Yellow fever is an acute viral hemorrhagic disease with a 40 to 50 nm enveloped RNA virus with positive sense of the Flaviviridae family. It is transmitted by the bite of female mosquitoes and is found in tropical and subtropical areas in South America and Africa, but not in Asia.
Primates and a few kinds of mosquitoes are the only known hosts. The origin of the disease is most likely Africa. From there it was introduced to South America through the slave trade in the 16th century. There have been several epidemics of the disease since the 17th century. It has been labeled as one of the most dangerous infectious diseases.
In most cases infection causes fever, nausea, and pain and generally subsides after several days. The toxic phase sometimes follows and can cause liver damage with jaundice and can lead to death. It causes increased bleeding in patients and is estimated to cause 200,000 illnesses and 30,000 deaths every year in unvaccinated populations. 90% of the infections occur in Africa.
Since the middle of the 20th century a safe and effective vaccine has existed and is required in some countries. Vaccines are the only measures to reduce yellow fever population amongst people since there is no known therapy. It is a re-emerging disease due to cases of yellow fever being on the rise since the 1980s.
It begins after an incubation period of three to six days and usually subsides after three to four days. Only about fifteen percent of cases enter the second, toxic phase. Surviving infection gives the patient a life-long immunity and normally there is no permanent organ damage.
After entering the host cells the genome replicates in the rough endoplasmic reticulum and in the so-called vesicle packets. It is primarily transmitted through the bite of the yellow fever mosquito. Like other Arboviruses yellow fever is taken up by the female mosquito and then replicates it in the stomach of the mosquito. Once injected the next time the mosquito feeds it passes the virus on.
There are three different infectious cycles in which the virus is transmitted from mosquitoes to humans or other primates. The Urban cycle is responsible for most major outbreaks of yellow fever that occur in Africa. There is also the Africa and South America cycle and the sylvatic cycle.
The virus replicates in the lymph nodes and infects dendritic cells in particular. If the disease takes a deadly course it can cause cardiovascular shock and multi organ failure with strongly increased cytokine levels follow.
Yellow fever is clinically diagnosed and often relies on the whereabouts of the diseased person during the incubation time. Mild cases can be confirmed through virologically. Even mild cases can cause a regional outbreak so each case must be treated seriously. The virus can’t be confirmed until six to ten days after the illness is suspected. Differential diagnosis allows for yellow fever to be distinguished from other feverish illness. Personal prevention includes vaccination as well as avoidance of mosquito bites. If traveling to a known affected area vaccination is highly recommended. In 20% of all cases flu-like symptoms may develop and in rare cases the vaccination can cause YEL-AVD.
The largest mass vaccination started in 2009 in West Africa and when it is completed in 2015 more than 12 million people will have been vaccinated against the disease. There are about 600 million people living in endemic areas worldwide. 90% of infections occur on the African continent. There are seven genotypes of yellow fever virus. Five of the seven occur solely in Africa. The virus most likely evolved in Africa, most likely the East or Central areas. Early on it was believed to be passed by direct human contact but Carlos Finlay first proposed mosquitoes were the carriers. Two vaccines were created in the 1930s.