NEW YORK (Reuters Health) - At least six people infected with the West Nile virus in Mississippi and Louisiana developed a polio-like syndrome with muscle weakness or paralysis in arms or legs, reported officials at the US Centers for Disease Control and Prevention (CDC) on Thursday.

The cases occurred in July and August and were sometimes misdiagnosed as either a stroke or Guillain-Barre syndrome, a temporary inflammation of the nerves after a viral infection that causes pain and paralysis.

Guillain-Barre syndrome is often treated with intravenous immunoglobulin, which are infection-fighting antibodies, or plasmapheresis, a process in which the fluid portion of the blood is removed and the remaining blood cells are returned to the patient.

"Many patients with West Nile virus infection who develop acute weakness are diagnosed with Guillain-Barre and receive treatments including intravenous immune globulin and plasmapheresis, which could have adverse side effects," said Dr. Jim Sejear, medical epidemiologist with the CDC, in a telebriefing with reporters Thursday.

"Our study suggests that many of these people may in fact have an illness similar to acute polio, which is not helped by these treatments and which could possibly do more harm than good," he explained.

In areas where West Nile virus transmission is occurring, the CDC urges doctors to conduct diagnostic tests for the virus in patients with weakness in the limbs that is painless--particularly if accompanied by fever and an increase in white blood cells in the cerebrospinal fluid.

"Physicians need to consider and test for West Nile virus and strongly consider performing tests to differentiate Guillain-Barre syndrome from the other causes of weakness before initiating therapy," Sejear said.

Sejear said the six patients are between 46 to 69 years of age and have been ill for about 45 to 60 days with "very little improvement." CDC is continuing to follow these patients, he added.