New research suggests individuals infected with hepatitis C virus may be more likely to develop Parkinson's.
Hepatitis C is a contagious liver disease caused by the hepatitis C virus (HCV), which is most commonly transmitted through contact with blood from an infected person, primarily through the sharing of needles.
While some people with hepatitis C may experience short-term illness that occurs within 6 months of exposure to HCV, around 70-85% of infected individuals experience chronic illness, which can lead to severe liver problems such as liver cancer or cirrhosis.
According to the Centers for Disease Control and Prevention (CDC), around 2.7 million people in the US have chronic hepatitis C infection, though the majority of these individuals are unaware they are infected because they have few symptoms.
Study coauthor Dr. Chia-Hung Kao, of China Medical University Hospital in Taiwan, and colleagues note that previous research has suggested HCV is neurotropic - meaning it can infect nerve cells, or neurons - and can replicate in the central nervous system.
Furthermore, the researchers point to a recent study that claimed HCV can trigger death of neurons that secrete the neurotransmitter dopamine, which is believed to be a key contributor to Parkinson's disease.
30% greater Parkinson's risk for hepatitis C patients
For their study, Dr. Kao and colleagues set out to investigate whether hepatitis C may be a risk factor for Parkinson's by analyzing 2000-10 data drawn from the Taiwan National Health Insurance Research Database (NHIRD).
- While the risk of Parkinson's increases with age, around 4% of people with the disease are diagnosed before the age of 50
- Men are 1.5 times more likely to develop Parkinson's than women
- In the US, the cost of medication for a person with Parkinson's totals an average of $2,500 per year.
The data involved 49,967 people with either hepatitis B, hepatitis C or both, alongside 199,868 people without hepatitis. All participants were followed-up for an average of 12 years in order to monitor any development of Parkinson's.
Of the participants who had hepatitis, 270 developed Parkinson's during the 12-year follow-up - of whom 120 had hepatitis C - compared with 1,060 participants who were free of hepatitis.
After controlling for potentially confounding factors, including participants' age, sex and diagnosis of diabetes or cirrhosis, the researchers found participants with hepatitis C were at 30% greater risk of developing Parkinson's than those who did not have hepatitis.
The team found that participants with hepatitis B and those with both hepatitis B and C had a similar Parkinson's risk as those without the viruses after possible influential factors were accounted for.
It is estimated that as many as 1 million people in the US are living with Parkinson's, and around 60,000 Americans are diagnosed with the neurodegenerative disorder each year.
The exact causes of Parkinson's are unclear, though past studies have suggested it may be triggered by a combination of environmental and genetic factors. Pinpointing the possible causes of Parkinson's is key to finding a much-needed cure, and this latest research may have uncovered another risk factor for the disease.
Commenting on the findings, Dr. Kao says:
"Many factors clearly play a role in the development of Parkinson's disease, including environmental factors. This nationwide study, using the National Health Insurance Research Database of Taiwan, suggests that hepatitis caused specifically by the hepatitis C virus may increase the risk of developing the disease. More research is needed to investigate this link."
There are some study limitations. The researchers note that hepatitis and Parkinson's were identified through International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes rather than through clinical assessment, neuroimaging or lab data, which may have led to less accurate results.
In addition, the researchers point out that the Longitudinal Health Insurance Database - part of the NHIRD - does not hold information on duration of viral hepatitis and risk factors for HCV infection, such as the sharing of needles and tattooing.
"These factors for HCV infection might have confounding effects on Parkinson's disease development, but could not be controlled for analysis in the current study," they explain.
Earlier this month, Medical News Today reported on a study that suggests a pesticide called heptachlor epoxide - found in milk in the 1980s - may contribute to Parkinson's development.