It is 1954, and about 300 children from the same neighborhood in Chicago enjoy swimming and other activities at the local YMCA. Over a three-week period, 17 of these children develop "flu-like" symptoms, with fevers up to 39.5ºC, sore throat, myalgias, headaches, and malaise. These symptoms persist for an average of 5 days, after which the victims begin to complain of stiff neck, markedly increased muscle pain, occasional muscle spasms, and elevated fever (to 41ºC). These symptoms are followed by flaccid paralysis that varies widely in its degree of severity. Some patients experience paralysis of only a few muscle groups (in one leg, for example), while others develop complete flaccid paralysis of all four extremities and are totally incapacitated. Four of the 17 victims eventually make full recoveries, but 8 of them recover with varying degrees of residual paralysis. Five of the victims experience a continuation of the illness, in which flaccid paralysis of the pharynx, vocal cord, and respiratory muscles develops. These victims are able to breathe only with the help of a mechanical device that provides external respiratory compression. All of them die within the next six months.

Fourteen of the 17 victims of this disease outbreak are boys, aged 12 to 17, all of whom are members of either the football or the soccer team at their respective schools. All of the victims who eventually died had tonsillectomies when they were younger, whereas only two of those who survived had undergone tonsillectomies. In a follow-up epidemiological study of the neighborhood, it was found that more than 50 children who used the pool at the YMCA experienced nonspecific flu-like symptoms during the same period in which the above cases developed, but their symptoms cleared up spontaneously within 1-3 days.

Question 7.1: What is your diagnosis?

The timing of the outbreak (1950s), the progression of symptoms, and the developing of varying degrees of flaccid paralysis all point to a classic epidemic of poliomyelitis (or "polio").

Question 7.2: What is the causative agent?

The causative agent is the poliovirus, one of the enteroviruses, which are member of the Picornaviridae family. The picornaviruses are plus-strand single-stranded RNA viruses with icosahedral capsids that are about 30 nm in diameter. Their virions do not have envelopes. Polioviruses are typically lytic viruses, and their replication results in destruction of the infected host cells.

Question 7.3: Why did the cases vary in severity?

In part, that is the nature of the poliovirus, which affects different individuals to differing extents. Most cases of polio are actually asymptomatic, with the virus limited to a brief period of replication in the oropharynx and gut. Some victims experience abortive poliomyelitis, which is a relatively minor, nonspecific, febrile disease that produces flu-like symptoms (sore throat, headache, etc.) for a short period of time. Many of the 50+ nonspecific flu-like cases cited in the follow-up epidemiological study probably were cases of abortive polio. Paralytic polio, as seen during this outbreak, is relatively rare, typically occurring in only 0.1-2% of infected individuals. However, it is more common among older individuals, pregnant woman, and people exercising strenuously or undergoing trauma when they are infected. In this case, all of victims were members of athletic teams that required strenuous activity. The most dangerous form of polio, in which paralysis of the respiratory muscles occurs, is called bulbar polio. Tonsillectomy is an important predisposing factor for this form of polio and may have been a factor in this outbreak.

Question 7.4: What is the pathogenic process?

The poliovirus initially infects the mucosa of the nasopharynx and GI tract. It then spreads to the CNS, where it replicates in the neurons of the gray matter of both the brain and the spinal cord. The virus travels both through the bloodstream and by migration along neural pathways. Flaccid paralysis of limb muscles occurs when anterior horn cells of the spinal cord are destroyed as a result of lytic viral replication. Bulbar polio, with paralysis of the respiratory muscles, results from viral invasion of the medulla oblongata.

Question 7.5: How is this disease transmitted?

Individuals who are infected with poliovirus shed the virus in their feces for weeks or months after all of their symptoms are gone. (Even asymptomatic individuals shed the virus in their feces.) Therefore, transmission is most often by the direct or indirect fecal-oral route. In this case, all of the cases developed within a relatively short period of time, implying that the outbreak was actually a point-source epidemic. If so, the most likely point source was the YMCA swimming pool that was shared by all of the victims. The water in most public pools is chlorinated to limit transmission of gastrointestinal diseases, but the poliovirus is quite resistant to chlorination and might have survived in the pool water for an extended period of time. Several of the outbreaks of polio that occurred in the U.S. during the 1950s were traced to neighborhood swimming pools.

Question 7.6: How is this disease prevented?

Poliomyelitis can be prevented quite effectively with a vaccine. The current vaccine of choice is the oral polio vaccine (OPV), which is based on an attenuated version of the virus.

Question 7.7: How prevalent is this disease today?

Polio cases in the U.S. are now exceedingly rare and virtually all of them are related to defective batches of vaccine, in which the virus has mutated back to its fully active form. There were four such cases in the U.S. during 1997 and 1998. Polio is also very rare in other countries that utilize the vaccine, but is still a problem in those that do not. The World Health Organization initiated a campaign to eradicate poliomyelitis worldwide in 1988. Between 1988 and 1997, the number of cases worldwide decreased by 89%, with 6,227 cases reported from 46 countries in 1998. More than 80% of the cases in 1988 occurred in India, Pakistan, Bangladesh and Nigeria, where efforts to distribute the vaccine continue.