Lepto is the most common zoonotic (transmitted between animals and people) disease in the world, causing illness in 7 to 10 million people, with nearly 60,000 deaths every year.
Living in the Desert or other arid areas, it’s easy to overlook a disease that only thrives in the presence of water. But in many parts of the world, where water is a more prominent feature of the landscape, Leptospirosis (Lepto) carries considerable risk, not only to dogs, but also to people. Dog owners in Southern California received a harsh reminder of this fact in 2021, when, over a few months, Los Angeles County experienced over 200 reported cases of Lepto, and it’s likely more cases went unreported.
In hindsight, it seems surprising that we could be caught so off guard. Lepto is not a new disease, and, viewed broadly, is not particularly rare. Epidemiologists estimate, in fact, that Lepto is the most common zoonotic (transmitted between animals and people) disease in the world, causing illness in 7 to 10 million people, with nearly 60,000 deaths every year.
In studying the Los Angeles outbreak, we learned that most cases were caused by the strain of Lepto most closely associated with dogs, indicating that most transmission was occurring from dog to dog. In the past, cases more often involved transmission from other species, typically livestock or wildlife to dogs, with the strain responsible often reflecting the source. This awareness is particularly frustrating when we acknowledge that a vaccination protecting against the dog strain has existed for decades.
Clearly, our complacence about Lepto has caught up with us, and it’s time to refresh our understanding of it. With that in mind, here are some key facts.
Transmission occurs through water or wet surfaces.
Lepto is caused by bacteria of the genus Leptospira. Transmissions for both animals and people usually occur by consumption of contaminated water, or through contact with open wounds or mucous membranes. Studies have suggested that infection can begin from only a few organisms in a water droplet splashed into an eye. The organism enters water in the environment through passage of urine from an infected animal and can remain viable there for months, as long as the water does not dry or freeze. It does not survive in seawater or chlorinated water, just in freshwater or on surfaces that remain wet or moist.
In dogs, as in other species, symptoms can vary.
Although cats can become infected with Lepto, they tend to recover without serious problems. Dogs, on the other hand, can get really sick. The organism has its greatest impact on the kidneys and liver, but the symptoms are often nonspecific—depression, loss of appetite, pain, weakness, vomiting, swollen eyes, drinking/urinating excessively— and their severity may range from mild to life-threatening. No one knows why some dogs react worse than others.
There are multiple strains of Leptospira.
Classifying Lepto species and strains (serovars) can get very confusing. For practical purposes, each serovar is linked to a particular species of animal that serves as its primary host. In theory, the primary host is the most likely to serve as a long-term carrier (and shedder) of its own serovar, but in reality many species of animal are capable of catching and transmitting more than one serovar.
There are seven serovars known to cause disease in dogs (see Table on following page). The last two cases I treated in the Desert were caused by L. icterohaemorraghiae , in a dog from Los Angeles, and L. autumnalis, in a local dog who had traveled only to Idyllwild. Both patients recovered well with treatment. As it did with these dogs, knowing the serovar gives us an idea about how a patient acquired their infection, but it doesn’t necessarily predict the severity of the disease or its response to therapy.
There are ways to protect against Lepto.
Until about 25 years ago, most veterinarians believed only dogs that lived around livestock, hunting dogs, or other dogs that spent a lot of time in ponds and streams were likely to catch leptospirosis. More recent evidence, like the 2021 outbreak, suggests that urban dogs may be equally vulnerable. We know that wild animals provide an ongoing reservoir for Lepto, and many species, particularly rodents and raccoons, can encroach on human living spaces, even in cities. Dog owners should, therefore, refrain from attracting wildlife into their yards and avoid leaving water bowls outside overnight, especially where rodent contamination may occur.
We now also know that other dogs can be a direct source of infection in surroundings where close contact occurs, particularly in the presence of water. For these risks, vaccination can be a key part of prevention. Lepto vaccines are highly effective and protect against the four most common serovars that affect dogs (see Table). Because bacterial vaccines (technically, called bacterins) never provide the duration of immunity we expect from viral vaccines, they have to be repeated every year. In the past, there has been reluctance among some veterinarians to give Lepto vaccines, since earlier vaccines were less refined than the ones we have currently, and the vaccines developed a reputation for causing a disproportionate number of reactions. Today, however, Lepto vaccines have very low reaction rates that are comparable to other vaccines, so for most dogs the risk of catching the disease far outweighs the chance of having a reaction to vaccination.
There are tests available to diagnose Lepto.
Veterinarians usually diagnose Lepto through blood tests, using either a Polymerase Chain Reaction (PCR) Assay, or by antibody testing with a Microscopic Agglutination Test (MAT). Which test they perform might depend on the situation at hand. The PCR provides a rapid diagnosis, but only if the patient has not received antibiotics, which can make it falsely negative, and it cannot distinguish between the various Lepto serovars. The MAT provides titer (blood test) readings that can help differentiate the serovars, but it is unreliable in the earliest stages of infection. It becomes more useful if the test is repeated 2 to 3 weeks later to measure changes in titers that develop after an immune response occurs.
Once diagnosed, affected dogs usually respond to treatment.
For reasons we can’t fully explain, a few dogs that contract Lepto will develop life-threatening symptoms, and some don’t survive. For most patients, however, appropriate antibiotics, anti-emetics, and fluid support effectively treat the disease. Hospitalization is recommended, although some patients can recover at home. Regardless, either a veterinary or home caretaker must remain aware of Lepto’s zoonotic potential, particularly before treatment begins. Urine from Lepto patients is highly infectious, so eliminations and contaminated surfaces must be managed carefully. Fortunately, the zoonotic risk seems to drop once antibiotics begin to have their effect.
As we can see from these facts, Lepto, as a disease in dogs, can be fairly well managed if we remain vigilant about it. Our greatest challenge may come not from an inability to prevent and treat it, but from our failure to recognize it as a threat. Many of the Los Angeles cases were associated with day-care and boarding facilities without any involvement from wildlife or livestock, and with little or no exposure to the outside environment. Perhaps, as our society has evolved, risk factors have changed, and it may be time to update our thinking about Lepto.