THE DOCTOR'S WORLD
THE DOCTOR'S WORLD; Encephalitis Outbreak Teaches an Old Lesson
By LAWRENCE K. ALTMAN, M.D.
Published: September 28, 1999
When you hear hoofbeats, don't think of zebras.
To doctors, the axiom is a call to focus on common ailments and not waste time on the exotic. But on those rare occasions when they do detect a zebra, doctors say they need to take extra steps in their investigation to make sure they have identified the right one.
A case in point is the encephalitis outbreak that is blamed for at least three deaths in New York City. Earlier this month, the Centers for Disease Control and Prevention in Atlanta and the New York City Health Department said the cause was the mosquito-borne St. Louis virus, which had never been identified in New York City before.
But last weekend the C.D.C., responding to findings from laboratory tests performed by Dr. Tracey McNamara, a pathologist at the Bronx Zoo, announced that the outbreak was caused by an even rarer zebra: the West Nile virus from Africa. Dr. W. Ian Lipkin, of the Emerging Diseases Laboratory at the University of California at Irvine, confirmed the findings.
The viruses are closely related, causing virtually the same type of inflammation of the brain. But the West Nile virus had never before been detected in the Western Hemisphere.
''C.D.C. would not have made the diagnosis of West Nile virus as quickly without Dr. McNamara's persistent medical sleuthing,'' Dr. Duane J. Gubler, the head of the C.D.C.'s arbovirus field station in Fort Collins, Colo., said in an interview.
The change in diagnosis, though not important in terms of the spraying and other public health measures taken to combat the outbreak, was scientifically embarrassing to the C.D.C., the premier Federal agency that is responsible for tracking infectious diseases in this country.
In recent years, C.D.C. and other health officials have led a campaign against the threat of new and emerging infections, warning doctors to expect more infectious disease ''zebras,'' like the sudden appearance of old microbes in new areas.
In fact, health officials thought they had scored a coup when they diagnosed the mysterious illness as the St. Louis virus. The disease is usually found in the Southeastern states, and humans are usually bystanders in such infections. Birds are the principal reservoir for the arboviruses, but they do not become sick. Instead, mosquitoes transmit the viruses to people, who may become ill.
In hindsight, the concurrent deaths of an unusual number of birds in the city has turned out to be an important but underestimated clue. Federal and local medical sleuths did not immediately relate the bird die-off to the human outbreak because West Nile, St. Louis and similar encephalitis viruses generally do not kill birds.
Now the C.D.C. is retesting blood and spinal fluid from patients who had symptoms of encephalitis and who did not show evidence of the St. Louis virus.
The new findings mean that health workers need to investigate a number of other possibilities, Dr. Gubler said.
A critical one is that the C.D.C. can no longer be certain that the West Nile virus has never been present in the United States, because the Federal scientists never specifically checked for it in earlier outbreaks and individual cases. In testing mosquitoes and specimens from humans with encephalitis, the C.D.C. routinely checks all types of viral encephalitis known to have caused infection in the Western Hemisphere, Dr. Gubler said.
There can be considerable overlap in findings from the laboratory tests unless extra steps are taken to distinguish between the many types of encephalitis-causing arboviruses. In a sense they are all one virus, but with many variations that can be detected in the laboratory. The encephalitis they cause is the same disease, although there can be subtle differences in the type of brain damage they produce.
The viruses occur in different geographic areas, and by scientific custom they are named for the area where they were discovered. They include Murray Valley encephalitis in Australia, Japanese encephalitis, and Rocio virus in Brazil and Argentina. They are distinguished in the laboratory by small differences in the proteins in the covering of the virus known as its envelope.
''We don't include West Nile, Japanese and other encephalitis viruses because they have never been known to be here,'' Dr. Gubler said, adding that in the New York outbreak, ''we had tunnel vision on St. Louis virus, because all the clinical, epidemiological, laboratory and geographic features pointed to St. Louis.''
''We've learned a lesson here,'' Dr. Gubler continued. ''We've got to be more open-minded.
''Once we get past this crisis, we are going to have to go back and check specimens from C.D.C. and state health departments to see if it has been here, and if so for how long.''
The West Nile virus was discovered in 1937 in Uganda. Since then ''it has rarely reared its head,'' Dr. Gubler said, though outbreaks and occasional cases have been reported from Israel, France, Romania and elsewhere in Europe. Presumably the virus was carried northward by birds migrating from Africa.
After a nearly two-decade silence, West Nile virus caused a large outbreak in Romania in 1996. This year, West Nile virus apparently caused illness among humans in Volgograd, Russia, though Dr. Gubler said he had received no reply to an inquiry he has made to Russian scientists about the outbreak. Lack of communication among scientists thwarts efforts to learn why a virus is spreading.
When birds began dying in and near the Bronx Zoo, Dr. McNamara, the pathologist there, initially thought the deaths might be a result of viruses that cause avian influenza, Newcastle disease, fowl cholera or Eastern equine encephalitis, Dr. Gubler said.
Dr. McNamara sent the specimens to the Department of Agriculture Laboratory at Ames, Iowa, where scientists eliminated those candidates. Because the Ames laboratory lacked the material to test for the pertinent arboviruses, they forwarded the specimens to the C.D.C., Dr. Gubler said.
Dr. Gubler said scientists also needed to investigate whether the deaths of the birds indicated that West Nile virus had become more virulent.
Also, the virus has been shown to stay in the blood of humans longer than other types of encephalitis virus, so that mosquitoes are more likely to pick it up and transmit it to other humans. But whether such transmissions occur on a regular basis is not known, Dr. Gubler said.
A main focus will be to prevent a recurrence of the outbreak in New York next summer. Scientists do not know whether the virus can persist in infected mosquitoes while they hibernate over the winter, Dr. Gubler said. In the absence of definitive information, health officials will consider the need to begin control of storm drains to decrease the probability of infected mosquitoes' surviving over the winter.