LEPTOSPIROSIS

You may have heard the word leptospirosis recently, and if you’re not familiar with it, it can sound pretty intimidating. Maybe it came up during a vet appointment, in a news story about wildlife, or even on signs posted around local beaches. It’s one of those diseases that shows up in headlines just often enough to raise questions, but not always often enough for anyone to explain what it actually is. So what exactly is leptospirosis, and why does it suddenly seem to be everywhere?

What Is Leptospirosis

Leptospirosis is caused by a group of spiral-shaped bacteria called Leptospira. The name comes from Greek roots meaning “narrow spiral,” which describes the corkscrew-like shape microbiologists see under the microscope. These bacteria thrive in warm, damp environments and are most often spread through the urine of infected wildlife such as rodents, raccoons, skunks, and other mammals. Many of these animals can carry the bacteria without appearing sick themselves. In epidemiology these animals are known as reservoir hosts, meaning the bacteria are able to live and reproduce in them without causing significant illness. Because the host remains healthy, the bacteria can continue circulating almost undetected in wildlife populations and the surrounding environment. When infected urine contaminates soil, puddles, creeks, or damp outdoor spaces, the bacteria can survive long enough for another animal to encounter them.

Dogs usually encounter it the way dogs encounter most things in the world - by exploring. Drinking from puddles, wading through standing water, investigating muddy areas, or passing through places where wildlife have been can all create opportunities for exposure. Dogs are particularly good at finding exactly the kinds of environments where these bacteria thrive. If the bacteria enters the body through the mouth, nose, eyes, or small breaks in the skin, infection can begin. From there, the bacteria travel through the bloodstream and most commonly affect the kidneys and liver. Some infections remain mild or even go unnoticed, while others can become much more serious. That range is part of why early recognition matters, since the infection responds best to treatment when caught early. Leptospirosis is also considered zoonotic, meaning it can spread between animals and people under certain conditions. Because of this, veterinarians approach suspected cases carefully and take appropriate precautions. The good news is that with modern treatment, vaccination, and an understanding of how the disease spreads, leptospirosis is something can be managed very effectively!

Risks And Exposure For Modern Day Pets

Dogs are among the  domestic species most commonly diagnosed with leptospirosis, and here's why:

1. Natural behavior increases exposure opportunities. Dogs explore the world with enthusiasm and curiosity. Sniffing, digging, drinking from puddles, splashing through water, or investigating areas where wildlife have passed through are all normal dog behaviors — and they also happen to be common ways dogs encounter the bacteria in the environment
2. Dogs are considered incidental hosts. This means dogs are not the bacteria’s long-term natural reservoir, but they can still develop illness if exposed. In incidental hosts, the immune system’s response to the infection is often what produces noticeable disease. Once infected, the bacteria may localize in organs such as the kidneys and liver, leading to symptoms that range from subtle lethargy and decreased appetite to more serious systemic illness. Because early signs can be vague, timely veterinary evaluation can make an important difference.
3. Dogs can shed the bacteria during infection. During active infection, dogs may pass Leptospira bacteria in their urine for a period of time. With appropriate veterinary care and routine hygiene precautions, the risk to families remains low. However, this zoonotic component is one reason veterinarians approach suspected cases carefully and emphasize prevention whenever possible.
   

The good news? Prevention and early treatment make a measurable difference. Vaccination has significantly reduced severe outcomes in many regions, and prompt medical care greatly improves recovery rates. Recognizing symptoms early is an important part of that.

EARLY SIGNS

Leptospirosis often begins with vague or non-specific symptoms that  resemble  other common illness. Dogs may appear tired, lose interest in food, develop a mild fever, or have vomiting or diarrhea. Because early signs can overlap with other conditions, persistent changes should be evaluated by a vet.

MODERATE ILLNESS

As infection progresses, symptoms may become more noticeable. Dogs may develop increased thirst or urination, dehydration, abdominal discomfort, muscle soreness, or continued vomiting or diarrhea. These signs can indicate that the infection is beginning to affect internal organs.

SEVERE DISEASE

In some cases, leptospirosis can lead to significant kidney or liver damage. Severe illness may include jaundice (yellowing of the eyes or gums), extreme lethargy, difficulty breathing, bleeding disorders, or reduced urination. Dogs showing these symptoms require immediate veterinary.

FELINE SYMPTOMS

Cats appear less likely than dogs to develop severe illness from leptospirosis, and documented clinical infections are relatively uncommon. Some exposed cats may show mild symptoms or none at all. Even so, exposure can still occur in environments where the bacteria are present. 

While dogs are the species most commonly diagnosed, cats can also be exposed to leptospirosis, but they appear to interact with the disease somewhat differently. Compared with many other species, cats tend to be more resistant to developing severe illness, and documented clinical infections are relatively uncommon. In some cases cats may test positive for exposure while showing only mild symptoms or none at all. Because the disease is rare and generally less severe in cats, there is currently no leptospirosis vaccine available for cats in the United States.

That said, cats are not completely immune. Rare cases of kidney-related illness associated with leptospiral infection have been reported. Outdoor roaming naturally increases contact with wildlife, shared water sources, and contaminated environments, which is one of many reasons veterinarians often recommend indoor living for cats whenever possible.

What That Means For You

Leptospirosis is something veterinarians take seriously, but it’s also something we know how to manage and prevent. Staying aware of environmental risks, recognizing symptoms early, and keeping your dog up to date on recommended vaccinations can dramatically reduce the chances of illness. If you’re ever unsure about your dog’s risk or have questions about prevention, our team is always happy to talk through it with you!

A Note To The West Coast

Because we practice here in California, leptospirosis is something we pay especially close attention to locally. Risk tends to increase after periods of heavy rain, when moisture allows the bacteria to persist in soil and standing water. In recent months, the disease has also received increased media attention following a small number of confirmed canine cases reported in parts of Southern California and the Bay Area since late 2025. However, it’s important to note that leptospirosis in dogs is not fully reportable statewide, so confirmed case numbers likely underestimate environmental exposure.

Exposure can occur in both urban and rural settings. Shared spaces like dog parks, boarding facilities, and neighborhood trails can play a role, and wildlife such as rodents, raccoons, and coyotes may contribute to environmental contamination. Beginning in late 2025, the disease has once again been affecting California’s sea lion population, with marine wildlife centers reporting large numbers of stranded animals during the most recent outbreak cycle.