MORBIDITY: +

MORTALITY: +

ETIOLOGY: PARASITIC

Trichinosis is caused by the parasitic roundworm Trichinella spp. In North America the three most common species involved are T. spiralis (found in pigs and rats), T. murrelli (found in wild game in temperate regions), and T. nativa (found in cold climate–adapted animals such as fox, wolf, walrus, and bear).

Adult Trichinella worms mate in the small intestine of a host. Instead of hatching from eggs, the young parasite develops in the adult female parasite until it reaches the larval stage; these larvae are then passed by the adult female. The male dies after mating; the female dies after passing larvae. Both are then passed in the feces. The larvae penetrate the intestinal wall, enter the lymph and blood systems, and are carried to various tissues throughout the body, including the striated muscles (meat). In the muscles larvae curl up, and an individual cyst is formed around each larva. In the cysts the larvae lie dormant until they are liberated by gastric juices when the meat is eaten by another host. Once the larvae are freed from their cysts by the digestion process, they are carried to the small intestine of the new host, where they mature to adults, mate, produce more larvae, and die.

If the meat containing encysted larvae is never eaten, the larvae will eventually die, and the cysts will become calcified. This is what happens in people and in animals that are not on the menu for human or animal consumption. Encysted larvae in pigs have been known to live 11 years.

HOSTS

Carnivores and omnivores are most susceptible to developing trichinosis because they are most apt to eat infected meat. Herbivores, such as horses, can become infected if they eat feed that contains scraps of uncooked, infected meat. Some animals that can be sources of infection are pigs, wolves, ground squirrels, bears (grizzly, polar, and black), walruses, seals, whales, dogs, cats (wild and domestic), rats, foxes, and horses. In the past pigs were the primary sources of infection, because their diet consisted of uncooked garbage containing raw, infected meat. Today, packing plants are federally inspected, and it is against the law to feed raw garbage to pigs intended for market. Homegrown pigs fed raw garbage and never passing through a federally inspected packing plant can still be a source of infection.

TRANSMISSION

Most cases of trichinosis involve eating meat. Transmission occurs when an animal or a person eats raw or undercooked meat from an infected animal. The dormant larvae that were encysted in the muscle tissue (meat) are released from their cysts by stomach juices when they are ingested. From there they complete their life cycles and infect the muscles of the new animal or human host (Figure 47). In North America, people are usually dead-end hosts, as are some animals, such as dogs and horses, as they are not eaten on a regular basis.

FIGURE 47 Trichinosis.

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