Shigellosis is an enteric infection with one of the species of Shigella bacilli, which may be asymptomatic or may cause dysentery.Bacillary dysentery is usually a self-limited, acute illness characterized by diarrhea with mucous-bloody feces, tenesmus, fever, and abdominal colic and tenderness. Symptomless infected persons and those convalescing from dysentery may harbor Shigella in the stool for several days, serving as a source of infection for others. The infection is worldwide and is most common among persons living in crowded unhygienic circumstances. Man is the principal host to the micro- organism, and the infection is usually transmitted from person to person, directly or indirectly.
Dysentery has been recognized for centuries. Delineation of dysentery attributable to Shigella, however, was not possible until 1896, when epidemic diarrhea in Japan was shown by Shiga to be caused by a specific micro-organism.Association of the bacillus isolated from feces of patients with the disease was possible by demonstration of rising titers of specific serum agglutinins in a significant proportion of patients. Subsequently, Flexner, Boyd, and Lentz identified other species of Shigella bacilli responsible for dysentery in man.
From the beginning, dysentery has been a nemesis to encamped military troops, persons living in unhygienic conditions, and patients in mental institutions. It has occasionally caused epidemic disease in hospitals, nurseries, arid schools. Currently, Shigella infection is responsible for a significant proportion of diarrheal disease particularly among children and older persons living in crowded urban areas. There was a progressive increase in the number of infections identified in England and Wales after 1940. Over 40,000 cases were noted in 1960.
Strains of Shigella can be characterized antigenically, and there are four serologic groups responsible for disease in man. These are usually identified as serotypes of Shigella dysenteriae (shiga), Shigella flexneri, Shigella boydii, and Shigella sonnei. Another group, formerly referred to as Shigella alkalescens, may also produce enteric infection in man, but biochemically and antigenically it is more closely related to coliform bacilli.
The Shigella are gram-negative, slender, non motile, nonsporulating bacilli, which on primary isolation resemble coccobacilli. They are aerobic and facultative anaerobic, growing readily on relatively simple media at 37° C. They can be selectively grown in the presence of various bile salts (SS and desoxycholate media), thereby being distinguished from most coliform bacilli. Species and strains of Shigella can be identified by carbohydrate fermentation and antigenic analysis with specific antiserum. As with other enteric pathogens, Shigella bacilli do not ordinarily ferment lactose.
Shigellosis is most’common in tropical countries under unhygienic, crowded conditions, but is endemic throughout the world.Young children appear to be more susceptible than adults to bacillary dysentery, but the disease is less common in the first six months of life than in older infants and children. Eighty per cent of criircfren under nine years of age infected with Shigella will develop dysentery, whereas only about 50 per cent of infected older persons develop illness. Under the age of 20 years the frequency of shigellosis in males is .greater than in females, but over the age of 20 years the reverse is true.
Nevertheless, the case fatality rate is higher in men at all ages. In some countries bacillary dysentery has its peak incidence in summer months, often following heavy rains, whereas in other areas, such as the United States, the highest incidence may-be in winter, spring, or autumn. The months when shigellosis is particularly prevalent in tropical and semitropical countries corresponds with the time of year when flies are prevalent. Shigella may be found in the intestinal tracts of flies having contact with infected human feces, but carriage of the bacilli on the insect’s feet is probably the means by which it may spread infection. In countries with a high standard of sanitation flies play an inconsequential role in transfer of infection.
Infection is usually transmitted by patients with dysentery, ambulatory persons convalescing from the disease, or asymptomatic carriers. Inadequately washed hands or contaminated inanimate articles are the principal means of transmission. The seats of toilets may become contaminated during flushing and serve as a source of infection. Epidemics of shigellosis have been related to milk, ice cream, and other foods and water contaminated by the hands, or feces of infected persons. Shigella in dust has also been incriminated in outbreaks of bacillary dysentery. These sources of infection are also important in perpetuation of endemic infection.
Outbreaks of shigellosis in winter usually spread from an infected person in a school or institution to other persons. Secondary cases may result in extension of the disease to other schools or institutions such as hospitals, or nurseries. Relapsing disease, reinfection, and chronic infection may enable perpetuation of infection among patients in nursing homes and inmates of mental hospitals. In this setting, outbreaks are often not explosive.
Endemic and epidemic shigellosis is commonly characterized by isolation of several different serologic types of Shigella. Major outbreaks caused by a single serotype are rare, but small outbreaks may be attributable to a single serologic type.
Shigella dysenteriae (shiga) is an uncommon cause of infection in Britain, Europe and North America, but is responsible for a significant proportion of cases in Asia. Shigella sonnei is the prominent cause of bacillary dysentery in countries where personal contact and endemic infection, rather than poor sanitation and unhygienic conditions, are responsible for spread of the disease
Following infection, the bacilli may be isolated from feces or rectum for only a week or slightly longer. Rarely does carriage of the bacilli persist as long as three months. Carriage of Shigella dysenteriae (shiga), however, persists longer than with other species, and carriage of Shigella flexneri may be intermittent during convalescence from infection. Generally, however, persistent carriers of Shigella dysenteriae remain ill, whereas carriers of Shigella flexneri are well. Convalescence from infection owing to Shigella sonnei is not associated with prolonged excretion of the bacilli in feces except in the very young and very old.
Although Shigella infection is largely confined to human beings, primates have been shown occasionally to be a source of infection in man. Shigella survive in eggs, oysters, clams, and shrimp for many days, bu: these serve as sources of infection only when contaminated by infected persons and their excreta.
Pathogenesis and Pathology.
Shigella species possess an endotoxin. similar to that of other gram- negative bacteria, but it seems unlikely that it plays a significant role in the pathogenesis of bacillary dysentery. Shigella dysenteriae (shiga) produces an exotoxin that car. exert a deleterious effect upon the nervous system. The occurrence of paralytic manifestations in dysentery produced by this species may be attributable to the exotoxin.
Bacillary dysentery is infrequently associated with bacteremia, and the infection is confined to the intestinal mucosa, occasionally invading mesenteric lymph nodes. Morphologic legions are most frequently observed in the colon, occasionally involving the terminal ileum. Ulceration of the mucosa develops, with intervening inflamed membrane but nc undermining of the ulcer edges as in amebic dysentery. The bowel wall is infiltrated with granulocytes, there is edema of the submucosa, and occasionally the involvement may extend to the serosa. If ulceration is not extensive, healing occurs without scarring, but when it is severe, fibrosis and even stenosis of the bowel may develop.
Clinical Manifestations of Shigellosis Infection.
Patients with shigellosis may have simple self-limited diarrhea, acute gastroenteritis, true dysentery, or no symptoms of illness. It has beer, proposed that bacillary dysentery may occasionally be responsible for chronic colitis, but it has been difficult to establish this relationship with certainty.
The incubation period of dysentery is usually about 48 hours, but it may be shorter. The illness begins abruptly with abdominal cramps, pain relieved by defecation, and watery diarrhea. This is followed promptly by the development of tenesmus, feverishness, and passage of mucous stool, occasionally containing blood. Abdominal tenderness, most pronounced in the lower quadrants, is found, and the bowel is hyperactive on auscultation. The temperature rarely rises very high, except in children, in whom it may become 104° F. or more. Without treatment the illness persists for a few days and then subsides.
Acute gastroenteritis, associated with nausea, some vomiting, and diarrhea, is seen particularly in outbreaks of infection by Shigella sonne:.
Shigella dysenteriae (shiga) infection is often more severe than that caused by the other species. The fever may be higher, hypotension develops more frequently, and the intestinal symptoms are more intense. Recovery may be delayed, and debility may persist for several weeks. Peripheral neuritis more commonly complicates infection by Shigella dysenteriae.
Conjunctivitis, iritis, and nystagmus are occasional complications of bacillary dysentery, appearing between the first and second weeks of disease. Nonsuppurative arthritis may also develop. Fluid and electrolyte depletion may be considerable, particularly in infants and children, producing severe dehydration and acidosis. Potassium deficit may be recognized, particularly as the patient’s fluids are replenished. Convulsions develop occasionally in children, but appear to be correlated to the degree of fever.
At times, bacillary dysentery seems to have been precipitated by onset of measles, and almost half the cases may be accompanied by infection with enteropathic viruses including adenovirus, echovirus, poliovirus, and Coxsackie virus. The association of a virus with the disease, however, does not influence its clinical manifestations. The frequency of serious underlying disease in persons with shigellosis is not as common as in salmonellosis. Massive intestinal bleeding occurs in severe bacillary dysentery when there is extensive intestinal necrosis and ulceration.
Diagnosis of Shigella Infection .
Isolation and identification of Shigella in the stool or from swab culture of the rectal mucosa are the only means of establishing the diagnosis of shigellosis. The serum agglutinin titer will rise in about half the patients with bacillary dysentery, but it is not ordinarily useful for diagnostic purposes. Shigella survive in feces for only a short while. Therefore, feces should be promptly cultured, or the specimen should be collected in a 30 per cent glycerol-saline solution for preservation. Examination of the feces microscopically in bacillary dysentery will usually reveal a large number of granulocytes, comprising about 90 per cent of all cells, apart from erythrocytes. As convalescence begins, mononuclear cells become predominant.– Specific immunofluorescence techniques have been developed for the rapid detection of Shigella in feces. This test agrees with cultural results in over 90 per cent of instances.
Viral enteritis caused by echovirus, Coxsackie, and poliomyelitis viruses may be epidemic and may be confused with shigellosis. Fever is uncommon, however, in viral enteritis unless there is severe dehydration, and the feces contain no blood or pus. Staphylococcal enterocolitis develops usually in hospitalized patients undergoing abdominal surgery who have received antimicrobial therapy. Amebic dysentery usually has a gradual onset, the diarrhea is ordinarily not severe, there is often little or no fever, and microscopic examina-
tion of the feces will reveal a predominance of mononuclear cells. Sigmoidoscopy reveals undermined ulcers with normal intervening mucosa. Salmonellosis is more often accompanied by nausea and vomiting at onset and occasionally by a chill and very high fever. Blood cultures are often positive. Staphylococcal food poisoning and many forms of viral gastroenteritis are more prominently associated with nausea and vomiting than with diarrhea.
Sulfonamides were effective in treatment of shigellosis when they were first used. Resistant bacilli emerged, however, and sulfonamides are not now considered to be the most effective drugs for the infection. Resistance of Shigella to multiple antimicrobials has been induced by episomal transfer from other enteric bacilli. Conceivably, therefore, drug-susceptible Shigella may become drug-resistant from other drug-resistant bacilli in the intestinal tract.
Tetracycline has been an effective drug against Shigella, but resistant strains have been identified. A few strains of Shigella are resistant to chloramphenicol. Colimycin and neomycin are almost always effective in vitro, but a few strains resistant to ampicillin have been found. Streptomycin given orally has been used to treat shigellosis, but strains resistant to this drug have also been identified. Cephalothin and massive doses of penicillin G may be effective.
At present, ampicillin in a dosage of 2 or more grams orally each day in divided doses for five days is the preferred agent in treatment of shigellosis in adults. Chloramphenicol or tetracycline may be effective alternatives if either the micro-organism is resistant to ampicillin or the patient is allergic to the penicillins. Oral streptomycin, 0.5 gram twice daily, is also recommended in Great Britain.
Fluid and electrolyte replacement should be given to the patient who is in collapse or who is dehydrated. Opiates such as paregoric or morphine may alleviate the abdominal discomfort and tenesmus but should be used cautiously.
The mortality rate in untreated bacillary dysentery is about 0.1 per cent or less, but it may be higher during famine or starvation. In addition, the fatality rate with dysentery caused by Shigella dysenteriae (shiga) is higher than with that attributed to other Shigella. Death rarely occurs when appropriate treatment is prescribed.