Unlike AIDS or Malaria, Trichinosis and Trichinella spiralis, the etiological agent of Trichinosis, no longer make the front cover of immunology or parasitology journals.  However, this lack of popularity does not mean that Trichinosis is a disease of the past or that Trichinella spp. are parasites not worth researching.  Approximately, 11 million individuals are infected with Trichinella spp. [1] Incredibly, once Trichinella spiralis larvae invade a muscle cell, they have the ability to induce angiogenesis in the muscle cells they reside in. [2] While most people shiver at the mention of avian flu or hemorrhagic fever, Trichinosis once was a disease that also inspired great fear. Trichinosis outbreaks sometimes had devastating effects on village populations. For instance, in Germany between 1860 and 1880 there were 8,491 cases of Trichinosis and in1865 in the small village of Hedersleden an outbreak had a mortality rate of 30%. [3]  

The history of the discovery of T. spiralis is just as fascinating as its biology. The history of the discovery is plagued with controversy and it involves a renowned doctor taking undue credit for its discovery and reveals the competitive nature of the scientific community.[3]  The discovery of this disease intertwined science and politics and allowed politicians to use health policy as a means to protect economic interests. For instance, by the early 1880’s the U.S. had become the world’s main pork exporter but since it was one of the few developed countries that did not inspect pork for Trichinella, most European countries used this as an excuse to set-up embargoes against American pork. [3] Then the biotechnological revolution of the 20th century led to the discovery that there are 8 different species of Trichinella. Therefore, Trichinosis is an interesting and important disease worthwhile studying from both a biological and historical lens. 



Trichinella spiralis is a parasitic worm and the etiological agent of Trichinosis.  [4] The only way to distinguish between species is to perform a biochemical analysis. [4] Molecular studies have found the greatest genetic variability for T. spiralis in East Asia, meaning that this species probably originated in this region.[4] Trichinella spp. belong to the phylum Nemathelminthes. Nematodes are more commonly known as roundworms because unlike the worms from the phylum plathyhelminthes nematodes are cylindrical and have a digestive tract.[5] Female nematodes tend to be larger than the males.[5] The worms that cause Trichinosis reside in the cells of skeletal muscle. [5]




“Trichinella larvae, freed from their cysts in the muscle tissue of an Alaskan bear.”

Source:Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved Retrieved February 22, 2009, from



Below is the taxonomy of Trichinella[4]:

Kingom: Animalia

Phylum: Nemathelminthes


Order: Trichocephalida

Family: Trichinellidae

Genus: Trichinella


-T. spiralis

-T. nativa

-T. britovi

-T. murrelli

-T. nelsoni

-T. pseudospiralis

-T. papuae

-T. zimbabwensis




Other names for Trichinosis include [1]:

-ICD9: 124
-ICD10: B75

History of Discovery



*London: On February 2, 1835 James Paget, a first year medical student, observes Trichinella spiralis while conducting a dissection on a cadaver. [3] The significance of this event is that he is the first to observe Trichinella and to microscopically analyze it.[3]


*Without informing Paget, Thomas Wormald sends Richard Owen a sample of the parasite from the same cadaver in which Paget had made the discovery. Owen then publishes a paper describing the parasite and calls it Trichina spiralis but does not give Paget the deserved credit. [3]


















James Paget.

Source: Sir James Paget, 1st Baronet. (2009). In Encyclopædia Britannica. Retrieved February 26, 2009, from Encyclopædia Britannica Online: 438256/Sir-James-Paget-1st-Baronet


*Philadelphia: Joseph Leidy finds Trichinella cysts in his pork dinner and hypothesizes that trichinosis is caused by consuming undercooked pork. [3] However, scientists disregard his hypothesis. [3]



*Germany: Rudolph Leuckart gives infected meat to mice and discovers that the ingested worms “had not only come out of their cysts into the gut of the mice, but also had become much bigger than they had been when encysted in the muscle of their former host.” [3]



*Germany: After dissecting a dog that had been fed  Trichinella infected tissue Rudolph Virchow finds that the Trichinella  larvae become adult nematodes that are not Trichuris. [3] Virchow hastily sends these findings to the Paris Academy of Sciences, but his messy handwriting delays the translation and publication of his letter. [3] Even though Virchow beats Leuckart to the experiment, Leuckart rushes to have the results from his own experiments with pigs published by the Paris Academy in September 1859.[3] However, his messy handwriting leads to the inaccurate reporting of the actual number of worms found in the pig’s intestine. [3] Leuckart’s paper claimed that Trichinella  became Trichuris.[3] In September 1859 Virchow’s paper is finally published and his findings contradict Leuckart’s hypothesis; after further experimentation Leuckart recognizes that Virchow is correct [3].



*Germany:  Friedrich Zenker performs autopsy on female servant who died of Trichinosis and recognizes the parasite [3]. Zenker’s contribution is “that he realized that the intestinal adults were the progenitors of the larvae in the muscle”. [3] He also discovers how Trichinosis is transmitted and established the pathogenicity of Trichenella spiralis. [3]

* Virchow, Zenker, and Leuckart made significant contributions to understanding the life cycle of T. spirela. Virchow discovered that the worms matured in the small intestine. [3] Zenker concluded that the work was parasitic and proposed that the larvae reached the muscles through the lymphatic system.[3] Leuckart discovered larvae in the uterus of the adult female worm. [3]



*Friedreich of Heidelberg is the first to perform a muscle biopsy to diagnose Trichinosis. [3]



*Name changed from Trichina spiralis to Trichinella spiralis. [3]



*T.R. Brown discovers that Eosinophilia is a clinical sign of Trichinosis. [3]



*The three Swedish explorers heading towards the North Pole might have died of Trichinosis from eating infected polar bear meat. [3]



*Kenya: Comparative infection studies reveal that there are different Trichinella species and that they each can only infect certain species of animals. [4]


History of Policy Concerning Meat Inspection and Trichinosis:

-1863: *Mandatory inspection of pigs in the Duchy of Brunswick. [3]




*Friedrich Küchenmeister’s Ueber die Nothwendigkeit und allgemeine Durchführung einer mikroskopischen Fleischbeschau (About the Need for and General Implementation of Microscopic Meat Inspection) is published. [3]



*Many European countries ban the importation of pork from the U.S. because trichinoscopy of slaughtered pig was not practice in the U.S. [3]



*President Harrison “approve[s] a bill for the introduction of trichinoscopy of export pork. This bill did not protect U.S. citizens consuming pork.” [3]



*“The incidence of Trichinella in swine in the U.S. in 1970 was higher than in Germany in 1870…” . [3]



* National Trichinae Certification Program (Pilot) is established in the U.S.


Clinical Presentation:

There are two main phases for the infection: enteral (affecting the intestines) and parenteral (outside the intestines).The symptoms vary depending on the phase, amount of encysted larvae ingested, age, gender, and host immunity.[6] An individual’s experience with symptoms can vary from having no symptoms to having symptoms that are “moderately severe.”[5] Symptoms also vary during the parenteral phase but eosinophilia (a high number of eosinophils) and fever are the most common symptoms.[5] Trichinosis can be fatal depending on the severity of the infection; death can occur 4-6 weeks after the infection.[5] Death is usually caused by myocarditis, encephalitis or pneumonia.[1]  Below is a table with the most common symptoms and signs observed and the time period and phase in which they are usually observed. 



                           Symptoms and Signs of Trichinosis

                      (Table based on Capo & Despommier 1996)

Weeks after Ingestion



% of people who develop symptom/sign

1 week


upper abdominal pain


1 week




1 week




1 week




1 week




2 weeks


myalgia (muscle pain)


2 weeks


paralysis state


2 weeks


perioribtal edema (accumlation of fluid around the eye)


2 weeks


facial edema (accumulation of fluid around the face)


2 weeks




2 weeks


skin rash


2 weeks


difficulty swallowing


2 weeks




2 weeks




2 weeks


weight loss


2 weeks


peripheral nerve sensation


2 weeks


hot flashes


2 weeks




2 weeks




2 weeks


splinter hemorrhages of the nail beds or retinae


2 weeks


visual disturbances


2 weeks


paralysis of ocular muscle


3 weeks (+)




3 weeks (+)




3 weeks (+)




3 weeks (+)







Wild carnivores and omnivores can become infected with Trichinella spp. when they eat carrion. [3] Humans become infected if they eat raw or undercooked meat from these infected animals. [3] Vertical transmission from mother to fetus has been reported.[6] Trichinella spp. are notorious for being able to infect a broad scope of animals including mammals, reptiles, and birds. Certain species of Trichinella can only infect a specific group of animals. For instance, T. pseudospiralis is the only species that can infect birds. [4] Only three Trichinella species are known to cause Trichinosis in humans: T. spiralis, T. nativa, and T. britovi. [4]



Trichinella spp. can infect mammals, birds, and reptiles; they affect carnivores, omnivores, and even marine mammals [1]. Swine are the best known reservoir and the main reservoir for T. spiralis. However, the main reservoirs vary across countries, for instance in Europe the red fox (Vulpes vulpes) tends to be the main reservoir [1]. However, the raccoon dog (Nyctereutes procyonoides) is the main reservoir in Finland and horsemeat tends to be a common source of infection.[1] T. nativa can be found in walruses and polar bears while T. britovi can infect foxes, dogs, cats, and raccoons [4]. The chart below indicates the animals that serve as a source of infection for humans and the table below shows a more detailed list of the reservoirs for Trichinella.



Distribution and Reservoirs of Trichinella spp. (Table based on Pozio& Murrell 2006)


Common Reservoirs

Geographic Distribution

T. spiralis

domestic pigs, wild boars , horses, and synathroopic rats

Europe, Egypt, China, Russia, Southeast Asia, Argentina, Chile, Mexico, New Zealand, U.S.


wildlife of arctic and subarctic including bears and walruses

Holarctic Region (Canada, Greenland, Alaska & New Hampshire, Byelorussia, Estonia, Finland, Latvia, Lithuania, Norway, Russia, Sweden, Siberia, China, Kazakhstan, Kyrgyzstan, and Tajikistan)

T. britovi

mustelids, viverridae, red foxes, jackals, wolves, brown bears

Palearctic region and Northern and Western Africa

T. murrelli

bob cat, black bear, coyote, raccoon, red fox, dog, cat, and horse

North America

T. nelsoni

hyena, jackal, bat-eared fox, domestic dog, lion, cheetah, bush pig and warthog

Eastern Africa (Kenya to South Africa)

T. pseudospiralis

birds, fox, Indian mole rat, and marsupials, wild boars

Palearctic, Nearctic, and Australia

T. papuae

sows, wild pigs, and crocodile

Papua New Guinea

T. zimbabwensis

crocodile and monitor lizard

Zimbabwe, Mozambique, Ethiopia




There are no known vectors for Trichinella spp.


Incubation Period

The incubation period is usually10 -20 days but can range from 1week -10weeks. [1] The duration period is a function of the number of larvae consumed. [6]



·         adult males: [3]


-length: 1.0-1.5mm

-width: 0.03 mm

            -smooth cuticle and exhibits pseudosegmentation

            -pair of flattened copulatory appendages and accessory papillae

            -copulatory bell

trichin2 male posterior.jpg

Trichinella spirals male posterior.”

Source:Dept. of Zoology, University of Manitoba (2000). “Trichinella spiralis.” Retrieved January 25, from


·         adult females: [3]


            -length: 2.5-3.5mm

            -width: 0.05mm

            - smooth cuticle and exhibits pseudosegmentation

            -no copulatory appendages and vulva is visible















Adult female of Trichinella spiralis.”

Source: Upton, S.J. “Adult female of Trichinella spiralis.” Online image. Kansas State University: Biology 625 Animal Parasitology. Retrived February 15, 2009, 625tutorials/ Trichinella01.html


trichin6 female.jpg







Trichinella spirals female posterior.”

Source:Dept. of Zoology, University of Manitoba (2000). “Trichinella spiralis.” Retrieved January 25, from


·         infective first stage larva: [3]

            -salmon colored

            -length: 1.0mm

            -width: 0.03mm

            -smooth cuticle and exhibits pseudosegmentation

            -no hypodermal glands

            -rounded ends, no appendages or projections




Global Perspective: Trichinella spp. are found worldwide. Approximately 11 million individuals are infected with Trichinella; Trichinella spiralis is the species responsible for most of these infections. [1] In 1993, there were 1,975 cases of trichinosis reported in Europe. [1]
China reports approximately 10,000 cases every year and is therefore the country with the highest numbers of reported cases. [5] In China, between 1964-1998 over 20,000 people were infected with Trichinosis and over 200 people died. [7] It is also important to keep in mind that major socio-political changes can produce conditions that favor the resurgence of Trichinella infections in swine and consequently humans. For instance, “the overthrow of the social and political structures in the 1990s” in Romania led to an increase in the incidence rate of trichinosis. [8] There is also a high incidence of Trichinosis among refugees from Southeast Asia. [5]


United States: The incidence of Trichinosis in the U.S. has decreased dramatically in the past century. For instance, in 1930, 1 out of every 6 persons in the U.S. had Trichinosis then by 1970 this incidence rate had decreased to1out every 25. [5] Between 1997-2001 there was an average of 12 cases per year. [9]



Life Cycle of Trichinella


Trichinella spp. is transmitted through two cycles: sylvatic and domestic. In the sylvatic cycle Trichinella spp. is transmitted through predator-prey interactions or scavengers eating carrion. [3] Predators and scavengers include foxes, bears, rats, and walruses. [3] Humans can become a dead end host in this cycle by consuming infected game meat. [3]


The domestic cycle involves humans, pigs, and rodents. Pigs become infected when they eat raw infected meat from rodents. Humans become infected when they eat raw or undercooked infected pork. After humans ingest the cysts from infected undercooked meat, pepsin and hydrochloric acid help free the larvae in the cysts into the small intestine [6]. The larvae then migrate to the small intestine and invade the columnar epithelial cells; the process of how the columnar cells are invaded is unknown. [3] In the small intestine, the larvae molt four times before becoming adults. [6] Thirty to 34 hours after the cysts were originally ingested, the adults mate and within five days to produce larvae. [6] The worms can only reproduce for a limited period of time because the immune system will eventually expel them from the small intestine. [6] Genetic studies with laboratory rats seem to indicate that the host’s genetic make-up can determine the duration of the intestinal phase and that “T-cell dependent antigen is necessary for protection against the intestinal phase of the infection”. [5] The larvae then use their piercing mouthpart called “sylet” to pass through the intestinal mucosa and enter the lymphatic vessels and then enter the bloodstream. [3] The larvae use the capillaries in striated muscle to arrive at their final destination: the muscle fiber cells. [5] It is believed that the larvae enter the muscle cells through mechanical means. [3] The muscle cell that a larva takes over is referred to as the nurse cell. In just three weeks the larvae induce dramatic changes in the muscle cells. [2] For instance, the larvae increase the size of the cell’s nucleus and create a “placenta” like structure around the muscle cell called a circulatory rete. [2] How can the larvae induce angiogenesis (formation of new blood vessels) around the muscle cell? It is hypothesized that the larvae’s genes activate certain genes of the host’s cell to induce these dramatic changes. [2] Because humans typically do not get eaten by other animals “humans are a parasitic dead end.” [2]


























Source: Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved January 26, 2009, body_Trichinellosis_il1.htm











“Trichinella sp. larvae encysted in muscle cell.”

Source: Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved February 22, 2009, body_Trichinellosis_il1.htm


Diagnostic Tests


Serological (blood) tests and skeletal muscle biopsy (2-4mm³) are the two main ways of diagnosing Trichinosis. [1] Eosinophilia is usually the earliest indicator of trichinosis.[6] Another indicator of being infected is a high level of muscle enzymes like creatinine phosphokinase. [6] Both means of testing were developed in the late 1800s. In 1898, Thomas R. Brown published an article in which he concluded the following: “[T]here is a marked increase in the percentage of eosinophilic cells in the blood in trichinosis. [T]his increase may be used as a diagnostic sign in this disease.”[10] A more modern but expensive means of diagnosis is the use of antibody detection technology such as enzyme-linked immunosorbent assays (ELISA). [6]
















Trichinella spiralis larvae in peripheral blood.”

Source: Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved Retrieved February 22, 2009, from




Management and Treatment

Corticosteroids can be administered but doctors have to keep in mind that corticosteroids suppress the immune system and thus can worsen the intestinal phase. [5] Mebendazole (200-400 mg three times a day for three days) or Albendazole (400 mg twice a day for 8-14 days) are used to treat trichinosis. [5] These drugs should not be given to pregnant women. [6] Mebendazole interferes with the parasites microtubule assemblage. This drug does not have severe side effects but “mild nausea, vomiting, diarrhea, and abdominal pain have been reported infrequently.” [11] Rare side effects, usually with high-dose therapy, are hypersensitivity reactions (rash, urticaria), agranulocytosis, alopecia, and elevation of liver enzymes.[11] Prednisolone (20-60 mg per day for the first few days) can be given to ease the side effects of inflammation[5]. Albendazole also inhibits microtubule formation and has “larvicidal effects”. [11] Albendazole does not normally cause severe side effects, but on occasion the following side effects have been reported: “Mild and transient epigastric distress, diarrhea, headache, nausea, dizziness, lassitude, and insomnia.” [11]



“Encysted larvae of Trichinella sp. in pressed muscle tissue.”

Source: Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved Retrieved February 22, 2009, from



Public Health and Prevention Strategies/Vaccines


Currently there are no vaccines for Trichinella spiralis. However, several mice studies aiming to produce vaccine candidates have yielded promising results. For instance, Dea-Ayuela et al. 2006 used extracts and excretory-secretory products from first stage larvae to produce an oral vaccine.[12] In order to prevent the gastric acids from dissolving the antigens before reaching the small intestine, scientists encapsulated the antigens in a microcapsule made of copolymers. This vaccine significantly increased CD4+ cells and increased antigen-specific serum IgGq and IgA, resulting in a statistically significant reduction in the average number of adult worms in the small intestine of mice. The significance of this approach is that if the white blood cells in the small intestine have been exposed to Trichinella antigens (through vaccination) then when an individual gets infected the immune system will expel the worms from the small intestine fast enough to prevent the female worms from releasing their larvae. Yuan Gu et al.2008 tested a DNA vaccine on mice which “induced a muscle larvae burden reduction in BALB/c mice by 29% in response to T. spiralis infection”.[13]  Researchers trying to develop a vaccine for Trichinella  have tried to using either “larval extracts, excretory-secretory antigen, DNA vaccine, or recombinant antigen protein.” [13]

The two most important ways of preventing Trichinosis are through legislative measures concerning meat production and increasing public awareness of properly cooking pork and game meat. [1] The Centers for Disease Control and Prevention (CDC) has several recommendations for preventing infection including freezing pork at low temperatures, fully cooking pork and wild game meat, and not feeding pigs raw meat. [14] For more information please visit the following website:

The United States Department of Agriculture (USDA) and Animal and Plant Health Inspection Service (APHIS) are responsible for the regulations concerning the importation of swine from foreign countries. The Foreign Origin Meat and Meat Products, Swine section covers swine meat (cooked, cured and dried, and fresh).  The USDA and APHIS developed the National Trichinae Certification Program. This is a voluntary “pre-harvest” program for U.S. swine producers “that will provide documentation of swine management practices” to reduce the incidence of Trichinella in swine.[15] The CDC reports that 0.013% of U.S. swine is infected with Trichinella. [15]



Useful web links


*For basic information about Trichinosis visit:

*For more information about the National Trichinae Certification Program visit:

*For more detailed information about the biology of Trichinosis visit:






[1] Gideon (1994). Gideon Informatics Inc. Retrieved January 31, 2009, from

[2] Comes, C. The art of being a parasite. (2005). Chicago, IL: The University of Chicago Press.


 [3] Campbell, W. C. (1983). Trichinella and Trichinosis. New York: Plenum Press.

[4]Pozio, E., & Murrell, D. K. (2006). Systematics and Epidemiology of Trichinella. Advances in Parasitology, 63, 368-439.

[5]John, D. T., & Petri Jr., W. A. (9th Ed.) (2006). Markell and Voge’s Medical Parasitology. St. Louis, MI: Elsevier Inc.

 [6] Capo, V. & Despommier, D. D. (1996). Clinical Aspects of Infection with Trichinella spp. Clinical Microbiology Reviews, 9, 47-54.

[7] Gu, Y., Li, J., Zhu, X., Yang, J., Li, Q., Liu, Z., Yu, S. & Li, Y. (2008). Trichinella spiralis: Characterization of phage-displayed specific epitopes and their protective immunity in BALB/c mice. Experimental Parasitology, 118, 66-74.


[8]Blaga, R., Durand, B., Antoniu, S., Gherman, C., Cretu, C.M., Cozma, V., Boireau, P. (2007). A dramatic increase in the incidence of Human trichinellosis in Romania over the past 25 years: impact of political changes and regional food habits. American Journal of Tropical Medicine and Hygiene, 983–986.


[9] Roy, S. L, lopez, a. S., Schantz, P.M., (2003). Trichinellosis Surveillance-United States, 1997-2001. Mortality and Morbidity Weekly Report. Center for Diseases Control and Prevention. 52(SS06), 1-8.


[10] Brown, T. R. (1898). Studies on trichinosis, with especial reference to the increase of the eosinophilic cells in the blood and muscle, the origin of these cells and their diagnostic importance. Clinical Laboratory of the Johns Hopkins University and Hospital.


[11] Rosenthal, P. J. (10th Ed.) (2007), Basic & Clinical Pharmacology, “Chapter 54. Clinical Pharmacology of the Anthelmintic Drugs.” The McGraw-Hill Companies, Inc, Retrieved February 19, 2009


[12] Dea-Ayuela, M. A., Iniguz, S.R., Fernandez, F.B. (2006). Vaccination of mice against intestinal Trichinella spiralis infections by oral administration of antigens microencapsulated

in methacrilic acid copolymers. Vaccine, 24, 2772–2780.


[13] Gu, Y., Li, J., Zhu, X., Yang, J., Li, Q., Liu, Z., Yu, S., Li, Y. (2008). Trichinella spiralis: Characterization of phage-displayed specific epitopes and their protective immunity in BALB/c mice. Experimental Parasitology, 118, 66–74.


[14] Centers for Disease Control and Prevention (2008). Trichinellosis Factsheet. Retrieved January 27, 2009,


[15] APHIS. USDA Animal and Plant Health Inspection Service APHIS - Veterinary Services. Retrieved February 11, 2009,


Picture references:


Centers for Disease Control and Prevention (2008). “Image Library: Trichinellosis.” Retrieved Retrieved February 22, 2009, from

Dept. of Zoology, University of Manitoba (2000). “Trichinella spiralis.” Retrieved January 25, from

Sir James Paget, 1st Baronet. (2009). In Encyclopædia Britannica. Retrieved February 26, 2009, from Encyclopædia Britannica Online: /438256/Sir-James-Paget-1st-Baronet

Upton, S.J.Adult female of Trichinella spiralis.” Online image. Kansas State University: Biology 625 Animal Parasitology. Retrieved February 15, 2009,