Marin/Sonoma Mosquito & Vector Control District
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Two new cases of hantavirus disease have been confirmed in Northern California, raising concern that a rare but deadly microbe may be spreading too fast for public-health authorities to keep pace.
    First recognized as a cause of human disease in the United States in 1993, hantavirus is borne by the common deer mouse and is often fatal. It was regarded until recently as a problem limited mainly to the arid Southwest.
    Now it's clear that hantavirus respects few boundaries. Statewide testing shows the infection rate among California deer mice averages 10.5 percent, but it can run much higher, above 70 percent, in some local areas.
    In California, the 31 confirmed cases of human infection with hantavirus are clustered mostly in rural areas east of the Sierra Nevada. But new cases keep cropping up in surprising places.
    The two new Northern California cases were confirmed last month in Yolo County, including what could be the first locally acquired infection in that area. Late last week, the first confirmed hantavirus case was reported in Ventura County in Southern California, where a 27-year-old groundskeeper has been hospitalized since May 17.
    A 27-year-old Contra Costa County woman died last year of an infection believed to have been picked up in Contra Costa or Alameda counties -- just how remains a mystery.
    Although few deer mice are found in cities, the virus has even been detected in the Bay Area and Los Angeles.
    Five mice among 77 trapped in Alameda County have tested positive, as have 17 of 495 trapped in Los Angeles. Only 30 deer mice, all negative, have been tested in San Francisco -- but that doesn't mean the city is immune.
    ``We have found hantavirus virtually throughout the state,'' said Vicki Kramer, chief of the vector-borne disease section of the state Department of Health Services in Sacramento. ``Virtually every county has mice infected.'
'     The virus causes what is known as hantavirus pulmonary syndrome, or HPS. Fever and muscle aches typically appear first, within one to five weeks after exposure, followed by coughing and shortness of breath. In the latter stages, symptoms can worsen rapidly, requiring hospitalization and ventilation within just 24 hours. Death comes when fluids accumulate in the lungs.
    There is no treatment. Some victims manage to survive with proper supportive care, but none of the current antiviral medications seem to have any effect.
    It takes a trained eye to tell the difference between ordinary house mice, which do not harbor the virus, and Peromyscus maniculatus, the species of deer mouse that can be infected for life without suffering any apparent ill effects.
    Infected mice can shed the bug in their feces, fur and saliva. The virus can be picked up fairly easily by breathing contaminated dust when cleaning or even walking through enclosed areas where deer mice have been.
    It's more difficult to acquire in open areas, but experts still advise against sleeping on bare ground in the backcountry or any avoidable contact with rodents or their leavings.
    The risk of human-to-human infection was considered nil until 1997, when the first case was reported in Argentina. In the United States, however, a study of health-care workers, who were known to have been exposed to infected individuals, has eased fears that the virus might spread from one person to another.
    The first mysterious surge in cases of lung disease was among the Navajo in the Four Corners area of Utah, Colorado, New Mexico and Arizona. Within a few months of frantic testing and lab analysis, the problem was traced to a newfound strain of hantavirus, called the Sin Nombre virus.

    The flare-up, it turned out, stemmed from a tenfold increase in the local rodent population after a period of heavy rain. Since then, hantavirus infections have turned up all over the country. Just last month, Vermont became the 31st state to report a case of human infection, one of about 250 deaths or illnesses to be confirmed since the virus was identified.
    But later studies confirmed that hantavirus is not really a new source of human disease. Tissue samples have turned up positive for hantavirus in cases of people who died years ago of unexplained lung disease, including a 39-year-old Utah man who died of hantavirus infection in 1959.
    The notion that close contact with mice is unhealthy was not new to the Navajo, whose medical traditions long included warnings about an ancient mouse-borne illness. The Navajo also tell of similar disease outbreaks in 1918 and 1933-34, when weather conditions also boosted rodent populations.
    Despite the rarity of human cases, hantavirus has clearly revealed some big gaps in the state's early-warning system for new and emerging agents of infectious disease. The front-line defense consists of just 16 state field biologists charged with covering all of California. Experts concede that many cases of disease may have been missed in the past.
    ``It's been here all along,'' said Curtis Fritz, a state epidemiologist working on the latest cases. The difference now is we are looking for it.'
'     The two recent Northern California cases were both in Yolo County outside Sacramento. An investigation is continuing in the latest case, in which a 20-year-old resident of Davis may have contracted the virus during a trip to a remote area of New Mexico where hantavirus is known to be endemic.
    That's the tentative conclusion based on the victim's own account of his activities during the weeks leading up to his illness, from which he has since recovered.
    Alfonso Alvarado, a hay hauler who lived in Woodland, was not so fortunate.
    Alvarado drove throughout the state to pick up or deliver hay and straw. So far, state biologists have had no luck tracking down the source of the infection.
    State disease sleuths, assisted by Yolo County health officials, recently donned face masks and protective jumpsuits for the unenviable task of collecting lab samples from 19 Peromyscus maniculatus.
    The deer mice, destroyed and sent to a hazardous-waste incinerator after their blood was drawn, had been collected from traps set out the night before at suspicious sites around Woodland, including haysheds and a weedy junkyard near Alvarado's home and haybarns where he worked.
    The samples all tested negative for hantavirus. Moreover, Alvarado's body was allowed by local officials to be cremated before the nature of his disease was confirmed -- and before sufficient tissue samples were taken for further investigation.
    Modern gene-sequencing techniques allow epidemiologists to closely track a pathogen, matching up the genetic fingerprint from virus found in the victim's system to virus isolated in the wild. But with no hanta-bearing mice and no more leads, state health authorities have just about given up the chase.
    ``It's frustrating,'' Fritz said. ``We have investigated all sites where he was known to have picked up or delivered in the weeks before he died. We trapped rodents at all those sites but none of those have tested positive. I'd say at this point it will probably remain a mystery. We just don't have anything that even looks suspicious.''

This article is available on the San Francisco Chronicle website
[http://www.sfgate.com], June5, 2000.