Trichinella spiralis

Historical aspect

  In 1835, a man died of tuberculosis in St Bartholomew’s Hospital, London. Dr Paget, a first-year student, carried out the autopsy and observed fine hard white inclusions in the muscles. Similar inclusions had been observed by doctors from time to time in the past, but were attributed to commonplace muscle calcification, which quickly blunted the dissecting scalpel. Dr Paget inspected the lesions with a hand lens and quickly recognised their worm-like structure. The name "Trichina
spiralis" was suggested.

   This name Trichina had already been given to a certain fly, however, and the name was later changed to "Trichinella". The discovery of the parasite was published by the famous biologist and palaeontologist Richard Owen, at that time assistant conservator of the museum of the Royal College of Surgeons. In 1859 Rudolph Virchow carried out transmission experiments in
which infected human muscle was fed to a healthy dog. After only 3 to 4 days adult Trichinella worms were found in the dog’s duodenum and jejunum.

   Life cycle

More than 100 species of mammals are susceptible to the infection. Infections with Trichinella spiralis affect chiefly carnivores and omnivores, although infection of horses has also been described. People become infected with this nematode by eating raw or insufficiently cooked infected meat, often pork or wild boar. The larvae of Trichinella spiralis which are in the meat develop in a few
days into adult worms (2-4 mm) in the wall of the small intestine. There they lay larvae (100 m m). These spread via the bloodstream to various muscles, including the heart, where they undergo encapsulation [Trichinella pseudospiralis does not form a capsule]. The larvae cannot continue to survive in the heart. The larvae are localised within the cells of the muscles, which is unique for a worm.

    After penetrating the muscle cell, a larva excretes a number of signal molecules and
proteins, which convert the cell to what is called a nurse cell. In the cell the behaviour of the worm is rather similar to that of a virus. Many of its proteins are glycosylated and often carry an unusual sugar (tyvelose). These proteins are excreted from a special organ in the larva (the stichosome). Various muscle proteins such as actin and myosin change or disappear, nuclear division is  stimulated and mitochondria are damaged. Local angiogenesis is stimulated by excretion of a blood vessel growth factor and new blood vessels, originating from nearby venules, develop and form a network around the infected cell.

  The metabolism of the nurse cell and the parasite is essentially anaerobic. After 1 to 4 months the adult worms die. The larvae in the muscles sometimes survive for years and can remain viable for a long time even in rotting flesh. Trichinella is unique among worms in that all development stages take place in the same host. There is never a free stage outside the mammalian body.

 Symptoms

Infection may be asymptomatic. In typical cases there is diarrhoea, abdominal pain, vomiting and fever a few days after eating infected meat. After 10 days the fever increases, the patient is very ill and debilitated, there are muscle pains and a typical peri-orbital oedema (differential diagnosis acute trypanosomiasis and nephrosis). This oedema is caused by invasion of the small muscles around the
eye. There may be signs of myocarditis, encephalitis, urticaria and asthma. There is often very significant eosinophilia. The myositis causes an increase in the muscle enzymes (creatine phosphokinase, CK). After a few months the symptoms are reduced or disappear. Mild infections are self-limiting.

 Diagnosis

Not many nematodes are found in muscle tissue. Occasionally a migrating third stage larva of Ancylostoma, Toxocara or Gnathostoma may be found (visceral larva migrans). Dracuncula medinensis may also be found in muscle tissue. Another, less common nematode which may be found here is Haycocknema perplexum (Tasmania).

 Prevention

Meat should be well boiled or roasted thoroughly.  Importance of meat inspection. The diaphragm of a slaughtered animal is inspected (the piece of muscle is flattened between two glass slides and examined using transillumination). This technique (trichinoscopy) is not so good for Trichinella pseudospiralis because it is not surrounded by a capsule and is easily missed.

Pig food (which may include infected rats) should be boiled for 30 minutes.

 
To store pork for 10 days at -25° C is generally impractical in developing countries. In the West meat is sometimes irradiated with high doses of gamma rays, which will kill any larvae. Trichinella spiralis nativa is cold-hardy.