eMedicine Specialties > Emergency Medicine > Infectious Diseases

Trichinellosis/Trichinosis

Author: L Kristian Arnold, MD, Chief Medical Officer, ArLac Global Health Services, Lexington, MA, Medical Director, Boston Police Department, Assistant Professor (retired), Emergency Medicine, Boston University School of Medicine
Contributor Information and Disclosures

Updated: Apr 30, 2009

Introduction

Background

Trichinellosis also called trichinosis or trichiniasis (Trich from Greek thrix meaning hair) is an infection due to nematodes of the genus Trichinella, most commonly T spiralis. Through historical, paleopathologic, and, most recently, genomic studies, the complex intertwined history of humans, their food, and this worm has become better defined. With genomic evidence suggesting the presence of Trichinella as a distinct species since some time in the mid to latter Miocene period (around 20 million years ago) and diversification into now 11 distinct species, the co-evolution of mammals, humans, and worms has become more apparent.¹

The first scientific descriptions of human Trichinella infection were presented by James Paget (of Paget disease) and Sir Richard Owen, then assistant curator at the Royal College of Surgeons, from an autopsy of a man with “sandy diaphragm.” The initial life cycle was worked out by Rudolf Virchow and associates over the years 1850-1870.²

By the 1860s, trichinellosis was well-recognized as a disorder spread through infected pigs, leading to a cultural aversion to certain pork products, particularly German and Dutch sausage.³  In the last 3 decades of the 20^th century, a variety of research efforts led to definition of 11 different Trichinella species infecting mammals, birds, and reptiles around the globe.⁴

Geopolitical and cultural factors evolving into the 21^st century are leading to a resurgence of human infections in areas that have been free of infection for decades.⁵ This resurgence has particular relevance to emergency medicine because early diagnosis is associated with improved treatment outcomes. Early diagnosis is, itself, dependent on clinician awareness and uncovering relevant patient history.

Pathophysiology

Infection is initiated by ingestion of viable larvae in raw or undercooked meat. Digestive action liberates the larvae. The liberated larvae develop into adults in the duodenum and jejunum, where they mate and bear offspring. The adult worms are expelled in the stool.

Newborn larvae penetrate the intestinal wall, enter the lymphatic system, and move via the bloodstream to areas of implantation. Although the exact mechanism has not been elicited, the newborn larvae have been implicated in cardiac and neurologic trichinosis. The life cycle is completed with the larvae invading a striated muscle cell. 

The larvae resist immunologic reaction from the host as they migrate out of the capillaries and penetrate muscle cells. Once in the cell, they alter cellular activity to turn the individual cells into "nurse cells." To nourish this activity, the larvae stimulate angiogenesis leading to formation of a capillary rete around the invaded muscle cell.⁶

A distinguishing feature between two general subclassifications (clads) of Trichinella species is whether the nurse cell forms a collagen capsule or not.⁷

Eosinophilia develops in response to the presence of the worm. Patients who develop neurologic and cardiac dysfunctions have marked hypereosinophilia associated with arteriolar microthrombi, often simply from numbers of larvae, leading to areas of cerebral and myocardial infarction.⁸ Immunologic reactions are also deemed responsible for one of the hallmark clinical findings—palpebral edema.

The direct trauma of the larva encysting in muscle cells, coupled with the immunologic response, is responsible for other clinical features (eg, fever, myalgias). Ultimately, the intramuscular cysts typically calcify.

Frequency

United States

Occurrence in the United States is largely limited to sporadic cases or small clusters related to consumption of home-processed meats from noncommercial farm-raised pigs and wild game. The US national surveillance system is a passive system with links to state and local levels. This means there is not active sampling of pork.⁹ Trichinosis has been a reportable disease since 1966. The Centers for Disease Control and Prevention (CDC) surveillance system has data as far back as 1947 demonstrating a significant decrease in cases from a peak of nearly 500 in 1948 to a total of 74 from 1997-2001, the most recent reporting cycle.¹⁰ Human migratory patterns risk to upset this improvement as demonstrated by a report of a Laotian immigrant to the US developing trichinosis after consuming lightly cooked commercially purchased pork.¹¹

The US Department of Agriculture conducts periodic surveillance of farm-raised pigs. In a 1999 study, the major risk factor for seropositivity in tested pigs was access to live wildlife or wildlife carcasses.¹²  Rats, raccoons, skunks, and opossums have been shown to be wild sources of domestic pork contamination. The decrease in infection of domestic pork in the United States has been associated with federal laws regarding commercial pork feeding practices directed at other pathogens, voluntary preharvest programs to control other risk factors such as exposure to rats in commercial herds, and attention to preparation guidelines.¹³

Hunters and others who eat carnivorous game continue to be at risk.¹⁴

International

Since the 1980s, in spite of much awareness of both the presence of this infection and the efforts in many countries to control it, an increasing number of outbreaks have occurred throughout developed as well as developing countries.¹⁵ Some authorities believe that trichinosis should be classified as an emerging/reemerging disease, particularly because of increasing reports of cases from some previously unaffected areas.¹⁶ Changes in ecosystem utilization, international trade of meats, and rising affluence in countries without well-established monitoring systems have all contributed to the increasing incidence.¹⁷

Although rare in countries with laws limiting the feeding of raw garbage or animal byproducts to commercially raised pigs and well-managed slaughterhouse surveillance systems, many countries have passed laws but fallen short on the implementation and quality assurance of the programs.¹⁸ The cost of surveillance programs can be high and complex to install because of education and cultural issues.⁹ An international commission with representation from more than 40 countries has developed a set of guidelines and holds quadrennial scientific and policy meetings.¹⁹   

Home raising of pigs, with feeding of raw garbage instead of grain, is still a common practice in a large part of the world at all levels of development. In many countries, pigs roam freely, leading to contact with sylvatic sources of infection. Celebration of the Thai New Year has been associated with 200-600 cases in a single year.²⁰ Group tourist travel has also been associated with cluster outbreaks.²¹ In several Southeast Asian countries, raw pork sausage and other dishes are considered delicacies and are regularly associated with outbreaks.

Trichinosis must be considered a risk when eating the flesh of any animal that might have fed on uncooked animal flesh. Many animals generally not considered carnivorous will eat meat products when presented in a form they can chew, that is, usually chopped, or ground and combined with vegetable matter. This most commonly occurs on small farms when animals are fed table scraps or the scraps from butchering another farm animal. Rats also represent a reservoir in settings where raw animal scraps are left out for domestic animals to eat.

Market changes with international movement of processed food ingredients through either commercial channels or individual transport of products have presented a new source of difficulty in control. Two case clusters from Germany in 1998 were related to commercially produced sausage produced from meat transported from several different countries in Europe.²² A 2007 outbreak in Poland with more than 180 confirmed cases at the time of this writing has been linked to one manufacturer of low price sausage. This outbreak even spread to Germany as shoppers returned with lower price sausage from the same Polish region.²³  Other clusters have occurred in relation to immigrant families bringing home-country delicacies back from vacation to their new countries of residence.^24,25,26

WHO reported that some swine herds in some of these countries had a 50% prevalence of trichinosis, and thousands of human cases had been documented. The report correlates this increase in prevalence with a breakdown in social structure in these countries (state veterinary services, state farms), coupled with economic hardship and war.²⁴ Travelers to these regions are strongly advised to avoid consumption of any pork sausage and to verify the adequate cooking of any pork prior to consumption. Pork products from this region should be considered suspect.

Although generally thought of as a disease of omnivorous or carnivorous animals, herbivores have demonstrated infection, most likely from prepared feed that contained remnants of infected animals. In France, horse meat, largely imported, has become the most common source with more than a dozen outbreaks involving more than 3000 human victims since 1976. Interestingly, the same meat exporting countries supply various other European Union countries that have no human trichinosis; unlike the French and Italians, those countries do not have the culinary habit of eating meat raw or minimally cooked.²⁷ Mutton and goat have become a recognized vector in countries where pig consumption is restricted for religious or economic reasons.

China has some of the highest recorded case numbers globally.²⁸ Serologic population surveys have revealed prevalence rates of between 0.66 and 12%, varying somewhat from among regions depending on eating habits. Yunan province was found to be the most significantly affected province. Pigs are the primary vector with prevalence rates as high as 50% in slaughterhouse surveys in some provinces.²⁹  The Western Region Development strategy of the 1990s led to many people from the endemic regions of central and eastern China migrating with infected livestock to regions with previously very low incidence. An increased demand from tourist industry development has led to an increase in small producers using uncooked swill to feed pigs and not confining the pigs from wild vectors with a concomitant increase in pig prevalence to as high as 100% reported in one region.³⁰

Arctic and subarctic mammals (polar bear, walrus, seal) have been identified as being vectors for Trichinella nativa. T nativa is resistant to freezing, even for months at -20^o C. Up to 60% of polar bears in Nunavik are infected.³¹ Some Inuit have adjusted eating habits to avoid old male walruses as they are primarily scavengers, whereas the young feed primarily on shellfish.³²

Consumption of wild boar prepared into sausage in Spain has led to human infection with T britovi in Spain and Sweden.³³ Wild boar have also been identified as a source of human infection in other areas in the Mediterranean basin, southeast Asia, and  Pacific Islands.³⁴

In recent years, the effect of consumer trends such as greater interest in "free ranging" and "antibiotic free" meats has been associated with an increase in trichinellosis among pigs destined to commercial markets in the US³⁵ and Europe³⁶ .

This rather extensive listing of international incidence of animal and human infection has been presented to offer some direct information regarding areas of particular concern as well as to further make the point of the importance of dietary and travel history in patients with diarrhea, particularly with associated myalgias and eosinophilia.

Mortality/Morbidity

Specific death rate information is not established. Death is rare without development of neurologic and cardiac involvement.

• The primary morbidity is persistent myalgia and fatigue in cases that do not develop neurocardiac involvement.
• Following neurocardiac involvement, persistent variable dysfunction of either system may develop, depending upon the distribution of lesions.

Sex

Incidence is equal in males and females unless particular culinary habits lead to higher exposure for one group.³⁷ Serological evaluation of a small case series of pregnant women suggests that transplacental migration of larvae is possible in humans.³⁸

Age

All age groups reportedly have been affected; however, trichinosis most commonly occurs in persons aged 20-49 years.

Although no good epidemiologic studies exist, there is evidence for potential transplacental infection of the fetus as it has been reported in different animals and in a fetus from a therapeutic abortion of an infected mother.^39,40

Clinical

History

The usual incubation period of trichinosis is 8-15 days. Following intestinal incubation, initial symptoms most commonly are gastrointestinal due to the invasion of the intestinal wall by the juvenile larvae. Diagnosis depends heavily upon a suspicion and obtaining the history of ingesting potentially infected meat that was not cooked enough to kill the larvae. With increasing global migration, global exotic food transport and exotic tourism, the emergency physician should be aware of this diagnosis and include appropriate travel and dietary questions in the history of patients with gastroenteritis or unexplained myalgias.

• Myalgia (75%) - Classically reported as most common in masseter, diaphragm, and intercostal muscles, though high percent reported in extremities and neck/shoulder girdle in one cohort from a large outbreak in Turkey⁴¹ ; may be severe to point of inability to ambulate or perform simple upper extremity or truncal tasks like feeding or sitting upright
• Fever (60-75%) - 38.5-40.5°C
• Weakness (75%)
• Diarrhea (40-60%) - Usually only in the acute intestinal proliferative and penetration phases of nematode infestation
• Facial edema (40-64%) - Usually considered one of the hallmark features, particularly when localized to the eyelids
• Headache (50-60%)
• Fatigue/malaise (up to 95%)
• Arthralgia
• Cardioneurologic syndrome - Onset of these symptoms has usually been reported to occur early following onset of general symptoms and prior to muscle invasion.^42,8 The syndrome includes varying combinations of the following:
  • Encephalopathy
  • Focal neurologic deficits
  • Acute myocardial injury (eg, myocarditis, sinus and atrial nodal dysfunction, congestive heart failure, infarction)
  • Hypereosinophilia (>4000 granulocytes/mm³)
• Rash - This may occur in several forms.
  • Urticaria (most common)
  • Petechiae
  • Splinter hemorrhages
  • Palmar rash - Peripheral palmar and volar digital edema and erythema; desquamation occurs (10% in one study⁴³ )

Physical

• Fever (71%)
• Palpebral edema (50-60%) - Usually considered one of the hallmark findings; may be associated with chemosis and proptosis.⁴⁴
• Generalized edema
• Muscle weakness and tenderness - Usually not true neurologic weakness but pain related
• Neurologic findings consistent with encephalopathy or focal deficits
• Cardiac findings of myocarditis, pericarditis, or ischemia

Causes

Trichinosis is a completely preventable infestation. The single most important causative factor is the consumption of inadequately cooked meat. Although most developed countries have some form of trichosis control program, these controls have been documented to fail.  

• Trichinella species of nematodes - Of the 11 recognized genotypes, the following are the most clinically significant, although others may not yet have been identified:
• T spiralis is the primary cause associated with domesticated animals.
• T britovi is seen frequently in wild boar, horses, and free-ranging swine. It has also been reported in bear in Japan where it has been given a separate classification, T9, because of minor genetic variations from the European T britova.¹⁰
• T murrelli has been identified in wild and domestic animals other than pigs only in North America.³⁴
• T nelsoni is seen in various large carnivores of sub-Saharan Africa.¹⁷
• T nativa has been documented in almost all land and marine mammalian carnivores in the arctic and periarctic regions around the globe. In humans, it has been associated with more prolonged diarrhea and fewer muscle symptoms. It is also more resistant to freezing than other species, having been documented to not be killed by prolonged freezing at -18^o C.³⁴
• T9, closely related to T britovi is isolated to Japan, existing in bear, fox, and raccoon dog.³⁴
• T pseudospiralis has been documented in birds and does not form a capsule in the muscle, thus leading to less muscle inflammation and pain. Conversely, the muscle phase seems to remain actively infective for a longer period, probably since, without cyst formation, ultimate calcification does not occur.⁴⁵
• T papuae in wild pigs has been identified in Papua-New Guinea as a source of infection among forest-dwelling hunters. It is a nonencapsulating form of Trichinella.³⁷
• T zimbabwensis has been identified as infecting reptiles in southern Africa. Particularly concerning for increasing human risk is their presence in farm-raised crocodiles.⁴⁶  More recent reports indicate a presence in sylvatic reptiles as well.⁴⁷

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