Where does Shigella Bacteria come from and How are Shigella infections treated?

Bacteriologists are quite proud when their name becomes attached to a disease causing organism.

This is not true of other people, American Legion members are not at all happy about having Legionnaire’s disease named after them, and homeowners and real estate agents in Lyme, Connecticut, could have done without having their city’s name permanently attached to Lyme disease.

But Kiyoshi Shiga, a Japanese bacteriologist, must have been extremely pleased to be immortalized in the name of the family of bacteria he discovered in 1870: Shigella. That the bacterium causes bloody diarrhea, fever, stomach cramps, and, in one of its forms, death from dysentery in no way diminishes the honor.

There are several different species of Shigella. The most common in the United States, causing about two-thirds of shigellosis, is Shigella sonnei, or Group D Shigella. Shigella flexneri, or Group B Shigella, accounts for the rest. Other types are rare in this country, including one unpleasant type called Shigella dysenteriae type 1, which causes deadly epidemics in many parts of the developing world. Although the illness Shigella causes is often referred to as “bacillary dysentery,” its effects in industrialized countries are mild, not at all resembling the severe and deadly dehydration characteristic of dysentery.

Shigella are gram-negative bacteria closely related to E. coli. Although there are only about 500 cases a year confirmed by the CDC, the actual number of infections is probably much larger, since most infected people recover without ever learning what organism made them sick. Infection is more common among children, particularly among the urban poor and children institutionalized in custodial care. Shigella affects only humans and some nonhuman primates, and is very infectious within families. In cases where one kid comes home with the disease, up to 40 percent of the other kids and 20 percent of the adults in the same household will become infected. Usually adults are infected without symptoms, probably because they have become immune.

Infants who are breast-feeding exclusively do not get infected, and this is particularly important in the developing world, where children are often the most common victims of the deadly S. dysenteriae type 1.

Shigellosis is frequently transmitted in food. The bacterium can survive the effects of stomach acid quite nicely, and they invade the cells of the colon, moving from cell to cell, destroying them as they go. They do this by producing a poison, shiga toxin, which causes ulcerous sores on the lining of the colon. Mild diarrhea is the most common symptom; severe or bloody diarrhea is rare with the types of Shigella common in the United States.

How do you get it? You can round up the usual suspects, people who don’t wash their hands after using the toilet, because the germ is present in stool. Vegetables can get contaminated by manure in the fields where they are harvested, flies can breed in infected feces and then contaminate food, and the bacteria can be transmitted by drinking or swimming in contaminated water. Kids who aren’t toilet trained are a good source, too.

Antibiotic treatment of shigellosis is problematic, and in most cases unnecessary. It’s hard to pick an antibiotic because by the time you know for sure by a lab test that you are infected with shigella, you’re already getting better. Antibiotics at that point wouldn’t help at all.

If they’re given early enough, antibiotics can shorten the illness and shorten the time you are infectious to others, but the germ is already largely resistant to ampicillin, tetracycline, and chloramphenicol. It still responds to a combination of trimethroprim and sulfamethoxazole and to norfloxacin, ciprofloxacin, and furazolidone, but no one wants to use these antibiotics unless absolutely necessary for fear of creating more resistant strains.

In most cases, the best treatment is just to try to get comfortable, drink fluids, and wait until it passes. Using antidiarrheal cures such as Imodium or Lomotil is a terrible idea, they slow the action of the intestines, encourage the growth of the organism in the stool, and prolong the fever.