How the vectors and ecology of infectious disease alter as the globe warms is one of the most poorly understood topics in climate change science, but most important for human health. Globally, infectious disease accounts for 1/3 of the 52 million people who die each year1, most of them in the lower latitudes. Recent experience with West Nile Virus in our own country reminds us how fast a new disease can spread, and the opportunities for it to do so in a warming world.
From arrival in New York City in the summer of 1999 — despite hundreds of millions of dollars in educational outreach and efforts to eradicate the mosquito that carries it —West Nile virus, a disease formerly confined to the Old World, became endemic in 48 states by 2003.2
Understanding the disease is the first step
Assistant Professor Maria Diuk-Wasser is an eco-epidemiologist and leading expert on infectious diseases at the Yale School of Public Health. She studies the ticks and mosquitos that transmit them, as well as relationships between those arthropods and the warm-blooded animals that they dine on and that serve as reservoirs to incubate the pathogens.
The virus that causes West Nile is zoonotic (i.e., it cycles back and forth between animals and may be transmitted to humans). In this case, the West Nile virus cycles between mosquitos that are the mechanism of transport, or vector, and birds that are the reservoirs in which the virus replicates itself, growing in number until it reaches infectious levels. Humans are incidental subjects of infection, not part of the organism’s normal development and hence, “dead end” hosts. Since the Culex pipiens mosquito that transfers the virus to birds is not prone to bite humans, different “bridge” species generally infect humans.
Such complex pathways of transmission make intervention a puzzle, says Diuk-Wasser. Do you target the species of mosquito that primarily spreads the disease among birds, or the ones suspected of sharing it to humans? Is intervention aimed at the vector or the host? Do you try to control the organism or modify the habitat that supports it?
Choosing which approach requires the sort of basic field study of insect vectors that has been on the wane for decades3 — largely replaced by laboratory efforts that develop therapeutics and vaccines. Improved diagnoses identify more human cases and medical treatments improve the health of the approximately 20% who will manifest symptoms of West Nile. But measures focused on people can’t contain a disease that’s incubated and amplified by animal hosts. For all the investment made over the decade and a half since West Nile arrived in the country prevention remains essentially unchanged from the 19th century perspective when Yellow Fever and malaria ravaged the nation — drain all standing water and avoid being bitten.
Where the science has advanced and where it yet might
The lack of interventions and programs, such as those that contained Yellow Fever and malaria outbreaks, partly explain how West Nile disease came to infect more than 5 million people in the US.4 However, the spread of the disease is largely a result of genetic variability and favorable environmental factors. These revelations suggest opportunities for future research and prevention.
The chief vector of the disease is a mosquito that lives about a week, and thus sets a limit for amount of time that the virus requires to reproduce. Natural selection favors genetic variants that reach infectious levels within that time frame. Extreme mortality suffered in Texas in 2012 led researchers to identify yet another and more lethal variant.5
On the ecological side of the research spectrum, researchers have looked at how hydrology and weather interact with the vectors, and how they’re affected by changing faunal populations of the warm-blooded animals on which they depend. At Yale, Maria Diuk-Wasser’s group used radio collars and field observations of thousands of birds to identify large roosts of American Robin as the local reservoir for the disease.6
Lyme Disease is another malady whose spread is frequently linked to a changing climate. Black-legged ticks, the disease’s vectors, depend on white-footed mice and deer during the various stages of their life cycle. Ticks appear to expand their range in response to warming temperatures and forest restoration.7 Recent years have seen increased endemism of vector ticks as far north as Canada.8
Assistant Professor Diuk-Wasser cites containment of a rabies epidemic in Ohio9 as an example of how knowledge of a disease’s ecology can provide an effective intervention strategy. Rabies is as lethal to raccoons as West Nile Virus is to crows. “It kills everything in its wake,” she says, wriggling her outstretched fingers to suggest the rabies virus spreading inexorably across the landscape. But surveillance of dead raccoons collected by animal control officials allowed public health officials to quickly map the progress of the disease across the region and identify its spread. Baits containing an oral vaccine were dropped throughout the forest in a ring around the advancing wave, inoculating and immunizing raccoons, and largely containing the outbreak.
West Nile and Lyme disease are trickier to combat than rabies. The vectors of these diseases travel farther and faster than raccoons and there are no vaccines for them. Fortunately, the illnesses themselves are relatively benign. Only about 1% of those infected by West Nile will develop a severe reaction, and nearly all cases of Lyme disease are successfully treated with a 10-day course of doxycycline. The same cannot be said with any assurance of several hundred other mosquito and tick-borne viruses that have yet to turn up, or turn deadly, on our shore.
The domestication of animals likely enabled the first great wave of epidemic disease around 10,000 years ago.10 A second wave followed the expansion of civilization and contact between Europe and Asia about 2,500 years later. Discovery of the New World prompted a third wave. Many feel that warming temperatures, changes in hydrology, and increasingly convenient travel and commerce put us on the cusp of a fourth wave, but the study of how changing climate impacts diseases and the vectors that carry them is poorly studied. Given that atmospheric CO2 levels are now as high as they were about 5 million years ago 11 and quickly moving beyond,12 fundamental research on the topic also should accelerate. An aggressive program of study of mosquitos and ticks and their relationships to this changing landscape are critical if we’re not to be caught flat-footed by nature again.
1. The Top 10 Causes of Death. (2011) The World Health Organization Media Centre.
2. The continuing spread of West Nile Virus in the Western Hemisphere. DJ Gubler 2007. CID Oxford Journal. cid.oxfordjournals.org/content/45/8/1039.long
3. Wanted: Medical Entomologist. Durland Fish. Vector Borne and Zoonotic Diseases. Editorial, 2001. Volume 1, Number 2.
4 Durland Fish. Personal communication
5. Maria Diuk-Wasser. Personal communication
6. Diuk-Wasser. Aian Communal Roosts as Amplification Foci for West Nile Virus in Urban Areas in Northeastern United States. Amerian Journal of Tropical Medical Hygiene. 2010.
7. Tokarevich, N.K., et al. (2011) The impactof climate chnge on the expansion of Ixodes persulcatus habitat and the incidence of tick-borne encephalitis in the north of European Russia. Global Health Action, 4:10.3042
8. Nicholas H. Ogden, Nicholas H. Ogden, L. Robbin Lindsay, Muhammad Morshed, Paul N. Sockett, and Harvey Artsob, The emergence of Lyme disease in Canada, CMAJ June 9, 2009 180:1221-1224.
9. Colin A. Russell, et al (Predictive Spatial Dynamics and Strategic Planning for Raccoon Rabies Emergencein Ohio, PLoS Biology, March 2005, Volume 3, Issue 3
10. World Watch Institute. (2005) State of the World 2005. Trends and Facts - Containing Infectious Disease.
11. Pagani, M., Liu , Z., LaRiviera, J., Ravelo, A. C. (2009) High climate sensitivity to atmospheric carbon dioxide for the past 5 million years, Nature Geoscience, 3, 27-30.
12. IPCC AR5 Summary for Policy Makers. (2013)