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What is BK virus?

BK virus (sometimes known as polyomavirus) is a common virus that most people get in childhood. Symptoms resemble a common cold, and once you’ve had a BK virus infection it remains silently present (dormant) in the kidney and the lining of the ureters (tubes between the kidney and bladder) and bladder forever.

Normally, BK virus can be  kept dormant by a healthy immune system, but it become an active infection if the immune system is weakened.

People with kidney transplants, who must take drugs to suppress their immune systems and stop organ rejection, seem particularly prone to reactivated BK infections.

This type of infection can affect the kidneys, ureter and bladder and can cause kidney damage. When this occurs, it is known as BK virus-associated nephropathy (BKVAN or BKN).

The virus gets its name from the initials of a kidney transplant patient who was the first to be diagnosed with the condition in 1971.

Causes of BK virus

BKN has increased over the years, probably because the more powerful anti-rejection drugs used today increase the risk of developing infections, particularly viral infections. Better diagnosis of the infection has also contributed to these increased rates.

Around six per cent of people who have had kidney transplants will develop BKN within five years of having the transplant, with the vast majority of cases within the first two years.

In BKN, the virus attacks kidney cells and causes inflammation, potentially damaging the kidney and leading to a reduction in kidney function. This can be mistaken for kidney transplant rejection, kidney biopsy is vital for accurate diagnosis. Sometimes, it can also cause the tube between the kidney and the bladder to narrow (ureteric stenosis).

We still don’t fully understand why some people are more likely to develop BKN than others. We know that the dose of immunosuppression is a major risk factor. Other factors may include the amount of dormant virus present in donor kidneys, changes in the virus that make it more likely to cause disease, or genetic factors, making certain people more prone to developing the viral infection.


BK virus symptoms

The vast majority of kidney transplant recipients who develop BKN have no symptoms at all, so renal teams will either screen for BK early after transplant or use evidence of a deterioration in kidney function to prompt tests. Rarely, a few people may experience symptoms including:

  • Changes in the colour of urine (urine that is brown or red in colour)
  • Pain when urinating
  • Difficulty urinating
  • Needing to urinate more than usual

BKN can affect the success of a kidney transplant, speak to your renal team immediately if you notice any signs of infection.

BK virus diagnosis

Doctors will check for signs of BK virus before and after a person has had a kidney transplant.

Some hospitals will test for it routinely, while others will only test if a problem arises, such as a reduction in kidney function. BK virus can be diagnosed through:

  • Blood tests
  • Urine tests
  • Kidney biopsies

BK virus treatment

BK infection is a sign of over immunosuppression. Currently, the main treatment is to carefully reduce the strength of the drugs a person takes to suppress their immune system, which can often control the virus without causing kidney transplant rejection. Anti-viral drugs may also be prescribed.

It can take weeks to see the effects of these treatments so blood tests may be increased to monitor progress.

If the virus is detected early, these treatments may be able to take effect before kidney function is adversely affected.

BK virus progression

BKN can be a serious problem if it doesn’t respond to treatment. In the worst cases it can lead to kidney failure and possible kidney loss if a reduction in immunosuppression causes rejection to occur.

Useful resources for BK virus

If you’ve had a kidney transplant and have any questions or concerns about BKN don’t hesitate to speak to your renal team. The virus can affect the success of a kidney transplant, so contact them immediately if you experience any of these signs of infection:

  • Changes in the colour of urine (urine that is brown or red in colour)
  • Pain when urinating
  • Difficulty urinating
  • Needing to urinate more than usual

The National Registry of Rare Kidney Diseases (RaDaR) is a database of patients with a variety of rare kidney conditions, including BKN. It aims to help improve understanding of these conditions and to find suitable participants for research trials into new treatments. Find out more about the BK Nephropathy (BKN) Rare Disease Group.

Reviewed March 2023

Simon Baker
Dr Simon Baker

"Our findings should help us to improve the care we give to kidney transplant patients but also have wider implications for those who develop bladder cancer.”

Dr Simon Baker

Researcher spotlight

Dr Simon Baker and his team at the University of York, will begin a study to understand whether a common childhood infection (BK virus) causes changes to the cells of the lower urinary tract that lead to cancer.

The research aims to highlight potential treatments and preventative measures to tackle cancer in kidney patients.

If Simon's theory is correct, he will show the importance of monitoring BK virus more closely in kidney transplant patients and provide evidence to support a new trial to protect against BK virus prior to transplant.

Our BK virus research

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