BK Virus Infection
BK virus infection is a common childhood virus that usually stays dormant in the body for life, but can reactivate in people with weakened immune systems, particularly kidney and bone marrow transplant recipients, potentially causing serious complications to the transplanted organ.
BK polyomavirus, Human polyomavirus 1, BKV
- Kidneys
- Urinary tract
- Bladder
- Ureters
- Renal tubular cells
- Uroepithelium
Table of contents
- What is BK virus?
- Who gets infected and how it spreads
- Symptoms of BK virus infection
- People at higher risk
- Complications in transplant patients
- How BK virus infection is diagnosed
- Treatment and management
- Monitoring and prevention
What is BK virus?
BK virus is a member of the polyomavirus family, which refers to a group of small viruses with double-stranded DNA. The virus was first discovered in 1971 from the urine of a kidney transplant patient whose initials were B.K., which is how the virus got its name.[2][16]
BK virus infection is extremely common. Most people become infected with this virus during childhood, typically experiencing symptoms similar to a mild cold or upper respiratory infection. In fact, a survey of 400 healthy blood donors showed that 82% were positive for antibodies against BK virus, indicating past infection.[2] It is thought that up to 80% of the adult population carries a latent form of this virus.[2][6]
After the initial infection, the virus does not leave the body. Instead, it remains dormant in the kidneys and urinary tract for the rest of a person’s life. This is known as a persistent infection, where the virus stays “asleep” and typically does not cause any problems.[4][13]
Who gets infected and how it spreads
The exact way BK virus spreads from person to person is not fully understood. It is known that the virus is transmitted from person to person, not from animals. It has been suggested that transmission may occur through respiratory fluids, urine, saliva, air, cough, blood, or sexual contact.[2][6]
Infected individuals periodically release the virus in their urine, which may be one route of transmission.[2] Because the initial infection typically occurs in early childhood and often presents with mild respiratory symptoms or no symptoms at all, most people don’t realize when they catch the virus.[6][13]
Symptoms of BK virus infection
When BK virus first infects someone during childhood, symptoms are usually very mild. These primary infections may cause a respiratory infection or fever, but many people experience no symptoms at all.[2][6]
For most healthy individuals, the virus remains dormant after the initial infection and causes no health problems. The majority of people with BK virus in their body will never have any symptoms from it throughout their lives.[4][13]
However, when the virus reactivates in people with weakened immune systems, particularly transplant recipients, symptoms can be much more severe. In these cases, symptoms may include kidney dysfunction with a progressive rise in blood creatinine levels, abnormal urine tests showing renal tubular cells and inflammatory cells, brown or reddish-colored urine, burning pain or trouble when urinating, passing more urine than usual, stomach problems, blurry vision, muscle pain or weakness, and in rare cases, seizures.[2][6]
Many transplant patients with BK virus reactivation will have no symptoms at all, and the infection is typically detected through routine blood or urine tests.[4][13]
People at higher risk
The BK virus can reactivate when the body’s immune system is weakened or suppressed. The people at highest risk are those who have received organ transplants, particularly kidney transplants, and bone marrow or stem cell transplants.[2][8]
After receiving a transplant, patients must take strong medications called immunosuppressants or anti-rejection medicines. These drugs are essential to prevent the body’s immune system from attacking and rejecting the new organ. However, these same medicines reduce the body’s ability to fight infections, allowing the dormant BK virus to wake up and become active again.[4][13]
Other factors that may increase the risk of BK virus reactivation include older age, male gender, and injuries to the kidneys.[6] Long-term illnesses such as diabetes or acquired immunodeficiency syndrome may also weaken the immune system enough to allow the virus to reactivate.[6]
Complications in transplant patients
In kidney transplant recipients, BK virus reactivation can lead to serious complications. Between 1% and 10% of kidney transplant patients develop a condition called BK virus associated nephropathy (BKVAN), where the virus causes damage to the transplanted kidney. Among patients who develop BKVAN, up to 80% may lose their transplanted kidney.[2][5]
BK virus reactivation typically occurs within the first year after transplant, though it can happen later.[4][13] The virus can cause damage to the kidney tubes, leading to kidney dysfunction. It is also associated with narrowing of the ureters (the tubes that carry urine from the kidneys to the bladder) and interstitial nephritis, which is inflammation of the tissue between the kidney tubes.[2][6]
In bone marrow or stem cell transplant recipients, BK virus is a major cause of hemorrhagic cystitis, a condition where the bladder becomes inflamed and bleeds.[2][10] Symptoms of hemorrhagic cystitis include blood in the urine, painful urination, burning during urination, increased urinary frequency, urgency, incontinence, abdominal or pelvic pain, urinary blockage, and potential kidney or bladder damage.[10]
BK virus affects up to 15% of transplant patients overall.[5] Studies have shown that 30% of pediatric transplant patients tested positive for BK virus in their blood, with BKVAN found in 6.6% of cases three months after transplant.[5]
How BK virus infection is diagnosed
BK virus infection is diagnosed through blood and urine testing. The most common method uses polymerase chain reaction (PCR) techniques to identify and measure the amount of virus present.[6] These tests can detect viral components in blood (called viremia), urine (called viruria), or tissue samples.
When BK virus reactivates, it first causes decoy cells and viral components to appear in the urine. As the infection progresses, the virus spreads across the tissue between kidney tubes and within a couple of weeks enters the blood vessels, causing viremia.[5]
In some cases, a kidney biopsy may be performed. This remains the gold standard for diagnosing BK virus associated nephropathy, as it can show actual damage to the kidney tissue caused by the virus.[5][7]
Regular screening is important for transplant recipients. Current guidelines recommend that kidney transplant patients be screened monthly for the first six months after transplant, then every three months until two years post-transplant. Screening typically involves testing blood or urine samples for BK virus DNA.[5][16]
Treatment and management
Currently, there are no specific antiviral medications that have been proven effective against BK virus. The main treatment approach is to reduce the dose of immunosuppressive medications, allowing the patient’s immune system to become strong enough to fight off the virus naturally.[3][4][13]
This reduction in immunosuppression must be done carefully by doctors to balance fighting the virus while minimizing the risk of transplant rejection. Most people with BK virus infection do very well once their immune system starts to attack the virus.[4][13]
Several medications and approaches have been studied for BK virus infection, with varying results:
Leflunomide is sometimes used because it has both immunosuppressive and antiviral properties. Some studies have shown benefit, though results have been variable.[3][6][14][16]
Intravenous immunoglobulin is becoming a popular option for treatment and prevention of BK infection. It works by increasing antibody levels against the most common BK virus types in the blood.[3][6][14]
Fluoroquinolone antibiotics were studied for prevention and treatment but were found to have no benefit in kidney transplant recipients.[3][14]
Cidofovir, an antiviral medication, has limited use because it can be toxic to the kidneys.[3][14]
Virus-specific T cell therapy is an emerging treatment option that shows promise. This approach uses specially selected immune cells to target the virus.[3][6][8]
For mild BK infections in otherwise healthy individuals, the infection may resolve on its own without any treatment. Additional supportive treatments such as pain medication, bladder irrigation, or increased hydration may be necessary depending on symptoms.[6]
Monitoring and prevention
Prevention of BK virus complications relies heavily on early detection through regular screening. Intensive screening programs that check for BK virus in blood and urine, followed by early reduction of immunosuppression when the virus is detected, have been shown to prevent loss of the transplanted kidney and reduce the amount of virus in the blood at one year.[15]
Transplant centers routinely screen patients to detect BK virus early, before it causes significant kidney damage. This allows for prompt intervention by adjusting medications.[5][7][16]
It is important to understand that nothing a patient has done causes the virus to wake up. Doctors do not know why BK virus reactivates in some transplant recipients and not others. People who did not catch BK virus in childhood might be at higher risk when exposed after transplant.[4][13]
Many transplant patients worry about reducing their anti-rejection medicines when BK virus is detected. However, if the immune system is not allowed to fight the virus, it can cause serious damage to the transplanted kidney. Doctors carefully balance reducing immunosuppression enough to control the virus while minimizing the risk of rejection.[4][13]