Trichinella spiralis - Introduction, Classification, History, Habitat, Morphology

Introduction to Trichinella spiralis

Trichinella spiralis, commonly known as trichina worm or pork worm, is a parasite that occurs in pigs (primary host), rodents, bears, hyenas, and humans. This nematode causes a zoonotic infection in humans called trichinellosis.

The parasite is typically present in undercooked pork products and must not be confused with pork tapeworm- which is distantly related.

Classification of Trichinella spiralis

The classification of Trichinella spiralis is done as follows:

Kingdom: Animalia

Phylum: Nematoda

Class: Enoplea

Order: Trichocephalida

Family: Trichinellidae

Genus: Trichinella

Species: T. spiralis

History of Trichinella spiralis

Historically, in 1821, Tidemann in Germany first discovered the encysted larval stage of Trichinella spiralis in the muscles of an infected man.

Peacock and Own, in 1835, also discovered the parasite.

Habitat of Trichinella spiralis

Both the adult and larvae stages of the Trichinella spiralis parasite can be found in the same host. The adult form of the parasite inhabits the small intestine of men, rats, and pigs while the encysted larvae live in the striated muscles of these hosts.

Morphology of Trichinella spiralis

Trichinella spiralis is one of the smallest nematodes that can cause parasitic infection in man. Since a single host can harbor both the adult form and the larvae, the single host can act as the definitive host. Also, unlike other closely related parasites, there are no free-living stages of Trichinella spiralis.

Adult worm

• adult Trichinella spiralis are very small and white, but visible to the naked eye

Male

• male Trichinella spiralis measures about 1.5mm in length and 0.04mm in breadth

• a short esophagus leads to a stichosome

• absence of spicule and copulatory sheath

• adult male worms have conspicuous conical papillae in the tail end which is an identifying feature

• the male is short-lived as it dies immediately, within a week, after fertilization

• as the male body degenerates, it is rarely seen in the feces

Female

• female Trichinella spiralis are larger than the males- about twice the length

• measures 3 mm to 4 mm in length and 0.06 mm in breadth

• presence of a single uterus which is filled with developing eggs in the posterior region and fully developed and hatched larvae in the anterior end

• larviparous – produces only larvae but not eggs

• larvae are passed out in the feces

• during its lifetime of 16 weeks, one female can produce 1000 to 10,000 larvae

Larvae

• measures 100 μm in length and 6 μm in breadth

• remains coiled inside muscle cysts

• highly infectious – cyst with a single larva can cause Trichinella spiralis infection

• present only in the striated skeletal muscles but not in the cardiac or smooth muscles

• inside the muscle cysts, the larvae develop and sexually matures

• in 30 days, Trichinella spiralis larvae differentiates and reaches 10 times its original size – becomes 1 mm long

• can remain viable for years before dying and getting calcified