Why leptospirosis incidence are on the rise

Lepto has a reputation for being a rural disease, but that is no longer the case

Once thought of as a rural disease, leptospirosis is on the rise and is now common in suburban and urban areas, particularly in small terrier dogs. Photo ©BigStockPhoto.com
Once thought of as a rural disease, leptospirosis is on the rise and is now common in suburban and urban areas, particularly in small terrier dogs.

Many misconceptions surround leptospirosis, a bacterial disease spread by contact with an environment contaminated with urine from a carrier animal. The types of dogs at risk, the types of areas where it typically occurs, and whether vaccinations are warranted are among the questions that vex veterinarians and pet owners alike.

The array of signs—and sometimes the lack of them—can present problems, making diagnosis difficult. Leptospirosis is a public health concern because it’s a zoonotic disease that can be transmitted from animals to humans. And for a host of reasons, including climate change, urban wildlife exposure, and reluctance to vaccinate, incidence of the disease appears to be rising. Here’s what you and your clients should know to make the best decision to safeguard patients.

Carriers and environment

The increased incidence of lepto, as it’s nicknamed, suggests it may be a “re-emerging” zoonotic disease, in the sense of increased awareness, more frequent diagnosis, and access to more types of testing. In the United States, greater incidence is probably associated with humans and their pets increasingly sharing their environments with carrier animals, such as raccoons, rodents, skunks, deer, opossums, mice, and rats, says Katherine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIM, associate professor in the department of clinical sciences at North Carolina State University College of Veterinary Medicine
in Raleigh.

All of these animals live not only in rural environments, but can also be found in urban and suburban backyards. And in rural areas, farm animals can also shed leptospires.

“In urban areas, rats and mice may be the more common carriers, but I think we have all seen raccoons or foxes, for example, in urban areas, and these can be nonclinical carriers and shed the organism into the environment,” Dr. Lunn says.

Lepto has a reputation for being a rural disease, but that’s no longer the case. It is now more common in city dogs—specifically, small terrier types, says Justine Lee, DVM, an emergency and critical care and toxicology specialist, who sees at least one to two cases of lepto per week in the ICU at Animal Emergency and Referral Center of Minnesota.

In his presentation on the disease at VMX in 2018, George E. Moore, DVM, DACVPM, DACVIM, of Purdue University College of Veterinary Medicine, cited research findings that dogs weighing less than 15 pounds were proportionately more likely to be diagnosed with the disease than dogs in other weight groups. “These small dogs in our experience have not been vaccinated against leptospirosis, leaving them susceptible to infection,” he stated in the proceedings.

City pets can acquire lepto as easily as licking a cement sidewalk where a mouse or rat may have urinated or drinking from a contaminated water dish left outside by a shopkeeper. The only place these days that lepto is uncommon is true desert environments, Dr. Lunn says. That’s because the organisms require moisture to survive. Anywhere there is water, including rainfall or man-made irrigation, there could be leptospirosis, she says.

Areas where lepto is more commonly seen include the Southeast and South, the Pacific Northwest, the Northeast, and areas prone to flooding, such as towns and cities located along the Mississippi River. Even areas thought of as deserts can harbor leptospirosis.

“There have been massive outbreaks even in the Phoenix area and that’s because of America’s love for lawns,” Dr. Lee says. “When they use sprinklers or have grass, it attracts wildlife to eat the grass and come to the watering hole.”

Is there a lepto “season”? It depends. In Dr. Moore’s lecture, he noted an increase in clinical cases in fall or early winter (September-December) could be attributable to greater raccoon activity during that period, due to dispersion of the year’s weaned offspring and the animals’ seeking habitat for winter. In warm areas such as south Florida, though, risk remains steady year-round.

Climate change may play a role as well. “We know rainfall and temperature affect survival of the organism, so weather patterns may also be affecting the incidence of disease,” Lunn says.

Tricky to diagnose

Lepto is one of those diseases with no signs or generic signs. Sometimes the only sign is sudden death.

“The classic cases have signs of vomiting, anorexia. and lethargy, due to acute kidney injury, and they may also have liver involvement manifesting as jaundice,” Lunn says. Other possible signs are muscle pain and stiffness, fever, abnormal bleeding, edema or effusions, PUPD without azotemia, and ocular signs such as uveitis or conjunctivitis. “Leptospiral pulmonary hemorrhage syndrome can cause respiratory signs, and this seems to be a common finding in some parts of Europe. When looking at lab test results, I would think of leptospirosis in patients with acute azotemia, cholestatic liver disease, mild to moderate thrombocytopenia, and glucosuria with normal blood glucose.”

In the U.S., greater incidence of leptospirosis is probably associated with humans and pets increasingly sharing their environments with carrier animals such as raccoons, rodents, skunks, deer, opossums, mice, and rats.
In the U.S., greater incidence of leptospirosis is probably associated with humans and pets increasingly sharing their environments with carrier animals such as raccoons, rodents, skunks, deer, opossums, mice, and rats.

Beyond overlapping signs, another reason it can be difficult to diagnose is that it’s not on the radar of many clinicians. They may assume certain dogs aren’t at risk or their locales are free of the disease.

And diagnostic tests for the infection are imperfect. In the early stages of the disease, a patient may test negative for antibodies, and in later stages may test negative by PCR if organisms are no longer present in blood or urine, Lunn says. “A combination of testing modalities that detect the organism itself, and the immune response, is the best approach. By using a combination of tests, and potentially testing more than once, the chance of confirming the diagnosis is improved.”

Be suspicious, Lee advises, especially if a dog has PUPD. “I always say, ‘When you see acute kidney injury, this should definitely be on your radar.’ And about 60 percent of the time when I see leptospirosis, dogs oftentimes have a low platelet count. So that should also trigger a veterinarian to be suspicious of it.”

Vaccination variables

Pet owners with toy breeds or small dogs have been advised for years by breeders not to vaccinate their pets for lepto. That’s because in the past, Lee says, the vaccine was very “hot.”

“It was a bacterin vaccine that had to stimulate the immune system, so it had more protein in it,” she says. “Now, all the veterinary companies ultra-purify the vaccine, so the risk of reaction is minimal.”

There are studies showing the risk of vaccine-related adverse events is greater in small dogs compared to large dogs, but that is not just for the leptospirosis vaccine, Lunn says. The risk of adverse events can also be greater if multiple vaccines are given during a single office visit.

“One study did show a small increase in risk of non-specific adverse events in dogs receiving a leptospirosis vaccine, compared to dogs that did not receive it, but the overall risk was still extremely low,” she says. “That risk should be weighed against the risk of exposure to leptospirosis for each individual dog.”

Manufacturers will typically recommend two initial vaccines, followed by annual vaccination. The newer vaccines protect against up to four strains of leptospirosis, and they protect dogs for at least one year after vaccination. “The old idea that immunity only lasts a few months after vaccination is not correct,” Lunn says.

Another reason few dogs are vaccinated for the disease is that the vaccine for lepto is classified as non-core, not one of the vaccinations all dogs should receive. Usually, it is recommended for “at-risk” dogs, but the determination of whether a dog is at risk has changed.

With the exception of dogs in desert environments, any dog is potentially at risk, Lunn says, even those who never go outside. “Most homes have mice, whether we like to believe that or not. And the dog that just goes outside for a few moments to urinate, if it sniffs wet grass or licks at a puddle contaminated with urine from a carrier animal, then it could be exposed.”

If nothing else, it’s important to advise clients of the potential risks in their locale and of the availability of a vaccine. Unlike vaccinations for diseases such as parvovirus or distemper, which protect a population of dogs through herd immunity and prevention of dog-to-dog spread, vaccinations for leptospirosis protect individual dogs as well. Lunn advises discussing the pros and cons of vaccinating for lepto with every dog owner, explaining adverse effects are rare and that the disease itself can be devastating. It can result in acute kidney injury, leading to chronic kidney failure and even death.

“As a clinician in a referral hospital, where leptospirosis cases are often referred, it is very difficult to diagnose leptospirosis in a dog and then honestly answer the owner who asks, ‘Why didn’t my veterinarian tell me there was a vaccine against this disease?’” Lunn says.

Kim Campbell Thornton is a frequent and longtime contributor to Veterinary Practice News. She is a Southern California-based freelance writer who specializes in pet-related topics.

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