An outbreak of poliomyelitis in Tajikistan has health officials concerned that unvaccinated children in the United States and elsewhere are at risk of contracting the paralyzing disease.
More than 560 cases of acute flaccid paralysis were reported in Central Asia's Tajikistan as of June 1, according to the Canadian Medical Association Journal.
Acute flaccid paralysis – the sudden onset of weakness or paralysis of the muscles – is an early sign of polio. Laboratory tests are needed to confirm the poliovirus as the cause; thus far, more than 230 of these cases have been confirmed as polio, according to the Global Polio Eradication Initiative.
This is a dramatic rise from the seven cases in that country reported by the World Health Organization (WHO) in April, according to the Canadian Medical Association Journal.
The Tajikstan outbreak is the first to occur in a country that was previously declared to be polio-free by the WHO. Tajikistan was declared polio-free in 2002.
Unlike Afghanistan, India, Nigeria and Pakistan – which all have endemic poliovirus in circulation – Tajikistan has not seen an outbreak in years, said Dr. Noni MacDonald, dean of medicine at Dalhousie University in Nova Scotia.
"This was a total surprise to us," MacDonald said. "We thought it was gone, and only in a few places like Nigeria and India. That was naïve."
The WHO recommends that, to remain polio-free, countries ensure that at least 90 percent of their populations are vaccinated. The rest of the population is then protected by herd immunity, the lowering of transmission rates seen when a high proportion of people who are vaccinated lower the likelihood that a susceptible person will come into contact with the virus.
Tip of the iceberg
Tajikstan had a vaccination rate near the 90 percent recommendation, MacDonald said, but there were pockets of subpopulations with vaccination rates that were lower.
"If the virus was imported to Canada or the U.S., where we also have pockets like those, the same thing could happen," MacDonald said.
Other health officials agree.
"I think it's worrisome," said Dr. Paul Offit, chief of infectious diseases and the director of the Vaccine Education Center at the Children's Hospital of Philadelphia. "This could be the tip of the iceberg."
"These fears are accurate and honest, and not overstated," Offit said. "Do I think poliovirus walks into this country? I do."
Several conditions are conspiring right now that could enable the spread of the disease, Offit said. International travel is common; many people with the virus will not develop symptoms; and there are growing populations within certain communities that are not getting vaccinated.
In the United States, 93.6 percent of children between 19 and 35 months old are immunized, according to the National Immunization Survey released in May by the Centers for Disease Control and Prevention (CDC).
But seven states and the District of Columbia have polio immunization rates that hover below the WHO's recommended minimum of 90 percent, according to the CDC's report: Arkansas (89.8 percent), Washington D.C. (87.7 percent), Montana (87.5 percent), Nevada (88.2 percent), New Jersey (89.1 percent), Utah (88.2 percent) and Virginia (88.8 percent).
The CDC's numbers describe the percent of children who received all three doses of the vaccine, and the numbers for school-age children tend to be slightly higher than these, because schools require the vaccinations, said Dr. Lance Rodewald, director of immunization services at the CDC.
The national vaccination rate in the United States has remained "rock steady" in recent years, Rodewald said, but there are communities in which the rates have been decreasing. The CDC, he added, provides assistance to states that request help in reaching out to those communities.
How polio attacks
The virus attacks the nervous system and is highly contagious. It spreads from person to person through water or food. While the polio vaccine prevents one from getting the disease, if an unvaccinated person develops paralytic polio, they cannot be cured.
However, most people who contract the virus do not get sick with the disease. Experts disagree on the exact number of people who will develop symptoms.
Fewer than 1 percent of people infected with the virus will develop paralytic polio, according to the Mayo Clinic. Offit said that about 1-in-200 people will develop paralysis. MacDonald said that both of those estimates are very conservative, and that it may be that only one person in 1,000 will develop paralysis.
And although that's good news, it also means that many more people in
Tajikistan are surely carrying the virus than have had symptoms so far.
If there are 500 cases of polio in Tajikistan, that could mean there are 500,000 people there who could transmit the virus to others, MacDonald said.
Several cases of acute flaccid paralysis have appeared in bordering Uzbekistan, and one polio case has been confirmed in Moscow, MacDonald said.
The last two outbreaks that occurred in Tajikistan were in the 1990s, with 111 and 26 cases reported in 1991 and 1994, respectively. Prior to the recent outbreak, the last confirmed case of polio was in 1997, according to a report in the journal Eurosurveillance.
The national vaccine coverage rate in Tajikistan was 87 percent in 2008 and 93 percent in 2009, according to the WHO.
During 2009, a total of 1,604 polio cases were confirmed globally, according to the Global Polio Eradication Initiative. Of these, 1,256 were in the four countries with endemic polio, and 348 were cases of polio that had been transported from one of these countries elsewhere.
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