Tweet
This week at Infection Landscapes I will cover trichinellosis, which is another roundworm infection. This roundworm does not cause a substantial burden throughout the world, but it is worth describing here because it does occupy some unique landsdcapes at the interface between human and animal ecology in both domestic and sylvan habitats.
The Worm. Trichinellosis is cause by nematode helminths of the genus Trichinella. While there are eight known species of the genus, only three are of major importance in causing human infection: Trichinella spiralis, T. nativa, and T. britovi. The majority of human infections are caused by T. spiralis.
Let's consider the life cycle. The defining feature of the T. spiralis life cycle is that larvae encyst in the skeletal muscle tissue of the host, wherein they await the consumption of their current host by another predatory or scavenging host to infect that new host and complete the life cycle. First, infection occurs when a new host ingests raw meat (i.e. the raw muscle tissue) containing the infectious encysted larvae. Upon reaching the stomach of the new host, the larvae excyst triggered by the acid and pepsin in the gastric environment. The free larvae subsequently make their way to the small intestine where they penetrate the mucous membrane of the epithelium and develop into adults after four molts. As adults, males and females mate and the females produce motile larvae after approximately one week. These larvae then penetrate the epithelium and ultimately gain the circulation (lymph and blood) to seek out skeletal muscle tissue. When the larvae reach the muscle tissue each larva will invade a single skeletal muscle cell wherein they encyst and become the infectious larvae capable of infecting new hosts. The process of encystation is unique in that the worm hijacks the host muscle cell and creates a new intracellular environment, call a nurse cell, which encapsulates and maintains the viability of the infectious larvae for many years until, one day, it finds itself in the stomach of a new host after being eaten raw along with its previous host:
Below is nice graphic from the Centers for Disease Control and Prevention (CDC) depicting the life cycle of this helminth:
Note that Trichinella does not require intermediate hosts. It is capable of completing its life cycle in a single species of either its primary hosts, such as rodents, pigs, or wild game, or incidental hosts, such as humans.
The Disease. The vast majority of Trichinella infections are asymptomatic. When clinical disease does occur, it follows one or both of two courses: enteral or parenteral disease.
Enteral disease will typically present in association with high volume infections and symptoms include diarrhea and gastric discomfort such as dyspepsia and nausea fairly soon (within ~ 1 week) after infection.
Parenteral disease is also associated with high volume infections and results from the migration of large numbers of larvae out of the small intestine and into the circulation with potential dissemination to many different organ systems. Fever, muscle pain, edema and vasculitis are typical presentations of parenteral disease, which follow from the systemic inflammation induced by the circulating larvae. Periorbital edema is considered a classic sign of parenteral trichinellosis, however it is not pathognomonic:
If the worms invade the central nervous system encephalitis can develop, as can other severe neurologic pathology involving pulmonary function and/or motor coordination. Although quite rare, when it does occur death is most often due to encephalitis, myocarditis or pneumonia following the extensive inflammatory responses associated with high volume infection.
The Epidemiology and the Landscape. As mentioned above, trichinellosis does not constitute a large global burden of disease. There are approximately 10,000 new infections each year and the vast majority of these are asymptomatic or of mild presentation. Currently, there are likely over 11 million people infected globally, with most infections occurring in China and Southeast Asia. The maps below published in Clinical Microbiology Reviews show the global distribution of Trichinella species:
Transmission to humans occurs within two fairly distinct landscapes, one domestic and one sylvan. The vast majority of infections in the world are transmitted by consuming inadequately prepared pork. Domestic pigs, and particularly the ways in which pigs feed under specific farming practices, are the common vehicle responsible for most human infection. You will recall from the description above that a critical and defining feature of trichinellosis is that an infected host must be consumed by a new host for transmission of the helminth to occur in the new host. As such, two additional features immediately emerge in shaping the ecology of transmission. First, the eating habits of key reservoir hosts (and the farming practices that maintain these habits for domestic animals); and, second, the eating habits of humans, particularly with respect to food preparation.
Domestic pig farming practices vary dramatically across the world. In many low and middle income countires, where most trichinellosis infections occur, as well as in some high income countries, garbage feeding is a common practice in raising domestic animals. As such, pigs will very often encounter dead rodents, which they eat, in these food sources. Rodents are a common reservoir host for T. spiralis and serve as important hosts in maintaining infection in pigs. Moreover, while sources of garbage are prime locations for rodents, and thus encounters with the pigs who might feed on the garbage, this rodent-pig cycle is by no means limited to such loci. Indeed, better resourced farms that can afford better quality feed can also be quite attractive sources of food to rodent species. Finally, the rodents are not essential to maintain infection in pigs. Pig cannibalism, either as it occurs naturally or due to specific farming practices of adding pig remains to feed, can be an important source of transmission for pigs, which can maintain infection within a pig community without any external hosts. Of course, if pig products used for human consumption are properly cooked then transmission is blocked and the pig infection cycle is irrelevant. However, in addition to varied farming practices throughout the world, there are also varied food preparation practices when it comes to pork, which brings us back to the second important feature in transmission.
So, the domestic landscape epidemiology of trichinellosis is defined by the intersection of farming and food preparation in the geographic spaces where T. spiralis is endemic. Below is a nice graphic developed by G.J. Jackson at the Division of Microbiology, US Food and Drug Administration and published by the Food and Agriculture Organization of the United Nations, which depicts this domestic cycle and the sylvan cycle to be described below:
In the developed world, many human infections result from contact with the sylvan cycle. Game animals, particularly bears, are frequently infected, albeit with different species of Trichinella. If these game meats are not properly prepared by cooking, then humans can ingest the intramuscular larvae and become infected. While this sylvan transmission cycle accounts for many infections in some developed countries, for example in the United States, this transmission cycle is by no means limited to developed countries. Indeed, the sylvan cycle can be an active transmission route in many developing regions as well. For example, the consumption of wild cat meats or feral pigs in sub-Saharan Africa can lead to human infection.
While the largest absolute number of incident and prevalent cases occur in Southeast Asia and are mostly transmitted via the domestic cycle, the largest density of human infection may be among arctic indigenous communities, where transmission occurs following consumption of polar and brown bears, foxes, and pinniped marine mammals:
This week at Infection Landscapes I will cover trichinellosis, which is another roundworm infection. This roundworm does not cause a substantial burden throughout the world, but it is worth describing here because it does occupy some unique landsdcapes at the interface between human and animal ecology in both domestic and sylvan habitats.
The Worm. Trichinellosis is cause by nematode helminths of the genus Trichinella. While there are eight known species of the genus, only three are of major importance in causing human infection: Trichinella spiralis, T. nativa, and T. britovi. The majority of human infections are caused by T. spiralis.
Trichinella spiralis larva
Let's consider the life cycle. The defining feature of the T. spiralis life cycle is that larvae encyst in the skeletal muscle tissue of the host, wherein they await the consumption of their current host by another predatory or scavenging host to infect that new host and complete the life cycle. First, infection occurs when a new host ingests raw meat (i.e. the raw muscle tissue) containing the infectious encysted larvae. Upon reaching the stomach of the new host, the larvae excyst triggered by the acid and pepsin in the gastric environment. The free larvae subsequently make their way to the small intestine where they penetrate the mucous membrane of the epithelium and develop into adults after four molts. As adults, males and females mate and the females produce motile larvae after approximately one week. These larvae then penetrate the epithelium and ultimately gain the circulation (lymph and blood) to seek out skeletal muscle tissue. When the larvae reach the muscle tissue each larva will invade a single skeletal muscle cell wherein they encyst and become the infectious larvae capable of infecting new hosts. The process of encystation is unique in that the worm hijacks the host muscle cell and creates a new intracellular environment, call a nurse cell, which encapsulates and maintains the viability of the infectious larvae for many years until, one day, it finds itself in the stomach of a new host after being eaten raw along with its previous host:
Trichinella spiralis larvae encapsulated in nurse cells within the skeletal muscle tissue of the host
Below is nice graphic from the Centers for Disease Control and Prevention (CDC) depicting the life cycle of this helminth:
Note that Trichinella does not require intermediate hosts. It is capable of completing its life cycle in a single species of either its primary hosts, such as rodents, pigs, or wild game, or incidental hosts, such as humans.
The Disease. The vast majority of Trichinella infections are asymptomatic. When clinical disease does occur, it follows one or both of two courses: enteral or parenteral disease.
Enteral disease will typically present in association with high volume infections and symptoms include diarrhea and gastric discomfort such as dyspepsia and nausea fairly soon (within ~ 1 week) after infection.
Parenteral disease is also associated with high volume infections and results from the migration of large numbers of larvae out of the small intestine and into the circulation with potential dissemination to many different organ systems. Fever, muscle pain, edema and vasculitis are typical presentations of parenteral disease, which follow from the systemic inflammation induced by the circulating larvae. Periorbital edema is considered a classic sign of parenteral trichinellosis, however it is not pathognomonic:
If the worms invade the central nervous system encephalitis can develop, as can other severe neurologic pathology involving pulmonary function and/or motor coordination. Although quite rare, when it does occur death is most often due to encephalitis, myocarditis or pneumonia following the extensive inflammatory responses associated with high volume infection.
The Epidemiology and the Landscape. As mentioned above, trichinellosis does not constitute a large global burden of disease. There are approximately 10,000 new infections each year and the vast majority of these are asymptomatic or of mild presentation. Currently, there are likely over 11 million people infected globally, with most infections occurring in China and Southeast Asia. The maps below published in Clinical Microbiology Reviews show the global distribution of Trichinella species:
World map showing the distribution areas of Trichinella spiralis (Tsp), Trichinella pseudospiralis from north America (TpsN), T. pseudospiralis from Europe and Asia (TpsP), T. pseudospiralis from Tasmania (TpsA), Trichinella papuae (Tpa), and Trichinella zimbabwensis (Tzi) (Clin. Microbiol. Rev. January 2009 vol. 22 no. 1 127-145)
World map showing the distribution areas of Trichinella nativa (Tna), Trichinella britovi (Tb), Trichinella murrelli (Tm), Trichinella nelsoni (Tne), Trichinella genotype T6 (T6), Trichinella genotype T8 (T8), and Trichinella genotype T9 (T9). In some regions, the distribution areas of these encapsulated species and genotypes overlap between them (Clin. Microbiol. Rev. January 2009 vol. 22 no. 1 127-145)
Transmission to humans occurs within two fairly distinct landscapes, one domestic and one sylvan. The vast majority of infections in the world are transmitted by consuming inadequately prepared pork. Domestic pigs, and particularly the ways in which pigs feed under specific farming practices, are the common vehicle responsible for most human infection. You will recall from the description above that a critical and defining feature of trichinellosis is that an infected host must be consumed by a new host for transmission of the helminth to occur in the new host. As such, two additional features immediately emerge in shaping the ecology of transmission. First, the eating habits of key reservoir hosts (and the farming practices that maintain these habits for domestic animals); and, second, the eating habits of humans, particularly with respect to food preparation.
Domestic pig farming practices vary dramatically across the world. In many low and middle income countires, where most trichinellosis infections occur, as well as in some high income countries, garbage feeding is a common practice in raising domestic animals. As such, pigs will very often encounter dead rodents, which they eat, in these food sources. Rodents are a common reservoir host for T. spiralis and serve as important hosts in maintaining infection in pigs. Moreover, while sources of garbage are prime locations for rodents, and thus encounters with the pigs who might feed on the garbage, this rodent-pig cycle is by no means limited to such loci. Indeed, better resourced farms that can afford better quality feed can also be quite attractive sources of food to rodent species. Finally, the rodents are not essential to maintain infection in pigs. Pig cannibalism, either as it occurs naturally or due to specific farming practices of adding pig remains to feed, can be an important source of transmission for pigs, which can maintain infection within a pig community without any external hosts. Of course, if pig products used for human consumption are properly cooked then transmission is blocked and the pig infection cycle is irrelevant. However, in addition to varied farming practices throughout the world, there are also varied food preparation practices when it comes to pork, which brings us back to the second important feature in transmission.
So, the domestic landscape epidemiology of trichinellosis is defined by the intersection of farming and food preparation in the geographic spaces where T. spiralis is endemic. Below is a nice graphic developed by G.J. Jackson at the Division of Microbiology, US Food and Drug Administration and published by the Food and Agriculture Organization of the United Nations, which depicts this domestic cycle and the sylvan cycle to be described below:
In the developed world, many human infections result from contact with the sylvan cycle. Game animals, particularly bears, are frequently infected, albeit with different species of Trichinella. If these game meats are not properly prepared by cooking, then humans can ingest the intramuscular larvae and become infected. While this sylvan transmission cycle accounts for many infections in some developed countries, for example in the United States, this transmission cycle is by no means limited to developed countries. Indeed, the sylvan cycle can be an active transmission route in many developing regions as well. For example, the consumption of wild cat meats or feral pigs in sub-Saharan Africa can lead to human infection.
While the largest absolute number of incident and prevalent cases occur in Southeast Asia and are mostly transmitted via the domestic cycle, the largest density of human infection may be among arctic indigenous communities, where transmission occurs following consumption of polar and brown bears, foxes, and pinniped marine mammals:
Prevention and Control. Prevention of trichinellosis in humans is comprised of 2 primary strategies: management of farming practices and preparation of meat products. Where domestic pigs are raised for human consumption care should be taken to employ some basic control measures. The adoption of the following control regulations should apply universally, and are especially important when garbage feeding is used: 1) prevent pigs from eating any dead animal carcasses, 2) strictly control rodent populations, 3) prevent contact between pigs and sylvan animal species, and 4) prevent pigs from eating other pigs, either dead or alive.
The second prevention strategy consists of proper food preparation. Thoroughly cooking all pork and game meats at 165 degrees F (74 degrees C) is the most effective way of killing the larvae in the meat and, thus, blocking transmission. Freezing pork (but not game meats) can also kill T. spiralis, however freezing must occur at 5 degrees F (-15 degrees C) for at least 20 days, or -4 degrees F (-20 degrees C) for at least 3 days, in cuts of meat that are less than 6 inches in thickness. Other species of Trichinella are much more resistant to freezing and, thus, this method is not useful for game meats.