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First Steps from Prevalence to Prevention
CHORI Researchers Identify High Virus Rates in Mosquitoes

Although mosquito-borne viruses, called arboviruses, represent a significant disease burden across the globe, resources are often limited to conduct the research required to adequately fight and prevent arbovirus outbreaks, which impact millions of people each year. A new study by CHORI scientist A. Desiree LaBeaud, MD, MS, and her colleagues, published in Emerging Infectious Diseases, Vol. 17, No. 2, February 2011, provides key data in the effort to help improve prevention and treatment strategies.

"Our study demonstrates the local abundance of mosquitoes that are able to propagate arboviral outbreaks in Kenya and the high prevalence of different arboviral infections within those mosquitoes."

“Our study demonstrates the high prevalence of different arboviral infections within mosquito populations in Kenya.”


The worldwide burden of arboviruses is staggering. There are over 50 million cases of dengue fever, for example, per year. Arbovirus outbreaks are sporadic, however, which makes it difficult to determine when the viruses are going to be circulating and plan a study accordingly.

In addition, diagnostic tools don't yet exist to accurately and rapidly differentiate one type of virus from another in the field. Dr. LaBeaud's study focused on identifying virus prevalence in mosquitoes during an outbreak of Rift Valley fever virus (RVFV) in Kenya, one of the first studies of its kind in this region.

"We wanted to determine the prevalence of RVFV in the mosquito population in Northeastern Province in Kenya, a hotbed for RVFV transmission," explains Dr. LaBeaud. "In addition, we also wanted to determine if other infections were circulating in the same region, so we tested the mosquitoes for West Nile virus as well, which is also known to circulate in Africa, and Kenya in particular."

Although the study was conducted during the 2006-2007 RVFV outbreak in Kenya, the prevalence numbers Dr. LaBeaud and her colleagues found were surprisingly high, with 22 percent of tested mosquito pools positive for RVFV, 18 percent positive for WNV, and 3 percent positive for both.

"The numbers of mosquitoes positive for these infections was really very high," says Dr. LaBeaud.
“We had expected the results to be positive, because there was an outbreak occurring at the time, but 22 percent positive is extremely high.”
"In addition, the minimum infection rates (MIRs), which are an estimation of the lowest proportion of the mosquito population that is carrying a particular virus, were also higher than expected.

"The MIRs were around two or three, which is really high as well, even during an outbreak," says Dr. LaBeaud.

Not only were the RVFV rates of infection high in the mosquitoes, but the mosquitoes had nearly equally high MIRs of WNV.

"That is perhaps one of the biggest findings," says Dr. LaBeaud.
"The clinical signs and symptoms of different arbovirus infections overlap. When an outbreak of a specific arbovirus occurs, everyone is focused on treating just that one infection, but in fact, what our study shows is that all patients with typical symptoms may not be infected by the outbreak virus. They may have a different infection, or a concomitant infection, which is important to know in terms of treatment and disease prevention in the community. Our study highlights the real-life simultaneous arbovirus circulation in endemic areas."

The other significant finding was a disparity in the rates of human infection based on the rates of mosquito infection for one of the two villages in the tested areas.

"We've been doing human studies in these areas for a long time, so we know that in one of the villages we have a significantly higher prevalence of RVFV human infection. Our study showed that there wasn't really a difference in the rates of mosquito infection for these two areas, however," explains Dr. LaBeaud.

“It hints that during outbreaks infected mosquitoes are everywhere, but infected humans aren't. This suggests that there is another component probably infected animals that makes a significant contribution to transmission to humans, but we have yet to identify the specific animal exposures that put humans at the most risk.”

Overall, the results of the study underscore the need for improved field diagnostics and better surveillance, as well as a greater clinical appreciation for the possibility of concomitant arboviral infections.

"Our current inability to rapidly detect arboviral infections in endemic communities has led to inaccurate risk assessments, underdiagnosis of clinical cases, and ineffective control measures," says Dr. LaBeaud. "Better detection methods in vector, animal, and human populations and recognition of arboviral circulation could lead to improved surveillance and better estimates of the true impact of arboviral disease on animal and human populations."

In the meantime however, the data provide a substantial first step in understanding the complicated relationship between mosquito prevalence numbers, animal infection and human disease outbreaks.

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Tuesday, May 17, 2011 8:19 AM

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