Papillomavirus Part 4: Equine Canker
From Guest Author Dr. Angelika Schoster, Department of Clinical Studies, University of Guelph.
Equine hoof canker (pododermatitis chronica verrucosa) is described as a chronic, proliferative dermatitis, beginning in the caudal part of the cleft of the frog and gradually expanding to the sole and wall. Equine canker is not lethal in and of itself, but because of where it occurs on the foot, and because it can be so difficult to treat and it recurs so often, it can severely compromise a horse’s ability to do its job (even if that job is just walking around in a field sometimes). The etiology of equine canker has been a topic of discussion for over 50 years, but so far no specific cause for the disease has been found.
Canker is usually diagnosed based on the appearance of the affected horny tissue and a distinctly fetid odour. The definitive diagnosis has to be confirmed by microscopic examination (histopathology) of biopsies collected from the affected tissue.
Treatment of canker has always been a dilemma for veterinarians and farriers because it is so difficult. Treatments used today can range from surgical intervention (removal of the affected tissue) to conservative medical management using topical medications of many different kinds.
The proliferative but non-metastatic nature of canker is similar to that of equine sarcoids. Like canker, sarcoids also tend to be difficult to treat and often recur. Both canker and sarcoids often include a mixture of proliferative and erosive changes in the skin secondary to overgrowth and thickening of the tissues. Due to these similarities, it has been speculated that bovine papillomavirus (BPV) might also be involved in causing canker. A large study in Europe (which will hopefully be published soon) has found evidence to suggest that a variant of BPV-1 plays a role in the development and/or maintenance of canker lesions in horses. Much more study is still needed, but this is another interesting example of how an infectious pathogen may ultimately play a role in a disease that’s origin has remained a mystery for decades.
More information about papillomaviruses is available in our archives.
Image: Severe canker in the frog of a horse's hoof (source: www.answers.com)
Equine sarcoids are skin tumours that affect horses, donkeys, mules and zebras. They are the most common type of tumour found in horses, and account for 35-90% of all equine skin tumours. Sarcoids are generally not life-threatening – unlike some other tumours, they do not metastasize (i.e. spread to other organs or tissues). However, they can be locally aggressive growths, meaning they may invade deep into the tissues immediately surrounding the primary tumour. They are also very difficult to treat, and therefore can affect a horse’s welfare or quality of life, depending on the size and location of the sarcoid. There is some evidence of a genetic predisposition to sarcoids in some horses as well. Involvement of bovine papillomavirus in the development of sarcoids was first suggested by Olson and Cook in 1951, who were able to reproduce sarcoid-like lesions in horses by inoculating them with tissue from bovine warts. Since then, researchers have used PCR to show that the vast majority of sarcoids contain BPV DNA, whereas normal horses and non-sarcoid skin tumours do not. Although BPV DNA has been found in healthy horses living in close contact with sarcoid-bearing animals, there is currently no evidence that BPV or sarcoids are transmissible horse-to-horse.
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).
Papillomaviruses (PVs) are small DNA-based viruses that are recognized causes of disease in many animal species and humans. More than 100 different human papillomavirus types (HPV) have been identified, which can cause a range of problems from annoying (but benign) warts to malignant cervical cancer. The ability of papillomaviruses to cause cancer was first recognized in animals, specifically with cottontail rabbit papillomavirus (CRPV), bovine papillomavirus (BPV) and canine oral papillomavirus (COPV). In general, PV infection causes benign disease in the vast majority of individuals, but the viruses have the potential to cause malignant disease in a small proportion of the mammals they infect as well.
