Still a Scourge
pdf
e-mail
print
Jim Gathany/CDC
Poliovirus is an Enterovirus, a genus in the picornavirus family of very small, single-stranded RNA viruses. Enteroviruses inhabit the intestines and circulate largely through fecal-oral transmission or in polluted water. Though many cause little disease, a few, such as Enterovirus 71, or several of the Coxsackie viruses, may cause meningitis and occasionally paralysis. But poliovirus is among the most dangerous.
Under an electron microscope, a polio virion (virus particle) appears as a bumpy sphere. Each of the three immunologically unrelated serotypes consists of an RNA genome enclosed in a capsid, or outer shell, by which the virion adheres to human cells via so-called polio receptors. These are found, among other places, on cells of lymphoid tissue lining the digestive tract. According to Harvard virologist James Hogle, the virion uses the receptor as a portal into the cell. There it hijacks the cell’s machinery to replicate itself. As long as the infection remains in the gut, however, it’s unlikely to harm its human host.
Sometimes, though, this intestinal virus produces paralysis. That, says Hogle, is because poliovirus receptors also exist in nervous tissue, including the anterior horn motor neurons of the brain stem and spinal cord. After it replicates in intestinal cells, the virus can enter the bloodstream, where it causes a mild but noticeable infection. Then it may cross the blood-brain barrier to reach the spinal cord.
Other times, circulating poliovirus finds a break in muscle tissue, makes its way to nerve cell endings and then shoots up the axons to the motor neurons in the spinal cord or brain stem. According to one theory experimentally proven by Wimmer, injections may sometimes be to blame, giving blood-borne poliovirus increased opportunity to ascend to the motor neurons. Kew believes that tonsillectomies may play a role; these operations, once very common, have been associated with bulbar polio, the most serious form of paralytic disease. Whichever way it gets to the anterior horn motor neurons, the poliovirus destroys them and may cause paralysis or death.
As devastating as the poliovirus can be, for half a century there have been two effective vaccines. The Salk version, approved in 1955, is known as inactivated polio vaccine (IPV). Consisting of a killed strain of the virus, it produces antibodies that circulate in the blood. Children vaccinated with IPV won’t get paralytic disease, though they could develop—and unknowingly pass along—polio-related intestinal infections if exposed to the live poliovirus.
In contrast, the oral polio vaccine (OPV), developed by Albert Sabin and approved in 1961, is made of live, weakened strains of the virus and is designed to produce a mild intestinal infection and induce long-lasting immunity. OPV has been the vaccine of choice for the eradication campaign. Volunteers can drop OPV in a child’s mouth and the infection it produces can spread beyond the vaccinated child to others in the community, widening the impact of a vaccination initiative.
As those on the front lines of the eradication campaign well understand, children in high-risk areas often need more than the usual three doses of oral vaccine to acquire immunity. Even then it doesn’t always work, Kew notes, especially during summer and autumn, polio’s high season. (Cold weather slows transmission of the virus.) Moreover, if a child has diarrhea, the vaccine can pass through the intestines before the live virus strain has a chance to replicate, and malnutrition can prevent antibodies from developing. Both diarrhea and malnutrition are common in Afghanistan, India, Nigeria and Pakistan, where polio has found its last redoubt. What’s more, these countries have high birth rates, continually providing a fresh supply of susceptible potential hosts.
Parents may not understand why their children must be vaccinated again and again, nor why, even after multiple vaccinations, a child sometimes develops paralytic polio. These failures lend credence to recent rumors circulating in northern Nigeria, where some mothers have refused to permit their children to be vaccinated. According to Kew, the parents have heard that the polio vaccine is a Western plot to sterilize their children. Their resistance to the push for universal vaccination is the reason the country remains a polio hot spot, and the consequences extend beyond its borders. Infections in Nigeria have spread westward to Ghana, the Ivory Coast, Mali and Guinea, and eastward to Sudan, Yemen and Somalia.
