Trichinellosis (aka Trichinosis)

• Reporting criteria
• The disease and its epidemiology
• Case definition

• Laboratory testing
• Case investigation
• Disease control measures

• References
• Contact

What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency  

• All specimens with a positive test for Trichinella (tissue biopsy or serum antibodies) should be reported to public health within four (4) days of the positive test result.
  • All positive Trichinella laboratory results should be reported to public health, regardless of symptoms. Public health will make the determination of whether the case meets the case definition.
• Cases should be reported using EpiTrax Reportal, fax, or telephone to CDPHE or local health departments (telephone and fax numbers are listed at the end of this document).
• All potential foodborne, waterborne, and enteric disease outbreaks should be reported immediately to CDPHE or local health departments and entered into EpiTrax, even if the causative agent is not yet known.
  • In general, an “outbreak” is defined as an increase in the number of illnesses above what is normally expected among a specific population, within a given area, over a certain period of time.
  • Specifically, a foodborne disease outbreak occurs when two or more people get the same illness from the same contaminated food or drink.

Purpose of surveillance and reporting

• To identify cases for investigation and potential outbreaks
• To monitor trends in disease incidence

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Etiologic agent

Trichinellosis (synonymously, Trichinosis) is a condition caused by an infection with nematodes (roundworms) of the genus Trichinella. The most common species that infects people is Trichinella spiralis. However, there are several other species demonstrated to be human pathogens, including T. psudospiralis, T. nativa, T. nelsoni, T. britovi, and T. papuae, all of which are associated with unique reservoir species and have distinct geographic ranges, including the United States.

Clinical description

The clinical signs, symptoms, severity, and duration of trichinellosis can vary depending on host factors and the quantity of larvae ingested (i.e., infectious dose). Mild infections can be asymptomatic or mild and “flu-like.” Symptoms often start with nausea, diarrhea, fatigue and abdominal discomfort, which corresponds with ingested larvae invading the intestinal mucosa (enteral or intestinal phase of infection). After approximately 1-2 weeks, symptoms can progress to include headache, fever, chills, cough, swelling of the face and eyes (periorbital edema), aching joints and muscle pain, itchy skin, continued diarrhea, or constipation. These symptoms can persist for days to weeks and correspond to the migration of Trichinella larvae through the body, via the blood (hematogenous spread), where they eventually enter skeletal muscle (parenteral or muscle phase of infection). Once encysted in skeletal muscle, larvae can persist for months to years. Larval penetration and tissue migration can also cause immune-mediated inflammatory reactions and stimulate the development of eosinophilia. Severe infections may result in cardiac complications (e.g., myocarditis), impacts to the central nervous system (e.g., encephalitis), renal and/or pulmonary involvement, thromboembolic disease, and death. 

Reservoirs

Trichinella species infect a wide range of vertebrates worldwide. Prior to improvements in pork production practices and regulations, pork was considered a major source of infection. However, currently in the U.S., wild game, particularly bear, wild boar, moose, wildcat, fox, seal, and walrus, are recognized as important reservoir hosts of Trichinella.

Modes of transmission

People become infected with Trichinella by consuming raw or undercooked meat (typically wild game) that contains encysted larvae. Digestion releases the parasite larvae from the meat, then larvae penetrate the intestinal mucosa where they mature into adult worms. The adult worms mate and produce new larvae, which migrate via the bloodstream to skeletal muscle throughout the body.

Trichinellosis is not transmissible from person to person, via environmental sources like water, or from the feces of infected animals or people; someone must consume Trichinella larvae, either within raw or undercooked meat, or foods that have been contaminated with larvae from contact with raw/undercooked meats, to become infected. 

Incubation period

The enteral or intestinal phase typically occurs between 1 and 2 days following ingestion, and the parenteral or muscular phase typically occurs between 2 and 8 weeks following ingestion.  

Infectious period

Trichinellosis is not transmissible from person to person. 

Epidemiology

In the United States, meat contaminated with Trichinella larvae is typically not sourced from commercially farmed food animals, but rather sourced from carnivorous and omnivorous wild animals, most often from game meat, including bear, wild boar/hog, wildcat, fox, wolf, seal, or walrus. Homemade jerky and sausage made from the listed meats can also be sources of Trichinella larvae. 

Trichinellosis is a rare condition in the United States, and the incidence has steadily declined over the past several decades due to improvements in commercial swine production regulations and practices; 34 cases were reported nationally between 2017 and 2022. From the most recent CDC Trichinellosis Surveillance Report, using data gathered between 2008 and 2012, approximately half of all cases were outbreak-associated (47%), 26% (22 cases) were associated with pork products, and 54% (45 cases) were associated with non-pork products, primarily bear meat. 

In Colorado, there have been eight reported cases (five confirmed, three probable) since 1999. Of these, three cases were associated with outbreaks attributed to the consumption of wild game during hunting trips. Only two cases have been reported in Colorado since 2016. 

Recent outbreaks of trichinellosis in the United States have been associated with consumption of undercooked game meat. In Alaska, two separate outbreaks with five cases apiece were detected between 2016-2017, both with an identified vehicle of raw or undercooked walrus meat. In 2022, an outbreak with a high attack rate occurred at a family gathering in Minnesota where eight of nine attendees fell ill with trichinellosis after consuming kabobs made from the meat of a black bear sourced from Canada. Interestingly, two of the infected individuals did not consume the meat but became infected presumably from cross-contamination. Similarly, another outbreak was detected in 2023 in North Carolina after 10 of 34 people attending a group gathering fell ill with symptoms consistent with trichinellosis after being served undercooked bear meat.

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Clinical description

A disease caused by ingestion of Trichinella larvae, usually through consumption of Trichinella-containing meat — or food contaminated with such meat — that has been inadequately cooked prior to consumption. The disease has variable clinical manifestations. Common signs and symptoms among symptomatic people include eosinophilia, fever, myalgia, and periorbital edema.

Laboratory criteria for diagnosis

Human specimens

• Demonstration of Trichinella larvae in tissue obtained by biopsy, OR
• Positive serologic test for Trichinella

Food specimens

• Demonstration of Trichinella larvae in the food item (probable)

Epidemiologic linkage

People who shared the implicated meat/meal should be investigated and considered for case status as described below. 

Case classification

Confirmed

• A clinically compatible illness that is laboratory confirmed in the patient.

Probable

• A clinically compatible illness in a person who shared an epidemiologically implicated meal or ate an epidemiologically implicated meat product OR
• A clinically compatible illness in a person who consumed a meat product in which the parasite was demonstrated. 

Suspect

• Instances where there is no clinically compatible illness should be reported as suspect if the person shared an epidemiologically implicated meal, or ate an epidemiologically implicated meat product, or has a positive serologic test for trichinellosis (and no known prior history of Trichinella infection).

Comments

• Epidemiologically implicated meals or meat products are defined as a meal or meat product that was consumed by a person who subsequently developed a clinically compatible illness that was laboratory confirmed.
• Negative serologic results may not accurately reflect disease status if blood was drawn less than 3-4 weeks from symptom onset.
• Case investigators may request a copy of the lab results prior to beginning the investigation. Trichinellosis/trichinosis (caused by Trichinella spp.) is sometimes confused with the sexually transmitted illness trichomoniasis (caused by Trichomonas vaginalis) when reported by providers or labs. 

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• The Colorado State Public Health Laboratory (“State Lab”) provides testing services to support public health investigations.
• The State Lab may test clinical and/or environmental samples when determined to be necessary for public health investigations. These services vary by pathogen but may include:
  • Testing for individuals who have been restricted or excluded from work or child care by public health
  • Testing for outbreak investigations, with prior approval from CDPHE
  • Other testing services as discussed with CDPHE
• The State Lab maintains a test directory, guidance for submitting samples, and other helpful resources on their webpage.
  • Additional questions can be directed to the Lab Coordinators by email (cdphe_labcoordinators@state.co.us; preferred method) or phone 303-692-3069.

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Note: Occasionally, providers have incorrectly reported trichomoniasis (a common sexually transmitted infection caused by the parasite Trichomonas vaginalis) as trichinellosis. CDPHE recommends confirming the trichinellosis diagnosis with the clinical lab or health care provider before proceeding with a trichinellosis case investigation.

Interview all cases of confirmed, probable, and suspect (including epi-linked probable and suspect) cases of trichinellosis to determine: 

• Potential source of infection, and implement disease control measures, as appropriate
• If others are ill (i.e., could this be an outbreak?) 

Local health departments have primary responsibility for interviews of sporadic cases in their jurisdictions. CDPHE is available to assist with case investigations upon request through CDPHE’s Enteric Disease Interview Team (EDIT). Interviews should be conducted and appropriate disease control measures implemented as soon as reasonably possible after the case is reported.

Forms 

For single cases, use CDC’s Trichinellosis Surveillance Case Report for your interview, and enter the information into the case’s morbidity record (CMR) in EpiTrax. Interview all cases, regardless of specimen source (serum or tissue biopsy). At a minimum, collect information about symptoms, hospitalization, outcome, exposure history, international travel, and demographics for all cases. A provider call may be necessary to obtain additional clinical details. 

Identify and evaluate contacts

• Contacts of a case who have clinically compatible symptoms and who shared an epidemiologically implicated meal or meat product are epi-linked probable cases and are treated the same as cases with a positive lab test.
  • Complete a case interview with all epidemiologically linked probable cases.
  • Epi-linked probable cases who have been interviewed should be entered into EpiTrax as a new, probable trichinellosis morbidity record (CMR). Link this newly created CMR to the lab-confirmed case’s CMR under the “contacts” tab in EpiTrax.
• Refer symptomatic individuals who have not previously been tested to their health care provider for testing and appropriate medical care.
• If a common source of exposure is suspected (e.g., a shared meal or social event), create an outbreak record in EpiTrax, and notify CDPHE as soon as possible.  

Reported incidence is higher than usual/outbreak suspected

If the number of reported cases of trichinellosis in your jurisdiction is higher than usual, or if an outbreak is suspected, investigate to determine the source of infection. Notify CDPHE as soon as possible to discuss the potential testing of untested, symptomatic contacts and enter the outbreak into EpiTrax. CDPHE staff can assist local public health agencies to investigate outbreaks and determine a course of action to prevent further cases, and can coordinate surveillance of cases that cross county lines.

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Treatment

Most Trichinella infections are uncomplicated and self-limited, and symptoms may resolve within a few months without treatment. However, prompt use of antiparasitics during the intestinal phase of mild infections can reduce the production and subsequent spread of larvae. Once larvae are established in the skeletal muscle (usually by 3–4 weeks post-infection), antiparasitics may not eliminate the infection and associated symptoms. Antiparasitics for the treatment of trichinellosis include albendazole and mebendazole. In severe cases, prednisone (or other corticosteroids) may be administered concurrently with antiparasitics.

Supportive care, including analgesics and antipyretics, is appropriate for patients with mild disease who have already progressed beyond the intestinal phase of illness. 

Prophylaxis

Post-exposure prophylaxis (mebendazole), as prescribed by a health care provider, may be effective in preventing trichinellosis if given within six days of ingestion of the implicated meat product. 

Education

• Always wash hands thoroughly with soap and water for at least 20 seconds before eating or preparing food and after handling raw or undercooked meats.
• Cooking game meat to > 165° F is the only way to reliably kill Trichinella larvae and make meat safe for consumption.
  • Use a food thermometer to measure the internal temperature of cooked meat; do not sample meat until it is properly cooked.
  • Freezing pork less than six inches thick for 20 days at 5° F (-15° C) will kill any worms.
    • However, freezing wild game meats may not effectively kill all Trichinella larvae, as some Trichinella species are more freeze-resistant than others.
• Additionally, safe handling of meat (i.e., separating raw or undercooked meats and their juices from other foods) is important to prevent cross-contamination with Trichinella larvae.
• Curing (salting), drying, smoking, or microwaving meat alone does not consistently kill the larvae.
• Always clean meat grinders and other meat-processing equipment thoroughly after each use to help prevent the spread of Trichinella larvae and other foodborne diseases.
• To help prevent Trichinella infection in animals (and subsequent spread to humans), do not allow pigs to eat uncooked meat, scraps, or carcasses of any animals, including rats, which may be infected with Trichinella.

Managing special situations

Trichinella infections are not known to be transmitted from person to person. In general, once the case’s symptoms resolve (including the resolution of diarrhea for at least 24 hours), the case may return to work or school and no further follow up is necessary.

Environmental measures

• Implicated food items and food prepared by cases must be removed from the environment.
• A decision about testing suspect/implicated food items must be made in consultation with CDPHE.
  • The general policy of the State Lab and the Communicable Disease Branch is only to test food samples associated with outbreaks, not in single cases.
• If a commercial product is suspected, CDPHE Communicable Disease Branch will coordinate follow up with the CDPHE Division of Environmental Health and Sustainability and relevant outside agencies.

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References

Cash-Goldwasser S, Ortbahn D, Narayan M, et al. Outbreak of Human Trichinellosis — Arizona, Minnesota, and South Dakota, 2022. MMWR Morb Mortal Wkly Rep 2024;73:456–459. DOI: http://dx.doi.org/10.15585/mmwr.mm7320a2.

Centers for Disease Control and Prevention (CDC). (2015, January). Trichinellosis Surveillance - United States, 2008-2012. https://www.cdc.gov/mmwr/preview/mmwrhtml/ss6401a1.htm 

Centers for Disease Control and Prevention (CDC). (2024, March). Clinical Care of Trichinellosis. https://www.cdc.gov/trichinellosis/hcp/clinical-care/index.html 

Centers for Disease Control and Prevention (CDC). (2024, March). How to Prevent Trichinellosis. https://www.cdc.gov/trichinellosis/prevention/index.html 

Centers for Disease Control and Prevention (CDC). (2024, May). Clinical Overview of Trichinellosis. https://www.cdc.gov/trichinellosis/hcp/clinical-overview/index.html

Centers for Disease Control and Prevention (CDC). (2024, June). Trichinellosis. https://www.cdc.gov/dpdx/trichinellosis/index.html 

Faber M, Schink S, Mayer-Scholl A, Ziesch C, Schönfelder R, Wichmann-Schauer H, Stark K, Nöckler K. Outbreak of trichinellosis due to wild boar meat and evaluation of the effectiveness of post exposure prophylaxis, Germany, 2013. Clin Infect Dis. 2015 Jun 15;60(12):e98-e104. doi: 10.1093/cid/civ199. Epub 2015 Mar 13. PMID: 25770171.

Gowler CD, Lee N, Morrison T, Mears V, Williams C, Fleischauer A, Wilson E. Notes from the Field: Suspected Outbreak of Trichinellosis Associated with Undercooked Bear Meat - North Carolina, November 2023. MMWR Morb Mortal Wkly Rep. 2024 Oct 10;73(40):906-907. doi: 10.15585/mmwr.mm7340a4. PMID: 39388384; PMCID: PMC11466378.

Heymann DL, ed. Control of Communicable Diseases Manual, 21st Edition. Washington, DC, American Public Health Association, 2022.

National Notifiable Diseases Surveillance System. Annual Summary Data 2018-2022 Request. (n.d.). https://wonder.cdc.gov/nndss-annual-summary.html

Springer YP, Casillas S, Helfrich K, et al. Two Outbreaks of Trichinellosis Linked to Consumption of Walrus Meat — Alaska, 2016–2017. MMWR Morb Mortal Wkly Rep 2017;66:692–696. DOI: http://dx.doi.org/10.15585/mmwr.mm6626a3.

Wilson M, Schantz P, Nutman T, 2006. Molecular and immunological approaches to the diagnosis of parasitic infection. Detrick B, Hamilton RG, Folds JD, eds. Manual of Molecular and Clinical Laboratory Immunology. Washington, DC: American Society for Microbiology, 557-568.

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CDPHE Communicable Disease Branch

• Phone: 303-692-2700 or 800-866-2759
• Fax: 303-782-0338
• After hours: 303-370-9395

CDPHE Lab Coordinators: 

• Email (preferred): cdphe_labcoordinators@state.co.us  
• Phone: 303-692-3069

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