Cats and bird flu

A news release about a just published article in the American Journal of Pathology (issue of January 16, 2006) has some mildly unsettling news. Dutch researchers have shown systemic spread of H5N1 in domestic cats (Rimmelzwaan et al., "Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts"). I haven't yet read the paper, but here is a summary, via innovations-report:

While spread of avian influenza from bird to man is known to occur, as first reported during the 1997 Hong Kong outbreak, human-to-human spread is extremely rare. Thus, the disease events that take place during mammal-to-mammal spread are not well characterized.

To assess the spread of H5N1 influenza virus in mammalian hosts, Rimmelzwaan et al. examined cats infected via the respiratory tract, via the digestive tract (by feeding on infected chicks), or by close contact with respiratory-infected cats. The researchers, led by Dr. Thijs Kuiken, then examined mucous membranes (throat, nasal, and rectal swabs) and organ systems (respiratory, digestive, nervous, cardiovascular, urinary, lymphoid, and endocrine) for the presence of virus and viral protein.

As expected, all cats were infected with H5N1 virus and exhibited clinical signs of disease (fever, lethargy, labored breathing, etc.), and virus was detected in throat, nasal, and rectal swabs, regardless of the original site of infection. Most interesting, virus spread throughout the organ systems with virus being found in respiratory and digestive tracts, liver, kidney, heart, brain, and lymph nodes. Furthermore, examination of infected tissues revealed cellular damage at sites containing viral proteins, providing an explanation for the increased severity of disease in humans.

These data underscore the potential for influenza virus to spread not only from the respiratory tract but also from the digestive and urinary tracts, greatly increasing the possible routes of mammalian transmission. Systemic disease has long been known to occur in birds, with the fecal-oral route of transmission being most important. However, this is the first demonstration of systemic replication in cats, providing a cautionary tale for humans regarding how influenza is spread and how the disease presents itself.

Rimmelzwaan and colleagues caution that because of the systemic nature of avian influenza, "H5N1 virus infection needs to be included in the differential diagnosis of a broader range of clinical presentations than is currently done." In addition better understanding of the mechanisms of spread, including possible fecal-oral route in humans, "may limit the risk of H5N1 virus developing into a pandemic influenza virus."

Cases of H5N1 in tigers and leopards in Thailand had already been reported, although cats don't usually get influenza.

Newswires are also carrying a report of another paper by the same group appearing almost simultaneously in the journal Science.

The team infected three laboratory cats with H5N1 taken from a human case in Vietnam. All got very sick with flu symptoms, and post mortems showed they had the same lung damage as people.

Cats given a human flu virus, H3N2, stayed healthy. Other cats studied caught H5N1 by eating infected birds, while two healthy cats housed with the sick animals caught the disease, showing it spreads among cats.

The results mean pet cats might give people H5N1 after eating one of the many wild birds or poultry still infected across East Asia. But more worrying than cats spreading the existing virus, says Kuiken, is how cats might change its evolution.

H5N1 was already known to infect mammals – 34 people, of whom 23 died, were confirmed to have the virus before the Asian poultry outbreak was largely controlled in March. (New Scientist)

And the BBC:

Writing in Science, the researchers say: "The implications are that, during H5N1 virus outbreaks, domestic cats are at risk of disease or death from H5N1 virus infection, either due to feeding on infected poultry or wild birds, or due to contact with infected cats.

"Second, the role of cats in the spread of H5N1 virus between poultry farms, and from poultry to humans needs to be re-assessed.

"Third, cats may form an opportunity for this avian virus to adapt to mammals, thereby increasing the risk of a human influenza pandemic."

Professor John Oxford, an expert in virology at Queen Mary College, London, told BBC News Online the study was "very significant and slightly alarming".

He said there was little evidence at present that cats could be infected with other forms of human flu virus, and so the possibility that H5N1 could mix with a human virus inside a cat and produce a deadly new strain was probably slim.

However, he agreed that it was possible that cats could be responsible for avian flu spreading from farm to farm - a phenomenon which has baffled scientists.

Professor Oxford said there was also work to suggest that pigs could be infected with H5N1, and, unlike cats, they could also harbour human versions of the virus.

"H5N1 is getting more and more worrisome," he said.

"If any virus is going to cause a great human pandemic in the near future, then it is likely to be H5N1."

This last seems to be a very reasonable judgment.