A parasite that is dangerous to people is being found increasingly in wild salmon from rivers and estuaries.

Eating a live worm on the fish can cause severe symptoms, including erratic heartbeat, unconsciousness, breathing difficulties and respiratory failure, possibly leading to death. Pregnant women and the elderly are particularly vulnerable.

The fish posing the greatest risk are likely to be those caught by anglers and netsmen who like to eat their own catch or sell it to local restaurants, shops or friends. The Food Standards Agency has issued a specific alert for fishermen to be aware of the dangers.

Wild salmon sold commercially is governed by strict hygiene and food-safety laws and is unlikely to pose a threat to human health. Most wild salmon on sale in supermarkets is from Alaskan waters. In advice to anglers the agency says that all fresh wild salmon must be either frozen or heated at 70C for at least two minutes to kill the parasite and its larvae.

In some cases, however, even after cooking there can still be a health risk and the parasite may cause severe anaphylactic reactions in some people.

Scientists at the Environment Agency and the Centre for Environment, Fisheries and Aquaculture Science (Cefas) are now urgently investigating the increase in the number of cases of wild salmon found with swollen and bleeding vents. Male and female fish are affected. The parasite, known as Anisakis simplex, has been known about for some years. Salmon ingest it when they are feeding at sea and then carry the parasite to fresh water.

Last year fishermen reported record numbers of affected salmon on some of the most popular salmon-fishing rivers.

Three years ago 5 per cent of salmon caught on the River Dee had the condition but last year the figure jumped to at least 15 per cent.

On the River Itchen the incidence was even greater with at least 30 per cent of salmon carrying the parasite. Affected salmon were also found on the Exe, Taff, Camel, Tamar, Eden, Tyne, Ribble, Exe, Wye and Lune, as well as in Scottish and Irish rivers. There is no evidence of the condition in wild trout or farmed salmon or trout.

The food safety watchdog is advising any consumers of wild salmon to inspect the fish and to remove any parasites. Any fish that is obviously contaminated should not be eaten. If wild salmon is to be eaten raw or almost raw it should be frozen for at least 24 hours at minus 20C or colder. This will ensure the destruction of any nonvisible parasites and larvae.

Any wild salmon destined for cold smoking or to be eaten after marinating or salting, such as gravadlax, must also be frozen. Wild salmon to be hot-smoked (in temperatures above 60C), however, is safe to eat without freezing. Cooking at 70C for two minutes will kill any parasites.

A spokesman for the agency said: “Heating or freezing should be enough to kill any parasite, but if anyone sees any sign of the parasite they should not eat it but should return it to their supplier.”

No wild salmon is being fished at the moment but the season begins soon in Scotland. In England and Wales the season begins on June 1 and salmon caught by rod and line can be removed from rivers after June 16.

Last year’s catch figures are not yet available but in 2006 49,389 salmon were killed for home consumption from rivers and a further 38,467 netted along the coast.

Experts are analysing the latest data and a new report is expected within weeks. There are particular concerns that, because salmon spawn through their vents, there could be a collapse in salmon numbers.

A spokesman for the Environment Agency said that the impact on salmon stocks and spawning was inconclusive.

The Scottish Executive introduced a ban last year on the sale of rod-caught wild salmon from rivers. Angling organisations are hoping that the Environment Agency will introduce by-laws allowing the same ban in England and Wales this year.

Anglers are still allowed to take some of their catch home to eat.

Times recommends

• Prince Philip attacks big families
• Take a dip in Dubai-on-Thames
• Eco Worrier: How can I persuade my husband to use a push mower?