Politico claims that deadly insect-borne diseases are “on the rise” in the U.S. due to “warming global temperatures.” Disease-carrying insect populations have increased greatly over the past several decades. But there is no decisive evidence that climate change is the reason. Instead, the surge of these insects corresponds to the banning of a highly effective insecticide.

Misrepresenting Its Source

For a special issue of Politico about “planetary health,” reporter Christina Animashaun made a graphic on “climate change and human disease.” It states:

Warming global temperatures are changing the range and behavior of disease-carrying insects like mosquitoes and ticks and extending the seasons in which they are active. As a result, incidence of the diseases they carry — including Lyme, spotted fever, West Nile and malaria — are all on the rise, despite yearly fluctuations.

To support that statement, she cites the “U.S. Global Change Resource Program’s Climate Health Assessment” but fails to provide a link to it. This is convenient given that the actual assessment does not support her claim. First, with regard to ticks, it states:

Though there are links between climate and tick distribution, studies that look for links between weather and geographical differences in human infection rates do not show a clear or consistent link between temperature and Lyme disease incidence.

Likewise, the assessment looks at West Nile Virus [WNV] and other mosquito-borne diseases. It says:

WNV is an invasive pathogen that was first detected in the United States just over 15 years ago, which is long enough to observe responses of WNV to key weather variables, but not long enough to observe responses to climate change trends.

Further conflicting with Politico’s story line, the assessment states:

Western equine encephalomyelitis virus (WEEV) and St. Louis encephalitis virus (SLEV) were first identified in the 1930s and have been circulating in the United States since that time. Like WNV, both viruses are transmitted primarily by Culex mosquitoes and are climate-sensitive. …

Despite climatic warming that would be expected to favor increased WEEV and SLEV transmission, both viruses have had sharply diminished incidence during the past 30 to 40 years. Several other mosquito-borne pathogens, such as chikungunya and dengue, have grown in importance as global health threats during recent decades; however, a link to climate change induced disease expansion in the United States has not yet been confirmed.

The assessment says nothing about the causes of spotted fever or malaria, except that malaria is “primarily acquired outside of the United States and based on travel-related exposures.”

In sum, the assessment cited by Politico as proof that “warming global temperatures” are fueling insect-borne disease rates says nothing of the sort.

Ignoring Key Data

Beyond misrepresenting its source, Politico ignores a seminal 2016 study of coastal U.S. mosquito populations in the journal Nature Communications. This study documents that “mosquito populations have increased as much as tenfold” during “the last five decades.” It also notes that “many studies have found positive correlations between temperature and insect populations.” However, none of these studies used “continuous datasets pre-dating the 1960s.” And “nearly all” of them “ignored the influence” of land use and the highly effective pesticide DDT.

Failing to account for DDT is a major oversight. The compound caused “drastic reductions in the abundance of many” types of insects from the 1940s through the 1970s.” Per a 2000 article in the British Medical Journal:

DDT was successfully used to eradicate malaria from some nations (United States, Europe) and to lower case rates by over 99% in others (Sri Lanka, India). In South Africa it was used to eradicate the two most dangerous species of malaria mosquitoes, Anopheles funestus and A gambiae, from the country. All this saved millions of lives.

The EPA banned DDT in 1972, but its effects sometimes continued for decades. In New York State, for example, “it took mosquito communities nearly 40 years to reach pre-DDT levels.”

The authors of the 2016 Nature Communications study addressed the above-listed shortcomings of earlier studies. They found that:

• Mosquito “species richness and abundance decreased, often precipitously, during the period of DDT use and then increased afterward, as the concentration of DDT in the environment decreased.”
• “Human population growth and resulting urbanization … was correlated with increased mosquito species richness and decreased relative abundance.”
• “Surprisingly, we found little evidence that mosquito abundance or diversity responded to year-to-year variation or long-term warming trends in temperature, despite the presence of significant warming trends over time.”

In brief, the study found that growth in mosquito populations did not correspond with rising temperatures but with decreased DDT. It is important to note that this study only reveals associations, which do not prove causation. Other factors may be at play. Also, statistical methods used to perform such analyses are prone to pitfalls. However, this study is far more telling than conclusions drawn from imprecise, erratic associations between temperatures and disease-carrying insects.

Flawed Assumptions

Public discourse on insect-borne diseases and global warming is plagued by junk science involving bogus associations and models that don’t reflect reality.

Likewise, a 2012 paper published in the medical journal Lancet regarding insect-borne diseases documents that “it is now well established in the scientific community that climate change has played and will play a mixed and minor part in the emergence of most vector-borne pathogens.” But “a persistent stream of reviews are published that claim that climate change is a primary driving force.” It then explains that such reviews are based on flawed assumptions from:

• “speculative reports that describe the general coincidence of increased disease incidence with warming in recent decades” while failing to account for the fact that “climate has not consistently changed in the right way, at the right time, and in the right places to account for the” changes in vector-borne diseases.
• “repeated publications of highly influential and visually arresting maps at the end of the 20th century that presented predictions of expanding malaria derived from mathematical models. Problematically, these models were not parameterized with data for key variables (e.g., vector abundance).”

In other words, public discourse on insect-borne diseases and global warming is plagued by junk science involving bogus associations and models that don’t reflect reality. Politico’s coverage of this issue is a prime example of this. Just Facts contacted Politico editor Stephen Heuser and informed him of these falsehoods he approved for publication. He never replied.

James D. Agresti is the president of Just Facts, a think tank dedicated to publishing rigorously documented facts about public policy issues. Rachel McCutcheon is a policy analyst with Just Facts.