Scientists believe that marine life is at growing risk from a range of diseases whose spread is being hastened by global warming and pollution.

An article by 13 biologists, 12 from the US and one from Holland, in the magazine Science outlines their concerns.

They cite a number of well-documented cases such as seals infected with distemper by sled dogs, sardines infected with the herpes virus caught from imported frozen pilchards, and corals killed by a soil-borne fungus.

They argue that many less obvious species may be disappearing without anyone noticing.

The lead author, Professor Drew Harvell of Cornell University, said: "The combined effects of rising temperatures, human activity and pollution are producing a volatile mix that may threaten tropical corals and temperate species alike".

Dramatic changes

The article says reports of marine disease are certainly on the increase, and actual incidents probably are as well.

"In the Caribbean, mass mortalities among plants, invertebrates and vertebrates have resulted in dramatic shifts in community structure.

Seals: In trouble from Siberia to the southern seas

"In the north Atlantic, frequency of mass mortalities of marine mammals appears to be increasing, particularly along heavily polluted coastal areas, suggesting human activity as a factor."

But the biologists say the problem is not that the emergence of new micro-organisms is causing apparently new diseases. It is rather that established diseases are finding new species to attack.

So not only did crab-eating seals in Antarctica apparently catch the canine distemper virus (CDV) from infected dogs used for hauling an expedition's sledges.

Similarly, they say, CDV isolated from Lake Baikal seals was genetically identical to CDV from domestic dogs in Siberia.

Whale mortality

A closely related but hitherto unrecognised agent, phocine distemper virus (PDV), was the killer of harbour and grey seals in northwestern Europe in the late 1980s.

Dolphins, porpoises and other cetaceans around the world soon afterwards fell victim to similar infections.

And influenza viruses that had spilled over from marine species or from migrating birds are also known to have killed seals and whales.

Coral stressed by warming oceans

The article says the emergence of new diseases is being helped by a long-term warming trend, by extreme El Nino-type events which affect the climate over huge distances, and by human activities that modify marine communities.

These activities include fish farming, and farming and development on shore.

The article says the coral bleaching of 1998 "was the most geographically extensive and severe in recorded history, causing significant mortality worldwide".

"The stress for many of these coral reef systems seems to be the result of long-term exposure to unusually high water temperatures resulting from a prolonged El Nino-Southern Oscillation event."

Reaching new areas

Habitat damage and pollution, including sewage discharges, also play a part in spreading and intensifying disease.

The recent mass die-offs of Mediterranean monk seals along the coast of Mauritania "may have been facilitated or caused by a toxic algal bloom".

The biologists say climate change and human activities can accelerate the transmission of harmful pathogens by forcing disease-carrying animals into areas where there are new hosts to attack.

And chemicals like organochlorides also weaken the immune systems of marine species, adding to the stress caused by the other factors.