Yellow Fever Virus

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Introduction

Yellow fever virus (YFV), an acute, viral hemorrhagic infection, affected settlers in the American colonies as early as 1699 (CPP, 2014). Today, over 900 million people that live in the forty-four countries in tropical and subtropical areas of South America and Africa are still at serious risk of the disease (CDC, 2014). With over 200,000 cases reported to the World Health Organization (WHO) each year, an average of 60,000 people will become severely affected with yellow fever virus and 30,000 people will die (WHO, 2014).

Yellow-Fever-Figure-1

Figure 1. Areas of risk: (A) South America & (B) Africa.
http://www.cdc.gov/yellowfever/maps/index.html

Factors like declining population immunity to infection, deforestation, urbanization, population movements, and climate change have actually caused the numbers of people infected with YFV to increase over time (CDC, 2011). When people move in and out of jungle areas, they carry the virus with them. Infected mosquitos who are endemic in the area act as vectors and move the infection through the population. To illustrate the multiple mechanisms of transmission, please review Figure 2. YFV can move through either the Sylvatic (jungle) cycle or the Urban cycle.

Figure 2. Transmission of Yellow Fever Virus http://www.cdc.gov/mmwr/preview/mmwrhtml/figures/r5907a1f1.gif

Figure 2. Transmission of Yellow Fever Virus http://www.cdc.gov/mmwr/preview/mmwrhtml/figures/r5907a1f1.gif

In the Sylvatic cycle, an infected mosquito will infect a monkey via biting. The monkey then serves as a reservoir for the virus, and if any other mosquito bites the monkey, it too will be infected with YFV.  The cycle continues and the number of infected mosquitos increases. Alternatively, a young man who works in the logging industry will come into the jungle to work. He may be bitten by an infected mosquito. When he goes home, he becomes ill. He is now infected with YFV. Once again, any mosquito that bites the young man will become infected, thereby creating an urban cycle of transmission (Brooks, G. F., Jawetz, E., Melnick, J. L., & Adelberg, E. A., 2013).

It is also known that infected mosquitos lay infected eggs, usually in the wet season. These eggs may stay alive all the way through a dry season and hatch when it once again becomes wet. This is vertical transmission of the disease, and this means that not only mosquitos, but also their eggs, need to be eliminated to break the cycle and reduce transmission risk. Similarly, other factors that make it difficult to completely eliminate YFV are that certain hosts (not humans) can have recurrent infections of YFV keeping the disease present in a community, and alternative vectors, like ticks, can acquire the infection at one stage of their development, and as they pass through other stages, the virus develops with them (Brooks, G. F., et al., 2013).

Because there is no specific treatment for the disease, the best course of action is prevention through vaccination. In this modern day, YFV vaccines are safe, affordable, and effective, offering life-long protection against the disease (CDC, 2011).