Dengue Fever, Malaria, Yellow Fever, West Nile, and Chikungunya; what do they all have in common?
You guessed it….MOSQUITOES! These are all mosquito-borne diseases that have plagued humans for at least the past 100 years.
Did you know that mosquitoes account for more human suffering than any other organism? They could become an even greater annoyance in 2016 due to the rapid movement of the Zika virus, which is primarily spread to humans through mosquito bites, from its endemic regions of Africa and Asia.
In the past nine months alone, 19 countries reported Zika virus transmission, specifically in the Americas and the Caribbean. The enhanced rate of transmission is most likely due to increased travel and urbanization. In fact, in Brazil, which is currently battling an outbreak, it is speculated that the virus was probably introduced during the 2014 FIFA World Cup (Marcondes and Ximenes 2015). It is therefore possible that the next summer Olympic Games scheduled for Rio de Janeiro in August could serve as a catalyst for increased viral transmission worldwide. Regardless, the Zika virus is certainly something to pay attention to this year.
So, you’re probably wondering exactly what the Zika virus is? In this blog post, we describe the history, epidemiology and biology of the virus.
The Zika virus was discovered in a rhesus monkey in the Zika Forest of Uganda on April 18, 1947 (Dick et al. 1952).
In early 1948, the virus was isolated from the Aedes (Stegomyia) Africanus mosquito in the same forest, implicating the Aedes genus as vectors of the virus. Its isolation from humans was first performed in clinical studies in 1968 in Nigeria, 21 years after its discovery (Fagbami AH 1979). However, from 1951-1981 there were several cases of human infection reported in Africa (e.g. Uganda, Egypt, and Tanzania) and Asia (e.g. Thailand, Malaysia, and the Philippines). By April 2007, the Zika virus had spread outside of its usual geographic range (Africa and Asia) with an outbreak on Yap Island in the Federated States of Micronesia (Pacific Islands). Six years later, the virus re-emerged in French Polynesia (including Tahiti and Bora Bora) causing an outbreak which lasted from 2013-2014. Also in 2014, there were outbreaks in New Caledonia and Easter Island. As mentioned, 2015 saw several outbreaks predominantly in the Americas and the Caribbean, with Brazil being the most affected country.
According to the United States Centers for Disease Control and Prevention, at least 34 countries have reported past or current evidence of Zika virus transmission.
The Zika virus is mainly transmitted by Aedes species mosquitoes, specifically Aedes aegypti, which is known to be prevalent in urban areas and is responsible for the current outbreak in Brazil. The virus breeds in these mosquitoes without causing harm, and remain for the entire life span of the insect. It is then transmitted whenever the mosquito takes its next blood meal (Ioos et al. 2014). The Zika virus can also be transmitted from mother to child, and one case of sexual transmission has been reported (Foy et al. 2011). Zika virus infection is mostly asymptomatic (approximately 80% of cases), but symptoms can range from mild, such as low-grade fever, skin rash, and achy joints, to serious neurological diseases such as Guillain-Barre syndrome and microcephaly, which is currently a major concern in Brazil. There is no treatment or prophylactic vaccine for Zika virus disease, and avoiding mosquitoes and managing symptoms are currently the main strategies for disease control.
The Zika virus is less popular than its high profile cousins of the Flaviviridae family of viruses, which include dengue, yellow fever, West Nile, and Japanese encephalitis virus; and as such the pathogenesis of Zika virus disease is poorly understood.
Zika virus has been classified as an arbovirus that contains positive single stranded genomic RNA. And a recent study by Hamel et al. (2015) has provided significant insight into its biology. Using the Zika virus strain isolated from the 2013/2014 outbreak in French Polynesia, it was demonstrated that human dermal fibroblasts, epidermal keratinocytes, and immature dendritic cells are the cell types primarily infected by the Zika virus. When a blood-feeding female mosquito of the Aedes genus injects the virus in the skin of the host, it then enters these cells via cell surface entry/adhesive factors such as DC sign, AXL, Tyro3, and Tim-1. Once inside the cell, the virus then induces apoptosis and autophagy, which allows its release from the infected cell for further infection of other cells and increases viral replication, respectively. Replication of the virus leads to activation of an anti-viral innate immune response by the host, which involves the production of type 1 interferons in infected cells (Hamel et al. 2015).
Currently, the Zika virus is relatively unknown, but it may soon rise to fame this year.