Polio returns to Minnesota

17 April 2009

amishPoliovirus has been isolated from a patient who died last month in Minnesota. Is this incident related to the outbreak of polio in an Amish community in the same state four years ago?

Here are the facts about this case that have been released by the Minnesota Department of Health: the patient, an adult, had paralytic polio, but it is not known if this played a role in death. Apparently the patient had multiple health problems, including a weakened immune system. The virus isolated from this patient is related to the infectious, orally administered poliovirus vaccine, OPV. They speculate that the patient was infected with OPV over nine years ago, because the use of this vaccine in the US was discontinued in 2000.

The fact that OPV use was discontinued 9 years ago in the US does not prove that this patient was infected with a vaccine virus at that time. The only way to answer this question would be to determine the nucleotide sequence of poliovirus isolated from the patient. From this information the number of years that the vaccine-derived virus has been replicating in humans could be determined. However, no sequence information has been reported by the Department of Health. It is likely that the patient was infected with poliovirus at any time in the last 9 years. If in fact the patient had an immunodeficiency, then infection could have persisted for at least nine years, as has been reported in other immunodeficient patients. However, it seems unlikely that the virus would replicate for 9 years in this individual, and then cause paralytic disease only recently.

I believe this individual was a member of a Minnesota Amish community and was therefore not immunized with OPV as an infant. The patient was probably infected recently with a strain of poliovirus derived from OPV. Because OPV has not been used in the US since 2000 and in Canada since 1995-96, the infecting virus was either imported from another country, where OPV is still used, or shed by an immunodeficient individual in the US. Such patients excrete poliovirus for years in the absence of clinical symptoms. A similar scenario has been invoked to explain poliovirus infection in 2005 of children in a Minnesota Amish community.

We will find out whether this speculation is correct when the Centers for Disease Control and Prevention release the complete data on this case.

You might be wondering why poliovirus has been isolated on two separate occasions in Minnesota. It so happens that the former Minnesota State epidemiologist was Dr. Harry Hull,  who previously worked on the polio eradication campaign at the World Health Organization. When he arrived in Minnesota after his WHO stint, he installed an excellent polio surveillance system in the state which remains in place to this day.

Odoom, J., Yunus, Z., Dunn, G., Minor, P., & Martin, J. (2008). Changes in Population Dynamics during Long-Term Evolution of Sabin Type 1 Poliovirus in an Immunodeficient Patient Journal of Virology, 82 (18), 9179-9190 DOI: 10.1128/JVI.00468-08