By Kelly A. Reynolds, MSPH, Ph.D.
Salmonellae are bacterial pathogens common throughout the world. More than 2,000 known species have been identified—all of which are pathogenic to humans—causing a range of symptoms from diarrhea to death. The most well-known serovars—subdivisions of salmonellae species based on distinct cell wall and flagellar characteristics—associated with human enteric illness include S. enteritidis, S. typhi and S. paratyphi. Readily found on raw poultry products and in human and animal wastes, these serovars are spread by food and waterborne routes of transmission. In fact, S. typhi and Vibrio cholera were the first recognized waterborne pathogens. Since their discovery in the 19th century, they have been responsible for tremendous morbidity and mortality worldwide.
In the United States, the number of cases of nontyphoidal salmonellosis (illnesses primarily caused by S. enteritidis) is between 2 and 5 million per year. The odds of contracting salmonellosis are 1:50 to 1:125 per year in the United States, making it the second most diagnosed gastrointestinal illness. More than 15,000 cases each year result in hospitalization.
Up to 95 percent of salmonella infections in the United States are due to foodborne exposures, mostly involving eggs. Although only 1 in 10,000 eggs are contaminated with salmonellae, many dishes made in restaurants and institutional or commercial kitchens are made from pooled eggs. Consider that if 500 eggs are pooled, 1 in 20 batches will be contaminated with the bacteria. In addition, a third of retail chickens have been found to harbor detectable levels of the pathogen.
Although food is a much more significant route of exposure, water may be directly or indirectly associated with salmonella transmission. Most salmonella are capable of prolonged survival in water and may even grow in heavily polluted water in warmer months. They may be found in any water source subject to fecal contamination that have been isolated from sewage-impacted surface waters as well as groundwaters. Groundwater sources of contamination may include sewage, percolation from agricultural land, leaking domestic drains, and seepage from septic tanks.
Indirect associations with salmonella infections may include avenues such as shellfish from polluted waters that are consumed or water used in food processing or preparation. Outbreaks have occurred from contaminated water used to cool cans of food after heat treatment. Contaminated water plays a role in the persistence and spread of the pathogens among domestic animals that may transmit the organisms via direct contact, food consumption, or by contributing to the level of environmental contamination.
Root of the cause
S. typhi causes typhoid fever. An estimated 16 million cases of typhoid fever and 600,000 related deaths occur worldwide. Although no longer common in developed countries, typhoid fever is widespread in developing countries and can cause serious or fatal disease. Unlike other salmonella, S. typhi only infects humans and is spread by food and water. Contamination of water with human feces is the primary transmission route. The serovars associated with typhoid and typhoid-like fevers (i.e., S. typhi, S. paratyphi) may infect any of the internal organs and have a fatality rate 10 times higher than those causing salmonellosis.
S. enteritidis is the most common serovar in developed countries, primarily causing a self-limiting stomach illness known as salmonellosis (i.e., food poisoning) but may also result in serious, life-threatening illnesses. Control of this organism in the environment is limited by the fact that there are many animal reservoirs contributing to environmental contamination including chickens, turkeys, pigs, cows, and other domestic and wild animals. Pets—including reptiles, birds, cats, dogs and many other warm and cold-blooded animals— can serve as carriers. Handling, consumption and co-habitation with animals contribute to individual exposures to salmonellae. To complicate matters further, infected animals such as humans may not show signs of illness.
Although cooking foods easily kill salmonella, many foods are eaten raw (i.e., fruits and vegetables), undercooked (i.e., eggs, poultry) or non-pasteurized (i.e., orange juice, milk). Numerous foods utilize raw eggs in their preparation (i.e., protein drinks, Caesar salad dressing, Hollandaise sauce, homemade ice cream and mayonnaise). Various salmonella species have been isolated from the outside of egg shells, while S. enteritidis has even been found inside the egg, suggesting that infection can be transmitted from the hen to her offspring.
Surface waters receiving discharge from domestic waste, meat processing facilities, or other animal wastes are likely to be contaminated with salmonella. Once in the environment, salmonella may survive in water, soil and on environmental surfaces for days to months and for months to years in feces.
All age groups are susceptible to salmonella infections but the symptoms are most severe in the elderly, infants and the infirmed. Patients with acquired immunodeficiency syndrome (AIDS) suffer from salmonellosis frequently and often experience recurrent episodes. The infectious dose is not well determined, as infection is highly dependent on the individual. Volunteer studies have shown that approximately 10 million organisms are needed to induce infection when ingested with milk. In other instances, as few as 15-24 cells cause infection.
Salmonellosis usually appears 12 to 36 hours following ingestion of contaminated food or water. The bacteria penetrate the gut to inflame the intestinal lining where enterotoxin is produced. Effects can spread to the blood stream, causing increased complications. Symptoms may last a week or more and include nausea, vomiting, abdominal cramps, diarrhea, fever and headache. Persons with diarrhea usually recover completely but it may be several months before the intestines and bowels function properly.
Approximately 2 percent of those infected will develop pains in their joints, irritation of the eyes, and painful urination—a condition called Reiter’s syndrome. This syndrome can last for months or years and may lead to chronic arthritis. Generally, salmonellosis isn’t fatal except in immunocompromised individuals; however, the disease can be serious enough that hospitalization and antibiotic treatment are required. Unfortunately, some salmonellae are resistant to antibiotics, largely as a result of their use to promote the growth of feed animals.
Relative to salmonellosis, symptoms of typhoid fever are much more severe and include fever, headache, anorexia, enlarged spleen, coughing and constipation, rather than diarrhea. Intestinal hemorrhaging and tearing may also occur. Illness usually occurs in one to three weeks but may not manifest for up to three months. Symptoms of S. paratyphi are similar to S. typhi infections but much milder and shorter in duration. Incubation is usually one to 10 days. A vaccine is available for the prevention of typhoid fever and may be recommended for travelers to endemic regions but it’s not 100 percent effective and requires frequent (every two to five years) booster shots.
Continued spread of salmonella is secured by the fact that a carrier state results in up to 4 percent of those infected. Carriers may be permanently infected and even transmit the organism, but don’t demonstrate any signs of disease. The organism is usually carried in the gallbladder and secreted in the stool. Because antibiotic treatments are thought to induce a higher rate of carriers, they are usually avoided except for severe or life-threatening infections.
In 1908, the introduction of chlorine treatment and filtration of water resulted in a 99.9 percent reduction of typhoid incidence in some U.S. cities. Eventually, chlorine disinfectants virtually eliminated typhoid, cholera and bacterial dysentery in the United States. Conventional water treatment processes, when properly administered, should eliminate salmonella contamination. Although they are more resistant to disinfectants than coliform indicators, the absence of coliforms and E. coli from treated drinking water appears to provide adequate ensurance that salmonellae are also absent. Contamination of drinking water after treatment, i.e., in the distribution system, is still a concern since re-growth of salmonellae during transport is possible.
The feces or urine of infected individuals may be directly analyzed to determine salmonella infection; however, isolation from water environments typically requires an enrichment and isolation step where a broth medium allows the target bacteria to multiply to numbers large enough to facilitate their isolation and identification. Numerous enrichment media are available for salmonella detection, and often contain inhibitors to interfering bacterial growth. Unfortunately, these inhibitors may also prevent the growth of stressed salmonella and lead to an underestimation of the true level of contamination in the test environment.
Chlorine disinfection is currently the most widely used, practical and effective disinfection technology available for water treatment. Although debates continue over the potential carcinogenic effects of chlorine exposure over time, application of this technology to water treatment has saved countless lives that would have otherwise been lost to infectious disease. In the absence of disinfection, drinking water can be boiled for 2-3 minutes to eliminate enteric pathogens. Although effective against all microbial pathogens, boiling isn’t the most efficient means of water treatment, requiring substantial time and energy. Point-of-use (POU) devices offer a means for routine removal of pathogenic bacteria and other harmful insults from drinking water. A variety of POU systems utilizing reverse osmosis, distillation, filtration and other purification methods are capable of bacteria removal. Salmonellae are relatively large enteric bacteria (0.6 micron) and can be effectively removed by POU filters. Disinfection treatments (ozone, chlorine, ultraviolet, etc.) provide alternative means of protection.
Proper hand washing is essential to reduce the spread of salmonella, especially following the use of the bathroom and prior to cooking or handling foods. Poultry, meat and eggs should be well cooked while raw or unpasteurized milk and dairy products should be avoided. Raw eggs may be unrecognized in certain foods (remember when you thought it was safe to eat cookie dough or cake batter?). To address this concern, a method for pasteurizing eggs within the shell has been developed. Cross-contamination of raw meats with produce and cooked foods should be avoided, keeping in mind that cutting surfaces and utensils, in addition to hands, can spread the pathogen from one food to another. Finally, infected individuals shouldn’t engage in the preparation, manufacture or handling of food or drink for the consumption of others.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds also has been a member of the WC&P Technical Review Committee since 1997.