The flu, or influenza, is primarily known as a viral infection that targets the respiratory system. However, the systemic stress caused by a severe case of influenza can have serious consequences for other major organ systems, including the kidneys. The flu can affect your kidneys, though this complication is usually an indirect result of the body’s severe reaction to the infection. Serious kidney damage, known as Acute Kidney Injury (AKI), is a potential complication of severe flu illness.
The Indirect Impact of Severe Illness
A severe influenza infection places significant strain on the kidneys through systemic effects, often leading to prerenal acute kidney injury. This injury is caused by decreased blood flow to the kidneys, not direct damage to the renal tissue. Common flu symptoms like high fever, vomiting, and diarrhea cause rapid dehydration. This reduction in body fluid volume lowers blood pressure and decreases circulation to the kidneys, which rely on consistent blood flow for filtering.
Another major contributor is the generalized systemic inflammation accompanying a serious viral infection. The aggressive immune response releases inflammatory signaling molecules called cytokines. This intense response stresses the vascular system, impacting the kidneys’ ability to regulate blood flow and function. This prerenal stress is generally reversible with prompt supportive care, such as aggressive fluid replacement, to restore normal circulation.
Acute Kidney Injury Pathways
Beyond reversible prerenal injury, severe flu can trigger specific pathways resulting in intrinsic Acute Kidney Injury (AKI). One pathway is rhabdomyolysis, where the viral infection leads to the breakdown of skeletal muscle tissue. This destruction releases large amounts of the protein myoglobin into the bloodstream. Myoglobin is toxic to the kidney’s filtering units (the tubules) and can physically clog them, causing direct tissue damage and acute kidney failure.
A second pathway is the development of septic shock or severe hypotension, often following a secondary bacterial infection. Extreme systemic infection causes blood vessels to dilate dramatically, leading to a critical drop in blood pressure (shock). When blood pressure falls critically low, the kidneys do not receive enough oxygen, resulting in ischemic injury to the renal cells. This lack of oxygenation causes acute tubular necrosis, a form of intrinsic kidney damage with a higher risk of lasting consequences.
While less common, evidence suggests the influenza virus may directly invade kidney cells. Studies have shown influenza A components in the kidneys of some critically ill patients. This direct viral injury, along with immune-mediated responses like acute glomerulonephritis, represents a serious mechanism for severe renal damage.
Recognizing Signs of Kidney Complications
Patients recovering from the flu should be aware of potential warning signs indicating a kidney complication. A significant decrease in urine output is a noticeable symptom of impaired kidney function. The urine may also appear darker than usual, reflecting a higher concentration of waste products.
Another concerning sign is edema, which is swelling caused by fluid retention. This swelling is often noticeable in the legs, ankles, and feet, or around the eyes. Unusual fatigue, confusion, or shortness of breath can signal a severe electrolyte imbalance or the buildup of waste products (uremia). The appearance of any of these symptoms requires immediate medical attention, as prompt intervention minimizes the risk of permanent kidney damage.
Individuals at Elevated Risk
Certain patient populations have a higher risk of kidney complications if they contract the flu. Individuals with pre-existing Chronic Kidney Disease (CKD) are vulnerable because their kidneys already have a reduced functional reserve. The stress of a severe flu infection can easily push their remaining function past its limit.
People with poorly controlled chronic conditions like hypertension and diabetes also face elevated risk. These conditions compromise the health of the kidney’s blood vessels and filtering units, making them less resilient to systemic stress. The elderly are also at higher risk due to a natural reduction in baseline renal function with age. Finally, individuals with immunocompromising conditions are vulnerable, as their compromised immune response increases the likelihood of severe multi-organ complications.