Two viral liver diseases could help us find the path toward the cause of Parkinson's disease.
Researchers from the University of Oxford and UCL Institute of Neurology in London have reported an association between hepatitis B and C infections and an increased risk of Parkinson's disease. Their findings were published early online in the journal Neurology.
The researchers examined records from 21,633 people with hepatitis B, 48,428 with hepatitis C, as well as people with autoimmune hepatitis (a non-viral type of hepatitis), chronic active hepatitis (constant liver inflammation), and HIV (a virus that doesn't cause hepatitis).
They compared the incidence of Parkinson's disease in these people with a group of nearly six million people with other minor health conditions, such as hernias, cataracts, and knee replacements.
People with hepatitis B were 76% more likely to develop Parkinson's disease than those in the comparison group—44 people with hepatitis B developed Parkinson's disease, where 25 cases would be expected in the general population.
People with hepatitis C were 51% more likely to develop Parkinson's disease: 73 of the people in the hepatitis C group developed Parkinson's disease, compared to 49 cases that would have been expected in the general population.
The rate of Parkinson's disease was not increased in people with autoimmune hepatitis, chronic active hepatitis, or HIV, which suggests there could be something about the hepatitis C and B viruses that's linked (somehow) to the Parkinson's, instead of it being the presence of liver disease or a viral infection.
This study made associations between the diseases, but could not determine the exact relationship between hepatitis B, C, and Parkinson's disease.
"It's possible that the hepatitis virus itself or perhaps the treatment for the infection could play a role in triggering Parkinson's disease or it's possible that people who are susceptible to hepatitis infections are also more susceptible to Parkinson's disease," study author Julia Pakpoor, of the University of Oxford, said in a press release from the American Academy of Neurology. "We hope that identifying this relationship may help us to better understand how Parkinson's disease develops."
Loss of dopamine-producing cells in the brain cause Parkinson's disease, but the reason for this loss is not known and there is no blood or lab test to diagnose it.
People with Parkinson's have four main symptoms: tremor, or trembling; rigidity in the limbs and trunk; slowness of movement; and balance problems. As the disease gets worse, the tremors interfere with daily life, causing: depression; difficulty in swallowing, chewing, and speaking; urinary problems or constipation; skin problems; and sleep disruptions.
About a million Americans have Parkinson's disease. On the other hand, the Centers for Disease Control and Prevention estimate that about 850,000 to 2.2 million people in the US are infected with hepatitis B and 2.7 million to 3.9 million people have hepatitis C.
Both hepatitis B and C are viral liver infections spread by contact with blood and body fluids of an infected person, such as unprotected sex, sharing needles, getting a tattoo or piercing with unsterilized tools, or sharing razors or toothbrushes with an infected person.
People with hepatitis are treated with antiviral drugs, but some are not cured and the virus remains in their body as a chronic infection.
So, what could explain the association the researchers found between people with hepatitis B or C and the development of Parkinson's disease?
One telling finding is that the association was only found in the viral hepatitis diseases, not in the chronic active hepatitis or autoimmune hepatitis patients. That may point to the hepatitis virus, or something involved in the disease or its treatment, as the cause of the increased risk for Parkinson's disease.
The researchers said that the association may involve shared disease mechanisms, shared genetic or environmental susceptibility, consequences of viral hepatitis, or its treatment. The cellular receptors that bind the hepatitis C virus have been found on the cells lining vessels into the brain—a critical location for entry of drugs and pathogens into the brain, but the significance of that as it pertains to development of Parkinson's disease is not known right now.
Whether any of these factors contribute to development of Parkinson's disease—and, if so, how—is a matter for future research. The new research findings have given us a connection and a direction to focus that research.