by Cindy Reich
Just the mention of this disease causes grave concern to horse owners in Africa. During a recent visit to Namibia, I saw firsthand the fear that surrounds any mention of African horse sickness. While driving down from the Caprivi region to Windhoek, news came out of some horses near Windhoek showing suspicious symptoms and high fevers. Word was that it could be African horse sickness. Immediately, plans went into action on the farm — setting up fans to discourage the midges that carry the disease. Putting on flysheets to keep the midges off the horses. Activating the fly zappers. Spraying the horses with insect repellent.
A horse show scheduled for the following month was canceled. This may all seem a bit extreme, given the fact that the diagnosis had not been verified, but horse owners in Africa have learned the hard way that you cannot over-prepare for African horse sickness.
What Is African Horse Sickness?
It is a viral disease, spread by the Culicoides imicola midge, a small biting insect. AHS is highly infectious, but not contagious — it requires the bite of the midge to transmit the disease — horses cannot transmit it to each other. The virus has nine different serotypes and it exists in three forms: the lung form, the heart form, and the mixed form. This makes it a very complicated disease to control. The mortality rate is 60 to 90 percent in domestic horses and in mules. Donkeys seem to have subclinical signs if infected. Zebras also do not appear to be affected by the disease which they can get, but do not have the symptoms that horses do.
What are the Symptoms?
In the lung form of the disease, the horse will have a very high fever, up to nearly 106ºF. This may be associated with coughing and sweating. The horse will stand with its head down and will have difficulty breathing. There may be a foamy discharge from the nose and mouth and the death rate can be as high as 90 percent.
The heart form of the disease has a lower death rate (60 percent) and is characterized by swelling of the eyes, especially the hollows above the eye, swelling of the lips, tongue, and head, preceded by a fever. Difficulty swallowing and colic symptoms are also present. Small, petechial (pinpoint) hemorrhages which can take up to eight days to occur, appear in the mouth and eyes in the terminal stages.
The mixed form can show a variety of the symptoms described above and is the most common form. The mortality rate for the mixed form is around 80 percent.
Horse Sickness Fever
This is the sub-clinical form of the disease that can appear in zebras and donkeys. Horses can also have this fever, which has a low mortality, but does result in a fever of 104-105ºF for several days. The horse will appear depressed and lethargic. Very few horses die from the horse sickness fever.
It is important to determine the diagnosis from a blood sample of the affected horse, preferably while it has a fever. Not only can a definitive diagnosis be achieved, but also the specific serotype can be identified.
There are other diseases that can show similar symptoms to AHS, so it is important to differentiate between AHS and other diseases by blood testing.
Treatment for AHS
There is no cure for AHS and the only treatment is supportive. Supportive treatment consists of anti-inflammatories, corticosteroids, and pain-killers. Sometimes antimicrobial treatment is also used, in case of a subsequent secondary bacterial infection.
Vaccinating for AHS
Modified live vaccines have been used with mixed results. The vaccine actually gives the horse a subclinical case of the disease. The premise being that the horse will have reduced symptoms and will develop antibodies to the disease. Horsemen have been advised in the past to vaccinate annually for this disease, but now this advice has become controversial. First of all, the vaccine doesn’t cover all of the serotypes (nine) of the virus. Furthermore, it has been found that the vaccinated virus may be mutating and infecting other horses with a form of the disease that is not recognized. However, the only licensed vaccine for AHS in Africa is the modified live vaccine.
This causes further problems in that horses that test positive for AHS cannot be distinguished from horses that actually had the disease and recovered, or horses that have been vaccinated with the live form of the disease. Many countries do not want horses that have subclinical AHS from a vaccination to enter their countries, as it might be possible for them to spread the disease. The vaccination period is usually from June 1 to October 31, which is before the hot, wet season in March and April when the midges are prevalent, and will hopefully insure the horses have the highest immune response at that time.
The horses do not transmit the virus to each other. A vaccinated horse will still have low levels of virus and it is possible for a midge to bite this horse and potentially transfer the disease to other horses or zebras.
Inactivated or killed virus vaccines would not have the ability to give the horse the disease, but may not be as effective in the antibody response. There is currently ongoing research to determine the best method of control via vaccination. In 2012, a killed virus vaccine was released, however it was not a registered vaccine, and the government still mandates that the modified live vaccine be used yearly in horses.
There is a killed AHS vaccine that is licensed in Egypt, but not in all of Africa.
There is also a concern about taking horses that have been vaccinated with the live vaccine into areas where AHS has not been reported. The thought process is that it may be possible for midges to bite these horses that basically have subclinical cases of AHS and transfer the disease to the native zebra population. Zebras do not develop clinical cases of AHS but are a reservoir whereby midges could potentially bite the zebra, pick up the virus and transmit it to other horses, thus spreading the disease.
To further complicate things, dogs have been found to carry the disease from eating infected horsemeat. They do not get the disease, but can be carriers. However, it is believed that the midges that transmit the disease rarely bite dogs.
Regulations Surrounding Reporting Cases
The Animal Diseases Act requires that all equines (horses, donkeys, mules) be vaccinated at least once a year with an approved AHS vaccine. Any confirmed cases of AHS must be reported by the veterinarian to the proper government agency.
As of this writing in April 2017, there have been nearly 100 cases of AHS in Africa reported so far, with the majority in the provinces of Kwal Zulu Natal and Mpumulanga.
Keep horses in stables at dawn and dusk when midges are most active.
If possible, have fans to direct air away from the horses — midges are too small to overcome forced airflow. Fans also disperse the carbon dioxide that horses exhale, which attracts the midges.
Cover openings in the stable (windows, doorways, etc.) with shade cloth, which has been shown to significantly reduce midge activity.
A valuable resource is the website: africanhorsesickness.co.za which provides an early warning map of emerging cases and other important information. It also updates where confirmed cases are woccurring.
Why Be Concerned About AHS If Not In Africa?
With the global transport of horses in the show, sale, and breeding trades, the equine world is really quite small. Since it has been shown that even within Africa the AHS virus has been able to mutate, it makes it a dangerous virus. Horses that have been vaccinated with the live vaccine in Africa could potentially transmit the disease if they are in an area where the biting midge that is the vector occurs.
Since the vaccine has not been shown to be totally effective against AHS due to the fact that it cannot protect against all nine serotypes, the continued use of a live vaccine is continually infecting horses, if you will, with subclinical cases of AHS. Right now the protocol for exporting horses that have been vaccinated is to test them pre-quarantine and then have them in a quarantine facility with insect control and test them again while in quarantine before export. However, there are not many countries that are comfortable with this level of control before allowing horses to be imported.
The very real concern is that there are 119 species of Culicoides midges in the U.S. alone, as well as species that already exist in Europe and South America. It is possible that these vectors that thrive in hot, damp climates, may well be able to ultimately transmit the disease in other countries.
The appearance of West Nile virus in New York in 1999 was completely unexpected and the U.S. was not prepared to control it. It took countless equine as well as human lives before a vaccine was developed. There is no reason to think that the rest of the world would be immune to this highly lethal horse disease. However, there is comfort in that it does not affect humans like West Nile did.
Another mode of transport for the disease could be found in the importation of zebras from Africa to other parts of the world, either for zoos or private collections.
Therefore, it is important to be aware of this disease whether you live in Africa or not. As a horse owner or breeder, it is crucial to be ahead of threats that may appear on the horizon, as we become an increasingly global business. As my friends in Namibia put it rather bluntly, “It is a terrible disease in that it kills horses in a particularly brutal fashion. Basically the horse suffocates in its own fluids.” Therefore it is understandable that when there were reports of horses with high fevers in the area when I was there in April, there was no time lost in canceling shows, clinics, and other gatherings of horses — even if it was a false alarm. Think equine influenza in Australia or West Nile virus in the U.S. and you begin to get the picture. Large congregations of horses are always more at risk than the horse in your backyard.
The weather is warming up now, and the grass is up. Mares have been bred that foaled earlier in the season, but really, May and June are big foaling months for most farms. As nature intended — because of mild weather and plenty of grass!