African horse sickness is a highly fatal, non-contagious, notifiable viral disease of horses and other equidae such as donkeys and zebras. The main route of infection is the bite of flies (midges) infected with the African horse sickness virus. Wageningen Bioveterinary Research (WBVR) carries out research into this disease.
Protection against one serotype does not protect against other serotypes. African horse sickness is a very serious disease of horses; 70 to 95 percent of infected horses will become diseased and the majority of infected horses will die. Donkeys and zebras are much less sensitive, only 10 percent of the animals get sick. In most cases, the infection in these equines is subclinical with hardly any symptoms. The susceptibility of mules is in between that of horses and donkeys, about 50 percent show clinical signs.
African horse sickness has never been reported in the Netherlands. Therefore, the Netherlands is historically free from this very serious disease.
African horse sickness is a notifiable disease. This means that suspicions of an African horse sickness infection must be reported to the NVWA (Netherlands Food and Consumer Product Safety Authority). It is a “Categorie A” disease (Animal Health Law).
The first clinical signs are observed after an incubation period of 2 – 14 days. Four clinical forms of African horse sickness are recognized: mild form, respiratory form, heart form and the mixed form.
The mild form is subclinical and mainly occurs in horses that have raised immunity against African horse sickness. This form is also more common in donkeys and zebras. The mild form is often overlooked. The clinical signs are limited to elevated body temperature up to 40°C for 3-8 days and in some cases dehydrated eye connective tissue and slight emaciation.
The respiratory form is characterized by problems with breathing. The symptoms are increased body temperature 40 - 41°C, irregular coughing fits, accelerated breathing and abdominal breathing (pumping) and red coloured eye connective tissue. Animals often die within a week as a result of oxygen deficiency due to fluid accumulation and foaming in the lungs.
The heart form is characterized by heart failure. The signs are increased body temperature 39 - 41°C, swelling of eyelids, facial tissue, neck, chest and shoulders. The animal usually dies within one week from heart failure.
The mixed form is a sequence of first the respiratory form followed by the heart form. These animals, too, usually die within a week of heart failure.
African horse sickness was first detected in Africa after importing domestic horses from Europe. The disease is endemic to tropical and subtropical Africa and occasionally spreads outside this area.
Small flies, midges, transmit the virus from animal to animal. Only specific midge species are biological vectors and spread the virus by ingesting infected blood and releasing the virus during the next blood meal. AHSV multiplies in both the equine host and the biological vector. Once infected, midges continue to shed virus for life. Direct contact between equines does not spread AHSV.
The Sahara appears to be a natural barrier to further expansion due to the lack of the right midge species. Via the Nile Delta and the African west coast, AHS can occur further north and even beyond Africa to the Middle East (1959-1963). Movements (import) of infected zebras caused short-term outbreaks of AHS in Spain (1987-1990) and Thailand (2020). The outbreaks in these regions did not last long, partly due to massive vaccination campaigns.
The midge species Culicoides imicola and C. bolinitos are the main biological vectors for spread of AHS in Africa.
African horse sickness virus (AHSV) is closely related to bluetongue virus, but this virus infects many ruminant species, such as sheep, goats and cattle. Bluetongue virus also consists of many serotypes and is also spread by midges. Other midge species, such as C. obsoletus (Europe), C. sonorensis (America) and C. brevitarsis (Australia) can spread bluetongue virus on the respective continents brevitarsis (Australia) can spread bluetongue virus on the respective continents. These species could probably also be biological vectors for AHSV under favourable conditions. Global warming may lead to these favourable conditions, but may also expand the range of known biological vectors.
In any case, global warming will increase the likelihood of introduction and spread of AHSV in the current temperate climate zones.
Africcan horse sickness is a notifiable animal disease. Wageningen Bioveterinary Research (WBVR) is the national reference laboratory for African Horse Sickness. WBVR maintains ISO-certified tests operational to diagnose suspicions of African Horse sickness (AHS).
Antibodies against African horse sickness virus (AHSV) after infection or vaccination can be detected with the ELISA and serum neutralization test (SNT). The ELISA detects antibodies against all nine serotypes, while the SNT detects antibodies that are specific for one serotype. Clotted blood (serum blood) is used in these serological tests.
Viruses can be detected with the PCR test for AHSV. The serotype can be determined with serotype-specific PCR tests or by determining the genetic code of the virus ('sequencing').
The unknown AHSV may also be cultured in the laboratory. Thereafter, the serotype can also be determined by the virus neutralization assay (VNT) using a set of standardized sera against each of the nine AHSV serotypes. Whole blood (EDTA blood) or organs (spleen, kidney lymph nodes) is used in these virological tests. Samples of blood or organs must be transported on ice (not frozen).
Live attenuated viruses of all serotypes are available as African horse sickness vaccines in Africa, except serotype 5, but are not allowed outside of Africa. These vaccines used in Africa are unsafe. In addition, these are not DIVA vaccines, which means that infections in vaccinated horse populations cannot be detected. Preventive vaccination in countries free of African horse sickness is therefore not considered. Until now, no safe (DIVA) vaccines for AHS have been registered.
To prevent is better than to cure. It is therefore important to quarantine and test all equidae from infected countries, which are imported to a country free of African horse sickness in order to prevent the introduction of the disease.
The quarantine period usually consists of two parts, namely before transport and after transport. The individual quarantine periods are usually 21-30 days. The quarantine facility must be closed to insects and the horses are often also treated with an insect repellent and/or with insecticides.
In addition to clinical monitoring, blood samples are regularly tested for the presence of African horse sickness virus and/or AHSV antibodies before the animals are transported or allowed into a country.
Containing or removing infected horses reduces virus uptake and thus also virus spread by midges. Midges can also be controlled to reduce the spread of the disease. After an AHSV infection has been detected, various zones are set up around the source of infection, in which measures apply, such as intensive monitoring and a ban on animal movements. This will help to control the outbreak.
In addition to laboratory diagnostics, Wageningen Bioveterinary Research (WBVR) also conducts research on new African horse sickness vaccines.
A new concept analogous to the DISA principle (Disabled Infectious Single Animal) as developed for a DISA/DIVA vaccine platform for Bluetongue was also developed for African horse sickness. Knockout of one protein in the vaccine platform results in attenuation, DIVA and the vaccine cannot be spread by midges. The AHS DISA/DIVA platform is applicable for all nine serotypes.
The first results in horses are very promising.
So far, this research has been funded by the Dutch government and will be continued within the European consortium SPIDVAC.
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