Just this fall, the Wildlife Conservation Society published its “Deadly Dozen” list, a report that provides examples of diseases that could spread as a result of climate-induced changes in temperatures and precipitation levels. In the report, Dr. Steven E. Sanderson, President and CEO of the Wildlife Conservation Society, is quoted saying, “The health of wild animals is tightly linked to the ecosystems in which they live and influenced by the environment surrounding them. Even minor disturbances can have far reaching consequences on what diseases they might encounter and transmit as climate changes.”
In what Dr. Alonso Aguirre, senior vice president of Conservation Medicine at the Wildlife Trust and chairman of the World Association of Wildlife Veterinarians, told this writer was a “political move”, West Nile virus was left off the Wildlife Conservation Society’s Deadly Dozen. Even Dr. Billy Karesh, Vice President of the Global Health Program at the Wildlife Conservation Society, admits than “this was by no means a comprehensive list.” He went on to explain that the purpose of the “Deadly Dozen” is to illustrate how wildlife health and human health and economies are interlinked and therefore both subject to changes brought about by shifting temperature and precipitation patterns.
The distribution and intensity of West Nile virus, and other vector-borne diseases, is most certainly influenced by temperature and rainfall, in addition to vectors and hosts. Temperature can determine the latitudinal boundary and upper elevation limit of the disease. Even a slight increase in global temperatures could greatly spread the areas susceptible to West Nile virus. One theory is that 1999, the year that West Nile virus first came to the United States, was one of the hottest and driest years in American history.
Temperature also has been linked to disease intensity and efficiency. Even relatively small changes in temperature (i.e. 2 degrees Celsius), have the potential to substantially increase transmission, the passing of a disease from an infected individual or group to a previously uninfected individual or group. Increased temperature also results in increasing the rates of infection, spread and transmission of the virus, and increases the virus’ ability to make copies of itself, which increases its survival. The time between when infection occurs and symptoms appear will be shorter under higher temperatures.
“As the shortening of this time period will allow a higher proportion of mosquitoes to transmit (despite the fact that longevity of mosquitoes decreases with higher temps) this could allow for more efficient transmission of the virus. In addition, mosquitoes will mature faster at higher temps and the virus can expand its range into areas that might have previously been restrictive for minimal temperature requirements for viral replication,” said Dr. Aaron Brault, of the Center for Vectorborne Diseases at the University of California, Davis campus.
What Brault is saying is that as our climate changes, more mosquitoes will be able to infect more people, more quickly and in more places.
Dr. Lyle Peterson, Director of the Division of Vector-borne Infectious Diseases at the Centers for Disease Control and Prevention, had explained the same concept of viral replication, stating that the virus replicates faster in the mosquito and the mosquito becomes infectious faster and more infectious when the temperature is higher.
West Nile virus affects birds and mammals worldwide and has become the focus of conservation, veterinary and human health concerns.
Several species of mammal have been found to naturally be exposed to West Nile virus, and can possible be used as indicators of transmission by seroprevalence testing, which is looking for antibodies in the blood to determine if an individual or population tests positive for a specific disease, such as HIV or West Nile virus. Previous scientific studies have shown patterns of antibody prevalence for West Nile virus across several mammal species, across several states.
Seroprevalence testing of mammals has been shown to be more intense in urban areas, which suggests a higher risk for exposure, indicating that factors that increase West Nile virus transmission, such as mosquito abundance, West Nile virus prevalence, and feeding on mammals are higher in urban areas.
Incidence of West Nile virus may be higher in urban areas, but the incidence in humans tends to be lower in areas with higher biodiversity, meaning more species of birds. The abundance of species most likely to transmit West Nile goes down when more species live in the area, which then decreases the chance of spread to humans. Higher biodiversity keeps the host populations of birds at a safer level, impeding their ability to infect humans. Increased biodiversity reduces the probability of disease transmission from he infected host to the vector, reduces the rate of encounters between infected vectors and hosts, the number of susceptible hosts, amounts of infected vectors, and also leads to a faster disease recovery rate in infected hosts.
A study by researchers from University of California at Santa Barbara and the College of William and Mary found that differences in bird biodiversity in neighboring counties, even with slight differences in species abundance, the counties with higher biodiversity had a lower incidence of human infection.
Looking at differences in bird biodiversity can help with public health, conservation and bioterrorism-preparation strategies, and be utilized to minimize health, ecological and economic impacts of the disease.
Brault said that the best way to predict an outbreak would be the ability to predict the weather. Perhaps more feasible options include monitoring mosquito density, and bird seroprevalence rates as well.
Not only can looking at the biodiversity of an area and testing for disease seroprevalence in birds and mammals be used to predict outbreaks, but migration patterns can prove to be an excellent indicator for when humans should take preventative measures to protect themselves from West Nile virus. The disease cycle starts in May, and the birds are the first to become infected by the mosquitoes. Gradually the horses become infected, and by September the first human cases are seen. The reason for this is that as birds migrate south in August or September, the mosquitoes switch their hosts and begin to feed on humans. As Aguirre said, “Right there is a very beautiful predictor for preventing disease.”