Trichinella spiralis - trichinosis

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Trichinella spiralis

Taxonomy, Common Name, Disease

 

* CLASS: ADENOPHOREA

* SUBCLASS: ENOPLIA

* ORDER: STICHOSOMIDA (TRICHURATA)

* SUPERFAMILY: TRICHOCEPHALOIDEA

* FAMILY: TRICHINELLIDAE

 

Scientific name - Trichinella spiralis

 

Disease - trichinosis

Historical

Trichinosis in man is associated with the consumption of pork

products, particularly uncooked or insufficiently cooked pork. The

worm was first reported in man in 1835 and was first recorded in the

United States in 1846. It was demonstrated that Trichinella caused

serious disease in man in 1865. This nematode was first discovered by

a medical student in London who noted during dissection of a cadaver

that muscle tissue contained gritty particles which tended to dull a

scalpel.

Hosts

Man, pig, rats and many other mammals. The natural hosts are flesh

eating animals.

Distribution

The worm was at one time widely distributed in Europe and the United

States. It is thought to have originated in the northern hemisphere,

but now is known from some areas in South America and Africa. It

occurs in New Zealand but not in Australia.

Life Cycle

This nematode has no stages outside a host. The adult worms are found

attached to or buried in the mucosa of the duodenum. After mating the

males die. Females (ovoviviparous) produce living young (approximately

1,500 per female over a period of 4 to 16 weeks) and then die.

Juveniles enter the lymphatics and mesenteric veins and are found

throughout the arterial circulation between the 7th and 25th day after

infection. They travel in the hepatoportal system through the liver,

then to the heart, lungs, and the arterial system, which distributes

them throughout the body. They are transported to striated muscles,

penetrate individual fibers, and cysts are formed around the

juveniles. Within cysts, juveniles remain viable for many years, up to

25 years in man and 11 years in pigs. There is agreement that there

are four juvenile stages but disagreement in the literature as to

whether or not nematode development occurs within the cyst. When

viable encysted juveniles are ingested, they are digested from the

cysts and pass to the duodenum where they mature. Humans are

essentially a dead end for the parasites.

Symptoms-Pathogenicity

 

1. Nausea, dysentery, colic due to invasion by adult worms.

2. Migrating juveniles cause pain as they invade muscle tissue;

there may also be edema (swelling), delirium, cardiac and pulmonary

difficulty, pneumonia, nervous disorders, deafness and delayed or lost

reflexes.

3. Many cases are never diagnosed because of the vaugeness of the

symptoms. Muscle biopsy can be conducted and involves pressing muscle

between glass plates to look for cysts. Xenodiagnosis involves feeding

suspected muscle to laboratory rats.

 

Management

Prevention -

 

1. Cook pork thoroughly (also flesh of bear, walrus, wild pigs).

2. Cook all garbage fed to hogs.

3. Proper meat handling, ordinary curing and salting of pork

products will not kill encysted juveniles.

4. Freezing is effective if carried out properly. The freezing

requirements differ with the size of the meat. Pieces not exceeding 6

inches in thickness require 20 days at 5F, 10 days at -10F, 6 days at

-20F. Larger pieces require longer periods. Quick freezing and storage

for 2 days is effective.

 

Although there have been marked reductions in the percentage of hogs

carrying encysted juveniles in the United States, there is

considerable variation from one part of the United States to another.

It is therefore a wise precaution to avoid uncooked pork products. In

the United States Trichinella occurred in 0.4 to 2.5% of the prepared

pork products examined in some areas (1961); however some areas had as

much as 10% infection. In Chile 0.22% of 500,000 hogs were infected

(1964). In Germany only 0.00026% of 14,000,000 hogs were infected

(1965). It has been estimated that 3/4 of the persons infected with

Trichinella are from North America.

 

Treatment of Host: No cure. Relieve symptoms with analgesics and

corticosteroids.

Characteristics

Characteristics: The males measure 1.4 to 1.6 mm long and are more

slender at the anterior than the posterior end. The anus is nearly

terminal and has a large papilla on each side of it. A copulatory

spicule is absent. Stichocytes are arranged in a row following a short

muscular esophagus. Females are about twice the size of males, also

tapering toward the anterior end. The anus is nearly terminal. The

vulva is located near the middle of the esophagus, which is about a

third the length of the body. The single uterus is filled with

developing eggs in its posterior portion, whereas the anterior portion

contains fully developed, hatching juveniles.

References

 

1. Schmidt, G.S., and L.S. Roberts. 1989. Chapter 24. Orders

Trichurata and Dioctophymata: Aphasmidian Parasites. in Foundations of

Parasitology. Times Mirror/Mosby College Publishing. St. Louis. 750 pages.

2. Chandler, A.C. and C.P. Read. 1961. Chapter 18. Trichuris,

Trichinella, and Their Allies. Introduction to Parasitology. John

Wiley & Sons, Inc. New York. 822 pages.

3. Noble, E.R. and G.A. Noble. 1973. Parasitology The Biology of

Animal Parasites. Lea & Febiger, Philadelphia. 617 pages.

4. Georgi, J.R. and M.E. Georgi. 1990. Parasitology for

Veterinarians. W.B. Saunders Company. Philadelphia. 411 pages.