Leptospirosis is a common waterborne disease caused by the bacteria Leptospira. Many strains of Leptospiraare found worldwide, but it is most usually found in warm areas with high rainfall. The bacteria can infect both humans and dogs, though is more common in dogs. Dogs at highest risk are those who routinely swim in stagnant bodies of water, rivers, lakes, and streams. Infection usually occurs when a mucous membrane or cut comes into contact with contaminated urine or water.


With a nickname like “swamp cancer,” you know it’s nothing good. Pythiosis is a rare but severe waterborne disease caused by a fungal-like organism called Pythium insidiosum. While more commonly known as a disease of plants, pythiosis can also infect animals—with terrible results.

Found in warmer climates with plenty of standing water, pythiosis is most commonly seen in the Gulf States (TX, LA, FL, MS, AL), southeast Asia, and South America. The organism attaches itself to small wounds in the skin or gastrointestinal tract and grows into large, often ulcerated lesions. If it starts in the skin, owners will notice large red itchy lumps. If it takes root in the GI tract, owners will notice signs such as vomiting, diarrhea, and weight loss. The disease is most commonly diagnosed in Labradors, a breed known for their love of swimming.

Because it is challenging to diagnose, pythiosis is often undiscovered until it is in an advanced stage. The organism is resistant to many treatments, and surgery is the treatment of choice.

Blue/Green Algae

Freshwater lakes and ponds are highly inviting to a water-loving dog, but beware of bodies of water with a dense buildup of blue-green algae. Under specific environmental conditions, most often during the summer months, photosynthetic bacteria can build up—a condition known as a blue-green algae harmful algal bloom (HAB). These algae can produce toxins with severe effects on pets and people.

Algal toxins come in a variety of forms and can affect any of the following systems: skin, GI tract, liver, and central nervous system. Depending on the type of toxin a pet is exposed to, symptoms can range from rashes, nausea and vomiting, respiratory failure, seizures, and death. Dogs should be kept from swimming in lakes with visible algal blooms as it is impossible to tell just by looking if the algae are producing toxins. Any illness after swimming in a lake with an algal bloom should be immediately reported to your veterinarian, as death can occur in severe cases.


One of several microscopic parasites known to cause diarrhea in both dogs and humans, Giardia lamblia is an organism many dog owners are familiar with. Infected animals shed oocysts in their stool, which are hardy and can persist a long time in cool, moist environments, where they can then pass into water sources and back into a host.

Long known to be a cause of traveler’s diarrhea in humans, Giardia also causes a sudden onset of diarrhea in dogs. While both humans and dogs can be infected, it is not considered a major zoonotic disease as most human cases are caused by other humans, and is not normally passed from dogs to people.

Although some dogs may experience severe dehydration and weight loss if infected for long periods, most cases of Giardia are mild and self-limiting. Symptomatic treatment and medications can hasten recovery in affected pets.


One of the nastier waterborne diseases, cryptosporidiosis is caused by the parasite Cryptosporidium. Both the parasite and the disease are often referred to as “crypto” by those unfortunate enough to have encountered it.

Multiple species of Cryptosporidium exist in different animal species and some can cross-infect humans. The parasite is protected in the environment by a thick outer shell, which makes it able to survive the environment for a long time and even resist chlorine disinfectants. It is one of the most common waterborne diseases linked to recreational water. Dogs are infected by ingesting the infected oocysts in contaminated food or water.

Crypto causes watery diarrhea, which can lead to severe dehydration. Fortunately for dogs, most cases are mild or subclinical and are rarely life-threatening. Symptoms usually resolve within two weeks, with appropriate treatment.


Dog owners in Texas and Louisiana have an additional organism to look out for in their water loving dogs: Heterobilharzia Americana, a flatworm that is the causative agent of canine schistosomiasis. The organism penetrates the skin of the dog while swimming or wading in contaminated freshwater, then migrates through the lungs into the liver. The adults produce eggs that penetrate the GI tract, which are shed in the feces and infect the intermediate host, freshwater snails.

The clinical signs are associated with the inflammation caused by the eggs, and usually manifests as vomiting, diarrhea, weight loss, and lethargy. The eggs can be found in the feces, though treatment can be challenging due to the chronic nature of the inflammatory lesions.

While this organism does not live in humans, the parasites can burrow into the skin and cause a rash known as “swimmer’s itch.”


Owners whose dogs are prone to ear infections can usually see it coming: the shaking head, the scratching at the ear canals, the stinky head. It is one of the most common reasons dogs are brought to the veterinary clinic. While otitis externa is a disease with multiple causes, such as bacteria, yeast, and underlying allergies, one of those causative agents can be found in the water.

Pseudomonas aeruginosa is the most common organism associated with chronic ear infections in dogs. It causes a smelly, oozy, purulent exudate and a substantial amount of painful swelling. Pseudomonas is frequently found in pools and is thought to be a common cause of “swimmer’s ear.”

Pseudomonas is diagnosed by an exam and culture of the ear discharge by the veterinarian. If the infection is limited to the external ear canal, it is usually treated with flushes and appropriate topical treatments. Dogs with extra floppy ears that trap heat and moisture, and those who love to swim, are at highest risk.