Yellow fever is a viral disease of typically short duration. It is a serious, potentially deadly flu-like disease spread by mosquitoes.
Yellow fever is most prevalent in parts of Africa, South America, Central America and the Caribbean. It isn’t curable, but you can prevent it with the yellow fever vaccine.
Transmission of Yellow Fever
The yellow fever virus is an arbovirus of the flavivirus genus and is transmitted by mosquitoes, belonging to the Aedes and Haemogogus species. The different mosquito species live in different habitats - some breed around houses (domestic), others in the jungle (wild), and some in both habitats (semi-domestic).
There are 3 types of transmission cycles:
Sylvatic (or jungle) yellow fever: In tropical rainforests, monkeys, which are the primary reservoir of yellow fever, are bitten by wild mosquitoes which pass the virus on to other monkeys. Occasionally humans working or travelling in the forest are bitten by infected mosquitoes and develop yellow fever.
Intermediate yellow fever: In this type of transmission, semi-domestic mosquitoes (those that breed both in the wild and around households) infect both monkeys and people. Increased contact between people and infected mosquitoes leads to increased transmission and many separate villages in an area can develop outbreaks at the same time. This is the most common type of outbreak in Africa.
Urban yellow fever: Large epidemics occur when infected people introduce the virus into heavily populated areas with high mosquito density and where most people have little or no immunity, due to lack of vaccination. In these conditions, infected mosquitoes transmit the virus from person to person.
Signs and Symptoms of Yellow Fever
Most people with yellow fever do not develop symptoms, or the symptoms are very mild.
Yellow fever has an incubation period of between 3 and 6 days, so it takes from 3 to 6 days for signs and symptoms to appear after a person is infected.
The main symptoms of yellow fever are a high temperature, a slow pulse, albuminuria, jaundice, congestion of the face, and haemorrhage, or bleeding.
Signs and symptoms are categorized into two stages:
In the early stage, the patient may experience:
aching muscles, particularly the back and knees
a high fever
loss of appetite
shivers, or chills
These symptoms usually improve and disappear after a few days (usually within 7 to 10 days), but around 15 percent of people enter a second stage, or toxic stage. The symptoms are more severe, and they may be life-threatening.
vomiting, sometimes with blood
tiredness, sluggishness, lethargy
jaundice, which gives the skin and whites of the eyes a yellow tinge
delirium, seizures, and sometimes coma
arrhythmias, or irregular heartbeats
bleeding from the nose, mouth, and eyes
Between 20 percent and 50 percent of patients who develop toxic stage symptoms die within two weeks.
Within 7 to 10 days, yellow fever is fatal in around half the patients who enter the toxic phase.
Diagnosis of Yellow Fever
Diagnosis will be confirmed after the doctor detects the signs and symptoms and carries out a blood test.
A blood test is necessary because other diseases have similar signs and symptoms.
some other viral hemorrhagic fevers
A blood test may reveal the virus, or it may detect antibodies that the body produces when the virus enters the body. A blood test may also reveal a drop in white blood cells, or leucopenia, another sign of infection.
The blood tests used are an enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR).
Test results may take several days.
Good and early supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever but specific care to treat dehydration, liver and kidney failure, and fever improves outcomes. Treatment is symptomatic. Rest, fluids, and use of pain relievers and medication to reduce fever may relieve symptoms of aching and fever. Associated bacterial infections can be treated with antibiotics.
Care should be taken to avoid certain medications, such as aspirin or other nonsteroidal anti-inflammatory drugs (e.g. ibuprofen, naproxen), which may increase the risk of bleeding.
Yellow fever patients should be protected from further mosquito exposure (staying indoors and/or under a mosquito net) for up to 5 days after the onset of fever. This way, yellow fever virus in their bloodstream will be unavailable to uninfected mosquitoes, thus breaking the transmission cycle and reducing risk to the persons around them.
The majority of infected persons will be asymptomatic or have mild disease with complete recovery.
In persons who become symptomatic but recover, weakness and fatigue may last several months.
Among those who develop severe disease, 20–50% may die.
Those who recover from yellow fever generally have lasting immunity against subsequent infection.
Prevention of Yellow Fever
A safe and highly effective vaccine prevents yellow fever. Yellow fever is known to be present in sub-Saharan Africa and parts of South America. If you live in one of these areas, talk to your doctor about whether you need the yellow fever vaccine. If you plan to travel in these areas, talk with your doctor at least 10 days, but preferably three to four weeks, before your trip begins. Some countries require travellers to present a valid certificate of immunization upon entry.
A single dose of the yellow fever vaccine provides protection for at least 10 years. Side effects are usually mild, lasting five to 10 days, and may include headaches, low-grade fevers, muscle pain, fatigue and soreness at the site of injection. More-significant reactions — such as developing a syndrome similar to actual yellow fever, inflammation of the brain (encephalitis) or death — can occur, most often in infants and older adults. The vaccine is considered safest for those between the ages of 9 months and 60 years.
Talk to your doctor about whether the yellow fever vaccine is appropriate if your child is younger than 9 months, if you have a weakened immune system (immunocompromised), if you're older than 60 years, if you are pregnant or breastfeeding.
In addition to getting the vaccine, you can help protect yourself against yellow fever by protecting yourself against mosquitoes.
To reduce your exposure to mosquitoes:
Avoid unnecessary outdoor activity when mosquitoes are most active.
Wear long-sleeved shirts and long pants when you go into mosquito-infested areas.
Stay in air-conditioned or well-screened housing.
If your accommodations don't have good window screens or air-conditioning, use bed nets. Nets that have been pre-treated with insecticide offer additional protection.
To ward off mosquitoes with repellent, use both of the following:
Nonskin repellent. Apply permethrin-containing mosquito repellent to your clothing, shoes, camping gear and bed netting. You can buy some articles of clothing and gear pre-treated with permethrin. Permethrin is not intended for use on your skin.
Skin repellent. Products with the active ingredients DEET, IR3535 or picaridin provide long-lasting skin protection. Choose the concentration based on the hours of protection you need. In general, higher concentrations last longer.
Keep in mind that chemical repellents can be toxic, and use only the amount needed for the time you'll be outdoors. Don't use DEET on the hands of young children or on infants under 2 months of age. Instead, cover your infant's stroller or playpen with mosquito netting when outside.
According to the Centers for Disease Control and Prevention, oil of lemon eucalyptus, a more natural product, offers the same protection as DEET when used in similar concentrations. But these products should not be used on children younger than age 3.
Quick Key facts
Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes. The "yellow" in the name refers to the jaundice that affects some patients.
Symptoms of yellow fever include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue.
A small proportion of patients who contract the virus develop severe symptoms and approximately half of those die within 7 to 10 days.
The virus is endemic in tropical areas of Africa and Central and South America.
Since the launch of the Yellow Fever Initiative in 2006, significant progress in combating the disease has been made in West Africa and more than 105 million people have been vaccinated in mass campaigns. No outbreaks of yellow fever were reported in West Africa during 2015.
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 little or no immunity, due to lack of vaccination. In these conditions, infected mosquitoes transmit the virus from person to person.
Yellow fever is prevented by an extremely effective vaccine, which is safe and affordable. A single dose of yellow fever vaccine is sufficient to confer sustained immunity and life-long protection against yellow fever disease and a booster dose of the vaccine is not needed. The vaccine provides effective immunity within 30 days for 99% of persons vaccinated.
Good supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever.