Tick-borne encephalitis

WHITE PLAINS, N.Y. -- West Nile virus infections should be back on the diagnostic radar screen for doctors through out the Northeastern United States.

And in all parts of the United States, arthropod-borne viruses should be in the differential diagnosis of any case of encephalitis of unknown etiology, said Dr. Roy Campbell of the division of vector-borne infectious diseases at the Centers for Disease Control and Prevention, Fort Collins, Colo.

Physician awareness of West Nile has led to increased reporting of other arboviral diseases. Several human cases of Powassan encephalitis, caused by a tick borne virus distantly related to West Nile, were reported last year in the Northeast, Dr. Campbell said at an international conference on the West Nile Virus sponsored by the New York Academy of Sciences.

The U.S. history of West Nile shows the importance of physicians' reports of unusual cases or clusters of cases to local health departments, said Dr. Marcelle Lay ton of the New York City Department of Health's division of communicable disease.

In late August, 1999, Dr. Deborah Asnis, an infectious disease specialist in the Queens area of New York, contacted Dr. Layton's office and described what turned out to be the first two documented human cases of West Nile virus infection in the Western Hemisphere. Dr. Asnis's index of suspicion was raised by the unusual finding of severe muscle paralysis that she had seen in two patients who appeared to have viral encephalitis.

If Dr. Asnis had not called about those cases, the entire outbreak might have been missed completely or its recognition delayed, Dr. Layton emphasized. Retrospectively, 15 other people hospitalized at that time were diagnosed with West Nile.

There will be good surveillance for infections in mosquitoes and birds this sum mer, but human cases could be the first sign of the virus in geographic areas where environmental surveillance may not be present, Dr. Layton said.

In updated guidelines released in April, CDC officials stated that enhanced surveillance of West Nile virus is a priority "for those states that are affected or that are at higher risk for being affected by the virus because of bird migration patterns and virus spread." In 2001, this includes the entire East Coast from Maine, New Hampshire, and Vermont to Florida and along the Atlantic and Gulf Coasts to Texas, as well as states "immediately adjacent to states with current West Nile virus activity, Canada, and countries in the Caribbean and Central and South America."

The most effective way to prevent transmission of the virus and other arboviruses "or to control an epidemic once it has begun, is to reduce human exposure via mosquito control," the CDC said.

Most human infections with West Nile virus are mild, with symptoms of headache and fever, or no symptoms. An estimated 1 in 150 people infected develops CNS disease, Dr. Layton said at the meeting, also sponsored by the CDC and the New York State Department of Health.

"I can't emphasize enough that the syndrome is relatively nonspecific," she added in an interview. No single sign "will let the physician know the patient has West Nile as opposed to an enteroviral or other cause of encephalitis."

A polymerase chain reaction test is available, but the enzyme-linked immunosorbent assay is the most sensitive screen for human West Nile infection. With CDC funding, almost all state public health departments can test serum and cerebrospinal fluid samples for West Nile virus. Positive tests must be confirmed, since the ELISA test can cross-react with dengue, Japanese encephalitis, and yellow fever. If a serum specimen is taken within 8 days of illness onset (when a patient could still be antibody negative), convalescent serums should be tested 2-3 weeks after illness onset.

Surveillance goals for 2001 should continue to focus on severe and hospitalized cases, in an effort to better understand the epidemiology and the clinical spectrum of West Nile disease in humans. "We continue to emphasize that this is more a disease of older adults," Dr. Layton said.

In 1999, most of the 69 laboratory-confirmed cases of West Nile virus were detected through physician-based reporting, Dr. Layton said. In 2000, there were 19 cases, 10 of which occurred in the outbreak's epicenter, Staten Island, N.Y. Most of the illnesses had an onset in mid to late August. In 2000, the earliest case was in mid-July.

The first eight patients identified in 1999 were aged 58-87 years, previously healthy, and living, at home. The clinical presentations were "basically identical," Dr. Layton said. All had a febrile illness followed by mild GI symptoms, then altered mental status pointing to a presumptive diagnosis of viral encephalitis. About half of the patients had the unusual finding of very diffuse muscle weakness, to the point of paralysis, including a need for ventilatory support.

All the patients spent a lot of time out-doors, especially in the evening. Later investigation revealed mosquito breeding sites in nearby yards and neighborhoods.

Dr. Asnis of Flushing (N.Y.) Hospital Medical Center described the eight patients, aged 29-87, who were treated at her hospital. Six older patients had encephalitis; two younger ones had meningitis.

All five who were admitted to the ICU were elderly; four had severe muscle weakness, four had respiratory problems, three were on respirators, and three died. The encephalitis patients had atypical bilateral weakness. All patients except for one had temperatures above 39[degrees] C; the meningitis patients had headaches; the encephalitis patients were confused; and almost all of the patients had GI complications.

The first admitted patient, a 60-year-old man, had had a fever for 3 days; was weak, nauseous, and confused; and could not lift his arms and legs. After 6 weeks, he was discharged to a rehabilitation facility.

Two men, aged 80 and 75 years, died after 3 weeks; both developed flaccid paralysis and reduced deep-tendon reflexes following fever and other symptoms.

A 79-year-old man with no previous medical history had become incoherent. He did not have much muscle weakness and was discharged. Later testing showed that he had West Nile encephalitis.

The last two patients admitted had meningitis. A 29-year-old woman had viral complaints, a rash, and weakness that resolved in about 3 months. A 49-year-old businessman had fever, headache, and weakness that resolved in about 2 weeks.

All but one of the spinal taps on these patients contained white blood cells, all but one had lymphocytosis, and most had red blood cells. Almost all of the patients were on third-generation cephalosporins; all of the ICU patients were on acyclovir, in case they had herpes encephalitis, Dr. Asnis said.

West Nile virus, first identified in Uganda in 1937, has caused epidemics in Romania in 1996, illnesses in humans in Algiers in 1994 and Russia in 1999, and outbreaks in Israel in the 1950s and late 1970s, with some reports in humans as recently as last year. The virus also caused illnesses in horses in Italy in 1998 and in Southern France last-year.

Tracking the West Nile Virus

The geographic extent of the West Nile virus was "tremendous" in 2000 based on the findings of Arbo-NET, a CDC surveillance for tracking viral activity in humans, other mammals, birds, and mosquitoes, Dr. Anthony Marfin said at the meeting.

Last year, evidence of the virus was found mostly in birds and nonhuman mammals. Reported human cases remained restricted to the metropolitan New York City area, said Dr. Marfin of the arbovirus disease branch in the division of vector-borne infectious diseases at the CDC, Fort Collins, Colo.

Infected birds, mostly crows, were found in 135 counties in 12 states and the District of Columbia. And 36 counties in five states reported that 480 pools of mosquitoes were positive for West Nile virus over a 4-month period.

States are stepping up efforts to monitor wild birds, which are the most important vertebrate host for the West Nile transmission cycle. In 2000, 135 counties in 12 states and the District of Columbia reported infected birds, starting with a report of a red-tailed hawk in New York in February 2000 and ending with an infected crow in Massachusetts in mid-November, Dr. Marfin said.

Nearly 90% of the 4,304 West Nile-infected birds in 2000 were American crows, which are highly susceptible to the virus and develop high levels of viremia, followed by blue jays (5%). Most (85%) of the infected birds were reported between July 1 and Sept. 30th.

Robert McClean, Ph.D., director of the U.S. Geologic Survey's National Wildlife Health Center, Madison, Wis., said that he sees "no barriers" to the spread of West Nile virus in birds, evenas far as the South American continent.

COPYRIGHT 2001 International Medical News Group
COPYRIGHT 2001 Gale Group