Yellow fever has an incubation period of five to six days. In the initial phase that lasts for three to four days, there is mild fever with rigors and chills and yellowing of the skin. The yellowing occurs as the disease affects liver causing hepatitis .
The patient also complains of headache, joint and muscle pain, loss of appetite, abdominal pain, back pain, nausea and vomiting, fatigue, weakness and dehydration. The next phase is remission, which lasts for 48 hours. Most patients recover after this phase.
If not treated completely, the disease becomes worse and the sufferers enter a more toxic, third phase of infection. It can lead to a condition called viral hemorrhagic fever with shock, jaundice and abdominal pain  .
In the toxic phase, there can be bleeding in the mouth, the eyes and the gastrointestinal tract. This will cause vomiting containing blood. It results in damage to the liver, kidneys, and the circulatory system. The World Health Organization estimates that about 50% of people worldwide who reach this severe phase of infection die, while half recover.
A diagnosis of yellow fever is established on the basis of complete medical history including history of travel and vaccination status.
The laboratory testing helps to confirm the diagnosis. These include:
There is no specific treatment for yellow fever. Treatment is supportive and effective treatment involves complete bed rest under mosquito nets.
Antipyretics and analgesics are helpful to control fever and pain respectively. Drugs like antacids and histamine blockers are used to control gastric bleeding. Blood transfusions can be given for severe bleeding.
Mortality due to yellow fever varies according to the severity of the disease. It varies from 50% without medical treatment to about 5% with complete medical care.
Mortality rate is higher with life-threatening hemorrhagic fever, jaundice, liver and kidney failure. Death usually occurs within 7 to 10 days of the toxic phase of yellow fever. Those who recover have life-long immunity. So overall, the prognosis is good if diagnosed and treated properly.
Yellow fever is caused by an enveloped RNA virus called yellow fever virus . The individuals who are more at risk of developing yellow fever include travelers to certain parts of South America and Africa. The risk of the disease can increase with various factors like travel to endemic areas, visiting rural areas or participating in outdoor activities such as camping, cycling, hiking or fieldwork.
Yellow fever is endemic in tropical areas of South America and Africa . About 15% of the cases who develop yellow fever can have serious complications. The World Health Organization estimates that there are 200,000 cases of yellow fever worldwide each year, resulting in 30,000 deaths.
Due to decreased immunity to infection, deforestation, climate changes and increased urbanization, yellow fever appears to be on the rise internationally. Yellow fever transmission with a high risk have been identified in 45 countries, many of them with tropical climates.
Yellow fever virus is transmitted to humans through the bite of an infected mosquito, Aedes aegypti. Some other mosquitoes such as Tiger mosquito can also serve as a vector for this virus. Yellow fever virus is taken up by a female mosquito by feeding on infected humans. The mosquito can then transmit the virus to other humans. Yellow fever virus can also be transmitted by mosquitoes from monkeys to humans by a sylvatic cycle . It mostly occurs in travelers while visiting or working in jungle border areas.
Mosquitoes that spread viruses usually bite during the day time, particularly around sunrise and sunset. Viruses reach the stomach of the infected mosquitoes and then reach the salivary glands via blood stream. When the infected mosquito bites a person, the virus is transmitted to blood system of the person through mosquito’s saliva.
After transmission of the virus from a mosquito to human, the virus replicates in the lymph nodes, particularly the dendritic cells. From there, the viruses reach the hepatocytes in liver. This results in eosinophilic degradation and release of cytokines . If left untreated, it can lead to complications such as cytokine storm, shock, and multiple organ failure, causing fatality.
Yellow fever is preventable by vaccination which is valid for ten years. It is effective in ninety five percent of the cases . The vaccination is not recommended for children under 9 months of age and immunocompromised individuals. Immunization should be done in travelers particularly who visit areas where yellow fever exists. The preventive measures should be taken to avoid mosquito bite by wearing full protective clothing, using insect repellant and living in screened areas.
Yellow fever is a viral infection caused by flavivirus that is transmitted by the bite of an infected mosquito. The virus is found in tropical and subtropical areas in South America and Africa .