Salmonella of Enteric Bacilli Infections
Members of the genus salmonella are ubiquitous pathogens found in humans, livestock, wild mammals, birds, reptiles, and even insects. Antigenic analysis has distinguished more than 1500 serotypes. About 10 serotypes make up most of the human isolates in a given year. A single serotype, S. typhimurium, is the most frequently isolated cause of Salmonella gastroenteritis. It also causes disease in many animal species. Other common human serotypes are S. infantis, S. heildelberg. The clinical pattern of salmonella disease can be divided into gastroenteritis, enteric fever (typhoid), bacteraemia, and the asymptomatic carrier state.
Salmonella is a rod-shaped gram-negative motile bacterium. There is widespread occurrence in animals, especially in poultry and swine. Environmental sources include water, soil, insects, factory surfaces, kitchen surfaces, animal faeces, raw meats, raw poultry, and raw seafoods etc.
Salmonella gastroenteritis usually follows the ingestion of food or drinking water contaminated by faces and accounts for 15% of foodborne infection in the U.S. Typically, the illness begins 12 to 48 hours after the ingestion and consists of nausea and vomiting, with abdominal pain and diarrhoea. Fever is present in about half the patients and a mild headache may be present. Diarrhoea persists as the prominent symptom for 3 or 4 days. Salmonellosis may be complicated by reactive arthritis and Reiter’s syndrome. The infective dose is small: as few as 15-20 cells. The pathogenesis is due to the invasion of the GI tract by the organism. There is evidence that an enterotoxin may be produced.
Associated foods include raw meats, eggs, milk and diary products, shrimps, yeast, coconut, sauces and salad dressing, cake mixes, cream-filled desserts and toppings, peanut butter and chocolate. Various salmonella species have long been isolated from the outside of egg shells. The present situation with S. Enteriditis is complicated by the presence of the organism inside the egg. Therefore, raw eggs may cause salmonella infection. Foods other than eggs have also caused outbreaks of S. Enteriditis disease. It is estimated that 2 to 4 million cases of salmonellosis occur in the US annually.
All age groups are susceptible, but symptoms are most severe in the elderly, infants, and the infirm. AIDS patients suffer salmonellosis frequently (20 fold greater than the general population) and suffer from recurrent episodes.
Enteric fever is caused by S. typhi. The bacilli enters the body via the Peyer’s patches and causes septicaemia. The first 7 to 10 days of the infection is usually asymptomatic. A high fever then develops with splenomegaly. Rose spots appear on the abdomen. Complications of typhoid fever include intestinal perforation and haemorrhage. Clinical improvement of untreated patients usually begins in the third week following infection. Typhoid fever is a severe diseases. When untreated, the average duration of fever is 30 days with a mortality of 20%; an additional 10% suffer a relapse of fever, intestinal haemorrhage or peritonitis. Because typhoid is a systemic disease, in the early stages, the organism should be cultured from the blood and not the stool. When the disease becomes established, both blood and stool may be positive, as is urine in 25% of cases. During the convalescent phase 4 to 5 weeks after infection, the blood will return to sterility but the stool may remain culture positive in half the patients. More than ¾ of patients will have high titres against O and H Ags. Those who recover may continue to excrete the organism for long periods of time. These chronic carriers serve as important reservoirs of infection.
The acute gastroenteritis caused by many Salmonella serotypes is also associated with transient bacteraemia. In humans, S. choleraesuis often presents as a focal infection without any obvious GI manifestations. In persons with sickle cell anaemia, skeletal infection is common. S. choleraesuis and S. typhimurium gastroenteritis may be complicated by endocarditis, especially in older patients with plaque or aneurysm, or meningitis in patients under 2 years of age.
About one-half of infected persons continue to excrete salmonellae 1 month after the symptoms have disappeared and 1 in 20 persons still do 5 months later. An unknown fraction of people become carriers after asymptomatic infection; the median carriage rate of Salmonella among healthy persons in developed countries is 0.13%.
Salmonella can be isolated on any of the common enteric media. Historically, rising titres of antibodies against Salmonella O and H Ags (Widal test) are used to diagnose typhoid. Methods have been developed for the detection of salmonella in foods.