By LTC Jason Richardson, Dr. Michael Turell, and CPT Elizabeth Wanja
Rift Valley fever death toll on the rise
(Pretoria News, May 7, 2010)
Economic losses from Rift Valley fever greater than previous documented
(ILRI Clippings, Oct. 10, 2010)
Epidemiologists now have a tool that might make such tragic headlines a thing of the past. The Military Infectious Diseases Research Program, part of the U.S. Army Medical Research and Materiel Command, and VecTOR Test Systems Inc. have developed a diagnostic device that can detect the presence of the Rift Valley fever (RVF) virus in mosquitoes, thus alerting deploying troops to its presence and enabling public health officials to prevent or mitigate the spread of disease.
A new handheld dipstick assay allows researches to test whether field-collected mosquitoes, like these collected in Kenya in 2006 during the RVF virus outbreak, are infected with RVF virus. (U.S. Army photo by LTC Jason Richardson)
RVF is a mosquito-borne viral disease that poses a significant health risk, primarily to livestock but also to humans. Epidemiologists have been aware of RVF since first isolating the virus in Kenya in 1931. Over the decades, the virus has spread throughout sub-Saharan and North Africa and beyond the continent’s borders to Saudi Arabia and Yemen. Health officials openly wonder whether RVF could strike Southern Europe or even the United States.
Scientists believe the virus persists in nature because infected female Aedes mosquitoes transmit it to their eggs. The infected eggs lay dormant until excessive rainfall, when they hatch and produce infected adult mosquitoes that can initiate an outbreak. Secondary vectors, such as Culex mosquitoes, also reproduce rapidly during heavy rains and contribute to disease outbreaks among animals (known as epizootics).
Forces at increased risk include personnel operating in the Horn of Africa as part of Operation Enduring Freedom to combat terrorism and to thwart piracy. The most vulnerable human populations are farmers and herders who live near canals, rice fields, and other wet zones where mosquitoes proliferate. In animals, RVF causes acute hepatitis and spontaneous abortion in infected livestock, leading to steep mortality rates and substantial economic losses. Once authorities declare an epidemic, they immediately prohibit all animal exports. Such bans cause further economic hardship, particularly for developing countries that depend on livestock for their livelihood. For Somalia, where livestock accounts for more than 80 percent of export earnings, extended bans can be devastating. Losses totaled $109 million during a ban there from February 1998 to May 1999 and another $326 million from September 2000 to December 2002.
Sick livestock infect other mosquito species, which then transmit the virus to humans. People can also contract RVF by coming into contact with the meat or blood of infected animals. Mild cases of human RVF present with flulike symptoms. In some cases, the patient can develop retinal degeneration, which may lead to blindness. In severe cases, the patient can develop encephalitis or even hemorrhagic complications, which kill up to half of those who contract the disease.
In 1997, 300 Kenyans died from what researchers suspect was a hemorrhagic form of RVF. Determining actual infection rates on the African continent can be challenging, as populations often live far from medical care. No treatments or licensed vaccines exist for human RVF, only palliative care; thus, accurate and reliable diagnostic methods are critical.
Laboratory evaluation of the RVF virus assay. (Photo courtesy of CPT Elizabeth Wanja)
The new test kit, developed by VecTOR with funding from the Small Business Innovative Research program and the Military Infectious Disease Research Program, is a handheld dipstick assay that determines whether field-collected mosquitoes are infected with RVF virus. Providing results in less than 20 minutes, it is easy to use and does not require the use of a laboratory with containment facilities. Another big plus is that the test does not require refrigeration, a particular concern in the tropics. As long as the dipsticks are kept within their original containers, they remain stable for two or more years.
The test kits allow early detection of infected mosquitoes, which enables military planners and public health officials to initiate preventive action, including mosquito control operations, administering animal vaccines, restricting the movement of livestock and warning against contact with infected animals. Officials can also distribute mosquito nets and repellents to the public and forewarn local hospitals. On the military side, relevant agencies can issue such proven preventive measures as repellent and insecticide-treated uniforms and netting.
The validation process for VecTOR’s RVF virus assay was a joint effort among researchers from the Entomology Branch, Walter Reed Army Institute of Research (WRAIR) Virology Division, U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID), the Kenyan Medical Research Institute, and the U.S. Army Medical Research Unit, Kenya.
- LTC JASON RICHARDSON is Director of the Entomology Branch at WRAIR. He holds a B.S. in biology from Vanderbilt University, an M.S. in medical entomology from Clemson University, and a Ph.D. in microbiology from Colorado State University. Richardson is Level I certified in science and technology and a member of the U.S. Army Acquisition Corps.
- DR. MICHAEL TURELL is a Research Entomologist in the Virology Division at USAMRIID. He holds a B.S. and M.S in entomology from Cornell University, an M.P.H in epidemiology from Tulane University, and a Ph.D. in epidemiology from the University of California, Berkeley. Turell is a board-certified medical entomologist.
- CPT ELIZABETH WANJA is a Research Entomologist in the Entomology Branch at WRAIR. She holds a B.S in zoology and an M.S. in applied entomology and parasitology from the University of Jos, Nigeria; an M.S. in medical entomology from Michigan State University; and a Ph.D. in environmental biology, majoring in entomology, from the University of Guelph, Canada. Wanja has completed the intermediate medical acquisition training.
