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Since July, there have been 19 reports of acute flaccid paralysis (AFP) in the Dominican Republic and one in Haiti. Though AFP can be caused by conditions other than polio, laboratory tests have confirmed that poliovirus type 1 caused at least seven of the recent cases.
As part of the World Health Organization's polio eradication campaign, the Western Hemisphere was certified as "polio-free" several years ago, a classification which appears to have been invalidated by the recent outbreak.
The new cases of poliomyelitis are unusual in that they appear to have been caused by a vaccine-derived strain of the virus. The attenuated virus strains used in the Sabin oral poliovirus vaccine (OPV) replicate in the intestine, but do not migrate to the central nervous system. This intestinal replication allows the vaccinated patient to develop immunity to the virus with almost no risk of contracting the disease. Because it is easily administered and induces lasting immunity, OPV has been the vaccine of choice throughout the developing world, and forms the backbone of the polio eradication effort.
Researchers have known for several years that OPV-vaccinated patients shed pathogenic virus strains in their feces, but in areas where vaccination is widespread these "revertant" strains pose little danger. In the Dominican Republic and Haiti, the local eradication of polio appears to have led to a relaxation of immunization standards. The recent outbreak apparently began when a vaccine revertant strain infected inadequately vaccinated individuals, leading to the spread of the pathogenic virus.
The Centers for Disease Control and Prevention (CDC) is currently working with the Pan American Health Organization and the Dominican Republic's Ministries of Health to investigate the outbreak further and bring the situation under control. The Dominican Republic has begun a mass immunization campaign with OPV to prevent the further spread of the disease, and Haiti plans to carry out three nationwide vaccination campaigns in the first three months of 2001.
State health departments in the US have also been advised to step up poliomyelitis surveillance, since inadequately vaccinated travelers could bring the disease into this country.
The US State Department has not issued specific travel restrictions as a result of the outbreak, but people traveling to the Dominican Republic or Haiti should contact their doctors if they are unsure about their polio vaccination status.
In addition to its immediate impact on public health, the outbreak raises serious questions about the ongoing polio eradication campaign. Since the eradication effort relies almost exclusively on OPV in developing countries, and since local elimination of polio is likely to lead to a relaxation of surveillance and vaccination standards, the experiences of the Dominican Republic and Haiti may be a preview of outbreaks in other developing countries in the next few years. Current WHO plans, which call for a cessation of vaccination once poliovirus has been eliminated from the wild, could exacerbate the problem.
Additional information on the recent outbreak is available from the
CDC.
-Alan Dove, Philadelphia