The presence of blood in the stool—medically termed hematochezia (bright red blood) or melena (black, tarry stool)—is an alarming symptom, especially when experienced alongside an infection like COVID-19. While the virus is primarily known for its effects on the respiratory system, a significant number of people also experience digestive tract symptoms. Gastrointestinal involvement with SARS-CoV-2 is well-established, ranging from mild discomfort to severe complications. Understanding the biological mechanisms and common indirect factors is important for linking this serious medical finding to a viral illness.
COVID-19’s Impact on the Gastrointestinal Tract
The SARS-CoV-2 virus gains entry into human cells by binding to the angiotensin-converting enzyme 2 (ACE2) receptor, which is highly abundant throughout the body. The lining of the small intestine expresses this receptor in high concentrations, sometimes surpassing the levels found in the lung tissue. This high density of ACE2 receptors provides a direct pathway for the virus to infect and damage the digestive lining.
Once the virus enters the cells of the gut, it initiates a local inflammatory response. This inflammation causes common digestive complaints, including diarrhea, nausea, vomiting, and abdominal discomfort. The virus’s ability to target the gut establishes the biological plausibility for digestive system complications, including gastrointestinal bleeding.
The Direct Link Between COVID-19 and Bloody Stool
Bloody stool is an uncommon complication of COVID-19, typically observed in a small percentage of hospitalized patients with severe disease. Studies indicate that gastrointestinal bleeding occurs in approximately 1.5% to 3.0% of individuals admitted to the hospital. This bleeding is rarely an initial symptom but usually manifests after the patient is already hospitalized.
A primary direct cause of bleeding is the virus’s systemic effect on the circulatory system, leading to hypercoagulability (excessive blood clotting). This increased clotting risk can paradoxically lead to a lack of blood flow in the intestines, causing ischemic colitis. When intestinal tissue is deprived of oxygenated blood, it becomes damaged and necrotic, resulting in lower gastrointestinal bleeding.
The virus can also cause direct injury to the intestinal mucosal lining, leading to erosions and inflammation that may bleed. In rare, severe cases, the inflammatory response can trigger conditions resembling severe colitis or enteritis. These direct mechanisms reflect a systemic, severe reaction to the infection and often require hospitalization.
Indirect Causes of Bloody Stool During Illness
For the average person recovering at home, bloody stool is far more likely to be an indirect consequence of the illness or its treatment. Common over-the-counter medications used to manage fever and aches can irritate the stomach and intestinal lining. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can cause gastritis or peptic ulcers, which may bleed and present as melena.
The use of blood thinners (anticoagulants) in hospitalized patients is another significant indirect factor. Because severe COVID-19 carries a high risk of dangerous blood clots, prophylactic doses of anticoagulants are often administered. While this treatment prevents severe thrombotic events, it inherently increases the risk of gastrointestinal bleeding.
The physical strain associated with being sick can also induce common, non-COVID-specific causes of rectal bleeding. Severe diarrhea, constipation, or straining can lead to the development or aggravation of hemorrhoids and anal fissures. These small tears or swollen blood vessels in the anus and rectum are the most frequent cause of bright red blood seen on toilet paper or streaking the stool. Immune system stress can also make patients susceptible to opportunistic infections, such as a flare-up of Cytomegalovirus (CMV), which causes inflammation and bleeding in the colon.
Identifying Different Types of Rectal Bleeding and When to Seek Medical Care
The appearance of the blood in the stool is an indicator of the potential source of the bleeding within the gastrointestinal tract.
Bright Red Blood
Bright red blood visible on the toilet paper or mixed in the toilet bowl water typically suggests a source in the lower GI tract, such as the anus or rectum, commonly from hemorrhoids or anal fissures. This type of bleeding still requires medical evaluation.
Dark or Black Blood
Bleeding that originates higher up in the colon may present as dark red or maroon-colored blood mixed with the stool. The most concerning sign is a black or tarry stool, known as melena, which indicates the blood has been digested. Melena usually points to a bleed in the upper GI tract, such as the stomach or small intestine, and necessitates immediate medical attention.
Any instance of bloody stool should be discussed with a healthcare provider, even if the amount is small. Seek emergency help if the bleeding is heavy, continuous, or accompanied by symptoms like severe abdominal pain, dizziness, fainting, or rapid breathing. These symptoms indicate significant blood loss or a severe complication requiring rapid intervention.