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

The finding lends credence to the idea that there´s a genetic componentto human-to-human transmission of this potentially dangerous virus, a newstudy reported.

"This suggests that there´s some genetic component to resistance in theperson who´s infected," said Philip Alcabes, an epidemiologist andassociate professor at the School of Health Sciences of Hunter College inNew York City.

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

A person´s ability to monitor foreign microscopic invaders such asavian virus is essentially genetic, Alcabes said. "That may account forwhy certain families seem to be susceptible whereas most people aren´t,"he added.

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

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

In December 2007, according to the Lancet report, a father andson in Nanjing, China were diagnosed within one week of each other asbeing infected with H5N1. Researchers from the Chinese Centre for DiseaseControl and Prevention in Beijing conducted field and laboratory tests ofboth 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 whenvisiting a poultry market six days before he fell ill. The 52-year-oldfather, who survived, had had substantial contact with his son whilecaring for him in the hospital. The father had no known direct exposure tobirds or other sick individuals.

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

The father received antiviral treatment as well as plasma from anindividual who had been vaccinated against the virus as part of a vaccinetrial. 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 coughedout by his son, or by contact with the son´s clothing that wascontaminated with fecal matter. The father wore a surgical mask during hislast visit with his son but had not worn protective gear before that, thereport stated.

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

According to the study authors, more than 90 percent of H5N1 clustersin 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 andInfectious Diseases, which funded the study, said, "This is not somethingnew. Limited human-to-human transmission has been seen intermittently foryears and years. This is just another example among a few. It´s what wecall 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 thecase here."

"I took great reassurance from this study," Alcabes added. "Very, veryoccasionally, you can get limited transmission of the avian virus fromone infected human to another human, but it seems only to happen infamilies. There´s nothing here to suggest new reasons to worry, let alonepanic."

Added Dr. Pascal James Imperato, chairman of the department ofpreventive medicine and community health at SUNY Downstate Medical Centerin New York City, "There´s no proof positive that the father was notindependently exposed to poultry."

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

Genetic-sequencing tests on bird flu virus samples collected from threeof the four brothers confirmed human-to-human transmission of the H5N1virus, the World Health Organization reported.

The northwestern region of Pakistan has 85 percent of the country´spoultry farms and was one of the regions hardest hit by bird flu last yearBBC News noted.

More information

The World Health Organization has more on avian flu.