Papillomavirus Part 2: Plaque Problems
From Guest Author Dr. Angelika Schoster, Department of Clinical Studies, University of Guelph.
No, not dental plaque like you get on your teeth - in this case we’re talking about aural plaques, which are a type of skin lesion that some horses develop on the inside surface of their ears. These plaques are usually depigmented, meaning the skin cells of which they are made do not have the same pigment in them as the cells of the surrounding skin, making the lesions appear pale grey or off-white. Other than being considered unsightly in some cases, aural plaques usually don’t cause problems for the horse – they’re not itchy, sensitive or painful. They can occur in any horse, no matter the age, breed or sex. Occasionally some horses may develop similar plaques on the udder or around the anus or vulva (under the tail). The diagnosis of an aural plaque is typically made based on the horse’s clinical signs alone (i.e. a raised, depigmented skin lesion inside the ear that is not associated with any inflammation or discomfort).
The exact cause of aural plaques is still not clear, but using extremely high-powered electron microscopy, researchers have been able to demonstrate that in some cases there is a papillomavirus infecting the cells within these plaques. No one has yet been able to isolate the virus to determine what type of papillomavirus it might be. Aural plaques are not known to be transmissible from horse to horse, but since there may be a virus involved, it's prudent to consider the possibility that virus transmission may play a role in their development (although there are likely many other factors involved as well). Unlike regular papillomas (i.e. warts, caused by equine papillomaviurs), aural plaques do not disappear on their own. It the past, the only way to remove the plaques was to actually cut them away surgically. In more recent years, successful treatment has been reported using an immune-response modifier cream such as imiquimod (Aldara), which is used in people to treat several different skin conditions, including genital warts.
Photo: Aural plaque on the ear of a horse (source: University of Minnesota - College of Veterinary Medicine)
