Norovirus, previously known as Norwalk-like virus, causes approximately half of all gastroenteritis outbreaks worldwide and is a common cause of sporadic gastroenteritis in developed countries. Local outbreaks are usually related to contaminated food or water, but person-toperson transmission underlies most sporadic cases. Infections spread easily within schools, hospitals, nursing homes and, on other densely populated communities including cruise ships. After a short incubation period, affected individuals develop nausea, vomiting, watery diarrhea, and abdominal pain. Biopsy morphology is nonspecific. The disease is self-limited.
The encapsulated rotavirus infects 140 million individuals and causes 1 million deaths each year, making rota-virus the most common cause of severe childhood diarrhea and diarrhea-related deaths worldwide. Children between 6 and 24 months of age are most vulnerable. Protection in the first 6 months of life is probably due to the presence of antibodies to rotavirus in breast milk, while protection beyond 2 years of age is due to immunity that develops after the first infection. Outbreaks in hospitals and day care centers are common, and infection spreads easily; the estimated minimal infective inoculum is only 10 viral particles. Rotavirus selectively infects and destroys mature (absorptive) enterocytes in the small intestine, and the villus surface is repopulated by immature secretory cells. This change in functional capacity results in loss of absorptive function and net secretion of water and electrolytes, compounded by an osmotic diarrhea from incompletely absorbed nutrients. Like norovirus, rotavirus infection becomes symptomatic after a short incubation period and manifests as several days of vomiting and watery diarrhea. Vaccines are now available, and their use is beginning to change the epidemiology of rotavirus infection. Unfortunately, oral rotavirus vaccines have been less effective in developing countries where they are most needed.
Parasitic disease and protozoal infections affect over half of the world’s population on a chronic or recurrent basis. The small intestine can harbor as many as 20 species of parasites, including nematodes, such as the roundworms Ascaris and Strongyloides; hookworms and pinworms; cestodes, including flatworms and tapeworms; trematodes (flukes); and protozoa.
Ascaris lumbricoides. This nematode infects more than 1 billion individuals worldwide as a result of human fecal-oral contamination. Ingested eggs hatch in the intestine, and larvae penetrate the intestinal mucosa. From there, the larvae migrate via the splanchnic circulation to the liver, creating hepatic abscesses, and then through the systemic circulation to the lung, where they can cause Ascaris pneumonitis. In the latter case, larvae migrate up the trachea, are swallowed, and arrive again in the intestine to mature into adult worms. Diagnosis is made by detection of eggs in the stools.
Strongyloides. The larvae of Strongyloides live in fecally contaminated ground soil and can penetrate unbroken skin. They migrate through the lungs to the trachea from where they are swallowed and then mature into adult worms in the intestines. Unlike other intestinal worms, which require an ovum or larval stage outside the human, the eggs of Strongyloides can hatch within the intestine and release larvae that penetrate the mucosa, creating a vicious cycle referred to as autoinfection. Hence, Strongyloides infection can persist for life, http://ebooksmedicine.net and immunosuppressed individuals can develop overwhelming infections.