Leptospira, a bacterial spirochete that is prevalent in animals worldwide, is the causative organism of leptospirosis, a zoonotic disease. Worldwide, about 300,000 to 500,000 severe human cases are recognized yearly, and the disease may be fatal in about 5% to 30% of human cases, according to Pedro Paulo Diniz, DVM, PhD, assistant professor in small animal internal medicine at Western University of Health Sciences in California.
Numerous Leptospira serovars have been isolated from companion animals (dogs, horses) and livestock (cattle, swine), and a wide variety of wild mammals, including mice, rats, moles, raccoons, opossum, deer, and skunks. Even marine animals, such as sea lions, have been found to be infected with Leptospira spp.
“In the United States, one of the biggest threats to dogs in terms of carriers of Leptospira is the raccoon,” Dr. Glickman says. “A very large percentage of raccoons may shed Leptospira in their urine despite appearing healthy. Such animals are often referred to as natural reservoirs of Leptospira infection. Rats are also an important carrier of Leptospira, particularly in more urban areas.”
The infection is typically transmitted when contaminated urine of a reservoir or clinically ill animal contaminates the environment, particularly water or wet soil. Canine infection can occur when a dog comes into contact with contaminated grass or soil or drinks from a puddle, pond, or other body of water.
People similarly can be infected by contact with a contaminated environment because the organism can enter the bloodstream through a break in the skin or can penetrate through mucous membranes. Cleaning up the urine of an infected dog, handling an infected pet, gardening, or swimming in a contaminated lake or pond can put a person at risk of infection. In one famous incident, more than 100 athletes participating in a triathlon became infected by swimming in a contaminated lake, according to Dr. Glickman.
Rainfall and flooding elevate the risk of Leptospira transmission, and the organism can persist in water and in wet environments for many months.
Some occupations, such as veterinarians and veterinary hospital employees, increase a person’s risk of leptospirosis. In a recent example, a small animal veterinarian in Washington state developed a high fever, pneumonitis, renal failure, and septic shock after a pet rat urinated on his ungloved hands, according to Dr. Diniz. “Twelve days of intensive care were required to save his life, and it took 2 months for him to recover and return to work,” he says.
Dogs are considered maintenance hosts for the L. canicola serovar and incidental hosts for other serovars. Dogs living in rural and suburban areas near agricultural land, bodies of water, and wetlands, as well as wooded areas, are at particular risk of disease. However, urban dogs are not without risk. According to a 2011 study in Preventive Veterinary Medicine, dogs living in urban areas are also at risk of infection. Even the pampered pooch living in an urban high rise can encounter an infected mouse or contaminated puddle on the street.
“It is a challenge for veterinarians to diagnose a dog with leptospirosis,” Dr. Diniz says. “Leptospirosis is not the first illness that comes to mind for most dogs with signs of refusal to eat, weight loss, fever, vomiting, diarrhea, stiffness, muscle pain, dyspnea, and weakness. To make it even more challenging, animals do not necessarily present with all of these signs at the same time. Some dogs present with increased drinking and urination due to kidney failure. Others may develop jaundice due to the liver failure.
“All of these signs can be associated with a large list of diseases,” Dr. Diniz adds.
Leptospira infection must be in the differential, because the veterinarian must request the laboratory to test the blood sample for it. The gold standard of diagnosis depends on the microscopic agglutination test (MAT), which detects IgM antibodies that typically appear about 6 to 12 days after infection, and some IgG antibodies, which appear much later, about 3 to 4 weeks after infection. Initial titers of 800 or higher suggest infection, which is confirmed if a 4-fold increase in titers is detected from the convalescent sample, according to Dr. Diniz. However, the magnitude of the titer does not correlate with disease severity.
PCR is a sensitive and specific test for Leptospira and can produce results in as little as 2 days post infection. However, a negative PCR does not rule out infection.
The MAT accurately determines the serovar in fewer than 50% of cases and PCR assays are unable to differentiate serovars; therefore Leptospira culture remains the most reliable technique to determine the serovar. However, knowing the serovar does not affect the therapeutic management of most canine patients, Dr. Diniz adds, because all serovars are sensitive to tetracycline or a penicillinase antibiotic.