BK virus infection is a common childhood illness that usually goes unnoticed, but it can become a serious concern for people who have received organ transplants, particularly kidney transplants. After the initial infection, the virus remains dormant in the body for life, only to potentially reactivate when the immune system is weakened by medications or illness.

Epidemiology

BK virus infection is remarkably widespread throughout the human population. Most people encounter this virus during their childhood years, often without ever knowing they were infected. Research suggests that a significant portion of adults carry antibodies against BK virus, indicating past exposure. One survey of healthy blood donors found that approximately 82% tested positive for antibodies against the virus, demonstrating how common this infection truly is.^[2]

The virus affects people across all demographics during childhood, but its clinical significance emerges primarily in specific adult populations. Among kidney transplant recipients, the virus poses a particular challenge. Between 1% and 10% of people who receive kidney transplants develop a condition called BK virus-associated nephropathy, which means kidney damage caused by the virus. Of those who develop this complication, up to 80% may lose their transplanted kidney.^[2] The condition can appear at varying times after transplantation, from as early as several days to as late as five years following the procedure.^[2]

The scope of organ transplantation worldwide makes BK virus a significant concern. In the United Kingdom, there were 2,263 kidney transplant procedures performed in 2021 and 2022. The United States reached a record high of 25,487 kidney transplants in 2021. When considering bone marrow or stem cell transplants, the numbers are equally substantial, with almost 5,000 procedures annually in the United States, approximately 4,000 in the United Kingdom, and over 32,000 across Europe as a whole. With BK virus affecting up to 15% of transplant patients, the public health implications are considerable.^[5]

Causes

BK virus belongs to a family of viruses called Polyomaviridae, which are double-stranded DNA viruses. The virus was first discovered and isolated in 1971 from a patient who had received a kidney transplant. This patient, a 39-year-old male with the initials B.K., developed a narrowing of the ureter after his transplant, leading researchers to identify this new virus.^[2]

The initial infection with BK virus typically occurs during childhood. At this stage, the virus may produce symptoms similar to a mild cold or respiratory infection, or it may cause no noticeable symptoms at all. Many children experience their first encounter with BK virus without their parents or doctors ever realizing an infection has occurred. After this primary infection, the virus doesn’t leave the body. Instead, it travels to the kidneys and urinary tract, where it establishes what is called a latent infection, meaning it remains present but inactive, like a sleeping presence that causes no harm.^[4]

The virus remains in this dormant state in the cells lining the kidneys and urinary tract for the remainder of a person’s life. It is thought that up to 80% of the general population carries this latent form of BK virus without ever experiencing any problems.^[2] The virus only becomes problematic when something disrupts the body’s ability to keep it under control, which typically happens when the immune system is weakened or suppressed.

Risk Factors

The primary risk factor for developing significant BK virus infection is immunosuppression, which means having a weakened immune system. The immune system normally keeps the dormant BK virus in check, preventing it from becoming active and causing problems. When this surveillance system is compromised, the virus can “wake up” and begin multiplying again.^[4]

Kidney transplant recipients face the highest risk for BK virus complications. After receiving a new kidney, these patients must take powerful medications called immunosuppressants to prevent their body from rejecting the donated organ. These medications intentionally dampen the immune response, which is necessary to protect the transplanted kidney from being attacked as foreign tissue. However, this same suppression of the immune system creates an opportunity for the dormant BK virus to reactivate. The virus most commonly reactivates within the first year after transplantation, though it can occur later.^[4]

Bone marrow or stem cell transplant recipients also face elevated risk for BK virus-related complications, though the manifestations differ from those seen in kidney transplant patients. In this population, BK virus is notably associated with a condition called hemorrhagic cystitis, which is inflammation and bleeding of the bladder.^[2]

Other factors that may increase risk include older age and male gender. Injuries to the kidneys may also trigger virus activation. People with long-term illnesses that weaken the immune system, such as diabetes or acquired immunodeficiency syndrome, may also be more susceptible to BK virus reactivation.^[6] Additionally, people who never encountered the virus during childhood and then receive an organ from a donor who carries the virus might be at increased risk for developing infection.^[4]

Symptoms

The symptoms of BK virus infection vary dramatically depending on whether it is a primary infection in childhood or a reactivation in an immunosuppressed person. During the initial childhood infection, symptoms are typically mild and easily overlooked. They may resemble a common cold or upper respiratory infection, with perhaps a low-grade fever. Because these symptoms are so nonspecific and mild, most people never realize they have been infected with BK virus.^[2]

In immunocompromised individuals, particularly transplant recipients, the picture changes considerably. Many people with BK virus reactivation experience no symptoms at all, and the infection is only detected through routine blood or urine tests during post-transplant monitoring. When the virus is discovered because kidney function begins to decline, doctors may perform additional testing even in the absence of obvious symptoms.^[4]

When symptoms do appear in transplant recipients, they can be quite varied and sometimes severe. Kidney transplant patients may experience signs of declining kidney function, which doctors detect through a progressive rise in a blood marker called serum creatinine. Laboratory analysis of urine may reveal abnormalities, including renal tubular cells and inflammatory cells, which indicate kidney damage.^[2]

Some patients develop urinary symptoms, including passing more urine than usual, experiencing a burning sensation or pain during urination, or noticing that their urine appears brown or reddish in color. These symptoms reflect inflammation and damage to the urinary system. Bone marrow transplant recipients may experience hemorrhagic cystitis, which causes visible blood in the urine along with significant bladder discomfort.^[6]

Additional symptoms that may occur in severe cases include stomach problems, muscle pain or weakness, blurred vision, cough, colds or trouble breathing, and in rare cases, seizures. The virus has also been associated with narrowing of the ureters, the tubes that carry urine from the kidneys to the bladder, and with a type of kidney inflammation called interstitial nephritis.^[2] There have been reports of BK virus being detected in specimens from women who experienced spontaneous abortion, though the virus was present without causing obvious clinical symptoms.^[2]

Prevention

Preventing BK virus infection in the general population is not currently possible because the virus is so widespread and initial infection typically occurs in childhood through person-to-person transmission. However, for transplant recipients, several prevention strategies have been developed and tested to reduce the risk of serious BK virus complications.

The most effective prevention strategy currently available is intensive screening and monitoring. Transplant centers have developed protocols that involve regular testing for BK virus in the blood and urine of transplant recipients. These screening programs typically begin one month after transplantation, with monthly testing during the first six months, followed by testing every three months until two years post-transplant. This vigilant approach allows doctors to detect the virus early, before it causes significant damage to the transplanted organ.^[5]

When screening detects BK virus in the urine, called viruria, or in the blood, called viremia, doctors can intervene early by reducing the patient’s immunosuppressive medications. This reduction allows the immune system to regain enough strength to fight the virus while still maintaining protection for the transplanted organ. Studies have shown that intensive screening programs followed by early reduction of immunosuppression can prevent kidney graft loss and significantly decrease the amount of virus in the blood at one year after transplant.^[3]

Some centers have investigated the use of antibiotics called fluoroquinolones for preventing BK virus infection. However, research has found that fluoroquinolone prophylaxis and treatment provides no benefit in preventing BK virus infection in kidney transplant recipients.^[3] Scientists continue to work on developing new preventive approaches, including the potential use of intravenous immunoglobulin, which contains antibodies that may help neutralize the virus.^[3]

Pathophysiology

Understanding how BK virus causes disease requires looking at what happens at the cellular and tissue level. After the initial childhood infection, BK virus establishes latency in specific cells within the kidneys and urinary tract. The virus particularly favors cells called renal tubular cells and cells in the lining of the urinary tract called the uroepithelium. In these locations, the virus remains dormant, with its genetic material present in the cell nuclei but not actively producing new virus particles.^[8]

When immunosuppression occurs, such as after organ transplantation, the balance between the virus and the immune system shifts. Without adequate immune surveillance, the dormant virus begins to replicate again. The virus starts producing new viral particles, which can be detected first in the urine as viral components are shed from infected cells. This stage is called viruria. As replication continues and intensifies, the virus spreads across the tissue layer called the interstitium in the kidney. Within a couple of weeks, viral particles can pass into the small blood vessels called capillaries, leading to viremia, where the virus is detectable in the bloodstream.^[5]

In the kidneys of transplant recipients, active BK virus replication causes direct damage to the tubular cells where the virus replicates. When pathologists examine kidney tissue under a microscope, they can see infected cells that have a characteristic appearance. These cells contain inclusion bodies, which are dense accumulations of viral particles within the cell nucleus. The infected cells are sometimes called “decoy cells” because their appearance can be mistaken for cancer cells due to their abnormal nuclear characteristics.^[2]

As the infection progresses, it triggers an inflammatory response in the kidney tissue, leading to what is called BK virus-associated nephropathy. The kidney tissue becomes infiltrated with inflammatory cells, and the normal architecture of the kidney tubules becomes disrupted. If left unchecked, this process causes progressive scarring and loss of kidney function. The damage can be so severe that the transplanted kidney fails completely, requiring the patient to return to dialysis.^[7]

In bone marrow transplant recipients, the pathophysiology differs somewhat. The virus primarily affects the bladder lining, causing hemorrhagic cystitis. The bladder tissue becomes inflamed and fragile, leading to bleeding that can range from microscopic blood in the urine to severe, visible bleeding that may require medical intervention. The damaged blood vessels in the bladder wall leak blood into the urine, and the inflammation causes pain and urinary urgency.^[10]

The virus can also cause narrowing of the ureters through a process of inflammation and subsequent scarring. This condition, called ureteral stenosis, can obstruct the flow of urine from the kidney to the bladder, potentially causing backup of urine and further kidney damage. The chronic inflammation and tissue injury triggered by the virus create scar tissue that progressively narrows these tubular structures.^[2]

At the molecular level, BK virus replication interferes with normal cellular functions. The virus hijacks the cell’s machinery to produce viral proteins and replicate viral DNA. This process eventually leads to cell death, releasing more viral particles that can infect adjacent cells. The cycle continues, spreading the infection through the tissue if the immune system cannot mount an effective response to contain it. The role of cellular immunity, particularly T cells, is crucial in controlling BK virus replication, which is why patients on immunosuppressive medications that target T cell function are at highest risk for severe disease.^[8]