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The West Nile Virus is heading for Northern California and will probably reach Alameda and Contra Costa counties within the next two years, according to an infectious disease expert with the U.S. Centers for Disease Control (CDC). 

Since its introduction to the eastern seaboard of the United States in 1999, the virus has spread westward as far as Imperial County in the extreme southeastern corner of California. The virus is mainly carried from location to location by migrating birds, and is passed on to humans primarily by bites from infected mosquitos. 

While 20 percent of individuals exposed to the virus only develop flu-like symptoms and 80 percent develop no symptoms at all, a small portion—less than one percent—develop severe, central nervous system diseases, some of which resemble polio, and some of which cause death. Last year 284 people died from West Nile Virus.  

Local, state, and federal health officials are currently conducting detailed monitoring of the virus’ presence in mosquitos, birds, humans, and other animals in every county in the nation. 

In an address last week to the Graduate School of Journalism at the UC Berkeley, CDC Acting Director of the Division of Vector-Borne Infectious Diseases Dr. Lyle Petersen said that with only four years of data to work from, U.S. health officials do not know how severely the virus will hit this country, or what kind of pattern it will develop. 

“It [the West Nile Virus] is worrisome because it can be spread over a prolonged season and a wide geographic range by a variety of hosts,” Petersen said, noting that 43 mosquito species, 170 bird species, and 18 other animal species—including cats, dogs, squirrels, chipmunks, rats, and zoo animals—have all been identified as carriers. 

He estimated that “in a couple of years, every county in the United States will have West Nile Virus.” He also predicted that the virus would spread to every area of the Americas “except for the extreme northern and southern portions.” 

Petersen also said he was “not hopeful” about prevention efforts that have surfaced so far. 

“You can’t get rid of every infected mosquito,” Petersen said, adding that widespread pesticide spraying of adult of larval mosquitos has both limited impact on the mosquito population and widespread detrimental effects on the environment. “The best thing you can do is lower your chance of infection.” To that end, researchers are looking at such therapies as antiviral drugs and the injection of serum containing the virus antibodies. “But so far,” he cautioned, “there’s no treatment on the horizon.”  

Petersen suggested that until something better is developed, the best defense was the standard mosquito bite prevention regimen: Wear protective clothing and use mosquito repellent if you have to be out at twilight or at night, when mosquitos are biting. The doctor should know. Chatting with his mailman out as his mailbox one evening this summer, he was bitten by a mosquito and contracted a mild form of the virus himself.  

So far this year, health agencies report that they have identified 127 deaths among the more than 6,000 individuals in 41 states who they have identified as having been exposed to the virus. The bulk of the deaths have occurred in Colorado (38), Nebraska (15), and Texas (13). There was one report this summer of a woman hospitalized in Alameda County with a severe neurological disease after contracting the virus during a visit to Colorado. 

Two of the major potential mosquito carriers of the virus are the Western Encephalitis mosquito (Culex tarsalis) and the Northern house mosquito (Culex pipiens), both of which are widespread in both Alameda and Contra Costa counties. 

The virus can also be contracted through infected blood transfusion. 

After the CDC confirmed 23 cases of blood transfusion infection in 2002, screening of the nation’s transfusion blood supply for the virus was instituted. While researchers have also detected what Petersen calls a small number of cases of transmission of the virus through breast feeding, he said that “the great benefits of breast milk for babies far outweighs any risk. I wouldn’t recommend that any woman stop breast feeding.” 

Petersen also said that researchers have uncovered one case of transmission of the virus from an infected mother to her embryo through her placenta. “We’ve also found three cases where the embryo did not contract the virus from the infected mother,” he said. “It looks like transplacental transmission is the exception, rather than the rule.” Petersen also said that he knows of one report of what he called “a claim” of the spread of West Nile Virus through sexual contact, which he discounted. He concluded that although the disease can be spread from human to human, he said such incidents are minimal.