Lyme Disease in Western US Could Rise as a Result of El Niño

Weather changes caused by El Niño could lead to a rise in cases of tick-borne diseases (TBDs), such as Lyme disease, in the Western United States, a new study published online in Proceedings of the National Academy of Science has suggested.

El Niño and La Niña are opposite phases of the El Niño Southern Oscillation (ENSO) cycle that occur periodically in the tropical Pacific Ocean. They cause fluctuations in temperature, with El Niño being the warm phase and La Niña being the cold phase. In the United States, El Niño is usually associated with wetter weather in the South and drier weather in the Pacific Northwest.

According to David N. Fisman, MD, MPH, from the University of Toronto, Canada, and colleagues, the irregular periodicity of the ENSO makes it useful for examining how changes in temperature and precipitation affect the risk of infectious diseases.

However, “evaluations of the potential impact of ENSO on changing infectious disease dynamics across multiple regions in high income countries remain limited,” they write.

With this in mind, Dr. Fisman and colleagues conducted a study to identify associations between ENSO and infectious disease hospitalization risk in different regions of the United States, and also to investigate differences in ENSO effect between the Western US and non-Western regions.

The researchers hypothesized that ENSO would be more associated with changes in the epidemiology of infectious diseases in the Western United States than in other regions.

They evaluated ENSO exposure data and National Hospital Discharge Survey data from 1970 to 2010. In particular, the researchers focused on five types of disease groupings that may undergo epidemiological shifts with changing climate. These were vector-borne diseases, pneumonia and influenza, intestinal disease, zoonotic bacterial disease, and fungal disease.

In the Western United States, the researchers found that ENSO was associated more with vector-borne disease (rickettsial diseases and TBDs) and less with enteric disease. These effects were most prominent during the 12 months after the ENSO.

“[W]e did observe a large (but imprecise) impact of ENSO on vector-borne disease risk in the Western region of the United States, without changes in disease burden in other regions,” the authors write.

ENSO particularly affected TBD-risk, and at a lag of 10 to 12 months. According to the authors, this finding “is consistent with potential effects of precipitation and elevated temperatures on tick vectors as well as increased rodent disease reservoirs, although such observations have not been universal.”

In contrast, ENSO was associated with more intestinal disease in non-Western regions as a whole. The authors add that this effect was driven mostly by a substantial increase in the risk of intestinal disease in the Northeast. This could provide important insight into the potential effect of environmental changes on infectious disease risk, they say: “A given environmental change may result in different effects on disease risk in different ecosystems.”

They also identified other regional effects of ENSO, including decreased zoonotic bacterial disease risk in the Midwest, and decreased fungal disease risk in the South. However, they found no effect of ENSO on pneumonia and influenza risk.

Discussing the decreased risk of intestinal disease in Western regions, and its increased risk in non-Western regions as a whole,

The results of this study highlight “the importance of better understanding the linkages between environment and infection risk in high income regions, and the need for investment in robust public health surveillance systems that are able to detect changing disease burdens,” the authors conclude.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.