• 09Feb

    The Zika Virus: Does it also affect the adult brain?

    Due to the recent epidemic outbreak of the Zika virus, spreading from Brazil to the rest of South America and even North America, the disease has become well known for causing birth defects when infecting pregnant women. However, new studies carried out at The Rockefeller University in New York, suggest that this virus may also cause damage to adult brain cells [1].

    The Zika virus was first discovered in 1947, and is of the same family of viruses that cause the diseases Yellow Fever and Dengue Fever [2]. This virus is mostly transmitted via mosquitoes, where it resides in the saliva of these insects and infects human skin cells, passing to the bloodstream and the lymph nodes. It can also be transferred through blood transfusions, sexual transmission and from mothers to their foetuses [3]. 80% of individuals that contract the virus do not present any symptoms whereas some patients experience mild symptoms such as headaches, fever, conjunctivitis and a rash. However, when pregnant women are infected with this virus, more serious implications occur to the unborn baby. These include microcephaly, which is where the baby is born with an abnormally small head and an underdeveloped brain, sometimes resulting in learning difficulties, seizures and a shortened life expectancy [4].

    During development, the brains of foetuses are mainly made up of stem cells called neural progenitor cells. These become neurones throughout a person’s lifetime which allows the brain to function correctly and communicate with the rest of the body. The Zika virus is thought to affect these cells providing the reason for the decrease in brain volume as shown in infants with microcephaly.

    Professor Gleeson and his team at the Rockefeller University used mice in their experiments as they share 97.5% of their genome with humans [5]. Adult mice brains contain neural progenitor cells in the anterior forebrain and in the hippocampus, both of which are highly important in learning and memory. Referring to his studies where the mice were made to mimic having the Zika virus infection, Gleeson commented; “It was very clear that the virus wasn’t affecting the whole brain evenly, like people are seeing in the foetus. In the adult, it’s only these two populations that are very specific to the stem cells that are affected by the virus. These cells are special, and somehow very susceptible to the infection” [1].

    Therefore, a loss of these stem cells caused by the Zika virus in adults could, over time, lead to decreases in cognition and long term memory, similar to that seen in Alzheimer’s disease [6]. However, Gleeson went on to say, “In more subtle cases, the virus could theoretically impact long-term memory or risk of depression but tools do not exist to test the long-term effects of Zika on adult stem cell populations” [1]. These damaging effects may be prevented in healthy human brain cells as the immune system is able to successfully stop the virus infecting, whereas individuals with less efficient immune systems may be more at risk.

    In conclusion, although this particular study provides evidence that points towards the Zika virus affecting adult brain cells, it is not conclusive. Further research is required as only one strain of mice and virus were investigated in this experiment. Other scientists in Florida are currently researching ways to eliminate this rapidly spreading disease by using genetically modified mosquitoes, hopefully paving the way to eradication.

    By Ellen Temple, 3rd Year Medical Sciences

    References:

    [1] Neuroscientist News, (2016). Zika infection may affect adult brain cells. [online] Available at: http://www.neuroscientistnews.com/research-news/zika-infection-may-affect-adult-brain-cells [Accessed 22 August, 2016].
    [2] Sikka, V. et al., (2016). The emergence of zika virus as a global health security threat: A review and a consensus statement of the INDUSEM Joint working Group. Journal of Global Infectious Diseases 8 (1) pp.3-15.
    [3] Hamel, H et al., (2015). Biology of Zika Virus Infection in Human Skin Cells. Journal of Virology. 89 (17) pp. 8880-8896.
    [4] World Health Organisation WHO, (2016). Microcephaly Fact Sheet. [online] Available at: http://www.who.int/mediacentre/factsheets/microcephaly/en/ [Accessed 22 August, 2016].
    [5] Coghlan, A. (2002). Just 2.5% of DNA turns mice into men. New Scientist. [online] Available at: https://www.newscientist.com/article/dn2352-just-2-5-of-dna-turns-mice-into-men/ [Accessed 22 August, 2016].
    [6] Li,H et al,. (2016). Zika Virus Infects Neural Progenitors in the Adult Mouse Brain and Alters Proliferation. Cell Stem Cell. 10, 1016.

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