Causes of Canine Leptospirosis
Leptospirosis is an infectious disease that is caused by a group of spiral-shaped bacteria collectively called Leptospira. These microorganisms, which are classified as “spirochetes,” are similar to other bacteria except that they are motile, which means that they can move around by whipping their tail-like membrane, called a “flagella.” Many different types of spirochetes exist in nature. Most of them live freely in the environment and do not disturb people or pets. However, two particular types of “spirochetes” have been identified as causing disease in domestic dogs. These are Leptospira, which causes Leptospirosis, and Borrelia, which causes Lyme Disease.
Leptospirosis is found all around the world, and is especially prevalent in wet, warm, tropical climates. Dogs living in icy winter conditions are unlikely to develop leptospirosis because the bacteria cannot survive freezing temperatures. The organisms also are killed by heat and prolonged dryness. Where Leptospira survive, and thrive, are in pools of standing water, puddles of urine, raw sewage, muddy patches, marshy areas and damp, neutral or slightly alkaline soil.
Wild and domestic mammals are reservoirs for the bacteria that cause leptospirosis. Carrier animals may or may not show any symptoms of being sick from their infection. Raccoons, skunks, opossums and rats are particularly common carriers. Dogs that come into contact with these animals, or their urine, have an increased chance of contracting leptospirosis. This is true not only in rural environments, but also in densely populated urban areas that are infested with rodents. Leptospira can be transmitted directly across the placenta, infecting unborn puppies. Dogs often become infected when they drink or walk through stagnant water that has been contaminated with the urine of infected animals. Leptospira can enter through a bite wound or other skin surface disruption, and it can also pass through intact skin. Dogs can become infected by eating part of a dead animal that carries the bacteria. The organisms can also be transmitted in semen. Once they penetrate abraded or intact skin or mucous membranes, Leptospira rapidly invade the dog’s blood stream and spread to all parts of its body. The organisms eventually settle in the liver and/or kidneys (and sometimes in other organs, such as the spleen, eyes and brain, or in the reproductive and gastrointestinal tracts). For some unknown reason, when they lodge in the liver and kidneys, Leptospira bacteria are sheltered from the dog’s immune system and are able to reproduce in large numbers.
Most of the animals that are infected with Leptospira do not become sick. They are called “carriers” of the organism, with an “inapparent” infection. However, when carriers urinate, they shed the bacteria and contaminate the surrounding environment, becoming a source of infection for other animals. They may shed the bacteria in their urine intermittently, or they may shed them for life. Dogs that spend a lot of time in wet, wooded areas frequented by wildlife, and those living in densely populated urban environments that harbor rats and other rodents, have an increased risk of contracting leptospirosis. Dogs that spend most of their time indoors, or in areas that are not likely to be contaminated by the urine of carrier animals, are much less likely to become infected. People can become infected through the same routes as dogs. Leptospirosis is a zoonotic disease.
The most effective way to prevent leptospirosis is to restrict a dog’s access to muddy marshy areas, ponds, irrigated pastures, raw sewage and standing pools of stagnant water. It is important to keep trash well-contained, especially in dense urban areas where rats and other rodents are common. Raccoons, squirrels and other wild animals should not be fed or otherwise encouraged to linger in areas frequented by domestic dogs.
Vaccination against Leptospira is another option for dog owners. Several vaccines are available to provide protection against several of the strains (serovars) of Leptospira. Unfortunately, no current vaccine protects against all of the bacterial subtypes. The conventional vaccine only provides protection against two of the common serovars: L. canicola and L. icterohaemorrhagiae. In 2004, the Ft. Dodge division of Wyeth Pharmaceuticals came out with a multi-strain leptospirosis vaccine produced from Leptospira sub-units, which reportedly provides protection against L. canicola, L. icterohaemorrhagiae, L. pomona and L. grippotyphosa. More recently, in 2010, Intervet/Schering-Plough Animal Health released a four-way canine leptospirosis vaccine that is designed to protect dogs against infection from the same four serovars as the earlier Ft. Dodge vaccine. This newer vaccine is claimed to provide protection from clinical disease and also to prevent colonization of the kidneys by the infective bacteria.
Many veterinarians, as well as the American Animal Hospital Association, consider the leptospirosis vaccines to be “non-core” vaccines for dogs. This means that they do not recommend routinely vaccinating against Leptospira, unless there is a good chance that the dog may be exposed to the infective microorganisms. The reason for this is that the leptospirosis vaccines tend to cause more allergic reactions than other common canine vaccines. These reactions can be as minor as a slight swelling at the injection site. However, they can be more serious, ranging from pain, facial swelling and hives to a potentially fatal anaphylactic reaction. There is no way to predict whether a particular dog will have an adverse reaction to any vaccine. Small breed dogs seem to be predisposed to having bad vaccine reactions. The immunity provided by current leptospirosis vaccines is short lasting; it may only protect the dog for one year, or even less. Most authorities recommend giving a leptospirosis vaccine only to dogs that live in endemic areas, or dogs that have a particular risk of contracting this infection.
No vaccine is one-hundred percent effective. However, most vaccines, if administered properly and given at the right time, will tend to make the disease much milder in the vaccinated animal, if it becomes infected by the organism which the vaccine protects against.
Leptospirosis is often seen in companion animals after heavy rains in areas frequented by wildlife. Dogs should not be permitted to drink from standing pools of stagnant water. Owners of infected dogs should be especially careful to avoid contact with their urine or other bodily fluids, until their infection is fully resolved.