By Alex Kirby BBC News Online environment correspondent

The mystery disease decimating two Indian vulture species now appears to be attacking two more.

Conservationists say they think it unlikely the birds will ever recover to their former levels.

They say the scale of the decline is so great that the extinction of at least two species is possible.

The implications for human health, both in south Asia and beyond, are disturbing.

The white-backed and long-billed vultures have for several years been succumbing to a disease which causes their heads to droop, leaves them lethargic and usually kills them within a month.

Massive decline

In some areas, numbers have fallen by more than 90% within a decade.

Dr Andrew Cunningham, of the Institute of Zoology (IoZ), London, UK, told BBC News Online: "We're almost certain a virus is responsible, and I'm more confident than I was a few months ago that this is an infectious disease, not pollution or poisoning.

Healthy long-billed vultures (Image by Guy Shorrock)

"The scale of what's happening is so great these birds are headed for extinction - yet until a decade ago the white-backed vulture was the world's most abundant bird of prey.

"Now, the disease appears to be spreading. I did a post mortem in June on a Himalayan griffon vulture, and the pathology was similar."

There are signs the disease is also claiming victims among the country's Eurasian griffons.

Dr Vibhu Prakash, of the Bombay Natural History Society (BNHS), which revealed the problem, said reports from Pakistan and Nepal indicated similar high mortality rates there.

There have been reports of sick vultures from as far away as Saudi Arabia, but they have not been conclusively linked to the Indian outbreak.

Supreme scavengers

The Himalayan and Eurasian griffons, like the two vulture species, belong to the Gyps genus. There are Gyps vultures in southern Europe and throughout Africa.

A drooping head presages death (Image by Vibhu Prakash)

Dr Debbie Pain, of the UK's Royal Society for the Protection of Birds (RSPB), told BBC News Online: "We're very worried the disease could spread to other griffon species in Europe and Africa, where Gyps are the main scavengers - they eat more meat than all other African predators combined.

"If it reaches Africa it could change the ecology entirely. There'd be a huge increase in animals like hyenas and jackals, and changes in disease patterns, with probable implications for people."

Trapped by dogs

The vultures' decline is already affecting people in India, where feral dog numbers have increased massively to exploit the carcases the birds are no longer eating.

Dr Prakash told BBC News Online: "There's a carcase dump where there used to be 70 or a hundred dogs. I was there recently, and we couldn't get out of the jeep because of the dogs. We counted 1,200 of them.

"They're a big threat to wildlife, and to people. There are reports of them killing children. We believe the incidence of rabies is increasing. And the stench of the carcases round some of the villages is unbearable."

The BNHS, RSPB, the IoZ and the UK's National Birds of Prey Centre are working together to open two centres in India.

One will investigate the disease, and the other will seek to care for vultures in captivity.