Pythiosis (Water Mold Infection) In Dogs

Causes, Treatment, and Prevention

dog in lake

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Pythiosis (otherwise known as water mold infection) is a rare fungal infection that can affect the skin or the gastrointestinal tract, causing sores and weight loss. It is caused by the aquatic mold Pythium insidiosum. Pythiosis can strike not only dogs, but many other animals as well, including cats, horses, and even humans. The infection is generally contracted when animals with open sores drink, stand, or swim in stagnant contaminated water. This serious condition has a poor prognosis, but with treatment, many dogs survive.

What Is Pythiosis?

Pythiosis, sometimes called water mold infection or swamp cancer, is a fungal infection caused by an organism called Pythium insidiosum. While not a true fungus, P. insidiosum is a type of water mold. Pythiosis is characterized by gastrointestinal signs and/or dermatological signs and can be life threatening. It is contracted when a dog swims in contaminated water and/or drinks contaminated water and not only affects dogs. Cats, horses, cows, and even people can succumb to pythiosis as well. 

Symptoms of Pythiosis in Dogs

Dogs can be infected with P. insidiosum for a long time before symptoms become severe. The most common symptom of this infection is weight loss, caused by gastrointestinal inflammation. Dogs with pythiosis that affects the skin typically develop sores that refuse to heal.


The most common sign seen in dogs that contract Pythiosis is extreme weight loss. This is usually is due to vomiting and diarrhea that is also caused by the disease process. Severe cases can lead to blockages in the dog's intestines.

Pythiosis can also affect the skin, causing ulcerative, draining lumps and fistulas that never seem to completely heal. These are typically located on the legs, abdomen, chest, face, and/or tail. Depending on the degree of inflammation associated with the nodules, the areas may either be a bit spongy feeling or quite firm to the touch.

Dogs with this disease often are lethargic and frequently develop a high fever as the infection progresses.

Cause of Pythiosis in Dogs

P. insidiosum is mostly found in unclean or stagnant water in ponds, bayous, or swamps. While not a very common pathogen, it exists in many places around the world, including Australia, Asia, and South America. In North America, it is mostly found in the states bordering the Gulf of Mexico, but on occasion is found in other states, as well.

The mold enters the dog's system through a break or sore on the skin—leading to the cutaneous type of infection—or when the dog drinks stagnant water contaminated with the mold, which causes the gastrointestinal version of the infection. In some dogs, both skin and GI tract can be affected.

While any dog is at risk, the most commonly infected dogs are young adults, and most often retrievers or other breeds that commonly swim or play in water or are used for hunting and retrieving.

Diagnosing Pythiosis

Many cases of Pythiosis don't ever develop clinical signs until the dog has been infected for a long time. This makes it difficult to diagnose based solely on a patient history of recently swimming in a pond. Instead, your vet may want to run various lab tests to check for signs of infection, including specialized tests that determine the presence of the P. insidiosum mold, such as biopsies.

Definitive diagnosis of the condition usually comes from the results of histopathology tests that examine tissue removed from the skin or intestines or swabs taken from secretions or pus draining from skin sores to determine if P. insidiosum is present. The veterinary pathologist might also attempt to culture, or grow, the mold. However, the genera Pythium as whole is difficult to culture, as it doesn't grow very well on the routine fungal culture media, and bacterial contamination that can skew results is common.

Generally, your vet will also order specialized blood tests to look for the presence of the mold, as well as general blood tests to check the dog's overall condition. Ultrasounds are frequently used to visualize your dog's intestines and check for any abnormalities in the thickness and integrity of the intestinal walls.

Treating Pythiosis in Dogs

Pythiosis can be difficult to treat, and it has often spread extensively through the dog's system before a diagnosis is made. Antifungals can be used, but only around 10 percent of cases fully recover from medications alone. The only truly curative measure for this condition is complete surgical removal of any and all infected tissue. For skin lesions, this may involve a limb amputation.

Any gastrointestinal involvement gets even trickier. Your vet may try removing pieces of intestine that are infected, but the prognosis is much graver if there is GI involvement. This is because the fungal infection can easily spread from the GI tract to other organs within the abdomen. The more it spreads, the more difficult it becomes to completely excise all infected tissue. Again, surgery is not curative if it does not completely cut out all infected tissue.

If your dog only appears to have skin involvement, once all the infected tissue is removed your vet will put your dog on a highly digestible, high calorie diet. This will counteract any weight loss your dog suffered. Your vet will also put your dog on antifungal medications for anywhere from three to six months to help prevent any recurrence. 

Dogs that survive the initial infection require checkups every two to three months to look for signs of recurrence.


While mild cases of pythiosis or cases that are detected early are generally treatable, the overall prognosis for this condition is poor. A true cure is only obtained by the removal of all infected tissue, which is not always possible in dogs with GI tract involvement or with extensive skin infection. Still, the earlier treatment begins, the better your dog's chances of survival.

How to Prevent Pythiosis

Pythiosis can be a very serious concern for you and your dog. Preventing your dog from swimming in and drinking unclean, stagnant water is the best way to protect your dog from contracting this
infectious water mold. It's particularly important to keep your dog out of swampy or stagnant water if you live in one of the Gulf states, where the mold is most common.

Article Sources
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