Leptospirosis in dogs is relatively uncommon overall, but its incidence appears to be increasing across the United States, and certain regions carry significantly higher risk. The disease is not one most dog owners will encounter, yet it’s serious enough that veterinary organizations now recommend vaccination for dogs in at-risk areas as a core part of preventive care.
Where Cases Are Most Common
Leptospirosis doesn’t affect dogs evenly across the country. Appalachia is considered the highest-risk region in the contiguous United States, with the upper Midwest and central Texas also carrying increased risk. A large-scale mapping study found that specific counties in Appalachia had some of the highest predicted probabilities of a positive test, while the Midwest, East, and Southwest were all more likely to yield positive results than other parts of the country.
The disease tends to spike after specific weather events. Hurricanes, heavy rainfall, and flooding all create ideal conditions because contaminated animal urine in soil washes into standing water, puddles, and streams. The bacteria responsible can survive in wet soil and water for weeks to months, which means a single flooding event can create lingering risk in a neighborhood or trail system long after the water recedes.
Why Cases Are Rising
Research tracking diagnostic test results across the U.S. has documented a clear upward trend in canine leptospirosis cases. Several factors are likely driving this. Expanding wildlife populations in suburban and urban areas mean more dogs are exposed to contaminated environments even in their own backyards. Rodents, raccoons, skunks, and other wildlife all carry and shed the bacteria in their urine. Livestock, including cows, pigs, and horses, are also common carriers.
Climate patterns matter too. More frequent extreme rainfall events create more opportunities for the bacteria to spread through water and saturated ground. A dog doesn’t need to drink from a visibly contaminated puddle to become infected. Walking through wet grass, swimming in a pond, or even sniffing contaminated soil can be enough if the bacteria enters through a cut, a mucous membrane, or the lining of the mouth or nose.
Despite the Risk, Vaccination Rates Remain Low
One striking detail about leptospirosis is how few dogs are vaccinated against it. During a 2023 outbreak in Wyoming, data from veterinary clinics showed that before the outbreak, only 5.4% of dogs seen for routine vaccination appointments received a leptospirosis vaccine. Even after public health outreach raised awareness, that number climbed to just 33.1%.
Owner hesitancy plays a role. Before the Wyoming outbreak, clinics estimated that only about a third of dog owners agreed to vaccination when it was recommended. After the outbreak made local news, compliance rose to roughly half. That still means nearly half of owners declined the vaccine even after seeing leptospirosis in their own community.
The American Animal Hospital Association recommends a four-serovar vaccine that covers the most relevant strains circulating in North America. These vaccines require an initial dose at or after 12 weeks of age, a booster, and then annual revaccination. Protection lasts roughly 12 to 18 months depending on the specific strain, which is why yearly boosters are necessary. The vaccine doesn’t cover every possible strain of leptospirosis, but it targets the ones most commonly causing disease in dogs.
Symptoms to Recognize
Early signs of leptospirosis are frustratingly vague. Fever, lethargy, loss of appetite, and muscle or joint pain can look like dozens of other conditions. Some dogs develop eye or nasal discharge, changes in urination patterns, or jaundice, a yellowing of the eyes and skin that signals liver involvement.
The disease can escalate quickly. Within a few days of initial symptoms, dogs may begin vomiting, become severely dehydrated, and develop back pain from kidney failure. The kidneys and liver are the two organs most commonly damaged by the infection, and the speed of that damage is what makes leptospirosis dangerous. A dog that seems mildly unwell on Monday can be critically ill by Wednesday.
How It’s Diagnosed
Diagnosing leptospirosis is trickier than many infections. The gold standard blood test, called the Microscopic Agglutination Test, is highly specific (meaning a positive result is very reliable) but has a sensitivity of only about 23%. That means it misses roughly three out of four infected dogs, especially early in the illness before the immune system has mounted a detectable response. DNA-based testing that looks for the bacteria’s genetic material directly performs better, catching about 54% of cases, but still isn’t perfect.
In practice, veterinarians often combine test results with clinical signs, bloodwork showing kidney or liver dysfunction, and information about the dog’s exposure history. A single negative test doesn’t rule out the disease, and vets may retest or begin treatment based on suspicion alone when symptoms fit.
Survival With Treatment
The good news is that dogs who receive prompt treatment have a reasonable chance of recovery, even with severe disease. Research from Tufts University followed 22 dogs with leptospirosis who had progressed to kidney failure serious enough to require dialysis. Of those 22, 16 survived and were able to leave the hospital, a survival rate of roughly 75% even among the most critically ill patients.
Treatment typically involves antibiotics and aggressive supportive care, including intravenous fluids to protect the kidneys. Dogs with milder cases caught early generally do better than those who arrive in kidney or liver failure. Recovery can take weeks, and some dogs sustain lasting kidney damage even after clearing the infection.
The Risk to People
Leptospirosis is zoonotic, meaning it can pass from animals to humans. During the 2023 Wyoming canine outbreak, at least one human case was identified in connection with the cluster of dog infections. Infected dogs shed the bacteria in their urine, and direct contact with that urine, or with surfaces and water contaminated by it, can transmit the infection to people.
If your dog is diagnosed with leptospirosis, practical precautions include wearing gloves when cleaning up urine, washing hands thoroughly after handling your dog, and keeping other pets away from areas where the sick dog has urinated. The bacteria is killed by common disinfectants and doesn’t survive well on dry surfaces, but wet or damp areas in your home or yard can harbor it for days.
Which Dogs Are Most at Risk
Any dog can contract leptospirosis, but some face higher odds. Dogs that spend time near ponds, streams, or marshy areas have more opportunities for exposure. Hunting dogs, farm dogs, and dogs in rural or semi-rural areas encounter wildlife urine more frequently. But urban dogs aren’t exempt. Rats and other rodents carry the bacteria in cities, and a dog nosing around a puddle in an alley or park can be exposed just as easily as one on a country trail.
Seasonality plays a role as well. Cases tend to cluster in late summer and fall, when warm, wet conditions favor bacterial survival. The Wyoming outbreak, for example, ran from August through October. If you live in or travel to a high-risk region with your dog, vaccination before the peak season offers the best protection available.