Viruses and Parkinson's Disease

Hepatitis C and other Viruses and Parkinson's Disease


Infection with the hepatitis C virus (HCV) significantly increases the risk for Parkinson's disease (PD), new research shows.


The risk of developing PD is obviously multifactorial, and study shows that hepatitis C virus infection can be one of the risk factors.


More detailed neurological tests and functional images might help us detect early PD in anti-HCV–positive patients.


Mechanism

In most cases, exposure to HCV, a small, enveloped RNA virus, leads to chronic infection, causing progressive liver disease, including hepatic fibrosis, cirrhosis, and hepatocellular carcinoma.

In developed countries, HCV is transmitted largely by injection from illicit drug use.


The investigators found a 2.5-fold increased risk for PD in patients with HCV infection.

After adjustment for age, sex, and comorbidities, including hyperlipidemia, hypertension, ischemic heart disease, epilepsy diabetes, cirrhosis, stroke, and head injury, the association between HCV and PD remained statistically significant .

An analysis stratified by age, sex, or comorbidity found that a positive association between HCV and PD was maintained in patients under age 65 years, men, or those with a combination of any of the comorbidities. Being male and having comorbidities could represent "hits" in the "second hit" theory of PD.

The theory, is that HCV enters the central nervous system (CNS) by disrupting the integrity of the blood-brain barrier, altering dopaminergic neuronal transmission in the midbrain, and triggering neuro-inflammation, which results in neuronal damage. This damage could be the first "hit," with second "hits" possibly including age, male sex, other environmental exposures (such as pesticides), and head injury.

The link between HCV infection and PD is supported by findings that this infection might release inflammatory cytokines, such as sICAM-1 and RANTES signaling, which may play a role in the pathogenesis of PD.

Dementia Link

HCV infection has also been linked to dementia, another neurodegenerative disease. Although the mechanisms here are not well understood, it's believed that a similar pathogenesis may be at play.

In contrast to HCV, there is no evidence that HBV, which belongs to the Hepadnaviridae family, is neuroinvasive. An earlier study on rats showed that HBV does not induce dopaminergic neuronal toxicity.

But other viruses have been linked to PD. One of the best-known examples is the type A H1N1 influenza virus that caused the 1918 influenza pandemic.

It has been shown that people born during the time of the pandemic influenza outbreak of 1918 have a 2- to 3-fold increased risk of Parkinson's disease than those born prior to 1888 or after 1924.

Another example is HIV, the retrovirus that causes AIDS. HIV can quickly infect astrocyte and microglia (monocytic lineage), causing CNS involvement, said Dr Tsai. "It has been reported that 5% to 50% of all AIDS patients have some motor dysfunction, such as bradykinesia, cogwheel rigidity, and tremor.


Several other viruses, including Coxsackie, Japanese encephalitis B, western equine encephalitis, and herpes, may also be linked to parkinsonism.




There's no cause for alarm for individuals with hepatitis C — for most people, several risk factors must be present for Parkinson's to develop.