Yellow fever is a viral hemorrhagic disease that is usually transmitted by mosquitoes that carry the infection. It gets its name from its characteristic symptom of jaundice that some people with the disease experience.

Approximately 200,000 people are infected with yellow fever throughout the world and 30,000 of those are fatal. It is endemic in several tropical areas in Africa and Latin America with 90% of cases being reported in African countries. Over the past twenty years, the incidence of yellow fever has risen due to reduced immunity and increased population and urbanization.

Although there is currently no specific treatment for yellow fever, prevention is particularly important with safe and effective vaccination available.

Symptoms

Initial symptoms usually present 3 to 6 days following the infection with the virus. There are two separate phases of the disease that are marked by a different set of symptoms.

The first phase, also known as the acute phase, includes symptoms such as:

• Fever and chills
• Muscle pain
• Body aches
• Headache
• Loss of appetite
• Nausea and vomiting

In most cases, symptoms resolve after 3 to 4 days, however 15% of patients relapse into the second phase within 24 hours. This phase is more toxic and includes symptoms jaundice, abdominal pain and vomiting. Bleeding from the mouth, nose or stomach may also occur, resulting in blood in vomit or feces.

The disease is usually fatal for 50% of patients that experience the toxic, while the remaining half recover with minimal ongoing effects.

Transmission

An arbovirus of the flavivirus is responsible for causing yellow fever and is usually transmitted among monkeys and humans by mosquitoes that act as carriers. Mosquitoes are known to breed in residential, wild or mixed areas, accounting for the three types of transmission cycles.
 

• Sylvatic yellow fever usually affects monkeys bitten by wild mosquitoes. The infection then spreads and humans that enter the forest may be bitten by an infected mosquito and become infected with yellow fever, although this occurs rarely.

• Intermediate yellow fever involves mosquitoes that breed in wild and domestic areas that infect both monkeys and humans. This often affects neighboring villages simultaneously and is the most common type of outbreak in Africa that often causes epidemics.

• Urban yellow fever occurs in densely populated areas with transmission occurring by mosquitoes that breed around households. This can cause large epidemics when a high proportion of the population has not been vaccinated and are not immune to the virus.

Treatment and Prevention

As a virus rather than a bacterial organism causes yellow fever, antibiotics are not an effective treatment. There is currently no specific treatment and symptomatic treatment to help minimize dehydration, respiratory failure and fever is recommended.

Instead, prevention of yellow fever is a far more promising solution. A vaccination is available that provides effective, lifetime immunity to yellow fever for the vast majority of people. In order to prevent an outbreak in an area, at least 60-80% of the population should be immunized.

Preventative vaccination can be offered to residents of high-risk areas and as part of the routine infant vaccination programs. It is also recommended that travelers to endemic areas be vaccinated against yellow fever.

Additionally, affected areas can control breeding grounds of mosquitoes to help minimize infections in the area. Insecticides are often used to eliminate mosquitoes during epidemics whilst populations are vaccinated to give them time for the vaccination to take effect. This technique is more useful for urban infections, however, and is not practical to target wild mosquitoes.

References

• http://www.who.int/mediacentre/factsheets/fs100/en/
• http://wwwnc.cdc.gov/travel/yellowbook/2014/chapter-3-infectious-diseases-related-to-travel/yellow-fever
• http://www.nlm.nih.gov/medlineplus/ency/article/001365.htm