Abscesses (cavities of pus formed from disintegrating tissue) in the perianal area are common complicating features of many diseases and disorders of the large intestine. Fungal infections of the moist and sometimes poorly cleansed area around the anus are common and permit the maceration (or gradual breaking down) of tissue and invasion by bacteria from the skin and colon. In diabetics, who are susceptible to skin infection, perianal hygiene is very important.

Bacterial infections

The colon may become inflamed because of invasion by pathogenic, or disease-causing, bacteria or parasites. A variety of species of Shigella, for example, attack the mucous membrane of the colon and produce an intense but rather superficial hemorrhage. In infants and in the elderly, the amount of fluid and protein lost by the intense inflammatory response may be fatal, but ordinarily such symptoms are less serious in otherwise healthy persons. Salmonella species, responsible for severe generalized infections originating from invasion of the small intestine, may damage the lymph follicles of the colon, but they do not produce a generalized inflammation of the colon (colitis). The cytomegalic virus, on the other hand, can cause a severe colitis, producing ulcerations. Lymphopathia venereum causes a more generalized and superficial colitis.

Food residues provide an excellent culture medium for bacteria, and the interior of the colon is a nearly ideal environment for their growth. The most widely distributed parasite producing disease in the colon is the protozoan Entamoeba histolytica. This parasite enters the digestive tract via the mouth and lodges in the cecum and ascending colon. This usually results in irritability of the ascending colon and failure to absorb water properly, so that intermittent, watery diarrhea ensues. The amoebas undermine the mucosal coat and may create large ulcerations that bleed excessively. Stools contain blood, but there is little pus or other evidence of reaction by the colon to the invading organism. In more generalized amoebic colitis, the rectum and sigmoid colon are invaded by E. histolytica, which manifest their presence by numerous discrete ulcerations separated from each other by a relatively normal-appearing mucous membrane. The amoebas may enter the portal circulation and be carried to the liver, where abscesses form and sometimes rupture into the chest or the abdominal cavity. Immunologic tests of the blood may help in diagnosis. After identification of the parasites by direct smear tests from the margin of the ulcers or from the stools, a combination of amoebicidal drugs and a broad-spectrum antibiotic—i.e., an antibiotic that is toxic to a wide variety of parasites, usually metronidazole and tetracycline—is administered.

Colitis

The most common form of chronic colitis (inflammation of the colon) in the Western world, ulcerative colitis, is idiopathic (i.e., of unknown cause). Ulcerative colitis varies from a mild inflammation of the mucosa of the rectum, giving rise to excessive mucus and some spotting of blood in the stools, to a severe, sudden illness, with destruction of a large part of the colonic mucosa, considerable blood loss, toxemia and, less commonly, perforation. The most common variety affects only the rectum and sigmoid colon and is characterized by diarrhea and the passage of mucus. Ulcerative colitis tends to follow a remitting-relapsing course. Diagnosis is determined by performing a colonoscopy or a biopsy.

Another type of colitis arises when antibiotic use causes the abnormal proliferation of certain types of bacteria in the colon, leading to inflammation. This disorder is treated by stopping the use of the causal antibiotic and administering others such as vancomycin or mexronidazole. About 15 percent of all cases of colitis involve extension of the disease beyond the area initially affected, with an increase in severity. Where the destruction has been extensive, there is a risk of malignancy 10 to 20 years after the onset of the disease.