Papillomavirus Part 3: Sarcoid Stresses
From Guest Author Dr. Angelika Schoster, Department of Clinical Studies, University of Guelph.
Papillomaviruses are generally strictly species-specific, meaning a certain type of virus will only infect one animal species. Infection of horses with bovine papillomavirus is the only known example (so far) of cross-species infection within this group of viruses. In cattle, bovine papillomaviruses (BPVs) cause warts, just like equine papillomaviruses cause warts in horses. It is now widely accepted that bovine papillomaviruses (BPV-1 and BPV-2) are also the primary cause of equine sarcoids. More recently, these viruses have also been implicated in some cases of equine dermatitis and equine hoof canker (more on that in another post).
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.
Six types of equine sarcoid are described based on their appearance: occult, verrucous, nodular, fibroblastic, mixed or malevolent. Each type tends to occur on different parts of the body, and they also differ in how they behave (i.e. how fast they grow). However, all six types share the tendency to recur and become more aggressive if they are disturbed accidentally (e.g. traumatized) or iatrogenically (e.g. surgically (but incompletely) removed). Unlike warts, sarcoids rarely disappear on their own (resolve spontaneously). Many different techniques have been used to try to treat sarcoids, including surgical removal, cryotherapy (freezing), carbon dioxide laser therapy, hyperthermia (burning), irradiation, photodynamic therapy, immunotherapy, and chemotherapy. Unfortunatley, to date there is no universally effective treatment for sarcoids (which is why so many different things have been tried and continue to be tried).
There is currently no effective vaccine against sarcoids, but knowing that a virus (BPV) is involved may ultimately allow researchers to one day develop a vaccine or other means of prevention. In the meantime, it probably wouldn’t hurt to keep your horse away from any warty cows that may be around.
Photo: A horse with recurrent aggressive sarcoids on the medial aspects of both forelegs (credit: M. Anderson)
