SARS agent still unidentified

Scientists at the National Centres for Disease Control (CDC) in Atlanta, Georgia, are working "around the clock" in collaboration with World Health Organisation researchers and international disease reference laboratories to identify the infectious agent responsible for an outbreak of a potentially deadly respiratory disease in south-east Asia.

CDC director-general Dr Julie Gerberding told a teleconference in the US early this morning, Australian time, that the mystery agent appeared not to be a common infectious agent "or we would have identified it by now".

CDC researchers are working on samples from individuals with the infection -- including post-mortem tissues from one of two victims in a Canadian family of seven, who developed symptoms of Severe Acute Respiratory Syndrome (SARS) after returning from a visit to Hong Kong.

One of the problems for researchers trying to identify the pathogen is poor access to tissue samples from Asian patients, particularly from the apparent epicentre of the epidemic, in China's Guandong province.

China initially refused CDC researchers access to patients or tissue samples from Guandong.

Worldwide effort

According to a WHO press release today, 11 laboratories in 10 countries are now involved in efforts to identify the infectious agent. Gerberding said CDC had deployed 12 experts to international locations to support efforts to manage the international outbreak.

The fact that SARS patients not respond to antibiotics is a strong indication that it is caused by a virus, not a bacterium -- antibiotics are ineffective against viral infections.

Like Australia, which has reported three suspected cases of SARS among travellers returning from Hong Kong, US authorities have received reports of 14 possible SARS infections. But medical authorities believe that the odds are against the infections being due to SARS; many travellers acquire common respiratory infections while overseas.

Geberding says that, of the 14 suspect cases in the US, 10 almost certainly do not have SARS infection because they "do not meet any of the relevant history" of travel to south-east Asia.

The other four meet some of WHO's criteria of having travelled recently to Asian countries where the SARS outbreak is centred -- China, Vietnam and Singapore -- and also have some of the symptoms of SARS: a high fever, a productive cough, and difficulty breathing -- as defined by WHO. These four patients are under closer scrutiny.

According to today's WHO press release, Chinese authorities have now issued a summary report on an outbreak of what may be the same or a related disease that began in Guangdong province in southern China, last November, and peaked in mid-February.

The Chinese report, which includes data on the diagnosis and management of more than 300 cases, is being analysed, and is expected to further contribute to understanding about the syndrome and possible links among the various outbreaks, according to WHO.

Because it covers the largest number of cases to date, it may also boost international efforts to establish effective treatment guidelines, should a relationship between the outbreaks be established.

Zoonotic suggested

While the identity of the pathogen remains unknown, CSIRO virologist Dr Bryan Eaton, of the Australian Animal Health Laboratory in Geelong, says the suddenness and rapid spread of the outbreak are characteristic of a zoonotic -- an animal-borne agent that has crossed into human beings.

Dr Eaton said AAHL, which has Level 4 biohazard containment facilities for dangerous animal viruses, would not become involved unless Australian health authorities sought its assistance.

Several other human health laboratories in Australia, including the Victorian Infectious Diseases Reference Laboratory at the Royal Melbourne Hospital, also had level 4 biohazard containment facilities.

Given that family physicians involved with SARS patents were now coming down with the same, potentially deadly infection, "this sounds like a level 4 virus", Eaton said.

Eaton said the reported symptoms of SARS did not sound like Nipah virus, a deadly zoonotic that caused the deaths scores of people and hundreds of pigs in Malaysia in 1998.

Like Hendra virus, which killed Brisbane racehorse trainer Vic Rail in 1994, Nipah virus is carried by Pteropus bats -- flying foxes. It is a paramyxovirus, distantly related to the human measles virus, and causes severe respiratory infection and meningitis. The lack of any reports of meningitis symptoms in SARS patients would seem to rule out Nipah virus, says Eaton.

Asked what other animal-borne virus might be the agent of SARS, Eaton said, "Flu is the one that comes to immediately mind -- Hong Kong and Guandong are the epicentre for new influenza outbreaks."

However, it was likely that current tests would already have identified identify any new serotype of influenza that emerged from local duck or chicken populations, he said.

A rodent-borne hantavirus was another possibility -- hantaviruses were found around the world, and caused 'flu like symptoms in humans, including bleeding in the lungs -- a symptom reported in some SARS patients.

Eaton said neither Australian animal health or human health authorities needed to be reminded that such outbreaks could happen.

"It's an acute reminder that we have to be on our guard constantly. With human migration patterns, modern transport systems, the movement of animals, and competition between humans and animals for living room, we're going to see more of these things," he said.