All content of this website is copyright by Mid-South Horse Review and may not be copied or reprinted without express written consent of the publisher and editor

Call Us: (901) 867-1755

The Mid-South Horse Review is available at over 350 locations throughout Tennessee, Mississippi, Alabama, Arkansas and Kentucky.
June 2021 issue is now available!


Pythiosis Insidiosum


Compiled by Nancy Brannon, Ph.D.

[Summary: the 3 Rs of Pythiosis]

We want to alert you to a serious disease that has crept its way into the mid-south: Pythiosis insidiosum.  The disease was brought to our attention by Jennifer Dunlap, DVM, who has recently been seeing cases of this disease in the mid-south area. If not properly diagnosed early, the disease can spread rapidly with fatal consequences. She referred us to Bob Glass at Pan American Veterinary Laboratories, who is the leading expert on this disease. Pan American Veterinary Labs has several white papers on the disease, as it affects horses, dogs, cats, and humans.

Kenneth Marcella, DVM, an equine veterinarian at KLM Equine in Canton, Georgia, also wrote an in-depth article on this disease for DVM360 magazine: “‘Swamp cancer’: the increasing threat of equine pythiosis.” (March 2011). Marcella wrote: “Know when to suspect this fungal infection so you can prevent its generally fatal consequences.” Jennifer Dunlap, DVM agrees!

Marcella wrote that “Pythiosis and infection by another member of the same class of organisms, Legenidium, are responsible for a number of infections in horses and other species.”

He also refers to Bob Glass of Pam American Veterinary Labs as the expert on this disease, and quotes Glass in his article. “Ten years ago, Pan American Veterinary Labs recorded fewer than 10 cases of pythiosis in dogs per year. Now they are identifying about 20 cases per month.”

Generally defined by Pan American Veterinary Labs:  “Pythiosis results from the infection with the fungal-like organism Pythium insidiosum , and occurs in Equines, Canines, Felines, Bovines, Humans and other species. This disease is also known as Phycomycosis, “Florida Horse Leeches,” Swamp Cancer, and other colloquial names. The disease is worldwide in distribution and is especially prevalent in tropical regions. Recently, numerous cases have been reported in the Midwest and Northeast United States and as far north as Wisconsin and Washington state.” (

Marcella also describes it as a “fungal-like infection causing cutaneous or subcutaneous, gastrointestinal, respiratory or multisystemic disease in many species of animals including humans. Horses are most commonly infected [with] devastating tumor-like nodular skin masses” (Marcella 2011). Ifnot caught early, these lesions grow rapidly and can lead to a fatal outcome.

Pan American Veterinary Labs describes the equine version: “Equine pythiosis is characterized by the development of cutaneous, subcutaneous, lymphatic, and intestinal lesions and less frequently by the involvement of bones and lungs (chronic pythiosis). Lesions caused by P. insidiosum can occur on any part of the horse’s body, but lesions of the lower limbs are more common due to more frequent contact with the organism in infested environments (stagnant water, grasses). The lesions often occur singly, but cases with multiple granulomatous lesions have been encountered. There are no reports of animal to animal, or animal to human transmission of this pathogen. If the disease is not treated in the early stages it is fatal in >95% of cases. In most cases treatment with antifungal drugs is not helpful.”

Jennifer Dunlap, DVM says the disease often appears to horse owners and veterinarians as a wound that just won’t heal, and it can easily be misdiagnosed. But, she says, if a wound has not healed in a few days, call your veterinarian! Dunlap says this is an opportunistic infection and the best way to assure a positive outcome for your horse is to catch it early. She says it used to be seen in tropical zones like Brazil, Florida, and other tropical climates. But now it has moved further north.

Marcella wrote: “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 is possibly because of changing climate and the increasing heat, humidity, and rain seen in this area.”

“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.” (Marcella 2011)

Glass says that there is no animal-to-animal transmission of pythiosis and he is highly confident that an infected horse cannot contaminate the environment. He says the majority of the cases in horses are dermal infections that start with a break in the skin.

“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 (itching) with the horse rubbing or even biting at the lesion.” (Marcella 2011)

It’s important to seek veterinary attention as soon as possible. When it grows larger and becomes tumor-like, “serum freely leaks from the raw, irritated skin. 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.” (Marcella 2011)

Pan American Vet Labs describes them:  “The formation of small hard coral-like masses termed ‘kunkers’ is an interesting characteristic of the disease in equines. These stony masses contain the viable hyphae [branching filaments that make up the mycelium of a fungus] of P. insidiosum surrounded by cell detritus from degranulated eosinophils [a type of disease-fighting white blood cell].

The best treatment is the enzyme-linked immunosorbent assay (ELISA) developed by Pan American Veterinary Labs that is specific for pythiosis fungus. The disease can be confirmed by evaluation of a simple blood sample. The test can also recognize the presence of Lagenidium.

“We have developed a ‘vaccine’ to pythiosis that can be used in confirmed cases, and this immunotherapeutic product works by helping the horse modulate the change from T2 helper to T1 helper cell response,” says Glass. This product has been shown to have an almost 100 percent cure rate for acute cases (< 15 days), but is less effective in chronic cases (> 60 days). The overall rate of cure is 75 percent for all cases, strongly suggesting that early diagnosis and treatment are crucial to success.” (Marcella 2011)

Dunlap uses the Pan American Veterinary Labs’ developed immunotherapy and Amphotericin (an antifungal medication used for serious fungal infections) in DMSO as a topical application. Dunlap also says that the lesion is very itchy and it’s important to keep the horse from chewing on it. Animax, or a steroid cream, can sooth the itching.

“If I could emphasize one thing to veterinarians,” Glass says, “it is to move pythiosis up on the diagnostic scale. If you see a horse that has a pythiosis-like lesion that does not respond to antibiotics and standard treatment in the first 10 days, you should think about pythiosis right away.” Dunlap agrees and emphasizes the need to seek appropriate veterinary treatment as early as possible in the stage of the disease.

We had the opportunity to talk with Bob Glass about the latest news on this disease, which has now been determined to be one of three related diseases/organisms. Glass said they have a blood test for Lagenidium and just last week (third week in August) developed a blood test for Paralagenidium. “We’re finding more and more that what was once thought to be pythiosis could actually be lagenidium. And now we’re seeing cases of paralagenidium, and there are probably more to come. A horse with a lesion could have any one of the three diseases. So accurate diagnosis is critical! The pythiosis treatment does not work, or does not work as well, on lagenidium. So we’re now working on developing a pythiosis-lagenidium combination treatment. We understand these two diseases fairly well.

“But paralagenidium is new and still poorly understood. We just validated our blood test for this disease last week (fourth week in August) and we have seen several animals infected with it.”

Another aspect that Glass said is that they’re looking at the level of cross reactivity among pythiosis, lagenidium, and paralagenidium. “They’re all in the family of Oomycetes,” in the kingdom Stramenopilia. They are not actually fungi, but are a closely related, although separate group of organisms. Therefore, anti-fungal medications are not effective with these diseases.

About the increased spread of the disease: Glass believes it is more a matter of increased recognition rather than increased incidence. Glass says there are more educated veterinarians who know what to look for and know to order a blood test for diagnosis. Glass said in the past year they’ve seen 1500 cases of this disease.

Other research that Glass is doing is, in cooperation with USDA, a clinical trial for a vaccine on the Chincoteague ponies. In addition to Pan American Veterinary Labs, he also has a vaccine company called Pythium Technologies. They’re in the process of, hopefully, developing a vaccine, trying to stop the disease outbreak in the Chincoteague ponies. Because of the large amount of rain last year, Glass says the disease really blew up among these ponies because they had to spend so much time in water. They had five horses die from it last year and have had one horse die from it this year. “It’s a very bad disease!” he said.

Pythium Technologies is a pioneer in applying immunotherapeutic and immunoprophylactic concepts to the treatment, control, and prevention of animal diseases including Pythiosis. Pan American Veterinary Laboratories is located in Lexington, Texas. Find more information at:
Glass, Bob. Pan American Veterinary Laboratories, Pythiosis Insidiosum
Glass, Bob. Pan American Veterinary Laboratories, Horse Pythiosis
Glass, Bob. Phone interview August 26, 2019.
Marcella, Kenneth, DVM. (2011) “Swamp cancer”: The increasing threat of equine pythiosis. DVM360 magazine. (March 1)
Additional resource:
Klinger, Sharon. (2018) “Equine Pythiosis: An Overview.” Today’s Veterinary Nurse. (Winter)
Sharon Klinger, RVT, VTS is with Premier Equine Veterinary Services, Whitesboro, TX, Winter 2018

Go Back »

Photo Gallery

Additional photos from this month's events.


Upcoming events for the next three months.

Media Kit

Advertising rates, display ad dimensions & photo requirements, mission statement & who we are, demographics of readership, and yearly editorial calendar.

Scroll To Top