By Laura L. Rayburn
Trichinosis is a disease acquired by eating raw or undercooked meat that contains the encysted larvae of the microscopic parasitic enteric nematode (roundworm) Trichinella spiralis. Alternate names include trichiniasis and trichinellosis.(9) Nematodes are unsegmented worms with cylindrical bodies, often narrowing at each end. The name nematode is from the Greek meaning “threadlike”. There are over 80,000 different described species, with over 15,000 of them being parasitic. Nematodes are found in salt and fresh water, on land and even in some vinegars and beer malts.(12) T. spiralis occurs in many species of carnivores and omnivores and is found worldwide except for some Pacific Islands and Australia, interestingly, it is found in Australia’s neighbor, New Zealand.(6,3) It is estimated that 2% of the world’s human population is infected with T. spiralis.
Carnivores/omnivores feed on infected rodents or meat from some other infected animal and pass it on in their tissues.(11) Some of the animal hosts that it has been found in are: pig (wild and domesticated), bear (including polar bear), horse, rat, walrus, seal, fox, wolf, hyena, jackal and wild felines such as cougars, lions and leopards.(1,2,5,9,10) Human to human transmission of the disease has not been proven.(5) Rodents are mainly responsible for maintaining the endemicity of trichinosis.(11)
Historically, infection with T. spiralis in man is associated mainly with the consumption of raw and undercooked pork. However, many recent cases have been connected to the eating of undercooked flesh-eating wild game, such as wild boar, bear and walrus.(1,3,10) The first report of T. spiralis in a human was in 1835 by a medical student in London who noted that during a cadaver dissection his scalpel blades were dulled by “gritty particles” in the corpse’s muscle tissue. The first US recorded case was in 1846, and by 1865 trichinosis was considered a serious disease.(3)
Infection by this metazoan parasite was once very common in the US, but now is relatively rare due to legislation prohibiting the feeding of raw meat garbage to commercially raised pigs and public awareness of eating raw or undercooked pork.(2) It is interesting to note that raw pork products that carry the United States Department of Agriculture (USDA) seal of having passed inspection are not inspected for infestation of the T. spiralis larvae.(6) Despite the fact that the USDA does not inspect pork for this parasite only 38 cases a year on average were reported during a five-year span from 1991-1996.(10) That number dropped dramatically from 1997-2001, with an average of 12 cases per year reported.(2) It is believed that many infections are underreported in the US, because patients with only a slight infection may be asymptomatic and trichinosis has a low mortality rate.(7) In Thailand there are between 200 and 600 cases of trichinosis reported around the time of the Thai New Year due to consumption of more pork at that time. The World Health Organization (WHO) has found that in Eastern Europe, swine herds have T. sprialis infection rates of more than 50%, and that there are correspondingly large number of human infections.(14)
T. spiralis has no stages in it’s development outside of it’s host.(3) The life cycle begins when meat (usually muscle) containing a viable larvae encased inside a cyst wall, known as a “nurse cell”, is eaten.(7) Gastric acid and pepsin inside the stomach dissolve the cyst wall, releasing the larvae. The larvae grow and reproduce in the host’s small intestine.(6) They attach to and penetrate the mucosa of the duodenum at the base of the villi. After 30-36 hours they mature and begin mating, after which the males die. The female parasite gives birth up to 1500 live newborn larvae about 5 days from the beginning of the infection until 4-16 weeks later, when she dies and is excreted out of the host’s body.(3,7) The juvenile larvae enter the lymphatic and mesenteric veins and travel in the hepatoportal system which transports them to the liver, heart, lungs. They then enter the arterial system which distributes them bodywide where they migrate to well-vascularized striated skeletal muscle.(3,7) The juvenile larvae can be found in arterial blood in weeks 1-4 of the infection.(3)
T. spiralis prefers to locate itself in the most metabolically active muscle groups. These include; the tongue, diaphragm, masseter (cheek) muscles, intercostals, pectorals, deltoid, gluteus, biceps, gastrocnemius and extraocular muscles. They do migrate to other tissue as well, such as the myocardium and the brain, but cannot live within that tissue and soon die, causing intense inflammation, then are reabsorbed.(7) Within the striated muscle fibers the larvae coil and develop the “nurse cell” or cyst wall around them. They will continue to grow until they increase their original size 10-fold. This cycle takes from 17-21 days.(7) Within these nurse cells the juvenile larvae may remain viable for up to 25 years in man and 11 years in the pig. Sooner or later the encysted larvae will calcify and die. Humans are the end of the life cycle for the parasite, unless an infected person is cannibalized.(3,7)
As an intracellular parasite, T. spiralis secretes protein kinases and phosphatases, endonucleases, and DNA-binding proteins which interfere with the host’s muscle cell differentiation. Other virulence factors
involved in the infectious process are serine protease, chitinase and collagenase which help make up the outer layer of the “nurse cell”.(13)
There are two stages or phases associated with the symptoms of trichinosis in humans. The first stage/phase is the intestinal phase that begins 1-2 days after eating the infected meat. This stage typically lasts up to a week, but may persist for several weeks.(6,7) The severity of the symptoms depends on the number of larvae ingested and the immune status of the host.(1,2,6) These symptoms include:
· Abdominal cramps
· Slight Fever
The second phase is the invasive phase during which the larvae migrate form the duodenum into the circulatory system and finally into the various muscles of the body. (8) This migration can trigger a diffuse inflammatory and allergic response. There can be many different symptoms associated with this stage because not only are muscles being invaded, so too can be many organs such as the heart and brain.(6,8) The duration of these symptoms may last from weeks to months.(7) Symptoms from the muscle and circulatory system invasion of the larvae are:
· Muscle pain (especially when breathing, chewing or using large muscles)
· High fever (up to 104 degrees is not uncommon)
· Profuse sweating
· Facial swelling
· Skin rash
· Abnormal fear of light (photo phobia)
· Muscle spasm
· Difficulty breathing and/or swallowing
· Cough and hoarseness
· Weakness and tiredness
Symptoms associated with the invasion of the heart and CNS during the second phase are:
· Congestive heart failure
· Delayed reflexes
· Vascular thrombosis
· EKG changes
Many cases of trichinosis are never diagnosed due to the vague and varied symptoms the parasite can cause, and unlike most other worm infections a microscopic exam of the patient’s stool will not reveal an infection.(3,8) Serum tests for antibodies will not usually return a positive result until 2-3 weeks after the onset of symptoms.(8) Usually trichinosis is suggested by the patient’s history of eating raw/uncooked pork or flesh-eating wild game (bear, wild pig, etc..) and an elevated level of eosinophils.(6,8) Other tests that facilitate diagnosing a T. spiralis infections are: the bentonite flocculation test (90% accuracy), ELISA latex agglutination, fluorescent antibody (after 3 weeks)and complement fixation tests.(6) Definitive diagnosis is achieved through the biopsy of a striated muscle where it inserts into a tendon, because this is a site where the larvae tend to concentrate. The biopsy should be taken after the second week of infection, when the larvae have had time to migrate to the muscle fibers.(6,8) The muscle biopsy should be sliced thinly and compressed between two microscope slides in order to find the encysted larvae. Also, some of the muscle biopsy can be soaked in pepsin and hydrochloric acid to free any viable larvae from their nurse cells, this too is examined microscopically.(6) Xenodiagnosis may be performed by feeding some of the suspect muscle to laboratory rats and then checking them for infestation within a few days to weeks.(3)
If an infection is diagnosed then the medications that are given are mebendazole or albendasole, taken by mouth, to purge the parasites from the intestine, and analgesics and corticosteroids to help with the pain and inflammation in severe infections.(8)
The best way to prevent trichinosis is to cook all fresh pork and wild game meat thoroughly. The meat should be cooked to at least 170 degrees F., until the juices run clear. Meat grinders should be cleaned thoroughly and pork and wild game should be ground last if processing other meats. If you raise your own pigs for consumption, then make sure that all meat fed to them has been cooked, do not allow them to eat uncooked carcasses of other animals, especially rats which are one of the main hosts of T. spiralis. Freezing meat at -13 degrees F. for 10 days or longer will also kill T. spiralis, but not some of the cold resistant strains found in some Arctic wildlife.(2,10,5)
1. ANON, “Trichinellosis”, Canadian Food Inspection Agency, June 6, 2006, http://www.inspection.gc.ca/english/anima/heasan/disemala/trich/trichfse.shtml accessed February 9, 2008.
2. ANON, “Trichinellosis” CDC Fact Sheet, July 15, 2004, http://www.cdc.gov/ncidod/dpd/parasites/trichinosis/factsht_trichinosis.htm, accessed February 9, 2008.
3. 3.ANON, “Trichinella spiralis” UC Davis, Dept. of Nematology, no date, http://ucdnema.ucdavis.edu/imagemap/nemmap/Ent156html/nemas/trichinellaspiralis, accessed February 16, 2008.
4. ANON, “Diseases and Conditions-Myocarditis", Johns Hopkins Heart Institute, 2007 http://www.hopkinsheart.org/diseases/mycarditis.html, accessed February 9, 2008
5. 5. ANON, “Trichinosis” New York State Dept. of Health, June 2004, http://www.health.state.ny.us/diseases/communicable/trichinosis/fact_sheet.htm, accessed February 9, 2008.
6. 6.Kelsey, D.S., “Enteric Nematodes of Lower Animals Transmitted to Humans: Zoonoses”, 2001, http://gsbs.utmb.edu/microbook//ch091.htm, accessed February 7, 2008.
7. 7. Murray, C., “Trichinosis” eMedicine website, May 10, 2007, http://www.emedicine.com/Med/topic2306.htm, accessed February 7, 2008.
8. 8. 8. Pearson, R., “Trichinosis” Merck Manual Home Edition, March 2007, http://www.merck.com/mmhe/print/sec17/ch196/ch196s.html, accessed February 9, 2008.
9. 9. Smith, S., “Medical Encyclopedia: Trichinosis” MedlinePlus website, October 9, 2006, http://www.nlm.nih.gov/medlineplus/print/ency/article/000631.htm, accessed February 9, 2008.
10. 10.ANON, “Trichinosis” Texas Dept. of State Health Services, October 1, 2005, http://www.dshs.state.tx.us/idcu/disease/trichinosis/faqs/, accessed February 24, 2008.
11. 11. Hunt, R., “PARASITOLOGY-CHAPTER FOUR NEMATODES (Round Worms)”, University of South Carolina School of Medicine, September 20, 2007
http://pathmicro.med.sc.edu/parasitology/nematodes.htm, accessed February 24,2008.
12. 12. ANON, “Nematodes”, Answers.com health, 2008, http://www.answers.com/topic/nematode?cat=health, accessed March 2, 2008.
13. Dzik, J., “Molecules Released by Helminth Parasites Involved in Host Colonization”, November 22, 2005, http://www.actabp.pl/pdf/1_2006/33s.pdf, accessed March 2, 2008.
14. 14. ANON, “Trichinosis”, Answers.com health 2008 http://www.answers.com/topic/trichinosis?cat=health, accessed March 2, 2008