Vectoring in on mosquitoes:
Breaking the cycle of vector-spread diseases
Vectors are the intermediary organisms that move diseases from one individual or animal to another. One of the most recognized vectors is the mosquito, which has played a part in shaping human history by spreading malaria, yellow fever, and many other diseases. Mosquitoes have made headlines recently for the role they play in spreading West Nile Virus.
Mosquitoes
are the special targets of separate projects by Shirley Luckhart (right),
assistant professor of biochemistry,
and Sally Paulson, associate professor of entomology.
Vectors are more than just vessels carrying disease agents from one point to another. They are fundamental components in the torturous life cycles of the parasites that cause diseases. During the process of blood-feeding, vectors pick up immature parasites from one host and deliver the fully formed, disease-causing parasites to another host. That is a very simplistic description of the complex relationships among vectors and parasites that result in the spread of many of the most important infectious diseases of humans and animals.
Luckhart's research takes her to Kenya, an African nation with the perhaps the highest percentage of people infected with malaria. Even with that distinction, only about 5 percent of the mosquitoes in Kenya are infected with malaria parasites; in most malaria-endemic countries the percentage of infected mosquitoes is measured in tenths of a percent."People would expect that percentage to be much higher just because of the reputation malaria has," Luckhart says.
Still, that seemingly small percentage is enough to infect millions of people around the world with a disease agent that saps the energy, productivity, and potential from people, families, and communities.
"We want to look at the genetic profile of mosquitoes to see if a population of mosquitoes is at risk for transmitting malaria," says Lockhart. Joining her in collecting mosquitoes from Lake Victoria to the coast of Kenya were Ed Lewis, assistant professor of entomology, and Andrea Crampton, a postdoctoral researcher in the Luckhart lab. Now, Luckhart's team is trying to determine whether genetic markers might be associated with the transmission of malaria parasites.
Luckhart hopes the research, conducted in collaboration with the U.S. Army, will result in a reliable field test to determine if mosquitoes associated with the spread of the disease are present. In theory, genetic testing may be quicker and less expensive than identifying infected mosquitoes through painstaking individual dissections and taxonomic evaluation.
"There are a number of things we've learned about the genetics of mosquitoes," Luckhart says. "Now we're trying to see if we can take what we've learned in the lab into the field and get meaningful results." What works in laboratories may not work in the field simply because the carefully controlled environments of laboratories may screen out unknown environmental factors that are important to the disease-propagation process or to the natural defense mechanisms in hosts and victims of malaria.
Meanwhile, Paulson is, in a manner of speaking, a component of a defense mechanism: she is a member of the Virginia Interagency Arbovirus Task Force, which was established by the state to combat West Nile Virus. The virus, which with the help of mosquito vectors can make the jump from birds, causes encephalitis in humans, horses, and other animals. Nearly a dozen people in the United States have died from West Nile Virus since it was first detected here in 1999. Though it is new to the United States, it has been recognized for years in the Middle East, Africa, and in Southern Europe.
The primary hosts for West Nile Virus are birds and mosquitoes. People and domestic animals are considered "dead-end" hosts; though they can become sick they apparently do not figure in the life cycle of the disease-causing parasite.
"Some species [of mosquitoes] may not be able to transmit the disease," Paulson says. "The only way to get a handle on that is to do lab work."
But before laboratory work can begin, field work is necessary.
Local health departments throughout Southwest Virginia identified likely mosquito breeding areas. Paulson and her students collected specimens from those areas, or worked with other people recruited by the health departments to collect specimens. Those specimens are being tested in Paulson's lab in Price Hall. That testing will result in information on which species of mosquitoes are infected and where those specimens were found.
"Very little information exists on vectors and their distribution in this part of the state," Paulson said. "We're pretty much starting from scratch. The information we develop in the lab this spring (2002) will allow us to focus on certain geographical areas during the summer."
Knowing the geographic distribution of mosquito species and knowing the susceptibility of those species to the West Nile Virus will allow public health agencies and homeowners in high-risk areas to take preventative measures.
Both Paulson and Luckhart agree that low-tech, tried-and-true control measures are effective in controlling mosquitoes. Measures that communities and homeowners can take include getting rid of breeding areas – specifically, standing water. Around homes, that can be as simple as tipping the water out of an old tire or changing the water in a birdbath every few days. Communities may use chemical or biological measures to control mosquito populations.
"The first step is to understand the problem, and that is what we're working on now," said Paulson. "After that, it is a matter of raising awareness that a problem exists and that there are things people can do to counter that problem."
Stewart Macinnis, College of Agriculture and Life Sciences
Also see Sally. Paulson's "10 basic facts about West Nile Virus"
Content of this column may be reused so long as credit is given to Virginia Tech and the researchers.