Chapter 6 - Epidemiology

Abstract

Nematodes of the genus Trichinella are perpetuated through a wild cycle and the epidemiology is closely related to the feeding behavior of host species. The domestic cycle is ancillary to the wild cycle and begins when humans raise domestic animals by feeding them with leftovers from Trichinella-infected domesticated or hunted animals. The main reservoir hosts of Trichinella are carnivores and only a few omnivore species with a scavenger behavior. In the muscle tissues of these animals, Trichinella larvae can survive for months up to several years depending on the species. After the host’s death, larvae remain viable and infectious in decaying muscles for weeks. Humans can strongly influence the sylvatic cycle, both favoring or reducing the transmission of these pathogens. The domestic cycles occur where there are high-risk practices on the farm. Industrial pigs marketed internationally have never been found to be infected with Trichinella. However, the illegal importation of pig, wild boar, or bear meat into a country by individuals in their luggage caused family outbreaks of trichinellosis. Trichinella infections in humans are related to cultural food practices, which include dishes based on raw or undercooked meat originating from several animals. Overall, the most important source of Trichinella infection for humans remains pork and its related products from backyard and free-ranging pigs, followed by wild boar and other game animals. Epidemiological information on Trichinella infections in animals and humans in the world is available for 95 countries. In the 21st century, the wild and domestic cycles were documented in 75 and 32 countries, respectively. Although trichinellosis has declined significantly, due chiefly to a reduction of the domestic cycle in developed countries, it remains a potential risk because of the continuous presence of most Trichinella spp. in a wildlife spillover into domestic animals.