Yellow fever virus (YFV) is an acute viral hemorrhagic disease transmitted by infected mosquitoes. The “yellow” in the name refers to the jaundice that affects some patients. Vector-borne transmission occurs via the bite of an infected mosquito, primarily Aedes or Haemagogus spp. Large epidemics of yellow fever occur when infected people introduce the virus into heavily populated areas with high mosquito density and where most people have weak or no immunity, due to lack of vaccination.
In these conditions, infected mosquitoes transmit the virus from person to another. Humans infected with YFV experience the highest levels of viremia and can transmit the virus to mosquitoes shortly before the onset of fever and for the first 3–5 days of illness. Given the high level of viremia, bloodborne transmission theoretically can occur via transfusion or needlesticks.
Asymptomatic or clinically inapparent infection is believed to occur in most people infected with YFV. For people who develop symptoms, the incubation period is typically 3–6 days. The most common symptoms some people experience are fever, muscle pain with a prominent backache, headache, loss of appetite, and nausea or vomiting. In most cases, these symptoms usually improve and are gone within several days.
But approximately 15% of patients enter a more toxic form of the disease within 24 hours of recovering from initial symptoms. In this phase, people are likely to develop jaundice, dark urine and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Almost 50% of patients who enter the toxic phase die within 7-10 days.
There is currently no specific drug to treat yellow fever; treatment is directed at symptomatic relief or life-saving interventions. However, specific care is taken to treat dehydration, liver, and kidney failure, aching and symptoms of fever. Good and early supportive treatment in hospitals improves survival rates. Infected people should be protected from further mosquito exposure during first few days of illness in order to not contribute to the transmission cycle.
Vaccination is the most effective way to prevent yellow fever. In high-risk areas where vaccination is low, it is critical to use mass immunization for controlling of outbreaks and preventing epidemics. It is important to vaccinate most of the population at risk to prevent transmission in a region where is a yellow fever outbreak. The yellow fever vaccine is safe and affordable and a single dose provides life-long protection against yellow fever disease.