Causative agent of trichinellosis

■ Humans can acquire an infection with larvae of various Trichinella species by ingesting raw meat (from pigs, wild boars, horses, and other species). Adult stages develop from the larvae and inhabit the small intestine, where the females produce larvae that migrate through the lymphatics and bloodstream into skeletal musculature, penetrate into muscle cells and encyst (with the exception of Trichinella pseudospiralis and some other species which does not encyst). Clinical manifestations of trichinellosis are characterized by intestinal and muscular symptoms. Diagnosis requires muscle biopsies and serum antibody detection. ■

Parasite species and occurrence. Eight Trichinella species and several strains have been described to date based on typical enzyme patterns, DNA sequences, and biological characteristics. The areas of distribution are listed in Table 10.5; several Trichinella species occur sympatrically, i.e., in the same geographic region (Table 10.5).

The most widespread and most important species is Trichinella spiralis, which develops mainly in a synanthropic cycle. Despite the generally low prevalence of Trichinella in Europe, a number of outbreaks have occurred since 1975 (e.g., in Germany, France, Italy, Spain, England, and Poland) affecting groups of persons of various sizes (the largest about 650). Worldwide, the annual incidences per 100 000 inhabitants (1991-2000) have varied widely, for example between 0.01 in Germany and the USA, 5.1 in Lithuania, and 11.4 in Bulgaria.

Morphology and life cycle (Fig. 10.18). Male Trichinella spiralis are approximately 1-2 mm long, the females 2-4 mm. A characteristic feature is the subdivided esophagus with a muscular anterior portion and a posterior part consisting of glandular cells ("stichocytes"). The other Trichinella species are of about the same length and do not show morphological differences, except T. pseudospiralis, T. papuae, and Trichinella zimbabwensis the muscle larvae of which do not encyst.

The life cycle described here refers to T. spiralis. Infection of humans and other hosts results from ingestion of raw or undercooked meat containing encysted Trichinella larvae (Fig. 10.16e). The larvae are released following exposure to the digestive juices, whereupon they invade epithelial cells in the small intestine, reaching sexual maturity within a few days after four moltings. The males soon die after copulation, the females live for about four to six weeks. Each female produces about 200-1500 larvae (each around 100 im long), which penetrate into the lamina propria. The larvae disperse

— Trichinella spiralis: Life Cycle

— Trichinella spiralis: Life Cycle

Trichinella Papuae

Fig.10.18 1 Male and female in the small intestine of a host; 2 larvae produced by female; 3 larvae penetrating muscle cell; 4 larvae encapsulated in musculature; 5 release of larvae from capsule following peroral ingestion by host; 6 infection of hosts with muscle larvae; 7 rodent hosts; 8 domestic animal hosts; 9 transmission of parasites to humans with trichinellous meat.

Fig.10.18 1 Male and female in the small intestine of a host; 2 larvae produced by female; 3 larvae penetrating muscle cell; 4 larvae encapsulated in musculature; 5 release of larvae from capsule following peroral ingestion by host; 6 infection of hosts with muscle larvae; 7 rodent hosts; 8 domestic animal hosts; 9 transmission of parasites to humans with trichinellous meat.

into organs and body tissues by means of lymphogenous and hematogenous migration. Further development occurs only in striated muscle cells that they reach five to seven days p.i. at the earliest.

The larvae penetrate into muscle fibers, which are normally not destroyed in the process, but transformed into "nurse" cells providing a suitable environment for the parasite. The muscle cell begins to encapsulate the parasite about two weeks p.i. by depositing hyaline and fibrous material within the sarcolemma. Encapsulation is completed after four to six weeks. The capsules are about 0.2-0.9 mm long with an oval form resembling a lemon. Granulation tissue or fat cells form at the poles (Fig. 10.16e). The capsule may also gradually calcify beginning at the poles.

The Trichinella larvae at first lie stretched out straight within the muscle cell, but by the third week p.i. they roll up into a spiral form (not observed in Trichinella pseudospiralis and some other species, see Table 10.5). They differentiate further during this period to become infective. The encapsulated Trichinella remain viable for years in the host (demonstrated for up to 31 years in humans). The developmental cycle is completed when infectious muscle Trichinella are ingested by a new host.

Epidemiology. In many countries trichinellosis exists in natural foci with sylvatic cycles involving wild animals, in particular carnivores. Such cycles are known to occur in most of the Trichinella species but T. spiralis is predominantly perpetuated in a synanthropic cycle (Table 10.5). Humans can acquire the infection from sylvatic cycles by eating undercooked meat of wild boar, bear etc. containing infective Trichinella larvae. Sylvatic cycles may remain restricted to natural foci without spreading to domestic pigs or other domestic animals. This is apparently the case with T. britovi in Switzerland, for example. Human infections are most frequently derived from the synanthropic cycle of T. spiralis.

Encapsulated muscle larvae are very resistant. They remain infectious for at least four months in rotting meat. Cooled to 2-4 °C, they survive in musculature for 300 days. They are generally killed by deep-freezing to -25 °C within 10-20 days, although muscle larvae of the cold-resistant species T. nativa may remain infective for many months at -20 °C (Table 10.5). Heat is rapidly lethal, but the larvae can survive drying and pickling.

The sources of human infection are raw and insufficiently cooked or frozen meat products from domestic pigs and wild boars, horses, and less frequently from bears, dogs, and other animal species. Dried and pickled meat containing trichinellae can also be infective.

Clinical manifestations. The severity and duration of clinical manifestations depend on the infective dose and the rate of reproduction of the trichinellae. As few as 50-70 T. spiralis larvae can cause disease in humans. The pathogenicity of the other species is apparently lower. Infections run a two-phase course:

Table 10.5 Trichinella species

Trichinella species and distribution

Cycle1 and important hosts (selection)

Characteristics of muscle larvae R: Resistance at -30 C, 12 h

T. spiralis Worldwide

Mainly synanthropic, also sylvatic. Domestic pig, rat, horse, wild boar, camel, dog, red fox, bear, humans

With capsule R: low

T. britovi

Temperate zone of the palaearctic region

Mainly sylvatic, also synanthropic. Red fox, wolf, jackal, raccoon dog, wildcat, bears, badger, marten, rodents, domestic pig, wild boar, horse, humans

With capsule R: moderate

T. murrelli Temperate zone in North America (US)

Sylvatic, also synanthropic. Bear, raccoon, red fox, coyote, bobcat (Felis rufus), horse, humans

With capsule R: low

T. nativa

Arctic and subarctic regions (north of the -6 °C January isotherm)

Mainly sylvatic, also synanthropic. Polar fox, red fox, polar bear, wolf, raccoon dog, jackal, dog, wild boar, humans

With capsule R: high

T. nelsoni

Sub-Saharan Africa, Asia (Kazakhstan)


Hyena, warthog, wild boar, domestic pig, humans

With capsule R: none

T. pseudospiralis Australia, India, Caucasus, Kazakhstan, US


Quoll (Dasyuridae), raccoon, korsak (steppe fox), rodents, bird species, (birds of prey and others), humans

Without capsule R: none

Trichinella papuae Papua New Guinea


Wild boar, domestic pig, humans

Without capsule R: ?

Trichinella zimbabwensis Zimbabwe

Cycle in farmed crocodiles Experimental hosts: pig, rat, monkey

Without capsule R: ?

1 Animals involved: synanthropic cycle: animals living in proximity to human dwellings (domestic animals, rats etc.); sylvatic cycle: wild animals.

■ Intestinal phase: incubation period of one to seven days. Symptoms: nausea, vomiting, gastrointestinal disorders with diarrhea, mild fever, and other symptoms. An inapparent course is also possible.

■ Extraintestinal phase: incubation period of seven or more days. Symptoms caused by invasion of body tissues by Trichinella larvae: myositis with muscle pain and stiffness, respiratory and swallowing difficulties, fever, edemas on eyelids and face, cutaneous exanthema. Feared complications include mycocarditis and meningoencephalitis. Further characteristic features are blood eosinophilia, raised activity of serum lactate dehydrogenase, myo-kinase and creatine phosphokinase, and creatinuria. This phase lasts about one to six weeks. It is frequently followed by recovery, but rheumatoid and other symptoms can also persist (e.g., cardiac muscle damage). Lethal outcome is rare.

Diagnosis. Diagnosis during the intestinal phase is difficult and only rarely trichinellae can be found in stool or duodenal fluid. During the extraintestinal phase, Trichinella larvae are detectable in muscle biopsies (either by microscopy in press preparations, histologically or by PCR-based DNA detection). Beginning in the third week p.i. serum antibodies appear (Table 11.5, p. 626). Clinical chemistry (see above) furnishes further diagnostic data.

Therapy and prevention. The recommended drugs for therapy are meben-dazole or albendazole in combination with prednisolone (to alleviate allergic and inflammatory reactions) (WHO, 1995). Heat exceeding 80 °C kills trichi-nellae in meat. The safest methods are to boil or fry the meat sufficiently (deep-freezing may be unreliable, see above!). Important disease control measures include prophylactic inspection of domestic and wild animal meat for Trichinella infection and not feeding raw meat wastes to pig livestock and other susceptible domestic animals.

Infections Caused by Nematodal Larvae

■ Larva migrans externa and Larva migrans interna are diseases caused by migrating nematode larvae. The first (externa) is a skin disease, usually caused by zoonotic hookworms or Strongyloides species. In the second type (interna), nematode larvae migrate into inner organs, e.g., in toxocarosis. In toxocarosis the infection results from peroral ingestion of infective eggs of roundworms of the genus Toxocara that are released to the environment in the feces of dogs, foxes, and cats (infection risk for children on contaminated playgrounds!). In humans, migrating Toxocara larvae can cause damage to liver, lungs, CNS, and eyes. Additional diseases caused by nematode larvae include anisakiosis, angiostrongylosis, and dirofilariosis. ■

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