It's possible that a serious mosquito-borne virus with no known vaccine or treatment could migrate from Central Africa and Southeast Asia to the United States within a year, new research suggests.
The chances of a U.S. outbreak of the Chikungunya virus (CHIKV) varies by season and geography, with those regions typified by longer stretches of warm weather facing longer periods of high risk, according to the researchers' new computer model.
"The only way for this disease to be transmitted is if a mosquito bites an infected human and a few days after that it bites a healthy individual, transmitting the virus," said study lead author Diego Ruiz-Moreno, a postdoctoral associate in the department of ecology and evolutionary biology at Cornell University in Ithaca, N.Y. "The repetition of this sequence of events can lead to a disease outbreak."
And that, Ruiz-Moreno said, is where weather comes into the picture, with computer simulations revealing that the risk of an outbreak rises when temperatures, and therefore mosquito populations, rise.
How the study was done
The study analysed possible outbreak scenarios in three U.S. locales.
In 2013, the New York region is set to face its highest risk for a CHIKV outbreak during the warm months of August and September, the analysis suggests. By contrast, Atlanta's highest-risk period was identified as longer, beginning in June and running through September. Miami's consistent warm weather means the region faces a higher risk all year.
"Warmer weather increases the length of the period of high risk," Ruiz-Moreno said. "This is particularly worrisome if we think of the effects of climate change over [average] temperatures in the near future."
Ruiz-Moreno discussed his team's research funded in part by the U.S. National Institute for Food and Agriculture in a recent issue of the journal PLoS Neglected Tropical Diseases.
CHIKV was first identified in Tanzania in 1953, the authors noted, and the severe joint and muscle pain, fever, fatigue, headaches, rashes and nausea that can result are sometimes confused with symptoms of dengue fever.
Signs and symptoms
Few patients die of the illness, and about one-quarter show no symptoms whatsoever. Many patients, however, experience prolonged joint pain, and there is no effective treatment for the disease, leaving physicians to focus on symptom relief.
Disease spread is of paramount concern in the week following infection, during which the patient serves as a viral host for biting mosquitoes. Infected mosquitoes can then transmit the virus and cause a full-blown outbreak.
The U.S. Centers for Disease Control and Prevention became aware of the growing threat of a global outbreak in 2005 and 2006, following the onset of epidemics in India, Southeast Asia, Reunion Island and other islands in the Indian Ocean. In 2007, public health concerns mounted following an outbreak in Italy.
To assess the risk of a U.S. epidemic, the authors collected data concerning regional mosquito population patterns, daily regional weather and human population statistics.
They ran the information through a computer simulation designed to conservatively crunch the numbers based on the likelihood that an outbreak would occur in the coming year after just one CHIKV-infected individual entered any of the three test regions.
The results suggested that because environmental factors affect mosquito growth cycles, the regional risk for a CHIKV outbreak is, to a large degree, a function of weather. The authors said that public health organisations need to be "vigilant," while advocating for region-specific planning to address varying levels of risk across the country.
Human behaviour a key factor
However, Dr. Erin Staples, a CDC medical epidemiologist based in Fort Collins, Colo., said that although the study was "carefully and nicely done" the investigation's focus on the role of temperature in CHIKV outbreak risk should not negate the importance of other key factors such as human behaviour.
"We're aware of the potential introduction and spread of this virus, as well as several other mosquito-borne diseases," she said. "We've been working to create and prepare a response to the risk that this virus could expand into the U.S."
"Similar to the messages we give for West Nile, another mosquito-borne disease, we believe that prevention is the most important thing to focus on," Staples said. "That means wearing long sleeves and pants, using air conditioning or making sure your screens are intact, avoiding standing water, and using mosquito repellant. Because if [CHIKV] were to be introduced into the U.S., the best way to prevent a spread is to avoid mosquito bites in the first place."
(HealthDay, January 2013)