The Brief Case: an Infectious Hazard of Hunting

A 38-year-old previously healthy man residing in British Columbia (BC), Canada, presented to medical attention with a 5-day history of fever, chills, night sweats, diffuse myalgias, and arthralgias. He presented no recent travel history. Several months earlier, he had successfully harvested a black bear (Ursus americanus) in BC, and the gathered meat was divided into several pieces and conserved frozen for later consumption. Two days prior to symptom onset, he and four household members ingested meat from the bear which had been thawed and cooked as meatballs. Three other household members also fell ill in the same time frame, but with milder symptoms that did not prompt medical assessment. Several meatballs were subsequently found to have not been thoroughly cooked. Two days after ingestion, the patient noted vague abdominal discomfort and nausea. Eight days after ingestion, he reported intense fever and chills. He also developed mild headache, severe prostration, myalgia in proximal limb muscles, transient abdominal pain, and pink-tinged urine that had resolved at the time of presentation. He denied any vomiting, diarrhea, chest pain, shortness of breath, adenopathy, or rash. The fever lasted for 9 days total, manifesting primarily at night. On examination, his vital signs and physical exam findings were within normal limits. Complete blood cell count was notable for a mildly increased white blood cell count (10.4*10⁹/L; normal range 4.0 to 10.0), and hypereosinophilia (3.3*10⁹/L; normal range 0 to 0.50). Aspartate aminotransferase (AST) (61U/L; normal range 15 to 45), creatine kinase (762U/L; normal range 55 to 170), and C-reactive protein (64.6 mg/L; normal <10) were all elevated. Bilirubin, creatinine, and international normalized ratio (INR) were within normal range. SARS-CoV-2 RT-PCR respiratory testing was negative, as was HIV screening, and blood cultures at 5 days of incubation. Trichinella serology by commercial indirect enzyme immunoassay (ELISA) targeting the excretory-secretory (ES) antigen performed at the National Reference Center for Parasitology (Montreal, Canada) on a sample drawn 1 week after ingestion of the bear meat was negative. Given the clinical suspicion for Trichinella infection, empirical treatment with mebendazole (400 mg po TID) was initiated on day 12 of illness, for a total of 13 days. The diagnosis of acute trichinellosis was subsequently confirmed based on repeat serological testing performed at the same reference laboratory with the same methodology as initially performed 6 weeks after having consumed the harvested meat which demonstrated a high positive response (optical density [OD] of 3.57; reference high positive cut-off OD ≥ 1.20).

Further microbiological investigations were performed to confirm the source and genotype of the Trichinella isolate linked to the infection. A 570 g sample of leftover frozen meat from the black bear was retrieved, and submitted to the British Columbia Centre for Disease Control (BCCDC) Public Health Laboratory for microscopic examination of Trichinella larvae. Several processing methods were pursued to investigate comparative diagnostic yield. No larvae were observed by microscopy using the paddle blender (Stomacher) technique. Examination of undigested meat processed with the blender revealed encysted and coiled larvae (Fig. 1A), with increased yield noted after centrifugation at 500 × g for 10 min. Examination of digested meat processed by concentration via Baermann funnel and centrifugation of settled material yielded both excysted coiled and uncoiled larvae (measuring between 1.1 and 1.5 mm), with enhanced sensitivity and clarity (Fig. 1B). Additional testing of leg muscle from the black bear was performed at the Canadian Food Inspection Agency’s Centre for Food-borne and Animal Parasitology (Saskatoon, Canada). Pepsin/hydrochloric acid digestion assay of a 50 g sample demonstrated a substantial parasite burden of 138 Trichinella first-stage larvae per gram (lpg); all recovered larvae were motile (live, and presumably infective). Multiplex PCR performed on DNA extracted from five individual and 10 pooled larvae identified all as genotype Trichinella-T6 (1). The patient fully recovered after treatment.

Trichinellosis, or trichinosis, is caused by infection in humans with the nematode (roundworm) Trichinella spp., and is acquired by consumption of raw or undercooked meat harboring first-stage larvae of the parasite. In the stomach, the meat is digested and the larvae are released to invade the small bowel mucosa. There, the dioecious larvae rapidly develop into adult worms, and after approximately 1 week, females shed newborn larvae that migrate via the bloodstream to striated muscle sites where they establish as infective first-stage larvae in the human host. The adult worms die within a few weeks, and are shed with the host feces. Infected animals do not typically exhibit clinical signs, and all Trichinella taxa are considered zoonotic. For over a century, all Trichinella infections were considered to be caused by Trichinella spiralis, which is the species most infective to domestic swine. However, current taxonomic classification separates Trichinella into two major clades, encapsulated and nonencapsulated. The encapsulated clade infects only mammals and includes several species such as T. spiralis and T. nativa, in addition to three genotypes with unresolved taxonomies: T6 (as in this case), T8, and T9. In contrast, the nonencapsulated clade may also infect avian or reptilian hosts, and encompasses three species including T. pseudospiralis (2). Previously, outbreaks of trichinellosis in North America were commonly linked to consumption of pork infected with T. spiralis. However, regulatory changes to reduce risk and increase public awareness, and controlled management practices implemented for commercial swine production have essentially eradicated this parasite from domestic pork, and outbreaks of trichinellosis in North America are now most commonly linked to consumption of wild game. In Canada, most infections have been attributed to T. nativa which is distributed across Arctic and subartic regions, and have been related to consumption of bear and walrus (3). The T6 genotype is distributed across several regions in Canada and northern United States; T6 and T. nativa are the most common sylvatic taxa in Canada. Trichinella pseudospiralis, T. murrelli, and the recently described T. chanchalensis are the other sylvatic taxa reported to occur in Canadian wildlife (2, 4).

Acute symptomatic trichinellosis is characterized by gastrointestinal symptoms such as nausea, vomiting, abdominal discomfort, and diarrhea, with typical onset between 1 to 3 weeks after exposure. A larval burden of ≥1 lpg is generally accepted as the food safety threshold for clinical disease (4). The severity of clinical symptoms varies proportionally to the ingested parasite load, and shorter incubation periods may be observed as in the current case, possibly due to a high infective dose, the type of meat consumed, and/or the implicated Trichinella species or genotype (5). Over subsequent weeks, larval migration into tissues may trigger an immune reaction that includes fever, chills, periorbital edema, myalgia, and conjunctival or subungual hemorrhage. Peripheral eosinophilia is prominent and appears early on in the infection during the parasite tissue migration phase. Elevated lactate dehydrogenase and creatine kinase may also be observed. As presented earlier, due to the absence of Trichinella parasites in the stool, stool microscopy provides little utility in this diagnosis. Rather, the diagnosis is most commonly confirmed by serological testing. However, antibodies in primary infection in humans are typically not detectable until 2 weeks (high larval burden) to 5 weeks (low larval burden) after ingestion, which needs to be considered for result interpretation early in infection (Table 1). In such cases, and due to diagnostic testing delays from turnaround time of send-out testing, the degree of clinical suspicion should dictate management (5). In this case, seroconversion was documented 6 weeks after ingestion of contaminated meat, and was strongly positive, confirming the diagnosis. In a recent outbreak of T. nativa in Canada, performance of a commercial Trichinella serological test markedly changed based on timing of infection, with a sensitivity of 40% in acute illness, and 87.5% during the convalescence phase (6). Even with a positive result, caution is warranted as cross-reactivity may occur with other nematodes and thus confound interpretation. This limits the utility of serological testing in individuals with previous exposure especially in the context of travel. Furthermore, serological testing cannot differentiate Trichinella species and first-stage larvae of all taxa are morphologically indistinguishable; as such, molecular testing is required for further characterization, although tissue biopsies are seldom available or practical for this purpose. Indeed, muscle biopsy in suspected Trichinella cases is rarely indicated unless an alternative diagnosis such as an inflammatory myopathy is considered. In these cases, histopathology stains such as hematoxylin and eosin may demonstrate Trichinella larvae preferentially located in skeletal muscle. Infection with larvae of encapsulating taxa of Trichinella leads to the transformation of skeletal muscle myocytes into what is referred to as a nurse cell to highlight the host-derived (rather than parasitic) nature of the capsule. The nurse cell-parasite complex may be accompanied by a surrounding variable inflammatory infiltrate. In the absence of biopsy specimens, if the implicated meat source can be retrieved for testing, confirmatory digestion assay and PCR can be performed as in this case for diagnostic and genotyping purposes, which provides additional insight into the biology and epidemiology of the isolate of concern. Larval burden is quantified manually using a stereomicroscope after filtration and sedimentation steps to isolate and concentrate any larvae, and is reported in lpg. The identification of motile larvae implies viability, which supports the transmissibility, freeze tolerance, and genotype of the isolate. Compared to biopsy specimens, the typically much larger amounts of infected meat available for digestion testing enable a much higher sensitivity of detection to be achieved, which is particularly relevant for low larval burdens.

When a tissue biopsy sample is available, meticulous tissue processing is required to enhance the microscopic detection of Trichinella larvae. Several processing methods have been used for the recovery of larvae, including paddle blender (extraction of larvae from solid material through stirring and extrusion), blender (homogenization by rotating metal blade in contact with sample in peptone), and enzymatic digest of muscle tissue from homogenized material with overnight incubation at 37°C with pepsin. Similarly, the “gold standard” for testing muscle tissue of domestic animals and wildlife for Trichinella for food safety purposes is the magnetic stirrer pepsin/HCl digestion assay which will reliably detect larval burdens of ≥1 lpg in a 5 g sample (4). Examination of undigested material via compressorium (squash preparation) provides a faster turnaround time, and enables the in situ visualization of larvae, with those of the encapsulating taxa appearing encysted and coiled. In contrast, digested material requires longer processing, but provides higher diagnostic yield. The digestion process releases any encysted larvae from their capsules, and these “free” larvae may appear as coiled or uncoiled, as shown earlier. PCR, or molecular methods, can then be performed to determine the species/genotype of any isolate(s) recovered.

Most cases of trichinellosis are self-limited and can be managed with supportive therapy alone; however, individuals who develop systemic symptoms may be treated with antiparasitics (albendazole or mebendazole), and prednisone. Additional epidemiological and laboratory investigations may provide important complementary information, and inform future prevention efforts in communities at high risk of infection. Importantly, as illustrated in this case, freezing alone is typically ineffective to prevent infection from wild game meat due to inherent cold tolerance of larvae of sylvatic taxa occurring in northern regions such as Canada, and thorough cooking in addition to general prevention recommendations should be followed (4). Similarly, drying, smoking or curing the meat alone are generally considered insufficient to kill the organism and indeed have been documented as the cause of previous outbreaks. These prevention strategies include cooking game meat to an internal temperature of 71°C or above, thorough handwashing when handling raw meat, and sanitizing all areas that may have come into contact with raw meat (4). Focusing these prevention efforts in individuals most likely to be at risk (e.g., hunters) is particularly important.

In summary, eliciting an exposure history in individuals with acute symptoms that may be consistent with trichinellosis, such as consumption of wild game, is important to assess for possible infection, and to guide diagnostic testing, clinical management, and public health prevention efforts.

We thank the staff from the British Columbia Centre for Disease Control Parasitology and Environmental Microbiology Laboratories for their contribution to specimen processing and testing. We also thank staff from the National Reference Centre for Parasitology (Montreal, Canada) for performing the serological testing, and Kelly Konecsni at the CFIA Saskatoon Laboratory’s Centre for Food-borne and Animal Parasitology for performance of the digestion assay and mPCR.

Answer: d. Consumption of wild game

The main source of Trichinella outbreaks in North America is consumption of wild game. Indeed, individuals who hunt wild game and subsequently consume improperly cooked meat are at highest risk. Several animal species are known to harbor Trichinella, including bears (black, grizzly, polar), walruses, foxes, wolves, and wolverines.

Answer: b. Serology

Diagnosis of trichinellosis is most commonly confirmed by serological testing, which is more accessible than invasive biopsy sampling. However, testing is frequently negative early in infection prior to development of an antibody response. Thus, empirical treatment if indicated should be considered early on given this limitation. Stool O&P is not a useful modality for the diagnosis of Trichinella.

Answer: b

Thorough cooking to an internal temperature of 71°C or above is required for all hunted wild game meat, and is the most important strategy to prevent acquisition of trichinellosis. Freezing alone, including flash-freezing, is typically ineffective to prevent infection from wild game meat due to larval resistance to cold. Meat should not be sampled until it is properly cooked. Finally, thorough handwashing should be used when handling all meat, especially when raw or at risk of improper cooking.