Path-o-what? How is Yellow Fever Transmitted?

The Yellow Fever Virus It is a single-stranded RNA virus that replicates at the site of transmission (usually cells in the skin). The virus spreads through the lymphatic vessels and lymph nodes into the bloodstream and internal organs. Lymphoid cells (cells of the innate immune system) appear to be the preferred cell type for primary replication. The liver and spleen produce large amounts of the virus once affected and release them into the blood.

An electron micrograph of Yellow Fever Virus virons

Humans infected with the yellow fever virus experience the highest levels of viremia (virus in the bloodstream) and can transmit the virus to mosquitos shortly before onset of fever and for the first 3-5 days of illness. During this time there is a high level of virus in the bloodstream and so bloodborne transmission can theoretically occur via blood transfusion or needle-stick.

The classical illness is characterized by 3 stages

The period of infection: consists of viremia (virus infection in he blood) and lasts 3-4 days. This is the period of time where signs and symptoms of infection is present but are nonspecific.

The period or remission: This period follows the period of infection and may last up to 48 hours. During this period of time fever and symptoms dissipate and a person appears to be getting better. In fact, patients with mild infections may recover at this stage.

The period of intoxication: 15% of patients progress to this stage, where the return of fever and other symptoms occurs. It begins on the 3^rd to 6^th day after onset of infection and is characterized by variable dysfunction of multiple organs including the liver, kidneys, and cardiovascular system.  Due to this, patients often see hepatic (liver) dysfunction, renal (kidney) dysfunction, hemorrhage (major bleeding), myocardial injury (changes in heart functioning), and central nervous system dysfunction.

There are three types of transmission cycles: sylvatic, intermediate, and urban. All three patterns of yellow fever are found in Africa, but only sylvatic and urban yellow fever occur in South America.

Yellow Fever Transmission Patterns

Sylvatic (aka Jungle) yellow fever: occurs in tropical rainforests where monkeys that got infected by sylvatic mosquitos, pass the virus onto other mosquitos that feed on them. These newly infected mosquitos in turn bite and infect humans who enter the rainforest. This type of transmission produces sporadic cases and mostly affects people who work in the forest.

Intermediate yellow fever: occurs in humid or semi-humid savannahs of Africa, where it can cause small-scale epidemics in rural villages. Affected semi-domestic mosquitos (mosquitos that breed in the wild and around households) infect both monkey and human hosts and increased contact between humans and infected mosquitos leads to the spread of disease. This is the most common type of outbreak occurring in Africa in recent decades.

Urban yellow fever: occurs when domestic mosquitos transmit the virus from person to person. This type of transmission results in large explosive epidemic outbreaks when travellers from rural areas introduce the virus into densely populated areas with a high number of non-immune humans and Aedes mosquitos.

To sum things up, there are three main take aways from today's post:

1. The Yellow Fever Virus replicates and travels through and within the lymphatic fluid and bloodstream to reach internal organs where it continues to replicate. 
2. There are three stages of illness with the final stage having the highest morbidity and mortality characterized by organ dysfunction and impairment.
3. There are three cycles of transmission, with urban yellow fever as the leading cause of explosive epidemic outbreaks.

Next week, we will go over how Yellow Fever is diagnosed. Have a wonderful week!

References

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

World Health Organization. (2014). Yellow fever (Fact sheet No. 100). Retrieved from: http://www.who.int/mediacentre/factsheets/fs100/en/