Seroprevalence of Trichinella sp. in Wild Boars (Sus scrofa) from Yanggu-gun, Gangwon-do, Korea

A total 7 outbreaks of trichinellosis have occurred in Korea, mostly as a result of consumption of raw wild boar (Sus scrofa) meat. Since only 1 serological survey on wild boars had yet been performed in Korea, the present study aimed to estimate the prevalence of trichinellosis in wild boars and some species of rodents by artificial digestion and serological examinations in Yanggu-gun, Gangwon-do, the endemic area of trichinellosis. Both the wild boar and rodent muscle samples revealed no Trichinella larvae by direct examination and artificial digestion method. However, serological examinations revealed that 4 wild boar sera samples out of 118 (3.4%) were positive to Trichinella antigen. Although the recovery of Trichinella larvae ended in a failure, it is proved for the first time that the sylvatic cycle of Trichinella has been maintained in wild boars of Gangwon-do, Korea.

Taxonomy of Trichinella and the epidemiology of infection in the Southeast Asia and Australian regions.

Seven species belonging to the Trichinella genus (five with encapsulated larvae and two with non-encapsulated larvae in host muscles) and three additional genotypes have been described to date: T. spiralis (genotype T1), a cosmopolitan species with a high infectivity to swine and rats; T. nativa (T2), etiological agent of sylvatic trichinellosis in arctic and subarctic areas of the Holarctic region, and its related genotype (Trichinella T6), detected in Alaska, Idaho, Montana, Pennsylvania, Wyoming, and Ontario; T. britovi (T3), etiological agent of sylvatic trichinellosis in temperate areas of Europe and Asia, and its related genotypes Trichinella T9 in Japan and Trichinella T8 in South Africa and Namibia; T. murrelli (T5), etiological agent of sylvatic trichinellosis in temperate areas of the USA; T. nelsoni (T7), etiological agent of sylvatic trichinellosis in Africa south of the Sahara; T. pseudospiralis (T4), a non-encapsulated cosmopolitan species infecting both mammals and birds; and T. papuae (T10), a recently discovered non-encapsulated species in sylvatic swine of Papua New Guinea. In the Southeast Asia and Australian regions, T. spiralis, T. pseudospiralis and T. papuae have been detected in sylvatic and domestic animals and in humans. A focus of human trichinellosis due to T. papuae was recently discovered in Papua New Guinea, with a prevalence of 28.9%. Trichinellosis has also been documented in domestic animals and/or humans in Cambodia, Indonesia (Bali and Sumatra), Lao PDR, Malaysia, Myanmar, Thailand, and New Zealand, and in wildlife of Tasmania.

Epidemiological survey of trichinellosis in Yugoslavia.

In a long term epidemiological survey that included different hosts for Trichinella spiralis among wild and domestic animals, the parasitological and serological screening revealed a high number of positive animals. The incidence of T. spiralis in grazing pigs was high, 21.76% (170 examined) but most of positive animals were lightly infected (0.016-0.02 l/g). While out of 2,394 pigs raised in small private farms, 1.67% were positive with high infection rates, none of the pigs raised on a modern breeding and fattening farm were seropositive. A repeated survey in grazing pigs performed five years later and in the same area, showed the prevalence of seropositive pigs to be still high (22% of 100 examined pigs). The same data were confirmed by detection of specific anti-Trichinella antibodies in Western blotting procedure. One of 2,031 serologically (IgG-ELISA) examined horses from Croatia reacted positively with very high OD values. Different characteristics of biological patterns (RCI, female fecundity, capsule formation, isoenzyme analysis) were found between two well distinguished groups of isolates. The first group consisted of well defined T1 isolates with patterns described by the Reference Centre for Trichinellosis, Rome, Italy. The data obtained by isoenzyme analysis of the representative sylvatic isolate were comparable to T3 characteristics of isolates from foxes in Italy.

The present status of trichinellosis in Thailand.

The epidemiology of trichinellosis in northern Thailand may be unique. Since 1962, outbreaks of the disease have occurred almost every year. The Department of Communicable Disease Control, Ministry of Public Health reported 118 discrete outbreaks of the disease, involving 5,400 patients, 95 of whom have died. Epidemiological investigations reveal that the outbreaks have occurred mostly in rural areas, associated with villagers celebrating local and traditional festivals such as the northern Thai New Year, wedding ceremonies, or other festive occasions. In almost all cases the source of infection has been traced to either hilltribe pigs, which are raised like wild animals, or to meat from wild boar. Infection is usually acquired through consumption of a local dish called "lahb," traditionally served during these festivities. This popular dish consists of raw finely chopped meat mixed with spices and chilies and is served nearly raw. Common clinical symptoms of trichinellosis in northern Thailand are fever and myalgia, presenting about two weeks after ingestion of raw meat containing encysted Trichinella larvae. Based on its infectivity for rats and pigs and on alloenzyme typing, the species appears to be Trichinella spiralis. The transmission cycle of trichinellosis in northern Thailand is primarily of a sylvatic type. Hilltribe pigs, wild boars, black bear and jackal are common reservoirs. Man intrudes into the cycle by eating the raw or under-cooked meat of these animals.

General introduction and epidemiology of trichinellosis.

All studies on trichinellosis have been based on the assumption that there is only one cosmopolitan agent of human and animal trichinellosis; Trichinella spiralis (Owen, 1835). In the past, swine and only a few synanthropic animals were considered reservoirs, and the presence of the parasite in sylvatic animals was considered rare. Breeding experiments before, and then biochemical approaches (isoenzymes and DNA analysis), showed the presence of a high genetic variability inside this genus, suggesting it be considered as polyspecific. At present, eight gene pools, named from T1 to T8, have been identified in the genus Trichinella. T1 (T. spiralis s str) is the etiological agent for the domestic cycle, with a cosmopolitan distribution related to swine breeding and human habits. However, in some areas this parasite passes from domestic to sylvatic animals and vice versa. Six gene pools are the agents for the sylvatic cycle at different latitudes, mainly in carnivores. T2 (T. nativa) is present in terrestrial mammals and seldom in sea mammals of arctic and subarctic regions. T3, T5, and T8 are present mainly in Canidae (T3), Ursidae (T5), Hyaenidae and Felidae (T8) of temperate-subtropical areas of Palearctic, Nearctic and South African regions, respectively. T6 is present in carnivores in the subarctic-temperate area of the Nearctic region; and T7 (T. nelsoni) in Hyaenidae and Felidae of Equatorial Africa. Only T4 (T. pseudospiralis) can infect both mammals and birds, and it shows a cosmopolitan distribution. Probably trichinellosis is present world-wide, but unknown in those areas where it has not been sought.

The significance of changing trends in trichinellosis.

Findings during the past two decades have indicated that there are changing trends in trichinellosis in terms of (1) new isolates of Trichinella, (2) new sources of meat, including meat of herbivorous animals and of a greater variety of wild animals, (3) increasing number of cases among new ethnic groups in the USA, and (4) the emergence of new foci in Asia. These changes offer new challenges that warrant our attention. The characterization of each of the new isolates of Trichinella, particularly with respect to its infectivity, has become critical. Since herbivorous animals have been implicated as the source for human infections, thorough cooking of all animal meat is compulsory. Ethnic groups who continue to prepare their food in traditional manner must be informed of the potential danger of trichinellosis and the need for modification in preparation. The emergence of new foci for human trichinellosis emphasizes the importance of direct communication and interaction on a worldwide basis regarding the parasite and the infection.