Summer, dogs and water seem made for each other. Once a four-legged friend catches the scent of a lake or the sea, most need little encouragement. Yet inviting-looking water and trouble-free swims in the past tell us very little about what may be living in that water today.<\/p>\n
When I first read Dr Karen Shaw Becker’s discussion of waterborne diseases, Giardia surprised me the most. I had associated it primarily with other dogs and contaminated environments rather than a swimming spot itself. But water also calls for awareness of leptospirosis, cryptosporidiosis and toxins produced by cyanobacteria. One less familiar condition is pythiosis: it is not a routine summer hazard in Estonia, although it is worth knowing about when travelling with a dog.<\/p>\n
These are very different problems. Cyanobacterial toxins may act quickly and demand immediate help. Giardia may cause prolonged or intermittent diarrhoea. Leptospirosis is a bacterial disease that can damage the kidneys and liver. A list of dangers is useful only if we understand the differences.<\/p>\n
What we call blue-green algae are actually photosynthetic bacteria known as cyanobacteria. Certain species and blooms produce toxins that can injure the liver or nervous system. Not every green patch is toxic, and colour alone cannot establish toxicity. But the reverse is equally important: clear-looking water is not automatically safe.<\/p>\n
The risk is particularly serious when a dog drinks contaminated water or licks it from its coat after swimming. Possible signs include drooling, vomiting, diarrhoea, weakness, loss of coordination, tremors and seizures. They can develop rapidly. If exposure is suspected, keep the dog out of the water, prevent licking, rinse the coat with clean water and contact a veterinarian immediately. Do not wait for the dog to become ill. There is no single specific antidote, and treatment depends on the clinical condition.<\/p>\n
If I see green scum, patches of discoloured water, an unusual smell or dead fish along the shore, the decision is easy: we find another place to swim. Such signs do not prove that the particular water is poisonous, but avoiding a risk we cannot assess at home is sensible.<\/p>\n
Giardia is a protozoan parasite whose infectious cysts enter the environment through faeces. Dogs may become infected by swallowing contaminated water, licking contaminated objects or grooming after exposure. Many infections cause no symptoms. Others can lead to soft stools, watery or intermittent diarrhoea, gas and weight loss.<\/p>\n
That does not mean every dog with soft stools has Giardia. Dietary changes, other parasites and intestinal diseases can cause similar signs. If diarrhoea persists or keeps returning, work with a veterinarian to choose the appropriate faecal tests. Because cyst shedding can be intermittent, one negative sample does not necessarily rule everything out. PCR and antigen tests also have limitations in their interpretation. PCR may remain positive for a time after treatment and does not, on its own, prove treatment failure.<\/p>\n
Integrative veterinary care often pays attention to the microbiome and intestinal barrier during Giardia recovery. Such support may be a useful addition, but probiotics or herbs alone are not a proven substitute for antiparasitic treatment when an infection is causing clinical signs. Treatment, hygiene and prevention of reinfection must work together. We have written about giardiasis<\/a> separately in more detail.<\/p>\n Cryptosporidium is another protozoan parasite. Its oocysts spread through faeces and may reach waterways. A healthy adult dog may have mild or unnoticed infection, while puppies and immunocompromised dogs can develop more severe watery diarrhoea and dehydration.<\/p>\n There is an important nuance here: calling an infection a \u201czoonosis\u201d does not automatically mean that dogs are as important a source for humans as other people or farm animals. Many Cryptosporidium species and genotypes are host-specific. Assessing risk depends on the exact organism and the person’s health. Nevertheless, careful disposal of a dog\u2019s faeces, particularly during diarrhoea, always makes sense.<\/p>\n Leptospirosis is caused by Leptospira bacteria, which can survive in damp contaminated environments. Infected animals shed the bacteria mainly in urine. The risk therefore extends beyond a suspicious pond to puddles, moist soil and places frequented by rodents. The organism can enter through mucous membranes or damaged skin.<\/p>\n Signs vary. Sudden lethargy, fever, vomiting, loss of appetite, muscle pain, changes in drinking or urination, jaundice and abnormal kidney results are reasons to seek veterinary help promptly. Leptospirosis is treatable, but severe cases need intensive supportive care. A single negative test cannot always rule it out: whether blood or urine testing is informative depends on the disease stage and any antibiotics already given.<\/p>\n Faddu’s leptospirosis is a very personal subject for me. He became ill despite having been vaccinated previously, and he received treatment. Thankfully, he recovered well. The experience made me ask what vaccination protects against, how long that protection lasts and why a vaccinated dog can still fall ill. From this one case we cannot determine the infecting serovar or how the illness might have progressed without vaccination.<\/p>\n The current veterinary consensus considers leptospirosis vaccination an important preventive measure. For example, the AAHA added it to its core vaccinations in 2024. Vaccines do not cover every possible Leptospira variant or guarantee absolute protection in every dog, but that does not mean they provide no protection. Adverse reactions are possible and individual risks should be considered. Some integrative veterinarians have suggested that vaccines generally \u201cweaken the immune system\u201d and could worsen illness, but available evidence does not currently support that broad claim.<\/p>\n If leptospirosis is suspected, protect people too: avoid contact with urine without gloves, clean contaminated areas as instructed and follow the clinic’s hygiene advice. Leptospirosis is not primarily passed between dogs through faeces, contrary to a common misconception. I have also shared Faddu’s experience in our article on kennel cough and leptospirosis<\/a>.<\/p>\n Pythiosis is caused by Pythium insidiosum, a water-associated oomycete rather than an ordinary fungus. It is diagnosed mainly in tropical and subtropical regions, including US Gulf Coast states, Asia and South America. There is no reason to treat it as a common swimming-related disease in Estonia.<\/p>\n In dogs, it may cause gastrointestinal inflammation and weight loss or skin lesions. Treatment can involve surgery and medication, and earlier diagnosis improves the outlook. If a travelling dog develops an unusual prolonged illness, tell the veterinarian about earlier travel and exposure to water.<\/p>\n I prefer thinking about health more broadly than one medicine or vaccine. A complete diet, appropriate exercise, sleep, stress management and control of existing illnesses all support health. But \u201cstrong immunity\u201d is not a laboratory result proving that a dog can defeat every pathogen. Even a very fit dog can develop a serious bacterial infection or poisoning. A healthy immune system cannot make cyanobacterial toxins disappear.<\/p>\n Prevention therefore has several layers for me. First we choose a safer environment. Then we consider the dog’s individual risks and discuss vaccination and other preventive measures. And when something goes wrong, we do not wait for days in the hope that the body will resolve it unaided.<\/p>\n CDC (2024). For Veterinarians: Harmful Algal Bloom-Associated Illnesses.<\/a><\/p>\n Companion Animal Parasite Council (2026). Giardia guidelines.<\/a><\/p>\n Companion Animal Parasite Council (2026). Cryptosporidium guidelines.<\/a><\/p>\n Sykes jt (2023). Updated ACVIM consensus statement on leptospirosis in dogs. Journal of Veterinary Internal Medicine. DOI: 10.1111\/jvim.16903.<\/a><\/p>\nCryptosporidiosis: another possible cause of diarrhoea<\/h2>\n
Leptospirosis: water, soil and the urine of infected animals<\/h2>\n
Pythiosis: rare, but real<\/h2>\n
Does a strong immune system protect against everything?<\/h2>\n
FendaF’s waterside checklist: before and after swimming<\/h3>\n
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References<\/h3>\n