News: Latest Research Cements Link Between Climate Change & West Nile Outbreaks

Latest Research Cements Link Between Climate Change & West Nile Outbreaks

A recent study underscores a connection between climate change and infectious disease, raising concerns about our quickly warming planet.

This research, published in the Proceedings of the Royal Society, took a closer look at factors driving the spread of West Nile virus. The lack of reliable predictive models for West Nile and other infectious diseases leaves the public, and healthcare providers, without warning of a potential outbreak.

The team of researchers, from all across the country, produced a new model that offers hope for predicting and alerting vulnerable regions throughout the country.

The Dangers of West Nile Virus

Isolated in humans in 1937 in Uganda, West Nile virus is a member of a large, particularly potent family of pathogens called Flaviviridae. The viruses in this group are transmitted mainly through mosquito bites or ticks. Flaviviridae infections vectored by mosquitoes include West Nile, Zika, yellow fever, and Dengue fever, among others. Across the globe, West Nile is the most widely dispersed of this pathogen family.

The virus emerged again in birds of the Nile river delta in the 1950s, and by 1997, a more deadly strain was identified as the cause of death in several types of birds in Tunisia and Israel. By 1999, the Centers for Disease Control and Prevention (CDC) responded to what was thought to be an outbreak of St. Louis encephalitis in New York City. Exactly how the virus got to New York City remains unknown.

Wild birds become hosts of the virus after they are bitten by infected mosquitoes. As birds become reservoirs of the virus, uninfected mosquitoes bite the birds and also become infected. Mosquitoes then feed on humans and mammals like horses. Humans are considered the "dead end" host, because the virus does not naturally go further. The virus can also be transmitted through organ transplant, blood transfusion, during pregnancy, and in a laboratory setting.

By January 2017, the CDC reports that 47 states and the District of Columbia have reported cases of West Nile virus in birds, mosquitoes, or humans. Of those infected with West Nile virus, 80% suffer no remarkable symptoms. In some people, the virus triggers potentially fatal encephalitis or meningitis. Between 1999 and 2015, the CDC received reports of 43,937 cases of West Nile virus, with 1,911 deaths. Roughly half of those infected, or 20,265 people, suffered neuroinvasive diseases like meningitis or encephalitis.

Cases reported in the US for West Nile virus in humans in 2016. Image via CDC

Because of the potentially fatal impacts West Nile virus can have on a population without immunity, it is critical to understand which regions might be hardest hit.

Factors That Influence Infectious Outbreak

Variability of environmental factors makes it hard to predict where West Nile will travel, and where it could lead to outbreak.

Like the Zika virus, West Nile depends on mosquitoes. Zika is expected to spread in the US, but likely only within the warmer range that supports the mosquitoes that carry it, Aedes albopictus and Aedes aegypti. West Nile, spread by mosquitoes in the Culex family, is already established across the country, north and south.

In the recent study, researchers sought to better understand what factors most prominently impact where the next serious outbreak of West Nile virus might occur. Criteria considered include:

  • Rainfall has an impact on mosquito populations. While precipitation collected in puddles, pools, around homes, and in equipment creates instant mosquito breeding conditions, heavy rains can flush out containers that would otherwise shelter mosquito larvae.
  • While dry weather reduces moisture, it can improve mosquito survival. Dryer weather can reduce predators that feed on mosquitoes and provide more stable breeding conditions.
  • Temperature impacts humidity and the abundance of mosquitoes and hosts.
Image by mrtwister/123RF

Using weather, temperature, precipitation, mosquito prevalence, and other data, researchers developed a predictive model that identifies the prime factors of regional incidence of West Nile virus in the US.

Drought & Immunity Impact Outbreak of West Nile Virus

Climate change impacts temperatures, rainfall, and incidence of drought. As far back as 2007, the Intergovernmental Panel on Climate Change reported that "climate change contributes to the global burden of disease and premature deaths." In 2016, warming and weather contributed to an explosive emergence of Zika virus.

In this study, researchers found drought and immunity were "the strongest predictors" of yearly cases of West Nile virus on the state and national level. First author on the study, Dr. Sara Paull, a disease ecologist from the National Center for Atmospheric Research, discussed the impact of drought on West Nile prevalence with WonderHowTo, stating:

Drought seems to be increasing the percent of mosquitoes infected with WNV, rather than affecting mosquito abundance directly. It is unclear exactly what mechanism is driving the pattern, but there are a few hypotheses that seem plausible (although there are no data yet to test them).

For instance, drought could cause birds to experience stress-related reductions in immune responses that could increase their likelihood of transmitting the virus to mosquitoes. Or if mosquito-bird interactions are increased by congregation, or if the vector-to-host ratio is increased by a reduction in bird breeding activity in drought years (since nestlings contribute quite a lot to WNV transmission).

— Dr. Sara Paull, NCAR

Drought is a combination of higher temperatures and lower precipitation which can boost prevalence of circulating a virus. For a human community with low immunity, the consequences could be an outbreak.

[I]ncreases in drought could potentially double West Nile virus epidemic intensity nationally, with epidemics in areas of low immunity being even larger.

The research finds that states with low human immunity, like Virginia, could experience an infection rate eight times higher in drought conditions. Climate changes that result in warming of northern states, like Idaho, Michigan, and Montana, could result in premium conditions for transmission of West Nile virus to populations with little herd immunity.

Also worrisome is the overall impact of climate change on the spread of infectious diseases.

Because drought severity is likely to alter transmission of other vector-borne diseases...variations and changes in drought severity should be examined as potential drivers of disease dynamics.

With global warming expected to increase the possibility of drought in the US, the reach of West Nile and other pathogens is likely to increase.

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Cover image by Galyna Andrushko/123RF

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