A polio outbreak spreading from a longtime hotbed of the disease in northern Nigeria has reached Chad, the fourth African nation this year to experience a resurgence of the disease, which public health authorities hope to eradicate globally by 2005.
In response, a $10 million emergency vaccination campaign that began last week in Niger, Burkina Faso, Ghana and Togo will be expanded to Chad at an additional cost of $3 million, World Health Organization officials said.
The Global Polio Eradication Initiative, launched in 1988, is in its final stages in more than 150 countries. Until the recent African outbreaks, only seven nations still had polio virus circulating in their populations this year.
The cases in Chad show that Nigeria is unwittingly exporting the virus to a bigger region of West Africa than previously believed.
Does this mean that every neighboring country around Nigeria is
infected? Will these countries be able to wall the virus off and keep
it from becoming endemic again? Those are the questions, said
David L. Heymann, a WHO physician helping to oversee the Global Polio
Until the new cases were diagnosed in August, Chad had not reported a case since June 2000. Ghana, Togo and Burkina Faso had also been considered polio-free for several years before the detection of seven new cases there over the past three months. Nearby Niger is one of the seven countries where polio is still endemic, but it, too, has experienced at least one case of polio in recent months arising from a strain of virus that originated in Nigeria.
Through genetic fingerprinting techniques, authorities determined that the new cases were caused by a strain of virus 99.9 percent identical to one that has long been circulating in Nigeria.
Epidemiologists know that for every polio infection that causes paralysis or permanent muscle weakness, there are at least 200 that cause no symptoms.
Consequently, they believe, the discovery of just a few paralytic cases means that hundreds or thousands of people in the region are carrying, and possibly transmitting, the virus.
The two cases in Chad turned up in August in two southern provinces, Mayo-Kebbi and Logone Orientale. The sites are about 150 miles apart but connected by a heavily traveled network of roads that also links them to northern Nigeria, said Christopher Maher, a scientist at WHO's Geneva headquarters who helps coordinate international support for eradication efforts. Several other cases are under investigation.
A three-day campaign to immunize 15 million children younger than 5 with oral polio vaccine took place last week in five west African countries. A similar campaign will be launched in Chad next month and continue through January. Cameroon, part of which separates Chad from Nigeria in the affected region, also will hold vaccination campaigns in December and January.
It takes more than three doses of polio vaccine to fully immunize children in countries where malnutrition and diarrhea are common. Repeated rounds of vaccination are needed to eliminate the virus from such populations—or stop it if it is reintroduced.
A country is considered polio-free when the chain of person-to-person transmission—the only way polio can spread—has been broken, but that doesn't mean it is immune to reintroduction to the disease. Like a forest in which a fire has been put out but much combustible material remains, a significant fraction of the population may still be susceptible.
The West African outbreak
points up the enormous risk to polio-free
countries as long as the virus is still endemic anywhere, Maher
Nigeria overtook India this year as the country with the largest number of polio cases, 178. Intensive eradication efforts are underway in India.
Smallpox is the only human disease that has ever been eradicated. The
last case of
wild smallpox was found in 1977, and the last case
ever—the result of a laboratory accident—occurred the