Trichinellosis in Humans
International Commission on Trichinellosis
Trichinellosis in Humans
Human exposure to Trichinella spp. can only occur by ingestion of raw or undercooked meat containing infective larvae. While pork has been the historical source of human infection, and continues to be an important source in some countries, in many countries effective control programs have dramatically reduced or eliminated pork as a risk for exposure. However, a variety of game meats continue to be a potential risk to humans if not prepared properly. These sources include, commonly, bear and other carnivores that are high in the food chain (e.g., fox, cougar), feral pig/wild boar, and walrus, and less commonly, smaller mammals (e.g., opossum, badger, squirrel), turtles and lizards. Hunter/consumer education and inspection of game meats is necessary to prevent infection from these sources. Other sources of infection include horses, for outbreaks in France and Italy, and dogs, for outbreaks in China, Russia and elsewhere. Because trichinellosis is not a frequent diagnosis in most countries, common source outbreaks are the most recognized occurrence of this disease. Outbreaks often occur during group events – holidays, weddings, etc. – where special or traditional dishes may be prepared that are not thoroughly cooked. Common source outbreaks also occur when Trichinella-infected game meats are distributed among a number of people and insufficiently cooked prior to being eaten. In common source outbreaks, identifying the meat which caused the infection may be used to confirm the presence of Trichinella larvae. Additional cases may then be identified from information on those exposed to the infected source meat.
Prevalence
As reported by Pozio (2007), the average yearly incidence of the disease in humans worldwide is probably close to ten-thousand cases, but with a low mortality rate (about 0.2%). It is likely that only a fraction of actual cases are reported due to the non-specific symptoms exhibited by this disease, as well as a lack of appropriate serological tests and a lack of knowledge of the disease on the part of physicians.
The prevalence of Trichinella infection in humans and the source of these infections vary greatly from country to country. Where pork production is tightly regulated and testing of pork for Trichinella infection has been required for many years, infections from pork are not found. However, in countries where pigs are raised by traditional methods (outdoors), human infections occur regularly. Human trichinellosis linked to consumption of infected pork occurs in Central (Mexico) and South America (Argentina and Chile), in Asia (the People’s Republic of China, Laos, Myanmar, Thailand, Vietnam) and Europe (Bosnia-Herzegovina, Bulgaria, Byelorussia, Croatia, Georgia, Latvia, Lithuania, Poland, Romania, Russia, Serbia, and the Ukraine) (Pozio, 2007).
Even in countries where Trichinella infection has been absent from the domestic pork supply for many years, human infections may still occur from imported meat products which have not been properly inspected. For example, more than a dozen large outbreaks involving over 3000 people occurred in France and Italy between 1975 and 2005, resulting from the consumption of horsemeat imported from countries of Eastern Europe and North America.
Game animals remain an important source of exposure to Trichinella spp. in all countries since infection cannot be controlled in wildlife. For that reason, hunters and consumers of game meats should either assure the safety of the meat through inspection methods, or prepare meats by methods which are known to inactivate the parasites.
Clinical Manifestations
Trichinellosis (the disease caused by Trichinella in humans) is manifested by symptoms associated with worms developing in the intestine (enteral phase) and in the musculature (parenteral phase). The occurrence of symptoms varies depending on the infecting dose, the susceptibility of the individual and possibly the species of Trichinella. In heavy infections, intestinal symptoms may develop as early as 1 week following ingestion of infective larvae. Intestinal symptoms, caused by worms invading and growing in the intestinal villi, typically include abdominal discomfort and diarrhea. The enteral phase generally only lasts a few days.
The onset of symptoms in the parenteral phase may occur within one-two weeks following infection or may be much later (4-6 weeks) as larger numbers of worms migrate and accumulate in the musculature. The clinical picture of the parenteral phase is much better defined although no signs or symptoms are pathognomonic. Symptoms include facial, and especially periorbital, edema, myalgia, headache, fever and in some cases a macro-papular rash. Conjunctival and sub-ungual hemorrhages are also frequently observed. Complications principally include myocarditis and encephalitis which result from larvae migrating in heart musculature or the central nervous system.
The most common laboratory findings in trichinellosis are high eosinophil counts and increased levels of muscle enzymes including creatine phosphokinase and lactate dehydrogenase. Eosinophil counts may reach 10,000 cell/mm3 and elevated levels may persist for several weeks to several months following infection. Muscle enzymes in serum typically increase several fold during active infection (larval migration and muscle cell penetration).
The case definition for trichinellosis, including classification of suspected, probable or confirmed cases, is based on a combination of signs and symptoms, laboratory findings, including biopsy (if performed), and epidemiological criteria. Several rubrics for case definition have been developed (see Dupouy-Camet et al. in Further Reading).
Diagnosis
Direct and indirect tests may be used to support the diagnosis of trichinellosis. Muscle biopsy is the only method for the direct verification of infection with Trichinella spp. A biopsy of 0.5-1.0 gram of muscle from the deltoid or gastrocnemius may be compressed between glass slides and examined microscopically for the presence of larvae. Alternatively, the muscle sample may be digested in acidified pepsin and larvae recovered for enumeration. Muscle larvae may be detected approximately 2 weeks after infection and numbers of larvae found in the musculature will increase over a period of several weeks. Microscopic detection of larvae in muscle biopsy compressions is more difficult when the infection is due to non-encapsulating species of Trichinella and therefore digestion of biopsy samples is the preferred method.
Detection of antibodies to Trichinella spp. is useful in confirming infection. The most commonly used test is the enzyme-linked immunoassay.(ELISA), with an excretory-secretory antigen. This test is available in commercial kits or may be performed according to various published protocols.
Further reading
Bruschi F. and Murrell K. D. 1999. Trichinellosis. In: Tropical Infectious Diseases. Principles, Pathogens & Practice. (Guerrant, R.L., Walker, D.H., Weller, P.F. eds.). Churchill-Livingstone, Philadelphia, PA. Vol. 2, pp. 917-925.
Dupouy-Camet, J. and Bruschi, F. 2007. Management and diagnosis of human trichinellosis. In, (Dupouy-Camet, J and Murrell, K.D. eds.), FAO/WHO/OIE Guidelines for the Surveillance, Management, Prevention and Control of Trichinellosis, Rome, pp. 37-68.
Gamble, H.R. and Murrell, K.D. 1988. Trichinellosis. In, Laboratory Diagnosis of Infectious Disease: Principles and Practice (Balows, W., ed), Springer Verlag, New York, pp. 1018 1024.
Pozio, E. 2007. World distribution of Trichinella spp. infections in animals and humans. Veterinary Parasitology, 149: 3–21.
Human exposure to Trichinella spp. can only occur by ingestion of raw or undercooked meat containing infective larvae. While pork has been the historical source of human infection, and continues to be an important source in some countries, in many countries effective control programs have dramatically reduced or eliminated pork as a risk for exposure. However, a variety of game meats continue to be a potential risk to humans if not prepared properly. These sources include, commonly, bear and other carnivores that are high in the food chain (e.g., fox, cougar), feral pig/wild boar, and walrus, and less commonly, smaller mammals (e.g., opossum, badger, squirrel), turtles and lizards. Hunter/consumer education and inspection of game meats is necessary to prevent infection from these sources. Other sources of infection include horses, for outbreaks in France and Italy, and dogs, for outbreaks in China, Russia and elsewhere. Because trichinellosis is not a frequent diagnosis in most countries, common source outbreaks are the most recognized occurrence of this disease. Outbreaks often occur during group events – holidays, weddings, etc. – where special or traditional dishes may be prepared that are not thoroughly cooked. Common source outbreaks also occur when Trichinella-infected game meats are distributed among a number of people and insufficiently cooked prior to being eaten. In common source outbreaks, identifying the meat which caused the infection may be used to confirm the presence of Trichinella larvae. Additional cases may then be identified from information on those exposed to the infected source meat.
Prevalence
As reported by Pozio (2007), the average yearly incidence of the disease in humans worldwide is probably close to ten-thousand cases, but with a low mortality rate (about 0.2%). It is likely that only a fraction of actual cases are reported due to the non-specific symptoms exhibited by this disease, as well as a lack of appropriate serological tests and a lack of knowledge of the disease on the part of physicians.
The prevalence of Trichinella infection in humans and the source of these infections vary greatly from country to country. Where pork production is tightly regulated and testing of pork for Trichinella infection has been required for many years, infections from pork are not found. However, in countries where pigs are raised by traditional methods (outdoors), human infections occur regularly. Human trichinellosis linked to consumption of infected pork occurs in Central (Mexico) and South America (Argentina and Chile), in Asia (the People’s Republic of China, Laos, Myanmar, Thailand, Vietnam) and Europe (Bosnia-Herzegovina, Bulgaria, Byelorussia, Croatia, Georgia, Latvia, Lithuania, Poland, Romania, Russia, Serbia, and the Ukraine) (Pozio, 2007).
Even in countries where Trichinella infection has been absent from the domestic pork supply for many years, human infections may still occur from imported meat products which have not been properly inspected. For example, more than a dozen large outbreaks involving over 3000 people occurred in France and Italy between 1975 and 2005, resulting from the consumption of horsemeat imported from countries of Eastern Europe and North America.
Game animals remain an important source of exposure to Trichinella spp. in all countries since infection cannot be controlled in wildlife. For that reason, hunters and consumers of game meats should either assure the safety of the meat through inspection methods, or prepare meats by methods which are known to inactivate the parasites.
Clinical Manifestations
Trichinellosis (the disease caused by Trichinella in humans) is manifested by symptoms associated with worms developing in the intestine (enteral phase) and in the musculature (parenteral phase). The occurrence of symptoms varies depending on the infecting dose, the susceptibility of the individual and possibly the species of Trichinella. In heavy infections, intestinal symptoms may develop as early as 1 week following ingestion of infective larvae. Intestinal symptoms, caused by worms invading and growing in the intestinal villi, typically include abdominal discomfort and diarrhea. The enteral phase generally only lasts a few days.
The onset of symptoms in the parenteral phase may occur within one-two weeks following infection or may be much later (4-6 weeks) as larger numbers of worms migrate and accumulate in the musculature. The clinical picture of the parenteral phase is much better defined although no signs or symptoms are pathognomonic. Symptoms include facial, and especially periorbital, edema, myalgia, headache, fever and in some cases a macro-papular rash. Conjunctival and sub-ungual hemorrhages are also frequently observed. Complications principally include myocarditis and encephalitis which result from larvae migrating in heart musculature or the central nervous system.
The most common laboratory findings in trichinellosis are high eosinophil counts and increased levels of muscle enzymes including creatine phosphokinase and lactate dehydrogenase. Eosinophil counts may reach 10,000 cell/mm3 and elevated levels may persist for several weeks to several months following infection. Muscle enzymes in serum typically increase several fold during active infection (larval migration and muscle cell penetration).
The case definition for trichinellosis, including classification of suspected, probable or confirmed cases, is based on a combination of signs and symptoms, laboratory findings, including biopsy (if performed), and epidemiological criteria. Several rubrics for case definition have been developed (see Dupouy-Camet et al. in Further Reading).
Diagnosis
Direct and indirect tests may be used to support the diagnosis of trichinellosis. Muscle biopsy is the only method for the direct verification of infection with Trichinella spp. A biopsy of 0.5-1.0 gram of muscle from the deltoid or gastrocnemius may be compressed between glass slides and examined microscopically for the presence of larvae. Alternatively, the muscle sample may be digested in acidified pepsin and larvae recovered for enumeration. Muscle larvae may be detected approximately 2 weeks after infection and numbers of larvae found in the musculature will increase over a period of several weeks. Microscopic detection of larvae in muscle biopsy compressions is more difficult when the infection is due to non-encapsulating species of Trichinella and therefore digestion of biopsy samples is the preferred method.
Detection of antibodies to Trichinella spp. is useful in confirming infection. The most commonly used test is the enzyme-linked immunoassay.(ELISA), with an excretory-secretory antigen. This test is available in commercial kits or may be performed according to various published protocols.
Further reading
Bruschi F. and Murrell K. D. 1999. Trichinellosis. In: Tropical Infectious Diseases. Principles, Pathogens & Practice. (Guerrant, R.L., Walker, D.H., Weller, P.F. eds.). Churchill-Livingstone, Philadelphia, PA. Vol. 2, pp. 917-925.
Dupouy-Camet, J. and Bruschi, F. 2007. Management and diagnosis of human trichinellosis. In, (Dupouy-Camet, J and Murrell, K.D. eds.), FAO/WHO/OIE Guidelines for the Surveillance, Management, Prevention and Control of Trichinellosis, Rome, pp. 37-68.
Gamble, H.R. and Murrell, K.D. 1988. Trichinellosis. In, Laboratory Diagnosis of Infectious Disease: Principles and Practice (Balows, W., ed), Springer Verlag, New York, pp. 1018 1024.
Pozio, E. 2007. World distribution of Trichinella spp. infections in animals and humans. Veterinary Parasitology, 149: 3–21.
