Scientists now look to insect world for villain in rapid spread of SARS

4/13/2003

Beware the bug.

Mosquitoes bring West Nile Virus, malaria, and dengue fever.

Ticks carry Lyme Disease and Rocky Mountain spotted fever.

Flies around the world carry any number of life-threatening infections, to say nothing of the deadly pathogens that fleas, midges, and other critters unload on hapless humanity.

Is it any wonder, then, that a few people engaged in unraveling the mystery of Severe Acute Respiratory Syndrome - SARS - may be turning to the insect world in their search for a culprit in the disease spread?

Reports this week said Hong Kong officials are testing cockroaches from a heavily infested apartment building to see if the insects bring dangerous baggage when they check in. It's just one of many leads investigators are tracking on this disease with global reach.

The most recent report shows there are 2,781 probable SARS cases worldwide since November, with 111 dead and 1,337 recovered. Most of those cases are in Asia. There are 166 suspected SARS cases in the United States, including six in Ohio and two in Michigan. There are no U.S. SARS deaths. Canada has 97 suspected cases, with 10 deaths.

Investigators are getting closer to naming the guilty virus. A mutant relative of the common cold, a coronavirus - so named for the sunny little projections that spike from its round body - looks increasingly like the cause of SARS.

Such viruses are “wobbly,'' said Julie Gerberding, Centers for Disease Control and Prevention director, in a press briefing Thursday. That is, they're inaccurate copyists at best. When they replicate their single-strand of genetic material, they don't care about getting things right. Evolution is a foregone conclusion.

As researchers come closer to confirming the pathogen's identity, there are still many unknowns about its spread. It is not easy to catch, it appears. Close contact seems to be a prerequisite for infection, with most cases creeping up among families or in health-care workers. Far fewer cases stem from community contact - such as through school or the workplace. In the United States, there are no cases of community infection, Dr. Gerberding said.

The virus appears to spread by droplets. Someone in a sneeze line-of-fire is most often the victim.

But there is much uncertainty, which probably contributed to roaches landing on someone's most-wanted list. Yet the insects' qualifications as a criminal are cant.

Despite their strong association with unsanitary conditions, roaches have seldom proved as vile as the places they chose to live.

Coby Schal, a professor of urban entomology at North Carolina State University, is a cockroach expert. Among the many things he researches about these insects is their ability to carry disease. Like you and me, they can, and they do.

“In my laboratory we have good evidence that roaches can carry bacteria associated with their environment,'' Dr. Schal said. He tested roaches from livestock feedlots that use lots of antibiotics. He also tested roaches from a public-housing project. All the roaches carried the ubiquitous E coli bacteria, but those from the feedlot carried an antibiotic-resistant strain - presumably one they picked up from feedlot practices.

Further testing in Dr. Schal's lab shows that a cockroach will retain a microbe in its gut for as long as two weeks, depending on which microbe it ingests. Theoretically, a cockroach could defecate a pathogen on food or a surface, and humans could take it from there.

So, a role for cockroaches isn't impossible. It's just not very efficient.

“Roaches probably represent a very low probability of transmitting diseases,'' said Donald Mullins, a professor of entomology at Virginia Polytechnic Institute and State University in Blacksburg, Va.

“If they can show a real high roach population and the incidence of disease, there might be a correlation, but it's not something you can count on,'' Dr. Mullins said.

“The efficiency (of disease transmission) very much depends on the roach population size,'' Dr. Schal said. If you have a huge, huge infestation, even an inefficient process can really get out of control.''

Still, there are no proven instances of cockroaches acting as disease vector, although there are suspicious cases.

Dr. Schal points to a study published in 1962 in the Journal of Tropical Medicine and Hygiene. The authors counted the number of infectious hepatitis cases in one part of Los Angeles County from 1956 to 1962. In 1956, the Carmelitos Housing Project had 27 cases of infectious hepatitis, while the surrounding larger area had 69 cases. In 1959, there was an aggressive campaign to rid the housing project of roaches. By 1962, with roaches reduced by 95 percent, there were no hepatitis cases in the project. But the surrounding region had 113 cases the same year.

Actually, roaches are strongly associated with only one known health threat: allergies.

As for SARS, the CDC isn't showing much faith in the roach theory.

“The primary means of transmission is face-to-face contact,'' said Dr. Gerberding. As for cockroaches, she said: “We look forward to learning where this hypothesis was generated.”