Understanding vasculitis in horses *H&H Plus*

“It started as a small patch on a hindleg and spread around the pastern,” says Megan Bills of the skin soreness and swelling that has grounded her showjumping gelding Solisto. “As that finally started to heal, scabs appeared on the other hindleg — which blew up like a balloon.” Solisto has been diagnosed with pastern and cannon leukocytoclastic vasculitis, a problem that is proving difficult to treat. [subscriber-content]“We’ve tried so many things, including two different courses of antibiotics and two courses of steroids,” explains Megan, who has owned the French-bred gelding, now 13, for eight years without previous issues of this kind. “Solisto has had blood and allergy tests, which came back normal, and we’ve changed his bedding type and kept him out of mud and sunlight. He has missed the whole summer season and both hindlegs are still swollen.” What is vasculitis? According to Horse & Hound vet Karen Coumbe MRCVS, the term vasculitis refers to inflammation of blood vessel walls. “Vasculitis can affect a vessel of any size, type or location in a horse of any age, breed or sex,” she says. “It is a veterinary description of a disease process, not a proper diagnosis in itself.” Karen says vasculitis cases can present considerable diagnostic and therapeutic challenges. “The damage causes the blood vessels to leak, which results in bleeding and loss of fluid into the body tissues, leading to inflammation and oedema [swelling],” she says. “In addition, the organ or tissue that these faulty blood vessels were supplying or draining may suffer further damage and loss of function. “The resulting disease process and prognosis will depend on the organ system involved and the extent of the problem,” adds Karen. “In horses, the tiny veins — called venules — that drain blood away are most commonly affected, resulting in a circulatory disruption, rather like a blocked plughole in a sink. In a few diseases, however, such as equine viral arteritis [EVA], the arteries are affected.” Karen points out that many forms of vasculitis involve the skin — meaning that the reaction is visible, at least, and the problem recognisable. “A vet may take small biopsy samples to aid the diagnostic process and check for infection,” she says. “With skin lesions, the lower limbs are more commonly affected. One diagnostic clue is that lesions are rarely symmetrical. “If there is no skin involvement, diagnosis may be more difficult. Oedema is another clue, but should not be overinterpreted as horses can develop swelling for many different reasons.” Among the recognised syndromes characterised by vasculitis are infectious conditions such as equine infectious anaemia (EIA), and the exotic diseases African horse sickness and equine ehrlichiosi (known as Potomac horse fever). Vasculitis may also occur due to drug reactions or immune system problems. “Of the serious forms of equine vasculitis, Purpura haemorrhagica [see box, right] seems to be the most widely recognised in the UK,” says Karen. “This is triggered by an allergic-type reaction, where the body’s immune system goes into self-attack mode and responds inappropriately. “The condition is most commonly seen following infection with strangles, but can also be an unfortunate complication of other infections such as equine flu — or, more rarely, other chronic bacterial or viral infections. “Some horses develop Purpura despite having had no known recent disease process. But as the condition tends to appear two to four weeks after the disease that started it, the connection may not be made. Whatever the cause, it would seem that it is the result of overproduction of specific antibodies. These deposit on to blood vessel walls and activate a strong immune response.” Karen adds that horses who develop Purpura after strangles have stronger antibody responses to the proteins from the strangles bacteria — which supports the theory of an immune-mediated mechanism for the disease. Confusing condition So where does Solisto’s lower limb problem fit into the picture? “Pastern and cannon leukocytoclastic vasculitis is a frustrating condition,” says Karen, who adds that the sore, scabby appearance is sometimes mistaken for mud fever. “It is the commonest vasculitis affecting the skin, yet its causes are not clearly understood. Treatment can be tricky, with regular relapses. “What we do know about leukocytoclastic vasculitis is that lesions are mostly found on the inside and outside of unpigmented legs. White skin typically develops an oozing discharge, with crusts that are often circular in shape. Cases that occur on coloured skin result in hair loss and scaling of the skin. “Hindlimb vasculitis has a less positive outcome than lesions on the forelimbs,” Karen adds, adding that the disease is more often seen in the summer months in areas with a lot of sunshine. “It is thought to be aggravated by sunlight, but experts vary in their opinion on how significant this is as a trigger. Certainly, many cases are improved by protecting the affected areas from direct or reflected ultraviolet light by stabling in daylight, using high-factor sunscreens and leg wraps or so-called sun socks.” Some cases worsen in winter, suggesting that reduced blood circulation in the extremities due to cold weather could be a factor. “Multiple factors may be involved, from buttercups to boots,” says Karen. “With any problematic skin condition, it is sensible to consult a specialist vet with expertise in skin cases. “It is important to differentiate this from other skin diseases that also affect the white parts of the lower limbs, which may be associated with liver disease,” she adds. “Your vet may run blood tests to rule out liver damage.” Additional physical conditions include trauma-induced focal or regional vasculitis, explains Karen, or light-activated vasculitis of non-pigmented skin known as photosensitisation. “Other types of vasculitis can affect different parts of the body,” she says. “A horse may develop skin rashes, oedema around the face and ears, difficulty in breathing due to swelling around the voice box or lung damage. “Treatment involves removing whatever is causing the immune system to misbehave, if this can be identified — which often is not possible. It is important to suppress the immune response and reduce inflammation, which may require judicious use of steroids, as well as providing supportive care and good nursing. “These are serious conditions requiring careful management and benefiting from being caught early and treated aggressively.” ‘His legs were like tree trunks’ When Claire Howie’s event horse didn’t come across the field when called as he usually does, she suspected something was wrong. “I saw he was covered in hives,” she recalls. “His face and legs were swollen and he was staggering, as if he was drunk.” Claire’s immediate thought was that Obi had suffered an allergic reaction to something he had eaten, but Campbell Thompson, of Nantwich Equine Vets, diagnosed a case of Purpura haemorrhagica. “Campbell walked into the stable and said: ‘That’s one sick horse,’” says Claire. “You could feel the heat on Obi’s body; he was white with sweat between his back legs. His limbs had doubled in size from the knee and hock downwards.” Immediate treatment included intravenous steroids and antibiotics, together with cold hosing to reduce Obi’s temperature. “I knew that the condition was incredibly rare and could be fatal, so the next 48 hours were touch and go,” says Claire, who endured a sleepless night. “Obi was still wobbly the next morning, but he whinnied when he saw me and wanted his breakfast. He continued to perk up over the next few days.” While she hasn’t been able to pinpoint a trigger, Claire is relieved that Obi is now fighting fit — describing the 15-year-old gelding’s success soon afterwards at the Retraining of Racehorses national championships at Aintree as “an emotional win”. Ref Horse & Hound; 21 November 2019 [/subscriber-content]