What is oomycosis?

Oomycosis (swamp cancer) is an infection caused by a water mold-like organism. The most common organism that affects dogs is called Pythium insidiosum. This organism is most commonly found in water, although it can be present in soil. This organism affects the gastrointestinal tract or the skin. An infection with Pythium insidiosum is called pythiosis. 

A similar organism, called Lagenidium giganteum forma caninum, can also infect dogs and that infection is called lagenidiosis. Like pythiosis, lagenidiosis can affect the skin but can also become systemic (body-wide), affecting the lungs, lymph nodes, or large blood vessels.

Large-breed dogs under six years old are most affected, perhaps because these dogs are most commonly involved in hunting or field trial activities in and around water. 

The most common sign of pythiosis is weight loss due to its effects on the gastrointestinal tract. These dogs can become emaciated (extremely thin). Vomiting and/or diarrhea may also occur. Often, affected dogs do not seem ill until they have been infected for a long time. The skin form of pythiosis is characterized by ulcerating nodules that drain and do not heal. Some affected dogs develop blockages in the gastrointestinal tract. 

“The most common sign of pythiosis is weight loss due to its effects on the gastrointestinal tract.”

Dogs with lagenidiosis may have progressive and invasive masses in and under the skin that may ulcerate and drain bloody, purulent (pus-filled) fluid. Dogs with systemic infection may experience sudden death from internal bleeding. 

How did my dog get this disease? 

Omycosis is typically acquired by drinking or swimming in water with zoospores. High-risk areas include swamps, bayous, and ponds; the most affected areas are U.S. states bordering the Gulf of Mexico. That said, many other states have seen documented cases of pythiosis in dogs, including Oklahoma, Arkansas, Missouri, Kentucky, Tennessee, the Carolinas, New Jersey, Kentucky, Tennessee, Virginia, California, and Arizona. Other countries have also reported pythiosis, including Australia, Brazil, Burma/Myanmar, Columbia, Thailand, and Japan. 

How are dogs with oomycosis treated? 

The most effective treatment for this condition is the surgical removal of all infected tissues. If a limb is involved, amputation may be required. Affected areas of the intestine will also need to be removed. Imaging (such as X-rays, ultrasound and/or CT) may be recommended prior to surgery to screen for non-removeable lesions in the abdomen or chest. 

Actively infected dogs may require hospitalization with intravenous fluid support and combination antifungal therapy, such as itraconazole and terbinafine, or intravenous amphotericin B. It is important for these dogs to receive high-calorie, highly digestible food to counter the weight loss that typically occurs in pythiosis. 

Antifungal medication is generally given for several months to reduce the risk of recurrence. There is a specific blood test for pythiosis and lagenidiosis that may be used to monitor the response to treatment. Abdominal ultrasound may be useful to monitor the health of the intestines, and other blood tests may be used to monitor organ system functions (e.g., liver and kidney). 

What is the long-term expectation for my dog with oomycosis? 

The prognosis for dogs with oomycosis is guarded to poor, and only 10%-25% of dogs are cured with medications alone. The outlook is somewhat brighter if all affected tissues can be removed surgically. 

The prognosis for dogs with systemic lagenidiosis is very poor as they often have infection in areas that can’t be surgically removed and the infection does not respond well to medications.