Shigella of Enteric Bacilli Infections
Shigella
The shigella species are the principal agents of bacterial dysentery. The disease consists of fever, abdominal cramps, and bloody diarrhoea with mucus. As few as 100 organisms can lead to infection and hence they are the most communicable bacterial agents of diarrhoeal disease. In developed countries bacillary dysentery is primarily a disease of children from 6 months to 10 years of age. The genus Shigella and Escherichia are much closer in DNA homology than other species. All shigella are non-motile and lactose-negative and they do not produce gas from fermentable carbohydrates. The genus Shigella is subdivided into four groups on the basis of their biochemical and serological reactions.
Group A - Sh. ysenteriae, 10 serotypes, do not ferment mannitol
Group B - Sh. Flexneri, 6 serotypes, mannitol fermenting
Group C- Sh. Boydii, 15 serotypes, mannitol fermenting
Group D - Sh. Sonnei, one serotype, mannitol fermenting
Pathogenesis
Shigellosis is acquired by ingestion. Because of its high infectivity, person to person spread is of great significance although food and water do play a role. The penetration of epithelial cells and subsequent intracellular multiplication of the microorganism depends on a plasmid of approximately 140 Md, which is present in all virulent Shigella stains. Some strains produce a Shiga toxin which is very similar to the verotoxin produced by E. coli O157:H7. It causes paralysis when injected into small animals and is cytotoxic to cell cultures. It is composed of an enzymatically active A subunit (32000 daltons) and several identical receptor-binding B subunits (7700 daltons). The toxin inactivates the mamalian 60S ribosomal subunit.
The incubation period is between 12 to 50 hours. Symptoms consist of abdominal pain, cramps, diarrhoea, fever, vomiting, blood, pus or mucus in stools. Severe infections result in mucosal ulceration, rectal bleeding and dehydration: the fatality may be as high as 10-15% with some strains. Reiter’s disease, reactive arthritis, haemolytic uraemic syndrome are possible sequelae.
Epidemiology
Sh. Dysentery type 1 is of great public health importance because it causes especially severe infections and may occur in explosive epidemics. Large epidemics had been reported from many parts of the world including Central America, Central Africa, and the Indian subcontinent. These organisms harbour an R plasmid encoding resistance to several antibiotics. Although there are more than 30 recognized Shigella serotypes, only a few predominate in any geographic area. Sh. Sonnei (which causes a relatively mild disease) predominate in industrialized countries.
Associated foods include salads, raw vegetables, milk and diary products, and poultry. Contamination of these foods is usually through the faecal-oral route. Faecally contaminated water and unsanitary handling by food handlers are the most common causes of contamination. Sh. Sonnei is usually involved in food borne outbreaks, whereas Sh. dysenterie is usually associated with contaminated water. Sh. flexneri is now thought to be in large part sexually transmitted.
Laboratory diagnosis
Shigella are easily identifiable in the clinical laboratory. In the first few days of illness, the stool contains large numbers of organisms. When shigellosis is suspected, a faecal sample should be cultured on differential media that inhibit the growth of gram-positive organisms and that also distinguish between most pathogenic and nonpathogenic enteric species (e.g. MacConkey agar). Suspect colonies are picked to other differential meida and subjected to a battery of biochemical tests. Those which are identified as Shigella may be further characterized by type-specific antisera.
Organisms are difficult to demonstrate in foods because methods are not developed or are insensitive. A nucleic acid probe to the virulence plasmid has been developed by the FDA and is currently under field test.
Treatment
As with all diarrhoeal disease, the cardinal consideration is to maintain fluid and electrolyte balance. Because the disease is self-limiting, most experts consider antibiotic treatment only for the young. It reduces the average duration of the illness from 5 to 3 dyas and reduces the excretion of viable organisms in the stool. Shigella frequently acquire R plasmids which render them resistant to many common antibiotics. Where the sensitivity is known, ampicillin or tetracyclin is usully effective.
Salmonella
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.
Gastroenteritis
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
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.
Extraintestinal Manifestations
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.
Carriers
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%.
Laboratory Diagnosis
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.