Equine cutaneous pythiosis is caused by the fungal-like organism Pythium insidiosum.
It is more common to tropical and subtropical climates, but it has also been found in Montana, Washington and New Jersey,
among other states.
It is most-often associated with horses exposed to lakes, ponds and swampy areas because the organism thrives in a warmer-climate
sand and requires an aquatic environment with organic substrates for optimal reproduction, says Michael Brashier, DVM, Dipl.
ACVIM, associate professor at Mississippi State University College of Veterinary Medicine.
The organism grows and thrives on grass and reproduces by oospores, which travel through the water to find new grass plants.
If it happens to find an animal in the process, infection can occur.
In horses, it can be an invasive, fairly rapidly progressive and proliferative pyogranulomatous disease, initially of the
skin. But the organism also can invade the subcutaneous tissues. The lesions are most commonly located on the distal extremities,
ventral chest and abdomen. Lesions on the lower extremities can encircle the limb and invade the tendons, ligaments and bone.
The infection may be aggressive, but depending on the horse, some have lesions that grow slowly.
With some horses, small lesions can regress on their own. Once the organism invades an open skin lesion, P. insidiosum encysts, further penetrates the tissue and can enlarge rapidly. It produces an ulcerative oozing lesion with a potentially
foul odor. The growing mass might be especially pruritic, and affected animals often are stressed and agitated, which might
lead to self-mutilation in an attempt to relieve the discomfort. Lesions of the limbs may involve both pain and lameness due
to greater tissue involvement.
"It's important to get a definitive diagnosis and not just treat empirically because there are other things that can look
the same as pythiosis that should be treated differently," says Amy Grooters, DVM, Dipl. ACVIM, professor at Louisiana State
University School of Veterinary Medicine.
Biopsy is important, and culture is something that is most successful using kunkers from the lesion as opposed to tissue.
Culture of the causative organism is difficult, but it is critical to a definitive diagnosis. At a minimum, the lesions should
be biopsied. ELISA blood tests are definitive, sources say.
"At this point in time, many equine practitioners are not aware of the disease, nor of the blood test, and therefore recognition
is important," says Dawn Logas, DVM, Dipl. ACVD, of the Veterinary Dermatology Center in Maitland, Fla. "If you've got a lesion
that you suspect is pythiosis, run the blood test to determine its diagnosis."
Culture is difficult because the organism does not thrive at low temperatures, and on biopsy you need a trained eye to identify
the hyphal structures of the organism. If in need, Grooter's lab at LSU has special antibody stains that will pick up the
hyphae and identify the organism.
"Once the lesions start, they can be locally quite aggressive and invasive and quickly invade vital structures making it impossible
to completely resect," Brashier explains. "During the summer, don't just assume it is granulation tissue. Have a good look.
If the granulation tissue has clefts in it with knobby pale yellow concretions in it, you need to assume pythiosis until proven
otherwise because you just can't let it get ahead of you. It also tends to be quite pruritic, which simple granulation tissue
Earlier the treatment, the better
It is best to catch pythiosis and treat it as early as possible. These lesions can become as large as 30 cm in diameter. A
small lesion may regress, but a larger lesion, especially on a limb, may be more difficult to treat because of the area it
covers and involvement of tendon and bone, once it's established. If you have a suspicious looking lesion, do the blood test,
and if it's positive, remove it, before it gets large enough to cause significant problems.
It may be treated by a combination of surgery and immunotherapy. A USDA-approved immunotherapeutic vaccine has been available
There are various forms of surgery depending on the extent and location of the lesion, i.e. laser surgery, Grooters says.
For horses, if the lesions are on the distal limbs, surgery may be difficult, where there may not be a lot of extra skin for