Am I at Risk For Getting Yellow Fever?

A quick overview of: who, where, and when Yellow Fever strikes.

Who is affected?

As mentioned last week, a virus that is endemic in tropical areas of Africa and Latin America causes yellow fever. There are an estimated 200,000 cases worldwide each year with 30,000 deaths, 90% of which occur in Africa. Approximately 1% of individuals with severe hepatitis in endemic areas of Africa might be caused by yellow fever.

Yellow Fever outbreak in Darfur

Where are people most at risk?

Yellow Fever epidemics occur in places where there are humans that can serve as the viremic hosts for infection of a species of mosquito vectors called Aedes (in Africa) and Haemagogus (in South America). The virology, pathophysiology, and players involved in the process will be discussed further next week. For now, it will be sufficient to understand that a “host” in this situation means a human who has been infected with the virus and the virus is being mass-produced in the humans’ body and circulating in his or her blood.  A “vector” in this instance means a mosquito that transmits the virus from an infected person (or animal) to another person (or animal).

Mosquito-borne epidemics in Africa occur in areas that are highly and densely populated by humans and where immunization coverage is low.  As mentioned last week, fewer cases occur in South and Latin America than in Africa. A part of this reason is due to the relatively high vaccine coverage in endemic areas of South America. Another part of the reason is that the vector density is relatively low in South America. This means, that the mosquito vectors, which primarily transmit the disease from infected monkeys to humans, are found in lesser amounts in South America than in Africa.

Since the introduction of vaccination after World War II, yellow fever has been rare among travellers to Africa and South America.

Where people can get infected by the Yellow Fever virus

When are people most at risk of getting yellow fever?

Human-to-human transmission in the absence of the mosquito vectors does not occur.

In Africa, a wide array of Aedes mosquitos is responsible for the transmission of the yellow fever virus. This occurs particularly during the end of the rainy season and beginning of the dry season (usually July – October). However, there can still be episodic transmissions during the dry season. The virus circulates via the mosquitos in the savanna vegetation zones in proximity to human settlements. Because both humans and monkeys can be hosts, the rate of virus transmission and infection can quickly reach epidemic levels.

In South America, the risk of infection is highest during the rainy season (January-May, with a peak incidence in February and March). Larval development of mosquitos occurs in areas that have reservoirs of rainwater, and higher instance of infection are seen in forested areas.

For those of you who do not live in the risk areas mentioned above, you might be thinking to yourself right now: "This is interesting and all, but how does it affect me when it does not affect the environment or country that I live in?" That is a valid statement, but I ask you to think beyond the box a little bit. In our lifetimes, it is probable that we will travel to these affected areas, meet people who are affected by Yellow Fever, as well as perhaps work to prevent the spread of Yellow Fever in affected areas. Even though the epidemiology of Yellow Fever might not seem to affect you right now, it has the potential to affect you and be important to you in the future. For that reason, it is important to be as informed as you can be regarding the disease. 

Next week we will go more in depth about the etiology (transmission) and pathophysiology of yellow fever. Have a nice day!

References

Barnett, E. D. (2007). Yellow fever: epidemiology and prevention. Clinical Infectious Diseases, 44(6), 850-856. doi:10.1086/511869

Gershman, M. D., Staples, J. E. (2013). CDC health information for International travel – yellow fever. Retrieved from

http://wwwnc.cdc.gov/travel/yellowbook/2014/chapter-3-infectious-diseases-related-to-travel/yellow-fever

Monath, T. P. (2015). Yellow fever. UpToDate. Retrieved from http://www.uptodate.com/contents/yellow-fever

United Nations News Centre. (2012). Yellow fever outbreak in Sudan’s Darfur region kills 67 – UN health agency. Retrieved from http://www.un.org/apps/news/story.asp?NewsID=43431#.VSWE2bpMH8E