Wednesday, December 07, 2005
Bacterial Pathogens in Food
The appearance of symptoms in less than 1 hour after ingestion of contaminated food or water is typically caused by a chemical intoxication. Symptoms occurring after an incubation period of 1 of 7 hours are most often caused by Staphylococcus.Those between 8 and 14 hours are likely to be caused by Clostridium perfringes. After 14 hours, other agents are more likely. We will review some of the more common bacterial pathogens causing food poisoning.
Salmonella spp. have been the leading cause of bacterial foodborne disease worldwide. There are there species: S. typhi, S cholerae-suis, and S. enteritidis. Animals (cows, pigs, sheep, cats, dogs, hamsters, mice, turtles, snakes, ducks, geese, chickens, turkeys, doves, pigeons, and parrots) provide the natural reservoir for nontyphoidal salmonella. Contaminated milk, poultry, and eggs have implicated in many outbreaks. The main mode of transmission of salmonella has been ingestion of contaminated food, the so-called Salmonella food poisoning. Five clinical syndromes have been described in persons with Salmonella infection: asymptomatic infection (includes carrier state), enteric fever, gastroenteritis, bacteremia, and focal infections.
The gastrointestinal illness is the most common of these syndromes; the symptoms are caused by mucosal invasion and inflammation. Nausea, vomiting, crampy abdominal pain, myalgia, headaches, and fever usually occur. The stools often contain white blood cells and blood. The illness is often self-limited with resolution in 5 to 7 days in the absence of antibiotherapy.
Persons in certain high-risk groups can have a more severe and prolonged illness. Blood cultures can be positive in up t0 10% of such cases. High-risk persons include infants less than 3 months of age, HIV infected individuals, and other immunocompromised hosts. Antimicrobial therapy is not required for uncomplicated gastroenteritis.
Therapy causes no clinical improvement and can prolong the excretion of the organism. Antimicrobial therapy is usually recommended for high risk patients although it is of unproven
Shigella spp. were the second most common cause of bacterial foodborne illnesses reported by the CDC from 1983 to 1987 and the leading cause in bacterial waterborne outbreaks during 1986 to 1992 in the US.
There are four species: Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei. Although this pathogen has been reported in contaminated food and water, the principal mode of transmission is person-to-person contact. There is no known animal reservoir. Shigella is capable of surviving in foods such as milk, whole eggs, flour, and shrimp up to 30 days. Foods incriminated in Shigella outbreaks in the US include shellfish, fruits, vegetables, chicken, potato salad, fried rice, and Mexican food.
Outbreaks of shigellosis have been reported in summer camps for the mentally impaired, among institutionalized persons, and on cruise ships. Waterborne outbreaks have been related to ingestion of water contaminated with human waste during swimming or bathing. Clinial manifestations include an abrupt onset with high fever, toxic appearance, and crampy abdominal pain. Profuse watery diarrhea may be followed by small amount of mucous and bloody stools whose passage is associated with urgency and tenesmus. Antimicrobial therapy of susceptible Shigella strains dramatically shortens the duration of diarrhea, fever, and period of cummunicability.
The choice of antimicrobial therapy can be difficult due to the frequency of antibiotic resistant strains. Nalidixic acid, TMP-SMX, oxyquinolones, ceftriaxone, and cefixime are considered adequate empirical therapy, but susceptibility testing is recommended for all isolated, and therapy modified accordingly. Complications include rectal prolapse, seizures, leukemoid reaction, hemolytic uremic syndrome, and Reiter’s syndrome. Strict attention to hand-washing and personal hygiene is necessary to prevent spread of shigellosis.
Preformed enterotoxins A to E have been related to enteric disease. Type A has been responsible for more than half of the reported outbreaks of staphylococcal food poisoning in the US. The growth of Staphylococcus is favored by the high-sugar, salt, and protein contents found in dairy products, egg salads, fish, poultry, and cream-filled foods. Asymptomatic individuals carrying Staphylococcus on skin or nose can contaminate foods during preparation. The illness is characterized by vomiting, diarrhea, and abdominal pain without fever lasting less than 24 hours. A minimum of 1 ng of toxin/g of food is thought to be required to cause clinical symptoms. Staphylococcal illness can be prevented by good hygiene and by keeping high-risk foods refrigerated until served.
Clostridium perfringes was the fourth most common cause of foodborne outbreaks in the US. Clostridium perfringes is a gram-positive, anaerobic, spore-forming bacillus. The spores can survive high temperatures during initial cooking and can germinate during the cooling process. The organism can multiply if food is held at temperatures between 15.6ºC and 51.7ºC. If the food is ingested without appropriate reheating, the enterotoxin is not destroyed.
In Europe and the US meat and meat products are the principal foods associated with outbreaks. There are five strains of Clostridium perfringes (A-E). Type A has been responsible for all cases of foodborne illness. Clostridium perfringes type C causes necrotic enteritis in malnourished children living in New Guinea who ingest undercooked pig meat. The onset of symptoms begins with watery diarrhea and severe crampy abdominal pain. Fever, nausea, and vomiting are unusual. Clostridium food poisoning is a self-limited disease and requires no therapy. Mortality has been commonly associated with Clostridium perfringes type C in Papua New Guinea.
There are seven toxin types (A-G) and three syndromes described with Clostridium botulinum: foodborne botulism, wound botulism, and infant botulism. Clostridium botulinum type E, the most common toxin type, is usually associated with fish consumption, especially in Alaska and Canada. Improper preparation of home-preserved canned vegetables has been the main cause of intoxication in the rest of North America. Honey has been implicated in cases of infant botulism in California. As little as 0.1 g of a food contaminated contains enough neurotoxin to produce illness. The incubation period is usually between 12 and 36 hours but can be as long as 8 days, depending on the dose of toxin ingested. Initial symptoms include acute symmetric descending flaccid paralysis with initial involvement of the cranial nerves but without sensory involvement.
Common symptoms include diplopia, dry mouth, dysarthria, and vision disturbances. Infant botulism occurs in children younger than 6 months old and is accompanied by lethargy, weak cry, poor feeding, constipation, generalized weakness, subtle ocular palsies, and hypotonia. For foodborne botulism, emetic and gastric lavage can be beneficial in reducing the amount of toxin absorbed. The trivalent antitoxin (types A, B, and E) is considered part of therapy for foodborne and wound botulism and needs to be given as soon as possible. The antitoxin is of equine origin, so hypersensitivity reactions occur in up to 20% of the cases. Infant botulism is managed with supportive care only.
There are five categories of diarrheogenic E. coli that cause foodborne and waterborne diseases and are defined by specific virulence properties; enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enterotoxigenic E. coli (ETEC), enterohemorrhagic E. coli (EHEC), enteroaggregative E. coli (EAggEC). The EPEC has been associated with outbreaks of infantile diarrhea, which often have occurred in hospital nurseries and in communities. Contaminated food and water have been the cause for outbreaks. The disease usually peaks in the summer months. The mechanism of disease relates to specific “attaching and effacing” adherence of the organism to intestinal epithelial cells and damage to the microvilli.
The most common cause of traveler’s diarrhea, ETEC, produces two types of enterotoxins, one of them resembles cholera toxin. Diarrhea is caused by ingestion of contaminated food or water rather than person-to-person spread. It is watery and associated with crampy abdominal pain, nausea, vomiting, and malaise. High fever is uncommon. Antimicrobial prophylaxis is not recommended for pediatric patients; although antibiotics shortens the duration of illness, the diarrhea is self-limited, and oral rehydration is considered the treatment of choice. A 5-day course of TMP-SMX or furazoline has been shown to shorten the duration of illness in pediatric patients. The EIEC produces invasive, dysenteric illness. It shares identical virulence genes with Shigella. EIEC is 10,000-fold less infectious than Shigella so foodborne and waterborne disease rather than person-to-person spread is more common.
The typical symptoms include watery diarrhea, followed by bloody, mucousy stools, high fever, severe abdominal pain, and a toxic appearance. If EIEC is suspected, TMP-SMX can be used. The EHEC produces Shiga-like toxins (verotoxins); more than 50 different EHEC serotypes have been described. Serotype E. coli 0157:H7 has been the cause of major foodborne outbreaks of hemorrhagic colitis and hemolytic uremic syndrome in the US. Clusters of the illness occurred in Washington, Idaho, Nevada, and California. The illness was traced to hamburger from a fast-food restaurant chain. Other food vehicles associated are roast beef, unpasteurized milk, and apple cider. The illness usually begins with a non-bloody diarrhea that may progress to bloody with severe abdominal cramps, with little or no fever (hemorrhagic colitis). The major complications are HUS and thrombocytopenic purpura.
Campylobacter species are carried by a wide variety of domestic and wild animals, including birds, cattle, sheep, pigs, dogs, and cats. Transmission has been associated with handling, preparation, and consumption of contaminated, raw, or undercooked animal meat and poultry. Outbreaks due to raw milk have been described. The disease has a bimodal age distribution with peaks in those younger than 5 years and between 15 to 29 years old. The spectrum of clinical manifestations is broad and depends on the species involved and characteristics of the host; Campylobacter jejuni causes watery, secretory, and inflammatory diarrhea which subsides without therapy within the first week in up to 60% to 70% of the cases. Campylobacter bacteremia is rare. Antibiotic therapy of gastroenteritis is controversial but is usually recommended for patients with severe diarrhea and those who are immunocompromised. Erythromycin can shorten the duration of excretion. Strains are usually susceptible to tetracycline, chloramphenicol, aminoglycosides and quinolones.
Bacillus cereus is associated with two syndromes: emetic syndrome in association with the ingestion of farinaceous foods, especially fried rice. It is manifested by an acute onset of nausea, vomiting, and malaise after ingestion of a preformed heat stable toxin. Symptoms last 6 to 24 hours. The less common diarrhea syndrome is associated with the ingestion of high-protein foods, vegetables, sauces and puddings. It is manifested by abdominal pain, watery diarrhea, and tenesmus. Nausea and vomiting are uncommon. The symptoms last 12 to 24 hours. Treatment is supportive.
Yersinia enterocolitica is responsible for illness mainly during autumn and winter months. The major reservoir for this pathogen is pigs, although birds, dogs, cats, cows, and rabbits can also carry the organism. Ingestion of contaminated pork is the major source Y. enterocolitica. Some outbreaks of yersiniosis have been caused by contaminated milk. Waterborne outbreaks die to contaminated spring and stream water have also been reported. The pathogenesis of yersiniosis is thought to involve invasion of the ileal mucosa (causing bloody, mucous diarrhea) and production of heat-stable toxin-like enterotoxin (causing watery diarrhea). Clinical manifestations depend on the age of the persons involved, with children younger than 5 years having a self-limited gastroenteritis. Fever, abdominal pain, and vomiting can occur.
Complications in adults include reactive polyarthritis, arthralgia, and erythema nodosum. The specific diagnosis is made by isolation of the organism from stool in standard media or on the selective medium cefsulodin-Irgasan-novobiocin (CIN). Yersinia is usually susceptible to TMP-SMX, aminoglycosides, chloramphenicol, tetracycline, quinolones, and third generation cephalosporins.
The Pediatric Bulletin