Nov. 2 (Bloomberg) -- Bird flu killed a dog in Thailand after infecting its lungs, liver and kidneys, highlighting the potential for pets to contract the lethal virus and spread it to humans, researchers in the country said.

The dog probably picked up the H5N1 avian influenza strain from infected duck carcasses in the central Thai province of Suphanburi two years ago, the researchers said in a study published in this month's Emerging Infectious Diseases journal. Five days after feeding on the carcasses, the dog developed high fever, panting and lethargy, and died a day later.

The dog's systemic H5N1 infection provides more evidence of the virus's ability to kill mammals. Disease trackers are monitoring for signs the virus is changing into a form more dangerous to people after it killed at least 74 people this year, as many as reported in the previous two years combined.

``Despite the low probability of H5N1 infection in domestic animals, the possibility of humans acquiring H5N1 infection from direct contact with infected cats and dogs warrants concern and highlights the need for monitoring domestic animals during H5N1 outbreaks in the future,'' said the authors, including Thaweesak Songserm from Kasetsart University and Alongkorn Amonsin from Chulalongkorn University.

Emerging Infectious Diseases is published monthly on the Internet by the National Center for Infectious Diseases in Atlanta.

Human Cases

The H5N1 virus is known to have infected 256 people in 10 countries in the past three years, killing 152 of them, the World Health Organization said on Oct. 31. Last year, 42 fatalities were confirmed, after 32 in 2004 and four in 2003. Millions could die if H5N1 becomes easily transmissible between people, sparking a lethal pandemic.

The H5N1 virus is reported to have spread in wild birds and domestic poultry to at least 38 countries in Africa, Asia and Europe this year.

Thailand recorded its first outbreak of H5N1 in poultry in January 2004, with human infections reported the following month. The last of the country's 25 human cases was reported in August. Seventeen Thais are known to have died of the disease.

Felines, including domestic cats, are at risk of infection from H5N1 if they prey on birds, studies published in March by researchers at two Thai universities and a government research center showed. A 2005 study showed H5N1 was probably transmitted between tigers in Thailand and German officials in March confirmed an infection in a stone marten, a type of weasel.

No Adaptation

Tests on the infected dog in Thailand showed it had a variant of H5N1 consistent with the strain circulating in the country at that time. Virus particles collected from the dog showed no sign of adaptation, the study in Emerging Infectious Diseases said.

The study is the first report of H5N1-related systemic disease in a domestic dog infected during the second wave of outbreaks in Thailand that occurred during October 2004, the authors said.

Two domestic cats in Iraq were infected with the same variant of H5N1 that was found in geese in China's Qinghai Lake last year, U.S. scientists said in a letter appearing in the August edition of Emerging Infectious Diseases.

The finding supported the notion that cats may be broadly susceptible to circulating H5N1 viruses and thus may play a role in re-assortment, antigenic drift, and transmission, said Samuel Yingst, Magdi Saad and Stephen Felt, researchers at the U.S. Naval Medical Research Unit No. 3 in Cairo, Egypt.

1918 Pandemic

Experts believe that a pandemic in 1918, which may have killed as many as 50 million people, began when an avian flu virus jumped to people from birds.

In uncomplicated human cases of seasonal influenza, disease is limited to infection in the nose, throat and lungs. In contrast, H5N1 was observed to have caused encephalitis, or swelling of the brain, in one patient in Vietnam, said virologist Menno de Jong, whose team observed 18 cases there.

Virus particles collected from numerous sites of infected patients could be cultured, including from the throat, rectum, blood and cerebral spinal fluid, said De Jong, who is head of the virology department at the Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam.

To contact the reporter on this story: Jason Gale in Tokyo at j.gale@bloomberg.net

Last Updated: November 2, 2006 00:59 EST