Trichinella spiralis, a nematode parasite, is the cause of the disease trichinosis, known to be found in undercooked pork products. Trichinella are the smallest known parasites to infect humans and have a strange life cycle.
Small adult worms mature within the intestines of an intermediate host such as a pig. An adult female creates live larvae in batches that bore through the wall of the intestines. The larvae the enter the blood and lymphatic system feeding off of it until taken to striated muscles. Within the muscles, the larvae encyst, where they become enclosed inside a capsule. A human is infected by eating the infected pork. Wild carnivores are often known to carry the parasite as well.
The life cycle of Trichinella spiralis completes all stages of development inside of one host, which is known as a direct life cycle. Small larval forms are encapsulated inside a small cystic structure within the host. When humans eat infected meat, the nurse cell releases the larvae which go straight to the intestines. After larvae have attached to the intestines, they switch from an anaerobic metabolism to an aerobic metabolism. It is at this time, the adult life cycle starts where the Trichinella burrow in the intestinal walls, mature, and then reproduce.
Female Trichinella have a life span of roughly six weeks and may produce over 1500 larvae. When the female dies, she is eliminated through rest of the digestive system. The larvae then begin to spread and run their migration to the striated muscle tissue. In cases of infection, fever and inflammatory pain will occur.
Nurse cell formation occurs in the skeletal muscle tissue where they thrive due to a hypoxic environment. This environment causes a stimulation of surrounding cells to secrete cytokines allowing the Trichinella to form nurse cells. Typically the cytokine VEGF is released assisting the parasite cells to thrive and mutate into adults, restarting the life cycle.
A male Trichenella spiralis is usually about 1.5mm long and have anterior flat bodies. Females are generally about twice the size of males at 3mm in length. The male has pseudobursa on each side of its body to copulate with the female. The female holds developing eggs in posterior portion of the body, while the anterior portion of her body contains fully developed larvae waiting to be released.
Symptoms of a trichinosis infection occur between 12 hours and 2 days after being ingested. The migration portion of the life cycle causes damage to the host's body tissues. The worms located inside the intestines release waste that is poisonous to the host and incites an immune response. After 5 to 7 days, facial edema. After 10 days, the host experiences extreme muscle pain, breathing difficulties, a slower pulse, and lower blood pressure. Heart and nervous system damage can occur, usually resulting in fatal heart failure, respiratory problems, or kidney damage. The average number of trichinosis deaths is quite low. There were no reported deaths from 1997-2001. The FDA is credited with stiff enforcement of swine inspection, resulting in the low number of infections.