Hematuria, the medical term for blood in the urine, can be an alarming sign. When a person has flu symptoms, they may wonder if the viral illness is the source of this unexpected symptom. While the influenza virus itself does not typically cause blood in the urine, its presence warrants investigation. Hematuria during the flu is almost always linked to a complication or another underlying condition.
How Influenza Affects the Body’s Systems
The influenza virus primarily targets the respiratory system, leading to symptoms like fever, cough, and body aches. The virus infects the cells lining the airways, triggering an immune response concentrated in the respiratory tract. Hematuria is not considered a direct or common symptom of a typical, uncomplicated flu infection.
The viral mechanism of influenza centers on replicating within respiratory cells, not on attacking the structures of the urinary tract. The urinary tract includes the kidneys, ureters, bladder, and urethra. Therefore, when blood is detected in the urine during the flu, it is usually a sign of an indirect consequence or a secondary issue.
Systemic Complications of Severe Illness
In cases of severe illness, systemic stress can lead to indirect causes of blood in the urine. High fever and reduced fluid intake during the flu can lead to significant dehydration. This fluid loss causes the urine to become highly concentrated, which may irritate the urinary tract lining and result in blood cells.
A more serious, though rare, complication is rhabdomyolysis, where severe viral infections cause the breakdown of skeletal muscle tissue. This releases a protein called myoglobin into the bloodstream, which is filtered by the kidneys. Myoglobin is toxic to the kidney tubules and can cause damage, leading to acute kidney injury.
The presence of myoglobin in the urine is known as myoglobinuria. Myoglobinuria can cause the urine to appear dark red, brown, or tea-colored, which may be mistaken for blood. This muscle breakdown highlights how a severe viral infection can trigger problems beyond the respiratory system. Recognition of this complication is important due to the risk of kidney damage.
Other Infections That Cause Blood in Urine
Sometimes, what is assumed to be the flu may actually be another type of infection that commonly causes hematuria. Urinary Tract Infections (UTIs) are a primary example, often presenting with fever and body aches that overlap with flu symptoms. UTIs are bacterial infections that cause inflammation of the urinary tract lining, frequently leading to visible or microscopic blood in the urine.
If a bacterial infection spreads from the bladder up to the kidneys, it is called pyelonephritis. Pyelonephritis causes pronounced flu-like symptoms, including high fever, chills, and flank pain, almost always accompanied by hematuria.
A separate mechanism involves post-infectious glomerulonephritis, a condition that occurs after certain infections, such as streptococcal infections. It has also been reported following influenza. This is an immune-mediated response where the body damages the filtering units of the kidneys, known as glomeruli, while fighting the infection.
Damage to the glomeruli allows red blood cells and protein to leak into the urine, which can result in urine that looks dark brown or like cola. This condition typically develops one to two weeks after the initial infection resolves. The distinction between a direct viral effect and a secondary immune response is important for diagnosis and management.
Recognizing Urgent Symptoms
Any time blood is observed in the urine, medical evaluation is warranted to determine the cause. However, certain accompanying symptoms signal a need for immediate attention. Gross hematuria, which is urine that is visibly red, pink, or brown, should not be ignored, especially if accompanied by signs of severe illness.
Other concerning symptoms include severe pain in the abdomen, lower back, or flank, which could indicate a kidney infection or kidney stones. Serious red flags include the inability to urinate, new swelling around the eyes or ankles, or the passage of dark, tea-colored urine. These signs may suggest acute kidney involvement, such as from rhabdomyolysis or severe infection, and require prompt medical assessment to prevent lasting damage.