Each year, I am one of thousands of horse owners in South Africa who spends six months anxiously awaiting the end of insect season. For us, biting midges are more than simply a nuisance---they are a mortal threat to our horses because they can carry the virus that causes African horse sickness (AHS), an untreatable and potentially fatal disease. AHS is a reality in this part of the world, so horse owners carry on with riding schedules and day-to-day responsibilities even when the risk is highest. Our daily routines do, however, revolve around the need to protect our horses from insects, and we are on constant alert for signs of AHS.
A few years ago, a particularly bad outbreak occurred in my area. It seemed like almost every day, from October to April, I would hear that yet another horse had succumbed to AHS.
The disease doesn’t discriminate. Top competition horses died along with school ponies and trail horses. The stories were enough to reduce you to tears: “My horse had eaten his dinner and was fine at 5 p.m.,” one dressage instructor told me, “but when I came back to do a nightly check at 9 p.m., he was dead.” Another woman lost an entire herd, despite her best efforts to save them. Stud farms closed down after losing valuable breeding stock. Newly imported horses were found dead in their fields.
Panic began to spread throughout the equestrian community. And I was soon to experience firsthand the awful challenge of fighting to save a horse from AHS.
African horse sickness virus is endemic to Southern Africa. Although it is not contagious, AHS is one of the deadliest equine viruses in the world, with a mortality rate of up to 95 percent.
Vaccines against AHS are available in South Africa, and most of us protect our horses with a series of two injections given sometime between July and December. The vaccine is an attenuated live vaccine that covers seven strains of the AHS virus, but it does not provide absolute protection. The resistance it confers to the different AHS strains depends on the horse’s own immune response; vaccinated horses can still come down with the disease. Of course we take other precautions, as well: We turn our horses out later in the day and bring them in before sundown during buggy months, avoiding dusk and dawn when midge activity is at its highest. We erect shade cloths over access points to barns and stables to discourage insects from entering those spaces.
We are especially vigilant when a dry spell is followed by heavy rains, because AHS transmission rates increase dramatically in these periods. Our horses often wear fly sheets even when temperatures exceed 100 degrees Fahrenheit. Some of us keep fans blowing over the stables at all times. And, even with all of these precautions, some horse owners take the temperature of all horses in their barns two or three times a day. Fever is one of the first signs of AHS, and any change in temperature will result in a call to the veterinarian.
You’d be forgiven for thinking these are the actions of overprotective owners, but these kinds of precau-tions are taken even by hardened South African horsemen with decades of experience.
So you can imagine what we thought when my sister’s pony---a little mare called Chilli---developed a fever and swelling in the depressions just above both eyes, which is a classic sign of African horse sickness. We were on the phone to the veterinarian straight away. A Welsh pony in the stable next door had contracted AHS days before and was currently fighting for her life. Her owners, tired and anxious, exchanged a look of worry with us as we brought Chilli in with her head hanging low.
Our family veterinarian drove out immediately and examined Chilli. To our relief, she told us the mare likely had the equine encephalosis virus (EEV), which is related to AHS but far less serious. EEV generally causes fairly mild signs---fever, loss of appetite and nasal congestion---and it has a mortality rate of less than five percent. The veterinarian left us with some anti-inflammatory medications and told us Chilli should be better in a few days. Relieved, we patted our horses goodnight and went home. We had gotten off lightly. Or so we thought.
A sudden illness
Two days later when I went to the paddock to bring in the horses, I noticed that my own horse, Peter Pan, had developed the same swelling above his eyes and fever. Peter Pan, a fine-boned gelding who carries blood of the native Basotho pony, had been perfectly healthy when I rode him earlier that day, so I knew this was a recent development. I called the veterinarian to come and treat him for encephalosis.
But after examining Peter Pan, the veterinarian decided to draw blood. That was the first sign that something was up. I tentatively asked, “Why are you drawing blood?” I already knew the answer, but I was hoping against hope that she wouldn’t confirm my fears.
“I don’t think this horse has encephalosis,” she said. Apparently, his clinical signs were more severe than Chilli’s. Left unsaid was what she thought he had. Peter Pan had been vaccinated against AHS, but that didn’t mean he couldn’t contract the disease. If he had, we’d caught it early, but that might not be enough to save him. I saw the color drain from my mother’s face. She knew, too, and all of the horror stories we’d heard came flashing back.
I swallowed hard and asked, “What form of AHS do you think it is?” AHS comes in three forms: cardiac, respiratory and mixed. The cardiac form has a slower onset of signs and a death rate of about 50 percent. The respiratory form has a 90 percent mortality rate. Signs of that form occur suddenly and death comes quickly. Horses can also have a “mixed” form of AHS, with the characteristics of both and a mortality rate of about 80 percent.
Blood tests would confirm the diagnosis, the veterinarian said, but she believed Peter Pan had the cardiac form. Swelling affecting the eyes and head is a typical a sign of cardiac AHS, she noted, and the gelding wasn’t coughing or showing signs of breathing difficulties----the first signs of the respiratory AHS. Further, the lack of respiratory-related signs meant that it was unlikely that he had the mixed form of AHS.
Although we wouldn’t know for sure until the blood tests came back, we would work under the presumption Peter Pan had AHS, providing him with supportive care. There is no cure or specific treatment for the disease, so care involves managing signs and keeping the horse hydrated.
The veterinarian prescribed non-steroidal anti-inflammatory medications to control Peter Pan’s fever and ease his discomfort, and then we began our long wait. The veterinarian cautioned against getting too hopeful and told me to watch the gelding as closely as possible over the next few days, taking his temperature several times a day.
She came out to check Peter Pan daily. Over the next two days, the gelding’s temperature would spike and then drop again as he fought the disease. Mostly, he would lie flat out on his side, lacking the energy to respond to my touch or the sound of my voice. I sponged cool water over him to help relieve the worst of the fever. When he did struggle to his feet, he would stand dead still in his stall, his head hanging to his fetlocks, the swellings in his jaw and eyes becoming more and more obvious.
Eventually, his tongue developed dark areas referred to as “bloodspots” because they are caused by bleeding just under the surface of the mucosa. I knew that if I started seeing red spots in his eyes, it was game over. Bleeding in the membranes and conjunctiva of the eyes is almost always a sign that the cardiac form of AHS is reaching a terminal stage.
But Peter Pan wasn’t ready to give in just yet. On the third day, the constant monitoring and nursing care seemed to have paid off: Peter Pan turned a corner and began to improve steadily. I knew he would make it when I came to check on him one morning and found him standing in his stall slowly eating his hay. It was the first time in days that he’d shown interest in anything, and a glimmer of the old Peter Pan was finally visible.
His temperature gradually returned to normal, and his swellings subsided over the next week or so, while the bloodspots also disappeared. Peter Pan had lost a lot of weight and condition, but that was something we could remedy in the coming months.
Looking back on our ordeal, I am thankful that Chilli had encephalosis. Without that scare, I might have missed the early signs of AHS in Peter Pan. And the fact that we caught his infection early may have helped tip the scales in his favor. From what I have seen of AHS, a delay of 12 or even six hours may have meant that the medications would not have been able to control the fever, and he may not have made it.
Though Peter Pan recovered from AHS, he has never been quite the same since. Before his illness, I had been riding him in dressage and competing occasionally. But after he recovered, he had a string of other viral infections that slowed him down. He is semiretired now and ridden from time to time, but he lacks the suppleness and fluidity of movement that he used to possess. Whether this has anything to do with his bout of AHS, I’m not sure, but I can’t help but wonder if his story would have taken a different path had it never happened.
Sidebar: Could it happen in the United States?
African horse sickness (AHS) hasn’t been found in the United States, but its arrival could be just around the corner. Outbreaks have occurred in Spain and Portugal as well as North Africa, and the nature of the virus means that North American equestrians would be wise to arm themselves with information about the disease.
The increased incidence of infections with the bluetongue virus, which is closely related to AHS and is transmitted by the same Culicoides insects, is a warning sign. Before 2006, it had not been seen in Northern Europe, but its global spread demonstrates the ability of these “exotic” viruses to move into areas previously free of the disease, even when these areas lack the usual vector. For example, during the Northern European outbreak, which included the United Kingdom and even Scandinavia, the vectors were indigenous midge species. That means that indigenous insects capable of transmitting AHS may well exist throughout most of the world.
Several factors contribute to the spread of these diseases, including the changing climate and the more frequent international transport of animals and animal products. As average temperatures rise, transmission and infection rates increase. In South Africa, C. imicola is the insect most commonly responsible for carrying AHS.
Global warming is changing the distribution patterns of C. imicola, so it could soon be seen outside of Southern Africa.
U.S. horse owners also have to worry about C. sonorensis, a species of midge native to North America that has been shown to have the characteristics necessary to serve as an AHS vector. Incidentally, this species is most often responsible for transmitting the bluetongue virus in North America.
Although an AHS vaccine is available in South Africa, bringing it to the United States, even in the event of an outbreak, isn’t a straightforward solution. As with any product made with an attenuated live virus, the AHS vaccine carries the risk that the organism introduced via the vaccine could revert to a virulent form and enter the insect population. In a naïve horse population, as would be found in the United States, such a development could be devastating, particularly given the mortality rate of AHS.
Ongoing research into different AHS vaccines may eventually yield better protection for horses, and South Africans have a wealth of experience with other control measures that can be shared among international veterinary agencies if the need arises.—Sophie Baker
Sidebar: African Horse Sickness
Definition: Highly fatal, insect-borne viral disease endemic to sub-
Saharan countries. Causes: AHS is caused by infection with one of nine strains (serotypes) of a virus in the Reoviridae family. Two insect vectors, both of which are biting midges, are known to carry the virus: Culicoides imicola and Culicoides bolitinos. The
disease is transmitted between horses, mules, donkeys and zebras through the bites of these insects, peaking during warm months of the year when insects are most active. Zebras are thought to be reservoir hosts, meaning they harbor the virus for long periods of time without becoming significantly ill. Signs: There are three types of AHS: A cardiac form, which leads to swelling in the depressions just above the eyes, along with the head, neck, throat and shoulder, in addition to a fever between 102 and 106 degrees Fahrenheit; a respiratory form, characterized by difficulty breathing, coughing, a frothy discharge from the nostrils and a fever of 104 to 106 degrees Fahrenheit; and a “mixed” form with characteristics of both. The mortality rate is between 50 and 90 percent depending on the form of the disease and severity of the case; horses typically die within a week of onset of illness. Diagnosis: The only conclusive test for AHS requires detecting the virus in a blood sample, which can take several days. Due to the fast-moving nature of the disease, most diagnoses are made on clinical signs alone, taking into account environmental factors such as insect activity and weather patterns.
Treatment: Intravenous fluids and other supportive care. Non steroidal anti-inflammatory medications and corticosteroids are used to address the fever and other clinical signs. Antibiotics may be administered to prevent secondary bacterial infections, particularly in respiratory cases. Horses may also be given intravenous fluids.
This article first appeared in the May 2017 issue (#476) of EQUUS magazine