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Trichinella infection in the human host can generally be divided into two distinct phases: an enteral (or intestinal) phase and a systemic (or muscular) phase. Infection with a low parasite burden may be asymptomatic, but when a large number of larvae are ingested, it typically presents with initial gastroenteritis.
Trichinellosis is a disease caused by small, parasitic intramuscular roundworms (nematodes) of the genus Trichinella. Among all the different species, Trichinella spiralis is implicated in a majority of human infections and subsequent deaths around the world. The minimum infective dose for humans is 70-150 ingested larvae of T. spiralis.
Symptoms of infection may range from asymptomatic or a relatively mild clinical presentation to quite severe disease. The manifestations are highly dependent on the number of worms present in the meat. A large number of mild trichinellosis cases are misdiagnosed as flu or other common diseases.
Following the ingestion of parasitic Trichinella larvae in humans, the incubation period begins, ending with the appearance of clinical symptoms. Several factors are responsible for the variable length of the incubation period: the infective dose (i.e. number of larvae ingested), the type of meat (raw or semi-raw), the frequency of meat consumption, and the implicated species and strain of Trichinella.
The length of the incubation period may range from 2 to 45 days, and it is generally thought that a shorter incubation period predicts a more severe course of the disease. However, this relationship is not always so clear-cut, as in certain outbreaks a prolonged incubation period (18-30 days) was followed by a severe illness.
Moreover, the incubation period is not necessarily asymptomatic. In approximately 10% of the affected individuals the development of the typical clinical presentation is preceded by loose stools without blood or mucus. Intestinal symptoms may also be present, most notably upper abdominal pain, flatulence, loss of appetite and, occasionally, vomiting.
In most patients the disease begins suddenly and dynamically, with general weakness, headache and chills accompanied by fever (up to 40 °C), with tachycardia (corresponding to the elevation in body temperature). Excessive sweating is also a typical manifestation of trichinellosis.
Muscle pain (myalgia), numbness or tingling sensations in various muscle groups, and itching of the skin often predominate, usually when the migrating larvae enter the patient’s skeletal muscles. The pain may be quite severe and limit muscular function of the limbs. These symptoms usually disappear during convalescence, but complete regression may be achieved by means of physiotherapy.
Furthermore, symmetrical facial or periorbital edema is a characteristic feature. In some patients there may be urticarial exanthems and bleeding under the fingernails due to vasculitis, which is the dominant pathological process in this disease. Conjunctivitis with subconjunctival bleeding can also be observed.
The major complications of trichinellosis are myocarditis and encephalitis – both life-threatening and often presenting simultaneously. Myocarditis is seen to complicate approximately 5-20% of all cases, and thus it should be systematically screened for even in the absence of specific symptoms. Secondary infections (such as bronchopneumonia and sepsis) also put the patient at risk for a lethal outcome.
Encephalitis occurs if the larvae successfully reach the brain. Other neurological disorders that may be observed include vertigo, convulsions and meningitis. In very severe cases facial nerve paresis, anisocoria (unequal pupils) and Babinsky’s reflexes have been noted, coupled with multiple cortical infarcts observed with magnetic resonance imaging.
Although there are clinical differences among individuals who are infected with various species of Trichinella, it is difficult to draw any firm conclusions as the number of infecting larvae is generally unknown. Thus such an approach to clinical differentiation should be made with caution. Nonetheless, it is thought that infections with Trichinella spiralis are more severe on average than those with Trichinella britovi, while Trichinella pseudospiralisinfection causes protracted clinical syndromes.