Does El Nino Impact Rates of Infectious Diseases in the US?

Climate change linked to El Niño has been found to impact rates of vector-borne diseases across the United States, according to research published in the Proceedings of the National Academy of Sciences.¹

David N Fisman, MD, MPH, of the division of epidemiology at the Dalla Lana School of Public Health and the division of infectious diseases at the University of Toronto, and colleagues set out to explore the link between infectious diseases and climate change in multiple high-income countries.

Using US National Hospital Discharge Survey (NHDS) data from 1970 to 2010, the researchers assessed 5 disease groupings that could be expected to experience epidemiological shifts due to the El Niño Southern Oscillation (ESNO): vector-borne diseases (arthropod-borne viral disease, tick-borne disease, and rickettsial disease), respiratory illness (pneumonia and influenza), enteric disease, zoonotic bacterial disease, and fungal disease, with cases grouped by month and region (Northeast, Midwest, South, and West) for assessment. ESNO data were extracted using the National Oceanic and Atmospheric Administration (NOAA) Multivariable ESNO Index (MEI), a bimonthly indicator of ESNO activity.

Dr Fisman and colleagues found that over time, hospital discharge due to pneumonia and influenza were most frequent; conversely, zoonoses were the least frequent. Throughout the study period, significant nonlinear multi-year temporal trends and seasonal oscillations were observed across all 5 disease groups.

In analyses focused on the Western region, researchers found that ESNO was linked to a cumulative increase in vector-borne disease risk and a cumulative decrease in enteric disease risk (relative risk [RR]: 2.96; 95% confidence interval [CI], 1.03-8.48 and RR: 0.73; 95% CI, 0.62-0.87, respectively). While no increased risk of enteric disease could be linked to ESNO in non-Western regions as a whole (RR: 1.12; 95% CI: 1.02-1.15), there was a “marked increase” in risk in the Northeastern region (RR: 1.15; 95% CI, 1.01-1.32). Additionally, a decrease in zoonotic bacterial disease risk was identified in the Midwest (RR: 0.36; 95% CI, 0.13-0.97) while a decrease in fungal disease risk was noted in the South (RR: 0.80; 95% CI, 0.66-0.97).

The researchers conducted additional exploratory analyses on the effects of Western region vector-borne diseases, and found that the increased risk was “likely attributable to changes in Rickettsioses and other tick borne infections” (cumulative RR: 3.27; 95% CI, 0.98-10.91) compared to arboviral disease (cumulative RR: 1.32; 95% CI, 0.03-61.38).

Direct links between the environment and infection “are evident in low-income countries with limited health-enhancing infrastructure,” the researchers noted. “The impact of the physical environment on infection risk has been less well-studied in high-income countries perhaps because of the perception that wealth, strong public health institutions, and strong infrastructure mitigate such risk.”

“This finding highlights the importance of better understanding the links 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 researchers concluded.

Study limitations

• The lack of clear effects of ESNO on other disease risk (ie pneumonia or influenza) could reflect either a true absence of effects or a lack of statistical power to find the effects
• The large geographic regions studied may have resulted in misclassification of disease exposure
• The use of data from hospitalized patients allowed researchers to study only a small group of patients who were of interest, and does not include those who were treated at outpatient facilities or who did not seek medical care

Reference

1. Fisman DN, Ruite AR, Brown KA. Impact of El Niño Southern Oscillation on infectious disease hospitalization risk in the United States. Proc Nathl Acad Sci USA. 2016. doi: 10.1073/pnas.1604980113