Pythiosis is a noncontagious disease caused by Pythiosis insidiosum, a fungus-like, protozoan organism. Until recently, it was considered to be a threat only to horses living in tropical or subtropical environments; however, it has been seen in many other areas of the United States, such as Illinois, New York, and even Wisconsin.1 Although dogs, cats, and even humans can be affected by pythiosis, horses are most commonly infected.
EXPOSURE AND INFECTION
P insidiosum feeds on plant material and was originally believed to live in ponds that were not spring fed. More recently, it has been discovered that the organism can be found in standing and stagnant water and in the moist soil left after the water dries up.2 Animals become infected when it finds its way into small scratches or wounds or when they inhale or ingest spores. To date, there are no documented cases of animal-to-animal or animal-to-human infection. There has been no correlation to breed, age, or sex, so all horses are equally at risk if living in an infectious environment.
Infection with P insidiosum can cause two types of disease: lesional and respiratory. The lesional form manifests as tumor-like masses and ulcerated lesions that can affect any part of the body. They can be cutaneous, subcutaneous, and/or intestinal. Rarely, with chronic pythiosis, lesions develop in the bones and lungs. Although relatively uncommon, lesional equine pythiosis can be fatal (>95% of cases) if it is not treated when it first develops.3
In horses, the most common lesion locations are the lower legs. This is logical because the legs are more exposed to organisms present in stagnant water and grasses. The next most common site of lesions is the abdomen. Generally, horses only develop one lesion. If multiple granulomas appear, they tend to be in the same general area. If not diagnosed and treated, the masses grow very large and ulcerate.
The lesions seem to be very pruritic. They have a strong odor, draining tracts, and a core lesion that is a hard mass called a “kunker.” They also produce a serosanguinous discharge that, along with loss of tissue, can lead to anemia. Lesions that erupt on parts of the body other than the limbs (eg, abdomen, chest, face) are usually circular and less lobulated.4
DIAGNOSIS AND TREATMENT
Histopathology of samples taken from the characteristic lesions can be helpful in diagnosing pythiosis by identifying microabscesses with eosinophils.3 However, the organism itself is very hard to identify in biopsy samples. A definitive diagnosis can be made using enzyme-linked immunosorbent assay (ELISA) testing of serum for pythiosis antibodies (Pan American Veterinary Laboratories; pavlab.com).5 The results are characterized as positive, borderline, or negative. Patients with borderline results should be retested in 30 days to confirm the diagnosis before treatment; treatment may also be started without retesting.
Because P insidiosum is not a true fungus, antifungal drugs have proven ineffective as therapy.6 Treatment of diagnosed pythiosis involves immunotherapy with a product that is described as a vaccine, although it cannot be used as preventive therapy. The full treatment protocol is a series of three 1-cc subcutaneous injections given on days 1, 7, and 21. The patient is rechecked on day 28, and if the lesion is smaller but not totally cured, a second 3-injection course can be administered.7 This immunotherapy protocol was developed in the 1980s, and the early form of the vaccine cured 100% of acute cases (horses infected for <60 days). It was less successful in chronically infected horses because of immunosuppression in those patients caused by loss of proteins and electrolytes through the open wounds.
A newer form of the therapeutic vaccine cured 50% of the chronic cases that had previously not been cured. The overall cure rate for acute and chronic cases combined is 75% with this product.6 Occasionally, surgical excision is needed to remove a larger lesion after it is shrunken by the 3-injection course of immunotherapy.
The respiratory condition caused by P insidiosum, Pythium-induced allergic syndrome, produces very mild clinical signs that include a sporadic, dry cough and clear nasal discharge. It is caused by ingestion of Pythium spores that have been trapped in hay that was grown on land where water sat stagnant, perhaps after a torrential rainfall or flood. The organisms still in the grass are picked up when the hay is baled. Horses may also inhale spores that are aerosolized when the wind blows up dirt that was once covered with stagnant water.
Bronchoscopic examination of horses with these clinical signs shows inflamed mucous membranes in the nose and throat. Definitive diagnosis is also done with ELISA testing, and treatment is the same as for the lesional disease. It is common to have multiple cases in the same training or boarding facility at the same time. In the practice where I work, most of the positive pythiosis cases we see are Pythium-induced allergic syndrome.
PRESENTATION: CASE EXAMPLES
Although pythiosis was once considered rare in the United States, in 2015, the practice that I work in, located 60 miles north of Dallas, saw more than 100 positive cases in a 6-month period. Approximately 40% of these cases involved cutaneous lesions, some that were originally diagnosed as summer sores. When these did not respond to steroid injections, we tested them for pythiosis, and the results were positive.
Included with this article are a few examples of how pythiosis cases present and respond to therapy. All cases described were treated with the newer form of the immunotherapeutic vaccine.