Epidemiology

Salmonella can be isolated from the intestinal tracts of humans and many of the lower animals. The prevalence of asymptomatic excretors of these organisms in the general population is approximately 0.2% (10). The most important reservoir of salmonella is domestic and wild animals in which the infection rates vary from <1 to >20% (14). An incomplete list of animals from which salmonellae serotypes have been isolated includes chickens, turkeys, ducks, pigs, cows, dogs, cats, rats, parakeets, as well as certain cold-blooded animals and insects. Animals sold as pets, especially baby chicks, ducks, turtles, and other reptiles, may also harbor salmonellae and serve as a source of infection.

Salmonella infection is usually acquired by the oral route, normally by ingestion of contaminated food or drink. Any food product is a potential source of human infection. The source of contamination may be asymptomatic carriers or persons with active clinical disease, but the greatest single source of human infection in the United States is the vast reservoir of Salmonella in lower animals. The high incidence of infection in domestic feed, and the present methods of processing foods and food products in bulk, results in foods with a potentially high incidence of contamination with Salmonella. For example, a significant proportion varying from 1 to >50% of raw meats purchased in retail markets is contaminated with salmonella (14). The most common source of contamination is natural infection of animals raised for human consumption and contamination of the carcass during slaughter and processing. Eggs or egg products, including dried or frozen eggs, are also a very common source of salmonellae. Of the various animal species, domestic fowl, including chickens, ducks, and turkeys, constitute the largest reservoir of infection and the source most often responsible for human infection. Adequate cooking of food before consumption decreases the possibility of infection, but salmonellae may survive cooking at low temperatures (<70°C) and cooked food can be contaminated after cooking by organisms from kitchen utensils or individuals. Food or drink may also be contaminated by rats, mice, insects, or other vermin harboring these organisms. Cross-infection also occurs by the airborne route from dried foods such as egg whites or dust that contain viable Salmonella.

Salmonellae also can be transmitted directly or by fomites from humans to humans or from animals to humans without the presence of contaminated food or water, but this is not a common mode of transmission. However, cross-infection of this type is responsible for a number of outbreaks in hospitals and nursuries. Nosocomial salmonellosis is particularly devastating in newborns, immu-nocompromised individuals, patients in burn units, and those receiving multiple broad-spectrum antibiotics. Recent evidence suggests that individuals receiving antibiotics are more susceptible to the newly emerging Typhimurium phage type DT104. Nursery outbreaks have been traced to newborn infants from mothers with recent Salmonella infections.

Fishmeal, meat meal, bone meal, and other by-products of the meatpacking industry are often contaminated with Salmonella organisms. These products are incorporated into animal and poultry feeds and apparently play an important role in the perpetuation of feed animal infections that can be spread to humans.

The true incidence of Salmonella infection is difficult to determine. Reported cases represent only a small proportion of the actual number. Normally only large outbreaks are investigated and documented; sporadic cases are underreported, mainly because only patients with protracted diarrhea report to a health care provider for microbiological evaluation. Although Salmonella infections occur throughout the year, the Salmonella Surveillance Unit of the National Communicable Disease Centers has observed a distinct seasonal pattern, with the greatest number of isolations reported from July through November of each year.

A close correlation exists between the Salmonella serotypes most often responsible for human infection and those isolated from animals in any one geographic location. These similarities document the importance of nonhuman reservoirs of Salmonella in the epidemiology of infection in humans.

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