MONDAY, April 7 (HealthDay News) -- Researchers have identified a probable case of human-to-human transmission of bird flu in China.

The finding lends credence to the idea that there's a genetic component to human-to-human transmission of this potentially dangerous virus, a new study reported.

"This suggests that there's some genetic component to resistance in the person who's infected," said Philip Alcabes, an epidemiologist and associate professor at the School of Health Sciences of Hunter College in New York City.

The finding follows reports of probable human-to-human transmission of the H5N1 bird flu virus in Pakistan.

A person's ability to monitor foreign microscopic invaders such as avian virus is essentially genetic, Alcabes said. "That may account for why certain families seem to be susceptible whereas most people aren't," he added.

According to background information for the new study, published online Tuesday in The Lancet, there have been 376 reported cases of infection with avian H5N1 virus around the world as of April 2, with 238 deaths since November 2003.

The H5N1 virus has infected poultry throughout Southeast Asia, Central Asia, Africa and Europe, prompting the destruction of millions of birds. The concern among health officials is that the virus will mutate and acquire the ability to jump easily between humans, leading to a pandemic and millions of deaths. Unlike the seasonal flu, humans have no immunity to bird flu.

In December 2007, according to the Lancet report, a father and son in Nanjing, China were diagnosed within one week of each other as being infected with H5N1. Researchers from the Chinese Centre for Disease Control and Prevention in Beijing conducted field and laboratory tests of both men, as well as 91 people who had had close contact with them.

It seems that the 24-year-old son, who died, was exposed to H5N1 when visiting a poultry market six days before he fell ill. The 52-year-old father, who survived, had had substantial contact with his son while caring for him in the hospital. The father had no known direct exposure to birds or other sick individuals.

The H5N1 viruses sampled from the father and the son were virtually genetically identical.

The father received antiviral treatment as well as plasma from an individual who had been vaccinated against the virus as part of a vaccine trial. The son was diagnosed too late to receive appropriate treatment.

All 91 exposed contacts, including the son's girlfriend and mother, tested negative for H5N1.

Transmission may have occurred if the father inhaled droplets coughed out by his son, or by contact with the son's clothing that was contaminated with fecal matter. The father wore a surgical mask during his last visit with his son but had not worn protective gear before that, the report stated.

Interestingly, no outbreaks of H5N1 in poultry were identified in Nanjing, a city in eastern China, before or after these two human cases. No sick or dead poultry were seen either, the report said, adding that it's also important to note that this case of son-to-father transmission has not been proven yet.

According to the study authors, more than 90 percent of H5N1 clusters in humans have occurred in blood-related family members.

And, experts said, this latest case is no reason for alarm.

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, which funded the study, said, "This is not something new. Limited human-to-human transmission has been seen intermittently for years and years. This is just another example among a few. It's what we call a dead-end person-to-person transmission."

He added, "What we worry about is secondary and tertiary transmissions, when someone gives it to someone who gives it to someone. That is not the case here."

"I took great reassurance from this study," Alcabes added. "Very, very occasionally, you can get limited transmission of the avian virus from one infected human to another human, but it seems only to happen in families. There's nothing here to suggest new reasons to worry, let alone panic."

Added Dr. Pascal James Imperato, chairman of the department of preventive medicine and community health at SUNY Downstate Medical Center in New York City, "There's no proof positive that the father was not independently exposed to poultry."

The Pakistan case involved a poultry worker who became infected and survived last year, while three of his brothers were also infected, and two died, according to a BBC News report on Saturday.

Genetic-sequencing tests on bird flu virus samples collected from three of the four brothers confirmed human-to-human transmission of the H5N1 virus, the World Health Organization reported.

The northwestern region of Pakistan has 85 percent of the country's poultry farms and was one of the regions hardest hit by bird flu last year BBC News noted.

More information

The World Health Organization has more on avian flu.