Proctocolitis is inflammation that affects both the rectum and the lower part of the colon, specifically the area extending about 12 centimeters above the anus. It shares symptoms with proctitis (inflammation limited to the rectum) but goes further, typically adding diarrhea and abdominal cramping to the picture. The term covers several distinct conditions with very different causes, from sexually transmitted infections in adults to food protein reactions in newborns.
Types of Proctocolitis
Proctocolitis isn’t a single disease. It’s a descriptive term for inflammation in a specific location, and the underlying cause determines how it’s classified and treated. The three main categories are infectious proctocolitis, food protein-induced allergic proctocolitis (most common in infants), and autoimmune proctocolitis, which falls under the umbrella of ulcerative colitis.
These types look different under a microscope, affect different populations, and require completely different approaches to treatment. An infant with bloody streaks in their diaper and an adult with rectal pain after a sexually transmitted infection both have proctocolitis, but the similarities largely end there.
Common Symptoms
Regardless of the cause, proctocolitis tends to produce a recognizable cluster of symptoms. The most common include:
- Tenesmus: a persistent feeling that you need to have a bowel movement, even when the bowel is empty
- Rectal bleeding or passing blood with stool
- Mucus or pus in the stool
- Diarrhea or, less commonly, constipation
- Cramping and pain in the anus, rectum, or left side of the abdomen, often during bowel movements
- An urgent, hard-to-control need to have a bowel movement
In infectious cases, rectal discharge is common. In infants with the allergic form, the hallmark symptom is blood-streaked or mucousy stools in a baby who otherwise seems healthy and is gaining weight normally.
Infectious Proctocolitis in Adults
Infectious proctocolitis is most often caused by sexually transmitted organisms, particularly in men who have sex with men. The four most frequently identified pathogens are gonorrhea (found in about 30% of cases), chlamydia (19%), herpes simplex virus (16%), and syphilis (2%). A specific aggressive strain of chlamydia called LGV can cause especially severe symptoms, including bloody discharge, deep mucosal ulcers, and perianal ulcers.
Several gut pathogens can also cause proctocolitis without being considered traditional STIs. Campylobacter, Shigella, Salmonella, Giardia, and Entamoeba histolytica all cause inflammation that extends into the lower colon. These infections spread through the fecal-oral route, sometimes through sexual contact and sometimes through contaminated food or water.
Treatment depends on the specific pathogen. Bacterial infections are treated with targeted antibiotics. For suspected chlamydia with signs of LGV (bloody discharge, ulcers, or tenesmus), a three-week course of antibiotics is standard rather than the shorter course used for uncomplicated chlamydia. Herpes-related cases require antiviral treatment. Because multiple infections can occur simultaneously, testing typically involves swabs for several organisms at once.
Food Protein-Induced Allergic Proctocolitis in Infants
Food protein-induced allergic proctocolitis, often shortened to FPIAP, is one of the most common causes of rectal bleeding in otherwise healthy babies. Prevalence estimates range widely, from 0.16% to 17% of infants depending on the study, but the condition accounts for up to 64% of cases where infants have bloody stools. Symptoms typically appear within the first month of life, with a median onset around 2 months of age.
Cow’s milk protein is the trigger in the vast majority of cases. In one Italian cohort of 69 confirmed cases, 98.5% reacted to cow’s milk. Other documented triggers include hen’s egg (22 to 37% of cases), beef (8 to 11%), wheat (about 5.5%), and less commonly chicken, fish, tree nuts, legumes, and rice. For formula-fed babies, cow’s milk and soy are the most frequent culprits.
More than half of reported FPIAP cases occur in exclusively breastfed infants, where the trigger proteins pass through the mother’s breast milk. This surprises many parents, who assume breastfeeding would prevent allergic reactions. The proteins from foods the mother eats, particularly dairy, can reach the baby in small but immunologically significant amounts.
Managing FPIAP
For breastfed babies, the first step is usually removing the suspected trigger food from the mother’s diet. In most cases, this means eliminating all cow’s milk and dairy products. Symptoms typically resolve gradually and completely within 72 to 96 hours of the mother cutting out the offending protein. If symptoms persist despite dietary changes, switching to an extensively hydrolyzed formula (where the proteins are broken into very small fragments) is the next step. A small number of infants who don’t improve on hydrolyzed formula need an amino acid-based formula, which contains no intact proteins at all.
The prognosis for FPIAP is excellent. Most babies can tolerate the trigger food by age 1, though some take up to 3 years to develop full tolerance. Some children even experience spontaneous resolution before any dietary changes are made. This is not a lifelong allergy in the traditional sense; the immune response that drives it matures and resolves as the infant’s gut develops.
Proctocolitis as Part of Ulcerative Colitis
When inflammation in the rectum and lower colon is driven by the immune system attacking the gut lining without a clear external trigger, it falls under the spectrum of ulcerative colitis. This form tends to be chronic and relapsing rather than self-limited. The extent of inflammation is mapped through colonoscopy. Some people have disease confined to the rectum and sigmoid colon, while others have inflammation extending through the entire colon.
Unlike infectious or allergic proctocolitis, the autoimmune form requires long-term management with medications that calm the immune response. Treatment goals focus on achieving and maintaining remission, reducing flare frequency, and preventing complications over time.
How Proctocolitis Is Diagnosed
Diagnosis starts with the symptoms and clinical context. A newborn with blood-streaked stools points toward FPIAP. An adult with rectal pain and discharge raises concern for an infectious cause. Chronic, relapsing symptoms in a young adult suggest ulcerative colitis.
For infectious cases, rectal swabs test for gonorrhea, chlamydia, and herpes. Stool cultures can identify bacterial pathogens like Campylobacter or Shigella. When the cause isn’t clear or symptoms are chronic, endoscopy (a camera examination of the rectum and colon) with tissue biopsies provides the most definitive answers.
Under the microscope, different types of proctocolitis leave distinct signatures. Ulcerative colitis shows chronic changes like distorted crypt architecture (the tiny glands lining the colon become irregularly shaped) and clusters of immune cells deep in the tissue. Infectious colitis may reveal pseudomembranes, viral particles inside cells, or parasites. Allergic proctocolitis in infants typically shows high numbers of eosinophils, a type of white blood cell associated with allergic reactions, clustered in the tissue lining the rectum. These biopsy patterns help clinicians distinguish between conditions that can look identical on the surface.