"Swamp cancer": The increasing threat of equine pythiosis - DVM
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"Swamp cancer": The increasing threat of equine pythiosis
Know when to suspect this fungal infection so you can prevent its generally fatal consequences


Why the increase in infection?

Photo 3: An angry, red, ulcerated lesion can be seen on the medial RF of this horse. It has been kept under a wrap but is not responding to treatment.
Climatic changes may have as much to do with increased pythiosis cases as any other single factor. In the United States, most cases of this disease come from two states: Florida is responsible for 60 percent of recorded infections, and Texas accounts for another 25 percent. Georgia, Louisiana, Mississippi and Alabama contribute another 10 percent, so the hot, generally wet and humid south is the ideal area for pythiosis and related fungal infections.

This area has experienced various stages and degrees of drought followed by wetter weather over the last few years. In times of low rainfall, lakes, ponds and streams recede, and plant growth occurs in these previously flooded locations. When wetter conditions follow and water covers this new vegetation, the ideal situation for fugal infection is created.

Photo 4: The same horse in Photo 3 after vaccine treatment. The lesion is much drier and not as red, moist or angry. The horse is also no longer pruritic, indicating an end to the allergic phase.
"We know that pythiosis and similar organisms parasitize plants, fish, algae and crustaceans," says Glass. "These organisms produce spores that move through the water looking for new plants to invade, and when horses, dogs or humans are in that wet environment, they are at risk of becoming infected."

Such situations are essentially "dead-end infections" because Pythium species cannot replicate outside of a plant environment. "We know that there is no animal-to-animal transmission of pythiosis, and we are highly confident that an infected horse cannot contaminate the environment," says Glass. "Ninety-nine percent of the cases in horses are dermal infections that start with a break in the skin." This explains the much higher incidence of pythiosis in hunting dogs and horses, both of which spend a great deal of time in wet grasses, swampy or boggy locations exposed to weeds, briars and other irritative objects that can cause small lacerations on the lower limbs.


Photo 5: An early red, uneven granulation bed is noted on the caudal heel and pastern of this horse, Ebony. She is slightly lame and pruritic as well.
Pythiosis typically begins as a small irritated area usually on the distal limb of a horse. This may be initially thought to be a sting, bite or small puncture, and the mild-looking lesion usually is not a cause of concern. Owners will generally begin cleaning the area and treating it with various topical antibiotic or anti-inflammatory creams. But within a few days, the lesion is markedly larger, red and irritated. It may also begin to be pruritic with the horse rubbing or even biting at the lesion.

Veterinary attention is sought at this point, and the lesion now looks more like a possible snake bite or foreign body puncture with significant reactive granulation tissue and necrosis. Radiography, ultrasonography and other diagnostic tests are unrewarding. Antibiotic and anti-inflammatory therapy is initiated at this stage, but the lesion continues to grow. It is tumor-like now, and serum freely leaks from the raw, irritated surface (Photos 1 and 2, p. 2E). Aggregates of necrotic cells form in the lesion, producing yellow to grey, pea-sized, gritty, coral-like bodies called kunkers. Although these structures are not specific to pythiosis, their presence is evidence enough to make one highly suspicious of fungal infection.

Histopathologic examination samples taken from the horse at this point may or may not be helpful. A report of "inflammatory response" with or without the presence of hypheal elements is usually returned. Special stains are needed to see the fungal hyphae in tissue, and even with correct staining the sensitivity is only 60 to 70 percent, so pythiosis can be missed unless fungal infection is suspected. If such an infection is suspected and antifungal therapy is started, the horse will likely still not respond. The lesion will continue to grow and eventually erode ligaments, tendons and bone and lead to death in 95 percent of cases within six months.

Photo 6: After vaccine treatment, the wound on Ebony as compared to Photo 5 looks drier, flatter and less irritated. The healing outer edges of the mass look almost burnt, and the central area is no longer oozing serum.
This rapid tissue destruction is solely the result of a massive allergic response to the presence of fungal hypheal elements on the part of the horse. T2 helper cells drive this reaction, and mast cells and eosinophils dominate the cellular population. Some horses (about 5 percent) are able to switch to a T1 helper cell response that effectively kills the organism and switches to a lymphocyte and monocyte population that promotes healing.

Glass notes that a serologic survey of hunting and retrieving dogs from at-risk areas showed that 15 to 20 percent of these animals have antibodies to Pythium, indicating previous exposure and successful destruction of the fungus. While no such serologic survey has been done in horses, Glass suspects similar findings, noting that it is "just not that easy for all horses to become infected with Pythium species since many more horses are exposed than become ill, and all the factors required for successful infection are not yet known."


Source: DVM360 MAGAZINE,
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