Editorial Note: Since 1975, the proportion of trichinosis cases associated with consumption of contaminated commercial pork has declined in the United States. This decline probably reflects a combination of factors, including laws prohibiting the feeding of offal to hogs, the increased use of home freezers, and the practice of thoroughly cooking pork. In recent years the relative importance of consumption of wild game (including bear, wild boar, and walrus) (2,3) as a cause of trichinosis has increased. Consumption of meat from any carnivorous animal that has fed on trichina-infested flesh poses a risk (Figure 2). In addition to the two multiple-case outbreaks in this report, 15 other cases were reported in 1990. At least three cases were sporadic; information on the remaining 12 is unavailable.
The outbreak in Iowa is the fourth since 1975 that occurred among the 900,000 Southeast Asian refugees who have immigrated to the United States (4,5). The three previous outbreaks were related to consumption of undercooked pork that was not obtained from a commercial producer (4). This outbreak is consistent with previous reports indicating that recent immigrants from Southeast Asia are at particular risk for developing trichinosis because of their dietary habits (4).
Based on serologic examination of hogs at abattoirs, the prevalence of Trichinella infection in commercial pork ranges from 0 to 0.7% (6,7). Approximately 80 million hogs are slaughtered commercially each year in the United States. About 40% of the pork produced is sold as "ready to eat" pork products; such products must be made with trichina-free pork or pork adequately cooked or treated to kill trichina larvae. Trichinella larvae in pork are killed by freezing at 5 F (-15 C) for 21 days (or longer if meat is greater than 15 cm thick); however, Trichinella larvae present in wild game are often relatively resistant to freezing (8). Cooking is one of the most common methods of assuring that Trichinella are destroyed; a temperature of 170 F (77 C) substantially exceeds the thermal death point and is usually achieved if the meat is cooked until it is no longer pink (9).
Public health officials in areas with large populations of immigrants from Southeast Asia should consider education programs directed at the prevention of trichinosis. Physicians need to be aware of the continued presence of T. spiralis in commercial pork in the United States and should consider the diagnosis in any patient with an illness compatible with trichinosis and whose dietary preferences put them at risk for infection.