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A Climatic Catastrophe: How Rising Temperatures Contribute to the Spread of Zoonotic Disease

By Amber Frantz

Fever, chills, and vomiting. These are the symptoms that led Hannah Heath to a 5-day hospital stay and an eventual malaria diagnosis in late June of 2023. The 35-year-old resident of Sarasota County, Florida was one of nine locally transmitted cases of malaria identified in the U.S. this past summer—a result of the first domestic outbreak in 20 years. Heath’s original symptoms were not dissimilar from a common case of food poisoning; however, blood tests soon revealed her alarmingly low platelet count, a more adverse effect that heightens the risk of internal bleeding. In an interview with Health News Florida, Heath explained that she struggled to get out of bed without assistance, and additional padding was placed around her bed to ensure she didn’t hit her arm, a simple accident that could have led to intense bleeding. A quick response from doctors on high alert at Sarasota Memorial Hospital led to Heath’s swift diagnosis and treatment after recognition of malaria parasites in her blood. Despite her full recovery, the malaria survivor ensures that she always keeps her mosquito repellent on hand and is now using her story to spread awareness about the uptick in vector-borne zoonotic diseases.

The rise in locally transmitted malaria cases is not an arbitrary event. Extreme heat waves across the continental U.S. this past summer not only broke record temperatures but altered the epidemiological model for the transmission of disease. Rising global temperatures, largely due to heat-trapping greenhouse gasses, have created a hospitable breeding ground for vector-borne zoonotic diseases, or zoonoses, which are transmitted to humans typically through the bite of a mosquito, tick, or fly. Defined by the World Health Organization as infectious diseases which have “jumped from a non-human animal to humans,” zoonotic diseases pose a dire public health problem that continues to grow in prevalence with an ever-evolving climate that is temperature-driven. Exacerbated by increased human vulnerability to infectious pathogens that are foreign to our immune systems, zoonoses continue to become progressively more complex and novel in their capacity to mutate, thereby creating a disease cycle that is difficult to predict and respond to. 

Epidemiological modeling works to predict the spread, mechanism, and range of infectious diseases in an effort to forecast and mitigate transmission. However, new temperature patterns have caused a deviation from predicted models due to increased seasonal variation in disease cycles. When considering the example of malaria, increased temperatures have a direct effect on vector viability. Mosquitos, the vector for malaria, demonstrate an increase in activity and reproduction in warmer climates. Rising water temperatures create a more hospitable environment for larvae development, ultimately increasing vector capacity for pathogenic spread. Ideal water temperatures for mosquito growth range from 22 to 32 degrees Celsius, and since common breeding grounds are typically found in shallow waters, this temperature is not difficult to achieve.

The authors of a systematic review published in the journal Travel Medicine and Infectious Disease comprehensively assessed the probability of future malaria spread in Europe as a result of rising temperatures due to climate change. After screening, data extraction from eligible papers was compiled in order to make a holistic prediction of how rising temperatures contribute to malaria spread in Europe—a region that once fully eradicated this disease. The reviewers reaffirmed the trend that malaria transmission in Europe is seasonally dependent as a result of varying temperature and climate conditions. This variation is most predominant in the summer months, denoted by the proportionality between increased temperatures and spread. Serving as evidence for future prevention, it is concluded that “unless socioeconomic factors remain [favorable] and appropriate public health and anti-vector measures are implemented and maintained,” increased spread is imminent. Europe, a Western civilization similar to the United States when it comes to modernization and seasonal fluctuations in temperature, is a strikingly comparable model for risk factors associated with disease spread. To put it plainly, if rising temperatures in Europe point towards an increase in malaria transmission, it is likely a similar scenario may transpire in the States. This trend in disease transmission between Europe and the U.S can be attributed to similar socioeconomic structures and temperature seasonality—two factors that are shared by the landmasses.

Malaria is not alone when it comes to upward case trends during the summer months. A late summer COVID-19 wave has made its annual appearance in each of the last three years, suggesting that there is a correlation between case numbers, temperatures, and human activity. This trend was clearly observed during the week of July 29, 2023, as the CDC reported a 12.5% increase in COVID-19 hospitalizations compared to the week prior. The increase can primarily be attributed to increased travel-based exposure that typically occurs during the warmer months of the year. COVID-19, caused by the virus SARS-CoV-2, has been traced to cases of zoonotic spillover in which pathogens spread from animal to human populations; the indicated linkage suggests that the virus originated by jumping from an animal to human host. This phenomenon has been identified as the “predominant cause of emerging infectious diseases,” further blurring the line between distinct animal and human pathogens. The impact warmer temperatures have on COVID-19 case numbers is only exacerbated by human behavior, particularly that of exposure as a result of factors like summer vacations and increased social interaction. In an interview with Scientific American, Dr. Archana Chatterjee, dean of Chicago Medical School and Vice President for Medical Affairs at Rosalind Franklin University of Medicine and Science, commented that COVID-19 is not yet considered a seasonal virus. This unpredictability complicates the epidemiological model for disease, which has struggled to account for the inconsistency of novel zoonotic diseases in response to an evolving climate.

 If there is one thing that has been learned from the COVID-19 pandemic and even the recent domestic malaria outbreak in the continental U.S., it is that response is crucial in order to mitigate spread. To further understand why specific zoonotic diseases respond positively to warmer temperatures, additional research is required. Predicting and preventing outbreaks before they occur is essential to the mitigation of disease and prosperity of public health. To that extent, it is time for the world to be put on notice—more epidemics driven by climate change are on the way. As summers get longer and winters grow warmer, the risks associated with temperature-dependent disease heighten. Combating climate change to ease the variation in disease seasonality calls for the reduction of carbon emissions and minimization of our ecological footprint. On an individual level, staying up to date on vaccines and educating oneself about vector unpredictability can assist in combating disease spread. If such measures are executed, the narrative of zoonotic disease and climate change can be rewritten from one of vulnerability to one of perseverance.