Saturday 29 August 2015

Man Has Polio Virus Living in Gut for 30 Years

by Rachael Rettner

A man in the United Kingdom experienced a very rare complication of the polio vaccine he received in childhood — he never cleared the virus from his body. As a result, the virus has been circulating in his gut for nearly 30 years, and is still being excreted in his stool today, according to a new report of the case.
Although it was known that some people could shed the virus from their bodies for long periods, the new case is by far the longest that the virus has stuck around in a person, the researchers said.

trivalent oral polio vaccine

Cases like these could potentially spread polio, and interfere with efforts to eradicate the virus, the researchers said.

The new findings "raise questions about how the population may best be protected from" these particular polio viruses, the researchers said. [7 Devastating Infectious Diseases]
Poliovirus has been eradicated nearly everywhere except for a few countries, including Afghanistan and Pakistan, although the virus has seen a re-emergence in countries such as Syria in recent years.
There are two types of polio vaccine used to prevent the disease: one type contains dead strains of the virus and must be injected, whereas the other type, the oral polio vaccine, contains a live but weakened virus.

The oral vaccine has a few advantages, including that it is easy to administer, and can more quickly stop the virus from replicating in a person's gut, if that person is exposed to the virus. However, there is a very small risk that the vaccine can cause the illness itself.
In addition, there is risk that the virus can linger in the gut of people who are given the oral vaccine. The vaccine works by prompting immunity to develop in the gut because the weakened virus briefly replicates there. People usually clear the virus from their gut in six to eight weeks after vaccination, but in very rare cases, people with immune system disorders cannot clear the virus, and it continues to replicate in their gut.
That's what happened to the patient in the new report, a 29-year-old white man who was given the vaccine as an infant. (The man has never developed the disease itself.)

The researchers analyzed more than 100 stool samples from the patient, which were collected between 1995 and 2015. They found high levels of the polio virus in the samples. What's more, tests showed that the polio viruses in the patient's gut were different from those in the vaccine, meaning that mutations had developed in the virus over time.
The viruses in the patient's gut were able to cause paralysis in a mouse model, suggesting that these strains are very virulent. However, tests with human blood from people who were vaccinated against polio showed that the antibodies in the people's blood were able to kill the viruses from the patient.
"These results are reassuring in that they indicate that vaccinated humans are well protected against infection" with these virus strains, the researchers said.
But the researchers noted that they used blood from people who had been vaccinated with the oral vaccine, and it's not clear if people vaccinated solely with the inactivated vaccine would show the same level of protection. In the U.S., children have been given the inactivated vaccine since the year 2000; prior to that, the oral vaccine was used.

There have been only 73 documented cases of people with immune problems who had the polio virus replicate in their intestines for prolonged periods. But it's possible that some cases were missed — strains of polio that differ from those in the vaccine have been found in sewage samples from Slovakia, Finland, Estonia and Israel, the researchers said. These findings suggest that "an unknown number of these chronic excreters exist elsewhere," the researchers said.
Surveillance of sewage and stool samples should be done to search for polio strains, the researchers said.

In addition, there is a need to develop antiviral treatments for patients like the one in the current study, because there is currently no effective way to stop the replication of the polio virus.
"These measures are needed to be able to identify and manage the possible risks of [divergent] strains spreading and causing disease in patients and the general population," the researchers said. Finally, new polio vaccines may be needed to fully eradicate the illness, they said.

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