MEDICINE / UNFINISHED BUSINESS : Polio’s End in Americas Seen by 1991 : Only three cases of the disease have been detected so far this year.

Buoyed by substantial progress in recent months, Pan American Health Organization officials remain cautiously optimistic that they can achieve their goal of eradicating polio from the Western Hemisphere by the end of this year.

So far this year, only three cases of the feared crippler of children have been confirmed--one in Mexico, one in Ecuador and one in Peru. Another 10 cases are classified as “polio compatible,” which means there is not enough evidence to definitely exclude the diagnosis. About 500 cases of polio-like illnesses remain under investigation; more than two-thirds are in Brazil, Chile, Colombia or Mexico.

“Things look good. . . . People are confident that the goal will be achieved,” said Dr. John Andrus, a medical epidemiologist with the Washington, D.C.-based international health agency.

The Pan American Health Organization launched its polio eradication campaign for the Americas in 1985. The group hoped that announcing a concrete goal would spur fund-raising efforts and motivate health officials.

The campaign could still be derailed by a number of factors, Andrus said. These include wars, ineffective batches of vaccine or persistent transmission of the virus in the areas of greatest concern--northwestern Mexico and the northern Andean regions of Ecuador, Colombia, Peru and Venezuela.

Through immunization, many Western nations, including the United States, appear to have eliminated the natural polio virus. But the virus is still responsible for more than 250,000 cases of paralysis worldwide each year, primarily in Africa and Asia, where the vaccine is not as widely used.

For an infectious disease to be eradicated, not simply controlled, the natural transmission of the virus that causes it must be halted. Then, this result must be confirmed over several years. So far, the only human germ eradicated worldwide is smallpox.

The World Health Organization’s goal is to eradicate polio from the world by the year 2000.

Smallpox eradication was aided by the fact that people infected with the virus almost always became sick and developed a rash. Polio cases, by comparison, must be confirmed in the laboratory, and most infections are asymptomatic.

The polio virus can easily be transmitted through food and water contaminated with human feces, as well as close person-to-person contact.

The first step toward global eradication of polio is success in the Western Hemisphere. The nearly $50-million campaign by the Pan American Health Organization has relied on several strategies:

National vaccination days, in which health workers try to immunize each child up to age 5 against polio and other childhood illnesses.

Intensive surveillance of potential cases, including comprehensive reporting of unexplained cases of paralysis, particularly in children.

“Mop-up” operations, in which public health workers go house-to-house in high-risk areas and immunize all children under age 5.

In 1989, 130 polio cases were confirmed in the Americas. But much of the decrease so far this year can be explained by a change to more stringent criteria for diagnosing cases.

If the new classification system had been used last year, only 24 of the 130 cases--those in which the virus was isolated from stool specimens--would have been classified as confirmed, Andrus said.

Even if success appears at hand by December, the Pan American Health Organization will not certify polio eradication until three years after the onset of the last case.