Corner 10/01: Warts & Aural Plaques
by Frosty Franklin, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069
Equine papillomavirus is
the virus that causes the development of benign, proliferative skin
tumors in horses. Microscopic evidence indicates that two different
clinical presentations occur in the equine from the papillomavirus:
(1) warts, and (2) aural plaques.
Equine warts are small,
gray to pink cauliflower-like growths that are usually found of the
muzzle, around the lips, nostrils, and eyes and occasionally on the
lower legs. Warts can also involve the penis and vulva. Lesions commonly
develop on young horses, 6 months to 3 years of age. The lesions range
in size from 5mm to 20mm and are generally multiple in numbers. Ten
to more than 100 warts are common. The incubation period is about 60
days. The warts reach maturity in 4 to 8 weeks. Then usually spontaneously
regress within 4 months. Some cases may last more than a year. Cases
that last more than 2 years may suggest an immune response deficit.
Warts are contagious. Transmission
occurs by direct contact (nose to nose) and indirectly via fomites like
fence posts and feed buckets. The virus remains vial in the environment
for up to 3 weeks at room temperature. Yearly infection of young stock
on large breeding farms has been reported. Lesions on the penis and
vulva can result in transmission of the virus by breeding. Affected
individuals should be kept away from the breeding herd. Disinfection
of the premises and equipment with lye, formaldehyde, iodine, and chlorhexidine
helps decrease spread of the virus.
Diagnosis is usually based
on the clinical signs, history, and appearance. A large wart on the
lower leg might be confused with a sarcoid. If the diagnosis is in question,
a biopsy specimen may be collected and submitted for histologic diagnosis.
Usually treatment is unnecessary.
The warts are harmless and almost always regress spontaneously. Management
practices to limit the spread of the virus include insecticides, isolation
of infected individuals, and disinfection of feed troughs, water buckets,
Under certain conditions
treatment is desirable. For instance, when a large mass of warts are
interfering with biting or other tack causing a delay in training. Surgical
excision and freezing with liquid nitrogen (cryosurgery) are often recommended.
Various topical ointments have shown some success, however, compounds
need to be very carefully applied and the horse prevented from licking
and chewing the treated area. EqStim (immunostimulant) given intravenously
has had reported success in both prevention and treatment of equine
warts. Any treatment of warts that creates an inflammatory response
may increase the risk of white hair and skin depigmentation.
Aural plaques are clinically
recognized as different from warts. They are benign, raised, white to
pink lesions that occur bilaterally on the inner surface of the ear.
They respond poorly to treatment and do not spontaneously regress. They
were thought to be caused by biting flies and are sometimes incorrectly
described as "ear fungus". Infrequently, these plaques appear on the
anus, penis and vulva. Aural plaques can be found on any horse older
than one year of age. These plaques can become severely irritated by
biting flies and horses become very defensive about having their ears
Treatment with a soothing
ointment, like Mentholatum, to the inner surface of the ear can be helpful.
I have also tried the various corticosteroid/antibiotic ointments like
Panalog or Otomax with a success. These ointments will relieve the inflammation
but the plaques remain. I am not aware of a consistent treatment reported
for aural plaques.