A man in Colorado became infected with West Nile virus through a blood transfusion, despite the fact that the blood he received was screened for the virus, according to a new report from the Centers for Disease Control and Prevention.

West Nile virus is most commonly transmitted through mosquito bites, and infection through blood transfusion is rare in the United States because all donated blood is screened for the virus, the CDC says. There have been just 12 reported cases of West Nile virus linked to blood transfusions over the last decade. [See 5 Things You Need to Know About West Nile Virus]

In August 2012, the man had received several units of donated blood as part of a bone marrow transplant for non-Hodgkins lymphoma, a type of immune-system cancer.

About 20 days later, the man developed a fever and breathing problems. His blood pressure dropped and his mental state became altered. He later died of encephalitis, or inflammation of the brain that can be caused by West Nile virus.

A postmortem blood test showed the man was infected with West Nile. Because the man had been in the hospital for about a month before he got sick, infection through a mosquito bite was unlikely.

When donated blood is screened for West Nile virus, portions of blood from several donors are sometimes pooled together.

The CDC investigation of the man's case revealed that the blood he received had indeed tested positive for West Nile virus when it was combined with six other samples for screening.

But when laboratory staff further tested each of the six samples individually for West Nile, none turned up positive. The individual test is generally better at detecting virus than the pooled test, so the results from the individual tests were considered correct, and blood from all six samples was released into the blood supply.

Its likely that the concentration of West Nile virus in the infected blood was low, leading to inconsistent results on the screening test, the researchers said. Because the man 's immune system was suppressed, he was probably more susceptible to West Nile infection, even with a low concentration of the virus.

The laboratories involved in screening the blood of the Colorado man for West Nile have since adopted a new policy to discard all blood samples that test positive for West Nile during pooled screening if an infected individual sample cannot be found, the CDC said.

Although West Nile infection from blood transfusion is rare, doctors should suspect the condition in people who show symptoms of the disease within a month of receiving a blood transfusion, the CDC said.

Most people infected with West Nile show no symptoms (this was the case for the donor), according to the CDC. About one in five people with develop a fever and body ache, vomiting, diarrhea or rash. Less than 1 percent develop encephalitis, the CDC says.

The case is described this week in the CDCs Morbidity and Mortality Weekly Report.

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