horse sickness is a dreaded, non-contagious, vector-borne disease with the potential to kill 50% to 95% of affected horses.
© 2020 by OIE Bulletin-John Grewar New window.
Of all known equine infectious diseases, AHS is the single most important in terms of devastating losses in naïve horses and economic impact on international trade. It is a dreaded, non-contagious, vector-borne disease with the potential to kill 50% to 95% of affected horses.
The etiological agent of AHS is an RNA virus belonging to the family Reoviridae, genus Orbivirus, that is transmitted naturally by species of Culicoides or midges. There are nine antigenically distinct serotypes of the virus.
While the primary host species are members of the family, Equidae, evidence of infec- tion has also been found in African elephants, black and white rhinoceroses, camels and dogs, none of which are considered epidemiologically significant.
Historically, AHS was considered restricted to tropical and subtropical regions of sub-Saharan Africa, where it had been known to occur regularly for over 200 years. Although infrequent, the disease has spread from west and north Africa to various southern European or Middle Eastern countries.
The most significant such event occurred between 1959 and 1963 when serotype 9 of AHS virus spread out of Africa into and throughout the Middle East, as far north as Turkey, and extended as far east as Afghanistan, Pakistan, and India.
Concern over the risk that AHS poses for horse industries in Europe has been keenly felt for some time. Southern European countries are faced with the potential threat of disease introduction from migration of the virus northwards from regions in Africa, where the disease is endemic.
Spread of the virus could result from the movement of nomads and their animals, passive wind-borne carriage of infected Culicoides over long distances, and legal or illegal trade in zebra from countries where the disease is currently active.
A similar if not greater risk exists for the Middle East which experienced incursions of AHS more frequently than any other region or country.
As the most recent occurrence of AHS in Thailand has shown, distance is no guarantee of safety from the risk of introduction of this disease. Thailand is approximately 6,000 miles distant from where the virus responsible for this event probably originated, and the furthest east in Asia where AHS has ever been recorded.
The most plausible explanation as to the source of virus lies in the fact that a shipment of zebra arrived in Thailand three to four weeks before the first AHS outbreak of the disease was discovered. Zebra are considered the natural reservoir of the virus, developing viremias lasting up to 40 days.
This would not be the first occasion that importation of zebra from a country in which AHS is a seasonal occurrence has been implicated in the introduction of disease into a disease-free country.
A shipment of zebra was the confirmed source of this virus for a major disease event that occurred in Spain in 1987 and subsequently involved Portugal and Morocco.
International trade in wildlife, both legal and illegal, is believed to have increased significantly in recent years.
What happened in the Iberian Peninsula in 1987 and Thailand in 2020 highlights the inherent risks of introducing a disease, such as AHS, into previously disease-free countries.
The influence of climate change and global warming on the epidemiology of AHS must also be considered vis-à-vis the threat it poses for a disease- free country. Increased ambient temperatures and reduced rainfall over a period of years has resulted in more widespread geographic distribution of some of the major vectors of AHS, especially C. imicola, in southern Europe.
An increase in ambient temperature can influence not only the life cycle of the Culicoides vector but also replication of the virus in the vector. As temperatures rise, the infection rate in Culicoides midges increases and transmission of the virus can occur sooner, however there is a concomitant decrease in the survival rate of the adult Culicoides.
The overall result of these changes is a higher transmission rate of the virus in a country possibly at risk of the introduction of AHS.
The occurrence in Thailand and the very recent confirmation of AHS in Malaysia underscore the importance of increasing awareness and familiar- ity with this dreaded disease among animal health officials, veterinarians and members of the equine industry around the world.
The potential consequences of AHS for the health of a country’s equine population and economy highlight the need for an adequate level of national preparedness in a) minimizing the risk of introduction of this disease, b) maintaining a program of active surveillance for the disease and c) having a response plan in place in the remote event of the introduction of the disease.
The take-home message from past and recent occurrences of AHS is that there is no room for complacency over the potential threat it represents for disease free-countries.
Press release by Equine Disease Quarterly - Article by Peter Timoney, MVB, MS, PhD, FRCVS - Maxwell H . Gluck Equine Research Center University of Kentucky