A recent outbreak of poliomyelitis in the Dominican Republic and Haiti appears to be caused by an unusual viral derivative of the Sabin type 1 oral poliovirus vaccine. The current outbreak raises serious concerns because a wild poliovirus has not circulated in the Western Hemisphere since 1991.

The Ministries of Health in the Dominican Republic and Haiti are currently working with the Pan American Health Organization (PAHO) and the Centers for Disease Control and Prevention (CDC) to understand the extent and reason for the outbreak, according to a PAHO press release.

In the Dominican Republic, three laboratory-confirmed cases of infection with derived poliovirus type 1 have been identified since July 12, 2000. An additional 16 patients who developed flaccid paralysis may also harbor the disease and are currently under investigation.

In Haiti, one laboratory-confirmed case of poliovirus type 1 has been identified. The onset of paralysis in this case occurred August 30, 2000. Thus far, despite aggressive investigative efforts, no other cases have surfaced.

According to Dr. Ciro de Quadros, director of PAHO's Division of Vaccines and Immunization, the key to controlling a poliovirus outbreak is "achieving and maintaining high vaccination coverage." This strategy applies to oral poliovirus vaccine-derived viruses as well as wild polioviruses.

"The governments are very committed to controlling the outbreak," Dr. de Quadros told Reuters Health. "All of the resources are available, the logistics are in place, and there is no lack of vaccines. The situation is under control."

The virus was first isolated by investigators at the PAHO Poliovirus Laboratory at the Caribbean Epidemiology Center and further characterized at the Poliovirus Laboratory at the CDC. The virus demonstrates 97% genetic similarity to the parent oral poliovirus vaccine strain, but it has assumed the neurovirulence and transmissibility of wild poliovirus type 1.

"The difference in nucleotide sequence," the researchers note, "suggests the virus has been either replicating for a prolonged period in an immunodeficient individual, or circulating for as long as 2 years in an area where vaccination coverage is very low, resulting in ongoing genetic changes in the original Sabin virus that gave it the properties of wild poliovirus."

From 1983 to 1993, a similar outbreak occurred in Egypt. In this setting, a type 2 oral poliovirus vaccine-derived virus caused more than 30 reported cases over the 10-year period. The investigators attributed this outbreak to low vaccination coverage. Once OPV vaccination was increased, the outbreak was quickly terminated.

(From Reuters)