Etiologic Agents

ETEC

The known causes of travelers' diarrhea are shown in Table 50-1. Heading the list in nearly all studies is ETEC. These organisms are the most common bacterial cause of diarrhea in children living in the developing world, and they are also the most commonly seen in travelers. These organisms produce either one or both of two enterotoxins: a heat labile enterotoxin (LT), a large molecule that is very closely related to cholera toxin, and a heat stable enterotoxin that is a small molecular weight polypeptide. They also possess colonization factors (CFAs), proteins that facilitate their binding to the small intestinal mucosa. Both LT and CFAs are antigenic, so that infection results in some degree of immunity.

Enteroaggrative E. coli

A more recently discovered type of E. coli, called enteroag-grative E. coli, has also been found to be common in travelers' diarrhea. This organism was first recognized by its ability to aggregate when placed in vero cell cultures. The mechanisms of virulence are not as well described as they are for ETEC, but they are also known to be common causes of diarrhea in children living in the developing world.

TABLE 50-1. Estimates of the Most Frequent Causes of Travelers' Diarrhea

EtioIogic Agents Frequency (%)

Enterotoxigenic Escherichia coli 25-40

Enteroaggregative E. coli 15-25

Shigella 5

Campylobacter jejuni 5*

Salmonella < 5*

Giardia lamblia 1-2

Entameba histolytica < 1

Cyclospora < 1f

Cryptosporidium < 1

Rotavirus 1

Norovirus < 1*

Other agents§ 1

Unknown origin 20-30

Data from Steffen and Sack, 2003; Ericsson et al (editors), 2003; and Sack, 1990.

*These organisms are isolated much more frequently in Thailand. Tound most frequently in Nepal. Tound frequently in passenger on cruise ships.

§These include Vibrio sp (including V. cholerae, V. parahemolyticus), Aeromonas, and other diar-rheagenic Escherichia coli.

Campylobacter

The one organism that is particularly environmentally specific is Campylobacter jejuni, which is found mostly as a cause of travelers' diarrhea in Thailand. The organism is frequently found in meats prepared in the markets there. Because it is also frequently antibiotic resistant, it may require some modification of antimicrobial therapy. Campylobacter sp are also the most frequent causes of infectious diarrhea in the United States among young adults, probably because they are widely distributed in commercially prepared (but highly contaminated) poultry sold in supermarkets across the United States.

Shigella and Salmonella

Shigella and Salmonella are found infrequently in travelers' diarrhea. The presence of bloody stools and/or fever should suggest the possibility of this diagnosis.

Other Bacterial Pathogens

Other bacterial pathogens are very infrequent. There are probably new agents yet to be found.

Viruses

Viruses as causes of travelers' diarrhea are infrequent, because most adults in the world have had rotavirus infections during their lives, they are relatively resistant to this infection, even though children living in developing countries are highly susceptible and the disease is frequent among them. Recently, however, the Norwalk agent virus (Norovirus) has been found to cause several outbreaks of gastrointestinal illness on cruise ships. The illness is characterized more by vomiting than diarrhea and has been called "winter vomiting disease."

Protozoa

With a few exceptions, protozoan parasitic infections are very rarely associated with acute travelers' diarrhea, they are more likely to be associated with persistent diarrheas. Giardia lamblia is probably the most frequently recognized organism in the persistent diarrhea of travelers. The organism is ubiquitous in areas where sanitation is less than optimal. Entamoeba histolytica is also found infrequently, but can produce severe persistent diarrhea. Cryptosporidium is a relatively frequent cause of acute childhood diarrhea in the developing world, but in the developed countries it is usually associated with chronic diarrhea in persons with immune deficiency disease. It may cause an acute episode of diarrhea in travelers which is short lasting. Cyclospora is a more recently described parasitic infection in travelers, particularly those visiting or living in Nepal. It rarely seems to be seen in other parts of the world in travelers. Usually producing a persistent diarrhea that may last months, it can be associated with considerable weight loss. Blastocystis hominis is frequently found in stools of returned travelers, but there is no evidence that this causes diarrhea. It should be thought of as a nondisease-associated protozoa, such as E.coli and Endolimax nana.

Constipation Prescription

Constipation Prescription

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