US researchers said Tuesday a rare case of raccoon rabies is responsible for killing both a US kidney donor in 2011 and his transplant recipient 18 months later.
The report in the July 24 edition of the Journal of the American Medical Association describes the final results of an investigation into the case, which was announced by US health authorities in March.
Doctors did not realize that the donor, described by US media reports as an Air Force mechanic in his 20s, had rabies when he died.
Instead, he was believed to have returned from a fishing trip with a lethal form of food poisoning.
The man's organs -- kidneys, heart and liver -- went to four different people -- three of whom survived and did not develop rabies.
The retired Army veteran who received the donated left kidney died in February 2013, a year and a half after his transplant.
Researchers went back over the donor records and found that the organ questionnaire asked whether he had been exposed to potentially rabid animals in the past six months. The answer was no.
In subsequent interviews with the donor's family, scientists learned he had sustained at least two raccoon bites seven and 18 months prior to his hospitalization.
Family members also reported he had "significant wildlife exposure," including trapping and keeping raccoons in North Carolina, "using them as live bait during dog training exercises, and preparing pelts for display."
The man did not seek medical care for the bites, and the animals were not available for testing.
"The researchers found that, in retrospect, the kidney donor's symptoms prior to death were consistent with rabies," said the study.
The remaining three recipients of his organs were notified and were given anti-rabies shots.
Researchers said the case shows that little is known about how long it takes for certain kinds of rabies to develop illness in people, and whether immune therapies to prevent organ rejection may play a role in slowing disease progression.
"To our knowledge, this is the first report in which un-vaccinated recipients of solid organs from a donor with rabies did not all develop disease," said the study.
A handful of such cases in the past -- organ donor infections with rabies via dogs or bats -- have resulted in the deaths of all the recipients who had not been vaccinated against rabies.
The case was also unusual in that it took a year a half between the transplant and the onset of fatal illness in the left kidney recipient, making it "the longest documented" incubation period.
The previous record was 39 days, from a cornea transplant that turned out to have a rabies infection, the study said.
Raccoon rabies cases have spread in the eastern United States in recent decades, but only one other case of human infection is known. That case also had an "uncertain" incubation period, the researchers said.
Human rabies infections more commonly result from dog or bat bites.
About 55,000 people die of rabies worldwide every year. The United States has reported about two human rabies deaths annually from 2000 to 2010.
Symptoms of rabies in humans may include seizures, partial paralysis, fever and brain inflammation, or encephalitis. There is no known treatment to cure rabies, once the infection has taken hold.
Researchers called for medical professionals to consider rabies as a potential cause of unexplained encephalitis -- of which there are 1,000 lethal cases each year in the United States -- in order to prevent future cases of rabies transmission by organ donors.
Routine tests are done for HIV and hepatitis but not rabies unless it is clinically suspected, US health authorities have said.