Trichinosis: symptoms, causes and treatment

Trichinosis is a zoonosis, meaning an infection that passes from animals to people. The cause is a roundworm called Trichinella. ECDC says several species occur in Europe, including T. spiralis, T. nativa, T. britovi and T. pseudospiralis.

The larvae live in the muscle of animals. ECDC says the most common reservoirs in Europe are pigs, horses and wild game such as wild boar. MSD adds that bears, foxes and walruses can also carry the parasite in other parts of the world.

The NHS website does not have a page on trichinosis. This guide draws on UK Food Standards Agency guidance, the European Centre for Disease Prevention and Control, and the MSD Manual, which is US-based. A doctor or infectious diseases specialist can advise on an individual case.

People become infected by eating raw, undercooked or processed meat that still holds live larvae. The FSA names pork, wild boar, horse and game. It is not passed from person to person.

MSD describes what happens next. Stomach enzymes dissolve the cyst wall and the larvae are released in the small intestine, where they become adult worms. The females burrow into the gut wall and release new larvae for about 4 to 6 weeks. The larvae travel in the blood and lymph to muscle, where they form cysts over one to two months.

ECDC says the highest risk in the EU and EEA comes from eating undercooked meat from pigs reared outdoors under uncontrolled conditions, or from hunted wild boar. The FSA says pigs can be infected when feed has been contaminated with dead wildlife such as rodents.

ECDC says symptoms depend on how many larvae were swallowed, from none at all to severe or even fatal illness. MSD says many infections are mild or silent.

The illness tends to come in two phases. In the first week the gut is affected. One to two weeks after infection, symptoms from larvae in the muscles begin.

• First phase, often within 1 to 2 days: nausea, vomiting, diarrhoea, abdominal cramps, tiredness and fever.

• Second phase: persistent fever, headache, chills, cough, aching joints and muscle pain and tenderness.

• Swelling of the face, especially around the eyes, and sometimes red, sore or light-sensitive eyes.

• Pain that is often worst in the muscles used to breathe, speak, chew and swallow.

• Itchy skin, and either diarrhoea or constipation, according to ECDC.

MSD says that when many larvae are present, the heart, brain and lungs may become inflamed. Heart failure, abnormal rhythms, seizures and severe breathing problems can result. Death can occur but is rare.

Without treatment, MSD says most symptoms fade by about the third month, which is when larvae are fully encysted in muscle. Vague muscle pains and tiredness can last for many months.

The professional edition of MSD says the dead larvae are eventually absorbed or become calcified. Most people with trichinosis recover fully, even though cysts may remain in the muscles for years.

There is no quick test for the early gut stage. MSD says doctors usually begin from the story, such as recent raw or undercooked meat, and from symptoms such as facial swelling, muscle pain and fever. A blood count often shows raised eosinophils, a type of white blood cell.

Antibody blood tests are used to confirm the diagnosis. MSD notes that antibodies may not be detectable until 3 to 5 weeks after infection, so a negative test is repeated at weekly intervals. A muscle biopsy can show larvae but is seldom needed. Muscle enzymes are often raised.

The condition can resemble other illnesses, including muscle inflammation, other fevers and some neurological problems, so specialist advice may be needed.

MSD says albendazole or mebendazole, taken by mouth, are used to eliminate adult worms from the intestine. These medicines do not clear cysts already formed in muscle. Painkillers such as anti-inflammatory drugs help muscle pain.

For severe illness, or when the heart or brain is inflamed, doctors may add a short course of a corticosteroid such as prednisone. The exact medicines and doses are decided by the treating doctor, and the professional MSD page notes that higher doses or longer courses have uncertain benefit and more side effects.

Because treatment works best before larvae reach the muscles, anyone who becomes unwell after eating raw or undercooked pork or game should say so when they seek care.

MSD advises cooking pork and meat from wild animals until it is brown throughout, to above 71 degrees Celsius. Smoking, salting and microwave cooking do not reliably kill the larvae. Meat grinders and other equipment used for raw meat should be cleaned thoroughly, and hands washed with soap and water.

Freezing can kill larvae in thin cuts of pork, but MSD says freezing is not recommended for wild animal meat because some Trichinella species survive low temperatures.

In the UK, the FSA says testing is required to stop infected meat from entering the food chain. Feral wild boar shot by hunters and supplied directly to consumers or local retailers should be tested, using guidance from the Animal and Plant Health Agency. Hunters who keep game for home use should take the same care in cooking.

There is no NHS trichinosis page to quote, so general advice applies. Contact NHS 111 or a GP if you have fever, muscle pain or facial swelling after eating raw or undercooked pork or game, especially if others who shared the meal are also ill.

Call 999 for severe breathing difficulty, chest pain, a seizure, confusion or collapse. These can reflect complications that MSD describes in heavy infections, though they are rare.

Care is delivered by NHS general practice and infectious diseases services. A private infectious diseases consultant can offer faster access to tests and review, but the treatment is the same.