Geographic Patterns of Canine Leptospirosis in the United States

In a study published in The Veterinary Journal, researchers constructed a predictive risk map for canine leptospirosis in the United States based on leptospirosis test results collected over 14 years. Positive tests were most likely to occur in the East, Midwest, and Southwest. Location (type of land cover) and climate affected the risk for canine leptospirosis.

Leptospirosis is a zoonotic bacterial disease that can cause liver and kidney failure in humans and other animals. It is transmitted via contact with contaminated water or urine and occurs throughout the United States.

“This analysis has produced the first comprehensive predictive risk map for canine leptospirosis in the contiguous USA and statistical support for some previously hypothesized risk factors,” wrote the authors.

The researchers analyzed the results of 87,355 microscopic agglutination tests for leptospirosis performed by IDEXX Laboratories from 2000 to 2014 on samples from dogs in the United States. The tests included all Leptospira serovars. The data did not include dogs’ leptospirosis vaccination status or indicate whether paired tests were conducted for individual dogs. To minimize the effect of vaccination status, the investigators considered titers greater than 1:800 to be diagnostic for leptospirosis (the same diagnostic value used by IDEXX).

Overall, 14.1% of the tests were positive. In general, tests from counties in the eastern, midwestern, and southwestern United States were more likely than others to be positive. Predicted probabilities of positive test results for specific counties ranged from 2% (Harney County, Oregon) to 37% (Webster County, West Virginia). The median overall predicted probability of a positive test was 12%.

The authors noted that many Appalachian counties have a high canine leptospirosis risk, adding that the Appalachian Mountains have high annual precipitation and large areas of deciduous forest. Their model, as well as other studies, also identified clusters of leptospirosis in California, Illinois, Kansas, Michigan, Nebraska, Texas, and parts of the upper Midwest.

The investigators examined a number of variables that could plausibly affect leptospirosis risk. They found that precipitation, temperature, proportion of low-density developed land (such as neighborhoods with houses built on large lots), and the presence of deciduous forests were useful in predicting a county’s leptospirosis test results. Because of the number of variables and the complexity of the statistical model, they could not predict risk based on individual variables.

The authors acknowledged some limitations to their statistical model. In some areas of the country, the number of tests submitted was low, potentially affecting the accuracy of risk predictions. The titer cutoff value used to diagnose leptospirosis may have missed some dogs with the disease and included some vaccinated dogs (although they note that relatively few dogs in the United States receive leptospirosis vaccines). Results were reported at the county level, but county lines are drawn arbitrarily. Still, they pointed out, identifying risk by county can be useful when deciding where to concentrate prevention and control measures.

“Our model can be used to characterize the risk of canine leptospirosis across the USA and can be applied to improve delivery of veterinary services, including diagnostics and vaccination, and to identify areas for increased research and surveillance efforts,” concluded the authors.

The study was funded by Zoetis, the manufacturer of a canine leptospirosis vaccine. Two of the study authors are Zoetis employees. The authors reported no other conflicts of interest.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.