When it starts raining crows and jaybirds instead of cats and dogs, it's high time to start thinking West Nile virus (WNV).

This East African, mosquito-borne, bird-transmitted, human-targeting pathogen landed suddenly on the Eastern Seaboard of the U.S. late in 1999. The West Nile virus' first prey consisted of seven American victims in New York City. (See BioWorld Today, Nov. 16, 1999, p. 1.)

"Birds are the natural reservoir for this virus," explained virologist/immunologist Michael Diamond of Washington University in St. Louis. "The cycle of nature is mosquito to bird. When humans get infected it's what we call a dead-end host," he continued, "meaning that the virus does not amplify enough in humans, or in many other mammalian species, so that the common Culex pipiens mosquito, which specializes in WNV, will not get reinfected.

"For a mosquito to be infected," Diamond continued, "it has to bite the bird, which has a much higher level of WSV in its avian blood. Then it would be able to transfer the virus to another bird or mammal. But if it transmits it to an animal, that recipient receives a very short-lived amount of high-level virus in its blood, and is not competent to get more mosquitoes infected. For example," Diamond went on, "the dengue and yellow fever viruses are close molecular cousins to the West Nile virus. They are carried by the Aedes aegypti mosquito, and not by Culex. Aedes can get infected by biting humans; Culex cannot."

In the four scant years since the West Nile virus invaded North America, it has advanced from the East Coast to the West Coast.

"As of the end of last year," Diamond observed, "which is the end of the mosquito season, 44 continental states had reported WNV activity either in mosquitoes, birds or humans. And as far west as the state of Washington, birds have been found infected."

California Holds Out For Clarity

"California had one WNV human patient, although it's not clear whether that case was imported and the person actually traveled with it, or whether that person got the infection endogenously. The person apparently had not traveled, although the Centers for Disease Control [CDC] could not find any infected mosquitoes in the California area where that person had resided. However, it's clear that in the 43 other states, mosquitoes and birds have been documented. And WNV has also traveled as far north into Canada as British Columbia, Ontario, Manitoba, Saskatchewan and some of the other provinces in the East as well. CDC fully expects that all 48 continental U.S. states will have West Nile activity by the end of this 2003 summer."

Diamond is senior author of a paper in the February 2003 issue of the Journal of Virology. Its title: "B cells and antibodies play critical roles in the immediate defense of disseminated infection by West Nile Encephalitis Virus."

"Probably the overall finding of that article for us," he told BioWorld Today, "is that T cells and antibodies play an extremely critical role in combating infection against West Nile virus very early in the course of infection. Although we were not surprised that antibodies did play a role - because they are an important part of the immune system - we were intrigued at how profound that role was when they were missing antibodies. At a time point, when inappropriate antibody responses did not generate adequate levels of antibodies, these animals were very susceptible to WNV infection. Just one viral particle - an exceedingly low dose - was enough to kill the animals."

Diamond recounted the in vivo experiments he and his co-authors conducted. "It's actually very simple," he began. "WNV is a mosquito-borne virus, so normally it gets inoculated by mosquitoes into the skin. That's exactly what we did. Not by a mosquito because we didn't want to have infectious insect vectors and infectious animals in the same place. We took a very fine needle and injected a very small amount of virus underneath the mouse skin, then watched them and waited to see what would happen. Some of the wild-type mice did fine," he recalled. "They got mildly sick with a fever-like illness, then recovered and were fully healthy. The other immunodeficient mice, with B cells and antibodies knocked out, did poorly, got sick and worsened. That was one of the outcomes that we looked at. In some cases we gave them antibodies a day or two before injecting them so they would have an extra supply of natural antibodies.

"We were able to reach the conclusion that if you don't have antibody produced between day 2 and day 4 after inoculation, you get a very high level of virus in the blood, and that ultimately causes the WNV to go into the central nervous system. Mice that lacked B cells and antibodies were completely unable to combat the virus. They developed serious brain and spinal cord infection, and ultimately died. These experimental results," Diamond pointed out, "may help explain why elderly humans and other people with weakened immunity, are at greatest risk of contracting serious diseases, notably viral encephalitis, when infected by WNV.

"However, in humans, West Nile virus causes such serious illness in only a small proportion of infected people. Last year, the CDC reported more than 3,500 cases of WNV, with only 5 percent to 10 percent of them resulting in serious illness or death."

NIH Plans Large-Scale WNV Clinical Trial

"The NIH is coordinating a clinical trial of West Nile virus to gain more information about the disease," Diamond noted. "They'll enroll patients very early on when they arrive in the ER for individuals suspected of WNV. They'll draw blood from these patients because they're in the hospital for various reasons. But some of the samples will be used to look at their antibody responses and see whether we can make predictions. Such as how does that correlate with what we see in our mouse model?

"And finally, based on all that data, we can begin to get a handle on what might be a novel approach to generating some sort of immunotherapeutic drug or vaccine to stimulate the immune system into generating a better protective response. Not something that will happen overnight, but over years," Diamond concluded.