Rat Boom and Leptospirosis Spike in NYC Linked to Climate Change

New York City faces a public health concern as leptospirosis cases surge alongside its booming rat population. Experts attribute both trends to climate change.

Leptospirosis, a bacterial infection transmitted through contact with animal urine, reached a record 24 cases in NYC in 2023. Already this year, there have been 6 reported cases. By comparison, the average number of locally acquired cases during 2021 to 2023 was 15 per year, and only 3 cases per year during 2001 to 2020. The NYC Health Department attributes this rise to increased exposure to rat urine in contaminated environments.

“Half of the locally acquired cases were reported in the months of June and October, months that were warmer and wetter with excessive rain and unseasonably warm days compared to prior years,” noted the department in a health advisory. “Excessive rain and unseasonably warm temperatures, factors associated with climate change, may support the persistence of leptospires in more temperate areas like NYC.”

Mahvash Siddiqui, a U.S. diplomat with expertise in climate change, highlights the connection between extreme weather events and the spread of leptospirosis.

“Leptospirosis, caused by bacteria in animal urine, thrives in tropical climates. Climate change-induced heavy rainfall and floods could worsen its spread, especially where rodents are prevalent. NYC’s high rat population, nearing 3 million, heightens the risk,” she explains (not speaking on behalf of the DOS, the views are her own). Siddiqui authored the Climate Change chapter of the State Department’s Quadrennial Diplomacy and Development Review.

Professor Rebecca Bratspies, director of CUNY Law School’s Center for Urban Environmental Justice Reform, emphasizes the urgency of addressing climate change.

“Climate change is a threat multiplier. We are just at the beginning of seeing what that means for zoonotic diseases. Human health risks from climate change run the gamut from heat-related, to flooding, to spread of disease. We are not prepared.,” she warns. “Climate change is the biggest challenge we face–the biggest human rights challenge, the biggest technology challenge, the biggest agricultural challenge, the biggest environmental challenge, the biggest health challenge and more.  The time for action is now (20 years ago would have been better).”

NYC’s estimated rat population has ballooned to 3 million—a threefold increase in a decade. Milder winters, driven by climate change, allow rats to reproduce year-round, contributing to the surge. Additionally, warmer weather leads to increased human activity, generating more waste that sustains the rat population.

The NYC Health Department has previously warned that excessive rain and unseasonably warm temperatures due to climate change is expected to support the persistence of leptospires in the NYC environment which may lead to an increase in cases in the future.

Scientists have long raised concerns about outbreaks of leptospirosis. A study was published in the Transactions of The Royal Society of Tropical Medicine and Hygiene (volume 104, issue 10) by Colleen L. Lau, Lee D. Smythe, Scott B. Craig, and Philip Weinstein, exploring this very topic.

“The situation underscores the need for a multi-pronged approach,” emphasizes Dr. Rajan Thapaliya (Ph.D.), an author, instructor and resident of NYC. “Climate change mitigation strategies are crucial to address the root cause. Additionally, NYC should explore improved waste management practices and stricter rat control measures. Public awareness campaigns can educate residents about leptospirosis and preventive measures.”

Siddiqui suggests that collaboration with other cities facing similar challenges  can be valuable. “NYC should leverage global partnerships with city leaders around the world to address leptospirosis and other climate-related risks,” she recommends.