Changing profile of causative agents

One hundred years ago, the roster of food-borne pathogens was very different from that of today. M. tuberculosis was transmitted by milk from infected cows, typhoid fever was spread by foods contaminated by cooks and food handlers infected with Salmonella typhi, and botulism was a steady risk from home preserved foods (Sobel, 2005). Brucellosis was transmitted through consumption of milk, meat, and organs from infected animals (Pappas etal., 2005). Trichinosis, acquired most often from consumption of inadequately cooked, infected pork products, affected one-sixth of the population (Schantz, 1983; Moorhead et al., 1999). Food-borne transmission of these pathogens has been virtually eliminated in the United States and, with the exception of botulism and trichinosis, clinical cases of these infections today are most often acquired overseas.

Even as these traditional pathogens faded in importance for their food-borne transmission, other additions were being made to the list of food-borne pathogens - particularly in the past three decades. The non-typhoidal salmonellae have emerged as common and serious causes of human illness throughout the industrialized world. While still associated with foods of animal origin (beef, poultry, and dairy products), the non-typhoidal salmonellae have increasingly been associated with eggs (Braden, 2006) and fresh produce. Highly drug-resistant strains have emerged which are associated with greater clinical severity in these infections (Helms et al., 2002). In a recent multi-state outbreak of S. typhimurium Definitive Type 104 (DT104) associated with ground beef, 41 percent of the cases in the case-control study had been hospitalized (Dechet et al., 2006).

Listeria monocytogenes was previously known as a cause of neonatal sepsis and meningitis in immunocompromised persons, and its route of transmission was obscure. It is now well recognized as a food-borne pathogen capable of causing febrile gastroenteritis in healthy persons (Ooi and Lorber, 2005; Schlech et al., 2005), in addition to severe disease in neonates and the immunocompromised host. The high-profile pathogen Escherichia coli O157:H7 is noteworthy for the severity of the acute colitis it causes and the post-diarrheal complication of the hemolytic uremic syndrome, the latter being the most common cause of acute renal failure in childhood (Tarr et al., 2005). Raw molluscan shellfish, such as oysters, clams, and mussels, have emerged as the distinctive food risk for illness with two Vibrio species, V. vulnificus and Vparahemolyticus. An unusual cause of sepsis and meningitis in premature and low birth-weight infants, Enterobacter sakasakii, has emerged as a risk associated with powdered infant formula (Drudy et al., 2006).

Viruses and parasites have also been newly identified as food-borne pathogens. Noroviruses, first discovered in 1972, are now recognized as probably the most common causative agent of gastroenteritis and of food-borne infections in the United States (Fankhauser et al., 2002; Glass et al., 2000). Cyclospora caye-tanensis and Cryptosporidium parvum, both intestinal protozoa of non-human mammals, have emerged from zoonotic sources as clinically and economically important causes of food-borne disease.

As noted earlier, the list of food-borne pathogens will continue to expand. The reasons for this include improved surveillance for both sporadic and outbreak-associated illness, and more sophisticated investigation using molecular biology techniques. Pressures for cost containment in the United States have de-emphasized coprodiagnostics, such as stool cultures, fecal microscopy, and antigen detection, since many enteric illnesses are self-limited. Since any apparently sporadic case may be part of a larger outbreak of food-borne illness, greater awareness of the public health benefit of coprodiagnostic studies is needed (Guerrant et al., 2001), and societal value should be accorded to finding and removing a contaminated food vehicle still in commerce.

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