African Horse Sickness is a notifiable disease in horses that is transmitted indirectly by mosquitoes. It is caused by a virus that has nine different serotypes that can cause four clinical forms: pulmonary, cardiac, mixed or febrile, causing various symptoms, devastating in some cases with high mortality rates in susceptible horses. Other species of equids can be affected, with donkeys and zebras being the most resistant to the disease, and the latter being considered the reservoirs of the virus. The control of this disease is through sanitary prophylaxis and vaccination.
What is African Horse Sickness?
African Horse Sickness is a non-contagious infectious disease endemic to its region of origin, sub-Saharan Africa, which causes fever, respiratory and vascular alterations that can take place in a peracute, acute, chronic or inapparent way. It affects equids, specifically horses are the species most susceptible to the disease, followed by mules and donkeys; in zebras the disease is usually subclinical or inapparent, considered the natural reservoirs of the disease Dogs can be infected experimentally or if they eat infected horse meat.
Its main importance lies in the high cost of its control, its high mortality (between 50 and 95% in horses) and limiting the movement of horses.
In Spain, African Horse Sickness has appeared twice: the first in 1966 in the field of Gibr altar and the second between 1987 and 1993 in Madrid due to the importation of zebras from Namibia.
Fortunately, African Horse Sickness, although dangerous, is not one of the most common diseases in horses.
Causes of African Horse Sickness
African horse sickness is transmitted by arthropods, specifically mosquitoes of the genus Culicoides, Culicoides imícola being the main vector of this disease, along with C. bolitinos. Other vectors also possibly involved are C. pulicaris and C. obsoletus.
The disease-causing agent is a virus of the Reoviridae family that belongs to the same genus as the one that causes deer hemorrhagic disease or Bluetongue, the Orbivirus genus. Nine serotypes of the virus are known. The highest incidence of the disease coincides with the favorable season for vectors, in summer-autumn, and in Africa due to its high temperatures, giving rise to major epizootics.
African Horse Sickness Symptoms
After the mosquito bite, the virus reaches the blood vessels of the horse, where it multiplies causing vascular fragility and blood extravasation, which causes pulmonary edema, small hemorrhages and subcutaneous edema, producing theclinical forms of the disease , which can be of four types:
Symptoms of the acute pulmonary form
This is the clinical form with the most fulminant evolution, with striking clinical signs where horses cannot breathe due to pulmonary edema and fluid in the chest cavity (hydrothorax). They usually die in a maximum of 4 days and symptoms include:
- High fever of 41 ºC.
- Tachycardia.
- Tachypnea.
- Profuse sweating.
- Superficial respiratory signs becoming deep.
- Painful, spasmodic cough.
- Strong frothy mucous exudation.
- Anguish due to respiratory distress (dilated nostrils, anxious eyes, open mouth, drooping ears, separated forelimbs, and extended head and neck).
Frequently, death occurs in apparently he althy horses during exertion. The animals appear with dilated nostrils, an open mouth, separated forelimbs and an extended head and neck indicating respiratory distress.
Symptoms of subacute cardiac form
This clinical form usually begins with fever of 39.5-40 ºC that lasts between 3 and 5 days. When the fever begins to subside, edema appears in:
- Supraorbital and periorbital fossae.
- Lids.
- Head.
- Neck.
- Shoulders.
- Chest.
In the terminal phase, they will present small hemorrhages (petechiae) on the conjunctiva and under the tongue. The horse will be very depressed and may prostrate at times. It can also manifest signs of colic and finally die prostrate due to heart failure. The mortality rate of this clinical form ranges between 30 and 50 %
Mixed form symptoms
In this form, clinical signs of the pulmonary and cardiac forms are observed, the latter being mostly asymptomatic followed by respiratory distress, with cough and foamy exudate. Other times, mild respiratory signs are followed by edema and death from heart failure.
It is the most frequent clinical form of the disease, which has a 70 % mortalityand is usually diagnosed when the equine is dead by necropsy.
Febrile Form Symptoms
It is the mildest form of the disease and most recover. It is more common in the most resistant equines, that is, zebras or donkeys, or in horses that have some immunity.
The clinical signs are mild, the fever is characteristic and lasts a maximum of one week, descending in the morning and increasing in the morning. the afternoon. It is usually accompanied by clinical signs such as:
- Anorexy.
- Mild depression.
- Mucous congestion.
- Supraorbital fossa edema.
- Tachycardia.
African Horse Sickness Diagnosis
This serious disease is notification required, since it belongs to the list of notifiable diseases of the World Organization for Animal He alth (OIE). Entering a non-endemic area is very serious and a great cause for concern, so making a correct diagnosis is key.
Although the clinical signs suggest this disease, it must be confirmed with tests at the laboratory authorized for this purpose in the country, after the obtaining samples by the official veterinarian.
Clinical and differential diagnosis
The clinical signs that the horse presents may suggest this disease, especially if we are in a favorable time and in an endemic area, and in case of performing a necropsy, the lesions may suggest this disease even more. illness. It must always be differentiated from other equine diseases, such as:
- Equine Viral Arteritis.
- Equine encephalitis.
- Hemorrhagic purpura.
- Equine piroplasmosis.
Laboratory diagnosis
Whole blood and serum samplesshould be taken during the fever phase in the live animal, or lung, spleen and lymph nodes at necropsy.
The tests will be to detect antibodies such as indirect ELISA or complement fixation, or to detect the virus such as RT-PCR or direct ELISA or virus neutralization.
The virus can also be isolated in cell culture (on BHK-21, MS and VERO cell lines).
African Horse Sickness Treatment
Being a devastating disease that requires notification to the authorities, treatment is not applied, but a series of measures must be taken to control possible outbreaks and the spread of the disease, through sanitary measures and vaccination.
Sanitary measures for African Horse Sickness
In endemic areas of the disease, when cases are reported, vector control should be carried out through disinsection with insecticides and larvicides, together with the vaccination of the animals.
In disease-free areas, equines that come from disease-endemic areas must be kept in quarantine for a minimum of 60 days, plus serological surveillance and mosquito control in animal transport.
If cases appear, do the following:
- Restrict the movements of the horse and equids that have had contact with it.
- Notification of suspected and diagnosed cases.
- Establishment of a 100 km protection area and a 50 km surveillance area around the area where the case is found.
- Stabling the animals during the hours of greatest mosquito activity.
- Disinsection and repellent measures against mosquitoes in transport and in the affected area.
- Implementation of serological, entomological, epidemiological and clinical surveillance programs around the foci for early detection of the disease.
- Vaccination of all equids belonging to the areas included in the protection zone.
African Horse Sickness Vaccination
Vaccination is the most effective measure to control a disease, interrupting the cycle between the infected horse and the mosquito to achieve the eradication of the disease. Vaccines for African Horse Sickness consist of:
- Live attenuated vaccines: they have the virus live but attenuated. These vaccines are only used in endemic areas or when the disease has appeared in a non-endemic territory by vaccinating for the serotype in question. These vaccines can be monovalent for a single serotype or polyvalent, specifically a trivalent (serotypes 1, 3 and 4) and another tetravalent (serotypes 1, 6, 7 and 8); serotypes 9 and 5 are not included because they are cross-protective with serotypes 6 and 8, respectively.
- Inactivated vaccine against serotype 4: Developed and used, however not currently available.
- Recombinant subunit vaccine: contains the viral VP2, VP5 and VP7 proteins, however it is still under study.
In addition to the African Horse Sickness vaccine, depending on the endemic area, it is important to know these other Vaccines for horses.
