Public Health Significance of Salmonella   by Dr Deryck D. Pattron, Ph.D.

Why is Salmonella important to public health safety?

Salmonella and the medical condition that it causes, Salmonellosis is one of the most commonly and widely distributed foodborne diseases. This disease in the past has caused tremendous cost to society in many countries worldwide. Millions (2-4) of cases have been reported annually and yet a significant number of cases has been unreported worldwide. In the United States it is estimated that the total cost of human salmonellosis is estimated at US$ 3 billion. In Denmark, the total cost of human salmonellosis is estimated at US$15.5 million. In general the costs of foodborne diseases are difficult to acquire and generally not available from developing countries.

One of the highlights that made Salmonella significant is that the European Commission has reported a 20-fold increase in Salmonellosis from 1980s to 1990s in a number of countries. This has resulted in an increased spread of two strains, namely Salmonella Enteritidis and Salmonella Typhimurium, causing much public health concern. Since as early as 1990s strains of Salmonella showed increased resistance to a range of antibiotics creating serious problems of treatment and recovery. This has led to tremendous human suffering, morbidity and mortality.

What is Salmonella?

Salmonella belongs to the genus of gram negative infective bacteria. The organism is transmitted through poultry, eggs, meat and other sources. Three species are recognized; salmonella typhi, salmonella choleraesius and salmonella enteriditis. It is estimated that over 2,500 serotypes exist. Salmonella typhi causes typhoid fever while salmonella typhimurium causes gastroenteritis or inflammation of the stomach and small intestines.

What is the incubation period for Salmonellosis?

The incubation period for Salmonellosis is between eight to forty-eight hours. The disease or infection lasts three to seven days if detected and treated promptly. The infection may be prolonged if diagnosis and treatment are delayed or if the bacteria become multi-resistant to antibiotics.

What are the symptoms of Salmonellosis? Symptoms of Salmonellosis include:  Fever.  Abdominal cramps.  Diarrhea.  Headache.  Chills.  Prostration.  Nausea.  Pain.

How is Salmonellosis diagnosed?

Salmonellosis is diagnosed by clinical observations and also by pathological confirmation of the pathogenic bacteria Salmonella in the blood and the feces. Blood and feces analysis are often considered essential tests in the positive identification of Salmonellosis.

How can Salmonella enter the food chain?

Salmonella can enter the food chain in three main ways: (i) Animals harbor Salmonella on their bodies and in their intestines. Animals that are reared under unsanitary conditions coupled with poor hygiene of food handlers provide an easy route for the transfer of Salmonella along the fecal-oral path. Allowing pets and other animals into food preparation areas may lead to cross-contamination of food being prepared. Petting or touching animals and pets without washing hands can lead to the transfer of Salmonella from animals to the food being prepared.

(ii) Manure, feces and litter are rich sources of Salmonella. Animals that are infected with Salmonella often pass out large amounts of Salmonella in their feces which can directly or indirect contaminate food.

(iii) Cross-contamination may occur through poor personal hygiene, poor sanitation, poor food handling and cooking practices. Of these practices hand washing is often the main culprit. Hands that are not washed before every new task, during the task, and after performing the task and when changing to another task, pose a serious risk of contamination. Cutting boards which are not frequently cleaned and sanitized between tasks and when changing use from meat to vegetables or from raw to processed or to cooked foods lead to the spread of Salmonella and cross-contamination of foods.

How is Salmonellosis treated?

Treatment usually takes the form of anti-diarrheal drugs and/or antibiotics such as fluoroquinolones, chloramphenicol, ampicillin and trimethoprim-sulfamethoxazole. How can Salmonella acquire antimicrobial resistance? Research suggests that Salmonella may acquire antimicrobial resistance through the following:  Uptake of new genetic material.  Mutation in the bacterial chromosome. How can the different types of antimicrobial resistance be detected? Different types of antimicrobial resistance have been detected using known "marker antibiotics". For example:  Uptake of new genetic material is displayed by Salmonella bacteria showing resistance to ampicillin, trimethoprin-sulfamethozazole and chloramphenicol.  Mutations in bacterial genome are displayed by Salmonella bacteria showing resistance to fluroquinolones. What are some of the common foods associated with Salmonellosis? Any food can become contaminated with Salmonella if prepared using poor hygienic practices, under unsanitary conditions and if not cooked thoroughly at the proper internal temperature for a specified time. Some common foods associated with Salmonellosis include but are not limited to:  Raw meats.  Poultry.  Eggs.  Milk and dairy products.  Fish.  Shrimp.  Yeast.  Coconut.  Sauces.  Salad dressing.

How eggs become contaminated with Salmonella?

Eggs may become contaminated with Salmonella enteritidis in two main ways: (i) Salmonella enteritidis may silently infect the ovaries of healthy appearing hens and contaminates the eggs before the shells are formed.

(ii) Salmonella enteritidis is normally present in the feces that can contaminate the outer egg shells and may penetrate cracks in the shell.

Who is at increased risk?

Healthy adults and children are at risk for egg-associated Salmonellosis, but the elderly, infants and persons with impaired immune systems are at increased risk for serious illness. In these persons, a relatively small number of Salmonella bacteria can cause severe illness. Most of the deaths caused by Salmonella enteritidis have occurred among the elderly in nursing homes. Egg-containing dishes prepared for any of these high-risk persons in hospitals, in nursing homes, in restaurants, or at home should be thoroughly cooked and served promptly.

What you can do to reduce risk?

Eggs, like meat, poultry, milk and other foods, are safe when handled properly. Shell eggs are safest when stored in the refrigerator, individually and thoroughly cooked, and promptly consumed. The larger the number of Salmonella present in the egg, the more likely it is to cause illness. Keeping eggs adequately refrigerated prevents any Salmonella present in the eggs from growing to higher numbers, so eggs should be held refrigerated until they are needed. Cooking reduces the number of bacteria present in an egg; however, an egg with a runny yolk still poses a greater risk than a completely cooked egg. Undercooked egg whites and yolks have been associated with outbreaks of Salmonella enteritidis infections. Both should be consumed promptly and not be held in the temperature range of 40 to 140 ºF for more than 2 hours.

What proactive measures can be taken to reduce Salmonellosis? (i) Vaccination of laying chickens. (ii) Avoid indiscriminate use of antibiotics as growth promoters that will minimize the appearance of new Salmonella resistant serotypes. (iii) Use probiotics to competitively exclude Salmonella form the small intestines. Feed should be supplemented with yeast or chickens should be sprayed with mucosal starter culture and/or feed the culture through water. This procedure will serve to competitively exclude Salmonella from the crop and ceca of the small intestines. (iv) Adopt and use HACCP food safety management system. (v) Use irradiation to kill Salmonella. (vi) Use heat processing to kill Salmonella. (vii) Reduce water activity to inactivate and kill Salmonella. (viii) Use low pH or acidification to inactivate and kill Salmonella. (ix) Use salt and sugar in processing to reduce water activity thereby reducing the activity of Salmonella. (x) Freezing may inactivate and destroy Salmonella. (xi) Adopt chemical disinfecting of seeds such as alfalfa sprouts to eliminate Salmonella and/or reduce to acceptable levels. (xii) Use recommended sanitizers such as chlorine used at a concentration of 20, 000 p.p.m. to sanitize floors, walls, ceiling, equipment and table tops to ensure Salmonella free environment. (xiii) Thoroughly cook foods at the proper internal temperatures. Use a food thermometer to verify that the correct internal temperature has been reached. (xiv) Keep eggs refrigerated at a temperature between 0 to 5 ºC. (xv) Discard cracked or dirty eggs as they may be a source of contamination. (xvi) Wash hands thoroughly with soap and then with sanitizer before, between tasks and after completing tasks. (xvii) Eat eggs promptly after cooking. Do not keep eggs standing for more than 2 hours after cooking. (xviii) Do not eat raw eggs because of the possibility of becoming infected with Salmonella.

What else is being done to curb the incidence and future outbreaks of Salmonellosis?

 U.S. Government Food Protection Agencies and other agencies worldwide have taken steps to reduce Salmonella enteritidis outbreaks. These steps include the challenging task of identifying and removing infected flocks from the egg supply and increasing quality assurance and sanitation measures.

 The Centers for Disease Control in U.S. and Ministries of Health in various countries have advised health departments, hospitals and nursing homes of specific measures to reduce Salmonella enteritidis infection. Some states in U.S. and other countries worldwide now require refrigeration of eggs from the producer to the consumer. The U.S. Department of Agriculture has implemented regular testing of breeder flocks that produce egg-laying chickens to ensure that they are free of Salmonella enteritidis. The U.S. Food and Drug Administration has issued guidelines for safe handling of eggs in retail food establishments.

 Research by these agencies and the egg industry is addressing the many unanswered questions about Salmonella enteritidis, the infections in hens and contaminated eggs. Informed consumers, food-service establishments and public and private organizations are working together to reduce and eventually eliminate, disease caused by this infectious organism.

 Increased training and awareness through the implementation of educational programs on television, radio, printed media and electronic media at all education levels (primary, secondary and tertiary, post-graduate) would all serve to reduce, eliminate and prevent Salmonellosis.

Summary

Salmonellosis is now being described by scientists as an emerging public health problem. Poor hygienic practices, poor sanitation, poor food handling practices and poor cooking practices have all contributed to Salmonella becoming uncontrollable in both developing and developed countries, leading to preventable morbidity and mortality. Increased resistance of Salmonella to known antibiotics have made treatments even more challenging. The best control of Salmonella is to adopt effective proactive measures that would eliminate, reduce or control Salmonella from farm to consumer in the food chain cycle. Such proactive measures would entail developing effective training programs that would educate the consumer, the public and householder about safe food preparation techniques such as the importance of and proper personal hygiene, good sanitation and good cooking practices. For the farmers good agricultural practices should be adopted and implemented together with vaccination, active surveillance and monitoring. Future research should focus on new, simple, realistic and practical methods of controlling multi-resistant strains of Salmonella. Heating processing and simply cooking eggs, poultry and meats at a safe internal temperature of 75 ºC still remain the only acceptable safe method of destroying Salmonella.

References

 Christian, J. & Greger, J. (1994). Nutrition for Living. Benjamin/Cummings Publishing Company, Fourth Edition.  Global Salm-Surv (GSS).  Jay, J.M. (2000). Modern Food Microbiology. Aspen Publication: Maryland.  Scientific Status Summary. Bacteria Associated with Foodborne Diseases. (2004). Institute of Food Technologists: Chicago.  The Medical Impact of Antimicrobial Use in Food Animals: Report of a WHO Meeting, Berlin, Germany. (1997). WHO/EMC/ZOO97.4.  U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Foodborne Pathogenic Microorganisms and Natural Toxins Handbook, The Bad Bug.

About the Author

Dr Pattron is a Public Health Scientist, Ministry of Health, Trinidad.