Trichinella is a genus of parasitic roundworms that affects mainly pigs, but also dogs, cats, horses, and numerous wild mammals (foxes, wolves, bears, wild boars, raccoons, etc.). It is also a human parasite. It is found worldwide but with varying prevalence depending on the countries. As a general rule prevalence is higher in less developed regions with predominant traditional small-scale pig farming.
Trichinella spiralis is the most frequent species on pig and other domestic animals worldwide, but other species such as Trichinella britovi and Trichinella nativa can have regional importance.
The disease caused by Trichinella worms is called trichinosis, trichiniasis or trichinellosis.
Are animals infected with Trichinella worms contagious for humans?
- YES, but not through simple contact (with hair, feces, etc.), but through ingestion of contaminated and insufficiently cooked meat or other tissues. For additional information read the chapter below on prevention and control.
Final location of Trichinella worms
Predilection site of adult Trichinella worms is the small intestine. Larvae are found mainly in the muscles.
Anatomy of Trichinella worms
Adult Trichinella worms are among the smallest parasitic roundworms. Females are not longer than 6 mm, males only half this size. As most roundworms, their body is covered with a cuticle, which is flexible but rather tough. The worms have no external signs of segmentation. They have a tubular digestive system with two openings, the mouth and the anus. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. The female ovaries are large and the uteri end in an opening called the vulva.
Trichinella females are viviparous, i.e. they do not lay eggs, but already developed L1-larvae, a total of about 2'000 during their short life span.
Life cycle of Trichinella worms
Trichinella worms have a direct life cycle, whereby it is one of the few species that can complete its life cycle without spending part of its development in the environment. Adult females in the small intestine of a final host release several thousand larvae (about 100 micrometers long) that penetrate into the gut's wall and reach the blood stream or the lymphatic system. They are passively carried forward to the muscles, where they encapsulate. They prefer blood-rich skeletal muscles, but may settle also in the tongue, the heart, the diaphragm or the eyes. In such capsules the affected muscle cell becomes a nurse cell for the larva: it nourishes and keeps it alive for years. In fact, it is a very special case of a multicellular organism living inside a single cell of another organism. After egg laying, the adult females are shed out with the feces of the host and die.
The encapsulate larvae remain there until the host is eaten by another potential host. In its stomach the nurse cells are digested, the larvae are released, migrate to the small intestine, burrow into its wall, complete development to adults and reproduce.
Harm caused by Trichinella infections, symptoms and diagnosis
Migrating larvae irritate the affected tissues and organs. Infections in pigs can cause unspecific signs such as fever, diarrhea, muscular pain and edema, but are usually rather benign. In dogs and cats most infections are benign and remain undiagnosed. In affected pigs larvae are particularly numerous in the jaw muscles and the diaphragm.
In dogs and cats most infections are benign and remain undiagnosed.
Human infections are substantially more dangerous. Fever, abdominal pain, nausea, vomiting and sweating have been reported. Other symptoms depend on the affected organs. If essential organs are effected (heart, respiratory muscles, brain) fatalities are possible in less than 2 weeks after infection.
Diagnosis through fecal or urine examinations is not possible, because infected hosts shed neither eggs nor larvae. Diagnosis in pigs is usually done after slaughter through microscopic examination of meat samples. Immunoassays (e.g. ELISA) are also available for early detection in humans and pigs.
Best prevention in endemic regions is to thoroughly cook whatever food of animal origin is fed to the pigs, dogs and cats. They should not be allowed to scavenge or hunt for whatever animals that could be infected (e.g. rats and mice).
To avoid transmission to humans, pigs slaughtered in abattoirs are routinely examined for Trichinella infections almost worldwide, and infected carcasses are confiscated. In rural regions, if home slaughter is practiced, samples of meat should be brought to a veterinarian for adequate examination. If this is not possible pork, chicken and whatever meat must be cooked at least at ~80°C: wait 3 minutes before eating. Freezing below -25°C during at least 20 days in pieces smaller than 15 cm thick shoul kill Trichinella larvae in pork, but not in meat of wild animals infected with otherTrichinella species. Unfortunately curing (salting), drying, smoking, or microwaving pork or other meat does not consistently kill infective worms.
The use of anthelmintics against Trichinella infections is seldom indicated in animals. Some classic anthelmintics (e.g. benzimidazoles such as albendazole or mebendazole, etc.) may kill adult worms in the intestine, but not the larvae in the muscles or other organs. Several macrocyclic lactones (e.g. doramectin, ivermectin, milbemycin oxime) have shown efficacy against Trichinella larvae in laboratory studies, but so far there are no commercial wormers approved for use against Trichinella infections on livestock or pets.
There are so far no true vaccines against Trichinella, neither for livestock, nor for pets. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Trichinella worms (i.e. using its natural enemies) is so far not feasible. Learn more about biological control of worms.
You may be interested in an article in this site on medicinal plants against external and internal parasites.
Resistance of Trichinella worms to anthelmintics
So far there are no reports on resistance of Trichinella worms to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it was not due to resistance but to incorrect use, or the product was unsuited for the control of these parasites. Incorrect use is the most frequent cause of failure of antiparasitic drugs.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Trichinella control.