The term “SWAMP CANCER” is used to describe a growth on horses that is not a cancer but in fact a fungal infection – Phycomycosis caused by the organism Hypomyces destrens. This infection probably acquired this name because they commonly occur on horses in swampy areas of the tropics and grow rapidly invading the tissue and developing a large fibrous reaction around them. Although they more typically occur in swampy, muddy or flood conditions they are also seen on horses in dry dusty paddocks. We do see an increase in number of cases during the warm humid conditions of the wet season.
The fungus is a normal soil organism that gains access through an injury, leech bite or possibly also insect bites. A firm swelling develops, ulcerates and exudes a blood-tinged fluid from holes [sinuses] in the growth. Characteristic granular yellow bodies called “kunkers” are found in the tumour. In long standing cases the local lymph vessels and nodes become enlarged and may become infected. Lesions are seen mainly on the legs and lower abdomen but may also occur on the head and recently noted inside the mouth of a horse. The horse often bites and kicks at the lesion and may resent examination.
Surgical excision is the most common treatment and general anaesthesia is usually required to allow careful dissection of the lesion from underlying structures + electrocautery of the underlying fibrous tissue. If the lesion is over / around tendons, joints, etc dissection may sometimes be very difficult if not impossible. In some cases a vaccine may be used to boost the horse’s immune system to fight off the infection. Topical sprays have been tried over the years with little success.
A similar growth that occurs with another fungus called Basidiobolus haptosporus. This infection is usually drier and slower growing. Unfortunately there is no vaccine against this organism so early excision is vital.
After surgical excision and electrocautery of the underlying tissue there will be an open wound approximately 1cm larger than the original lesion. This should be hosed twice daily and wound spray applied. Antibiotics and anti-inflammatories may be dispensed depending on the location and size of the lesion.
If the horse is not vaccinated against tetanus a short-acting anti-tetanus injection will be given. Good quality feed is essential after surgery as the horse has usually lost a significant amount of blood due to continual seepage from the growth and is often anaemic. The lesion should be watched closely for signs of re-growth and the clinic contacted.