The highly pathogenic flu strain, which has killed 60 percent of humans infected, can also spread to organs other than the lungs in adults, including the intestines, raising concerns about how the disease might spread, according to a study published in the British journal The Lancet.
A team led by Jiang Gu of Beijing University studied post-mortem tissues of two adults -- one man and a pregnant women -- who had died of the disease.
Since it was first identified in 1997, H5N1 avian flu is known to have infected 328 people worldwide, killing 200, according to the World Health Organisation.
There have been 25 cases and 16 deaths in China, where the virus is thought to have first emerged on poultry farms in the southern province of Guangdong.
Almost all those who contracted the disease dealt extensively with infected fowl, though a few cases of human-to-human transmission have been reported as well.
Scientists fear the virus will mutate into a form that could spread easily among humans, unleashing a pandemic similar to the 1918 outbreak that killed at least 20 million people across the globe.
The researchers detected viral genetic material and antigens not only in the lungs, where the H5N1 strain was known to lodge, but also in the trachea, in disease-fighting T cells of the lymph node, and in brain neurons.
The doctors also found traces of the virus in the placenta, as well as in the lungs, immune cells and liver cells of the fetus.
Antigens are toxins that cause an immune reaction in the body, stimulating the production of antibodies.
This "vertical transmission" of the H5N1 virus from one part of the body to another and into the womb "warrants full investigation, since maternal infections with common human influenza virus are generally thought not to infect the fetus," Gu said.
In humans, H5N1 mainly affects the lower respiratory tract, crippling the lungs ability to take in oxygen and causing respiratory failure. It also causes diarrhoea in 70 percent of patients.
The fact that viral genetic material was found in the intestine and in faecal samples "suggest viral replication occurred in the intestine," noted pathologists Wai Fu Ng and Ka Fai To of Princess Margaret Hospital in Hong Kong.
"This finding could have important implications for infection control," they said.
The pathologists cautioned that laboratory experiments are needed to confirm Gu's conclusion that the virus is able to replicate outside of the lungs.