PAHO Member Countries on Alert for New Virus Threats
SARS,West Nile Prompt Steps for Preparedness and Response

Pan American Health Organization (PAHO) experts briefed health officials from throughout the Americas on preparedness efforts for possible new outbreaks of Severe Acute Respiratory Syndrome (SARS) and the spread of West Nile virus, two of more than 30 new disease threats to appear in the past two decades. The briefings took place during the 44th PAHO Directing Council meeting held Sept. 22–26 in Washington, D.C.

Preparing for SARS

On July 5, the World Health Organization (WHO) announced that the last human train of SARS transmission had been broken in Taiwan. The last of the epidemic's 8,422 cases worldwide was reported on July 13 in the United States, and the only post-epidemic case was a laboratory worker in Singapore who was likely infected on the job.

Marlo Libel, PAHO advisor on communicable diseases and the organization's leading expert on new pathogens, told visiting health officials he and other experts believe the disease could reappear.

"Despite all that we have learned in such a short time, there are still many unknowns about SARS," said Libel. "PAHO and WHO are advising countries to remain vigilant and build capacity to detect and respond to any new outbreak that might occur."

The risk of SARS' reemergence is seen as highest in China, where the first cases occurred. At secondary risk are countries such as Singapore and Canada, where local transmission followed importation of SARS.

Latin America and the Caribbean, along with the United States (which had no local transmission), are in the lowest-risk category. Therefore PAHO is advising the region's health officials to integrate their SARS surveillance with that of other acute respiratory illnesses and to focus detection efforts on health care settings and clusters of cases.

"The key is to be able to detect the unusual," Libel said in an interview. He added that this can be difficult with SARS, given the similarities between its symptoms and severe cases of influenza and other acute respiratory infections.

Nevertheless, he said officials should be on the alert for two or more health care workers in the same setting who present SARS-like symptoms (fever over 100.4° F and respiratory illness) or three or more symptomatic patients, visitors or health care workers in the same hospital in a 10-day period. "This would warrant a laboratory investigation of the suspect cases," he said.

PAHO has distributed reagents for laboratory diagnosis of SARS to Argentina, Brazil, Chile, Colombia, Mexico, Panama, Venezuela and the Caribbean Epidemiology Center (CAREC) in Trinidad. Libel cautioned that current tests tend to generate false positives. "They pick up other coronaviruses that are not SARS," he noted. "There are many coronaviruses circulating. The common cold can be caused by a coronavirus."

Should cases be confirmed, said Libel, the proper response would be to rapidly implement infection control measures, expedite subsequent diagnoses and activate public health response measures, including contact tracing of patients and screening of outbound travelers. He emphasized that the evidence does not support health screening of incoming international travelers. "What is really helpful is finding out where they've come from and where they will be staying in the next two weeks," he said.

Libel noted that Canada had effectively utilized public health structures and processes in place for influenza preparedness in dealing with its SARS outbreak.

West Nile watch

While Latin American and Caribbean countries are considered low-risk areas for SARS, West Nile virus is a more likely threat. Otavio Oliva, a PAHO advisor on communicable diseases, told the region's health officials that the virus is now established in North America, affecting 44 of 50 U.S. states and five Canadian provinces.

The human form of the disease has spread rapidly in the United States since July of this year, with a cumulative 4,666 cases (including 88 deaths) reported as of Sept. 19, more than double the number reported by the same time last year. The largest outbreak has been in Colorado, where an explosion of Culex tarsalis mosquitoes this summer is believed responsible for the large number of human cases: 1,542, including 27 deaths in the state as of Sept. 23.

The virus has already been detected in birds and/or horses in the Dominican Republic, El Salvador, Jamaica, Guadeloupe and particularly Mexico, said Oliva. As of early September, Mexican officials had identified 623 infected horses and 30 birds in 19 states.

In an interview, Oliva noted that a key question is why no human cases have appeared in Mexico, even though there are people with the disease in three of four bordering U.S. states.

Vector experts in Mexico's Secretariat of Health recently offered two possible explanations. One suggests that "the current panorama of West Nile virus in Mexico is a stage previous to the appearance of outbreaks and that the virus is molding itself to the ecology of the country's different regions."

The second hypothesis points to widespread exposure of the Mexican population to as many as four strains of dengue virus, a member of the same family of flaviviruses, which share a similar structure. In contrast, the U.S. population is nearly free of antibodies to these viruses. Conceivably, prior exposure to flaviviruses could confer a degree of immunity not only on humans, but on birds and horses as well. In Mexico, only two of the more than 650 identified infected horses and birds have fallen ill, a rate dramatically lower than among U.S. animals.

Oliva pointed out that "in Latin America, from Mexico down to Argentina, dengue virus is highly endemic, and there are areas of vaccination for yellow fever [also a flavivirus]." He added: "We don't know how West Nile is going to behave in this environment."

PAHO support

To date, several PAHO member countries have established surveillance systems for West Nile virus, and others have asked for PAHO support in developing theirs.

PAHO has now organized a task force on West Nile surveillance, laboratory diagnosis and prevention, and has incorporated West Nile virus into its existing Regional Surveillance Network. It is developing guidelines for detection as well as promoting communication and coordination among and within countries, and training public health professionals in laboratory procedures, surveillance, vector control, veterinary public health, immunization and disaster response.

Oliva noted that prevention planning is also a key part of preparedness. "It will be important to inform the population how to protect itself against the disease," he said, adding that mosquito-control programs are already in place throughout the region as part of efforts to fight dengue, providing a starting point for fighting the species of mosquitoes that carry West Nile virus.

For any emerging disease, noted Libel, "the issue is response—how do we respond? But for that you need preparedness. You have to have things in place before such a disease breaks out."