In allergic colitis, the immune system overreacts to the presence of specific proteins, often through breast milk or infant formula. It causes inflammation in the colon, leading to symptoms such as rectal bleeding.

Allergic colitis is one of the most frequent symptoms of cow’s milk protein allergy (CMPA), a condition in which people have an allergic response to a particular protein present in cow’s milk.

CMPA is also known as eosinophilic proctocolitis or food protein-induced allergic proctocolitis.

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Allergic colitis is an immune response to the presence of specific proteins, leading to inflammation in the end section of the colon.

Allergic colitis typically affects babies, and one older study in the United States reported that it affects around 1.6 per 1,000 infants. However, the true incidence is unclear, partly because no diagnostic test is available.

It typically occurs in infants younger than 12 months and tends to resolve with dietary changes.

Allergic colitis frequently occurs as part of CMPA, which appears to affect 2% to 3% of infants. By the age of 6, however, this rate falls to less than 1%.

The most common symptoms of allergic colitis are bleeding from the rectum and bloody stool. The blood can give the poop a red color.

Other symptoms that may occur include:

Rectal bleeding can occur in infants for several other reasons, such as infection, anal fissure, and intussusception.

Learn more about blood in baby stool.

Allergic colitis may occur due to an immune response to certain proteins.

Cow’s milk and soy are the most common triggers for allergic colitis. Infants can come into contact with proteins from these triggers by drinking breast milk or formula.

Other factors that could lead to allergic colitis include issues with the immune system and possibly an imbalance in the microbes living in the infant’s gut, also known as the gut microbiota.

Doctors generally consider a person’s symptoms to determine whether they have allergic colitis. There is currently no specific test for the condition.

The clearest sign that an infant has allergic colitis is if their symptoms improve with dietary changes.

However, some doctors may use tests to get more information about a person’s situation or rule out another condition. If so, they may use some of the following tests:

Learn more about allergy testing.

The main way to treat allergic colitis is to remove the suspected triggers from the infant’s diet.

If the infant’s diet includes breast milk, the nursing person will also need to eliminate the potential trigger from their diet. In most cases, this is cow’s milk protein.

After the protein causing allergic colitis is removed, bleeding should stop within 2 weeks. However, most cases resolve within 3 to 4 days.

If an infant still has symptoms after 2 weeks of removing cow’s milk protein from their diet, doctors will suggest removing soy and then egg.

Doctors may also recommend using a hypoallergenic infant formula instead of breast milk or regular infant formula.

Some researchers have suggested that probiotics and fecal transplants can help, but these are not common treatment options.

Learn more about elimination diets.

The most important thing when managing allergic colitis is to stick to any recommended dietary changes. If someone does not follow them and symptoms do not improve, doctors will not know whether or not the infant has allergic colitis.

It is also important for people to monitor an infant’s diet closely. Milk can appear in many different foods — and sometimes in surprising places — so caregivers should be mindful of ingredient lists for older infants with more varied diets.

If a person is breastfeeding and needs to remove specific foods from their diet, a nutritionist can provide advice to ensure they continue to eat healthily.

Most cases of allergic colitis should fully clear up within 2 weeks of removing the trigger food from the infant’s diet. Up to 20% of cases affecting breastmilk-fed infants resolve without treatment.

Most infants develop a tolerance to their trigger food over time. By the age of 3 years, most can tolerate the food that triggers their allergic colitis.

Some research suggests that infants who experience allergic colitis may go on to develop disorders affecting the gut in later life, such as inflammatory bowel disease.

Around 50% of children who had CMPA as infants go on to develop adverse reactions to different foods, and between 50% and 80% develop adverse reactions to environmental substances they might breathe in.

What does baby poop look like with a milk allergy?

If a baby has a milk allergy, their poop may contain mucus or even blood. This can make it appear red. In some cases, they may have diarrhea, although some babies will continue to produce poop with regular consistency.

What happens if CMPA goes untreated?

If an infant with CMPA does not receive treatment, they will continue to experience the symptoms of this allergy. Eventually, they will likely be able to tolerate cow’s milk.

However, some symptoms, such as diarrhea, may lead to complications, including dehydration and hypovolemic shock. Hypovolemic shock is an emergency that happens when the body loses a lot of fluid, leading to low blood volume.

Do babies with CMPA cry a lot?

Some babies with CMPA will fuss and show signs of irritation, though this is not always the case. Many will appear otherwise healthy.

Caregivers should contact a doctor if an infant experiences rectal bleeding or has blood in their poop.

Although allergic colitis can resolve on its own, it can be dangerous in infancy without treatment. Allergic colitis symptoms can also be due to other conditions that require urgent attention.

Allergic colitis is a condition where the immune system responds to a protein — typically a cow’s milk protein — and causes inflammation in the colon. This often leads to bleeding from the rectum and bloody poop.

Nearly all cases of allergic colitis affect infants. The condition can resolve in time, but the most effective treatment is to remove triggering foods from the infant’s diet. If the infant is receiving breast milk, the person providing it will also need to remove the triggering food from their diet.