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Trichinellosis

6 March 2013, by BOURNERIAS I. & DUPOUY-CAMET J.

Trichinellosis, also known as trichinosis, is a cosmopolitan zoonosis caused by a nematode of the Trichinella genus in which man is the accidental host. Seven species of Trichinella cause the disease in man: T. spiralis (cosmopolitan and restricted to pigs), T. pseudospiralis (cosmopolitan), T. nativa (arctic regions), T. nelsoni (tropical Africa), T. britovi (temperate regions of Europe and Africa), T. murrelli (North America) and T. papuae (rare cases in the Far East caused by consuming reptiles). In France, the rare, sporadic cases reported are caused by eating wild boar meat or are in subjects returning from abroad (caused by eating pork, warthog or bear, etc.).

DIAGNOSIS

The diagnosis is suggested by a combination of often debilitating muscle pain, facial oedema and a high fever (often in excess of 39° C). These signs are present in about 80 percent of cases. The inaugural signs of the disease are gastrointestinal in 40 percent of cases (enteral phase). This stage precedes the acute signs (parenteral phase, invasion of the muscles by the larvae) which arise around about the 10th day after infestation. Mylagia and muscle weakness affect preferentially and progressively the extra-ocular muscles (photophobia), the masseters (pseudo-trismus), the neck muscles (pain on swallowing and speaking), the tongue muscles, flexor muscles of the hands and feet and the low back muscles. The fever lasts less than a week; the muscular signs reach their peak towards the 2nd to 3rd week of the infection, last 1 to 2 months and regress in 3 to 6 months.

Eyelid, facial and/or conjunctival oedema is characteristic, arises early (7th to 25th day) and lasts a few days. The oedema respects the general facial structure and differs entirely from angioedema and other non-histamine induced oedema. Conjunctivitis, which may sometimes cause sub-conjunctival haemorrhage, regresses more slowly. A hives-like or morbiliform rash (//www.cmaj.ca/content/176/4/449/F1.expansion.html) on the torso, abdomen and limbs is sometimes observed during the invasion phase. Itching is sometimes present but can be observed months after the initial infestation. Distal subungueal splinter haemorrhage is characteristic (although nonspecific) but rare. Peripheral oedema has also been described. Asymptomatic or pauci-symptomatic forms are possible, particularly in children. Although this disease is rarely fatal, its severity lies in its potential neurological and/or cardiac complications. The intensity of the symptoms is dictated by the quantity of larvae ingested but also by the Trichinella species responsible for the infestation, T. spiralis being the most pathogenic.

Its psychological impact varies, some of the subjective signs being persistent (itching, dermatographism, asthenia and myalgia).

Non-specific laboratory test results can help orient the diagnosis: blood hypereosinophilia (early onset, more than 1000/mm3) combined with an increase in muscle enzymes (CPK and LDH).

The main differential diagnoses are other protozoan infestations (invasion phase of bilharzia) and medical conditions: dermatomyositis, Churg and Strauss disease, polyarteritis nodosa, muscle toxicity induced by certain medicines (statins, for example) or food supplements, Shulman syndrome, etc.).

The diagnosis is put forward on the basis of epidemiological data (ingestion of raw or improperly cooked meat containing the Trichinella larvae encased in a cyst).

The meats most commonly responsible are pork, wild boar, bear, occasionally walrus or dog, warthog, omnivorous animals which have become infected by ingesting contaminated rodents or members of their own species.

Horsemeat has been a major source of epidemics in the past.

The diagnosis is confirmed by blood test.s The tests used are ELISA and indirect immunofluorescence, and the results are confirmed using the Western blot technique. The antibodies emerge 10 to 15 days after contamination (a few days after the clinical signs of the acute phase) and detection of seroconversion is of major diagnostic value. However, a positive blood test does not necessarily indicate a recent infection and, if the clinical symptoms and signs give rise to doubt, the test must be done again one to two weeks after the first sample is obtained. The blood test may continue to be positive for many years.

A muscle biopsy (larvae will be present from the 3rd week after infection) allows a direct parasitological diagnosis but is only justified when the diagnosis is uncertain.

TREATMENT

Treatment is symptomatic (analgesics, antipyretics and corticosteroids) and anthelmintic. Systemic corticosteroids (prednisone, 1 mg/kg/day for a week) are currently recommended in all cases; resuscitation measures may be required in the severe forms.

The earlier an anthelmintic treatment is prescribed, the more effective it will be. The advantage of such treatment is that it destroys the adult worms in the enteral phase and reduces muscular dissemination, but its effects on the larvae in the established stage have not been demonstrated.

The anthelmintic used is albendazole (Zentel®,Eskazole®,hospital-use only) at a dose of 15 mg/kg/d, i.e. one 400 mg tablet twice a day for ten days to two weeks; a 4 percent oral solution form for children also exists. It is contraindicated during pregnancy and in children aged less than 2 years old. This medicinal product is currently available from pharmacies and is 65% reimbursed by the French national healthcare system.

PREVENTION

Trichinellosis is rare in countries in which basic health, hygiene and food safety rules are applied. In regions or circumstances (hunting) in which these rules are not respected, it is essential for the individual to apply preventive measures, for instance, ensuring that pork or wild animal meat is correctly cooked. The larvae will not survive cooking temperatures of more than 65°C for 5 minutes (meat brown in the centre). Microwave cooking is not recommended. Trichinella spiralis larvae will not survive 10 days in a freezer at -20°C but T. britovi and especially T. nativa larvae resist freezing. Home-curing, particularly smoking, will not kill the larvae.

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