YELLOW FEVER: “Brazil is running to prevent a potentially devastating scenario”

According WHO, large epidemics of yellow fever occur when infected people introduce the virus in densely populated areas with high mosquito density and where most people have little or no immunity due to lack of vaccination." 

"It is under these conditions that infected mosquitoes successfully transmit the virus from person to person and now Brazil is running to prevent a potentially devastating scenario from developing." 

Yellow fever is an acute viral haemorrhagic disease transmitted by infected mosquitoes. 

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.

Good supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever.

Signs and symptoms

Once contracted, the yellow fever virus incubates in the body for 3 to 6 days. Many people do not experience symptoms, but when these do occur, the most common are fever, muscle pain with prominent backache, headache, loss of appetite, and nausea or vomiting. In most cases, symptoms disappear after 3 to 4 days.

A small percentage of patients, however, enter a second, more toxic phase within 24 hours of recovering from initial symptoms. High fever returns and several body systems are affected, usually the liver and the kidneys. In this phase people are likely to develop jaundice (yellowing of the skin and eyes, hence the name ‘yellow fever’), dark urine and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Half of the patients who enter the toxic phase die within 7 - 10 days.

Yellow fever is difficult to diagnose, especially during the early stages. More severe disease can be confused with severe malaria, leptospirosis, viral hepatitis (especially fulminant forms), other haemorrhagic fevers, infection with other flaviviruses (e.g. dengue haemorrhagic fever), and poisoning.

Blood tests (RT-PCR) can sometimes detect the virus in the early stages of the disease. In later stages of the disease, testing to identify antibodies is needed (ELISA and PRNT).

Transmission

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.
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• * 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.
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• * 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.

Vaccination include:

• * infants aged less than 9 months, except during an epidemic when infants aged 6-9 months, in areas where the risk of infection is high, should also receive the vaccine;
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• * pregnant women – except during a yellow fever outbreak when the risk of infection is high;
• people with severe allergies to egg protein; and
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• * people with severe immunodeficiency due to 
• symptomatic HIV/AIDS or other causes, or who have a thymus disorder.

Historically, mosquito control campaigns successfully eliminated Aedes aegypti, the urban yellow fever vector, from most of Central and South America. However, Aedes aegypti has re-colonized urban areas in the region, raising a renewed risk of urban yellow fever. Mosquito control programmes targeting wild mosquitoes in forested areas are not practical for preventing jungle (or sylvatic) yellow fever transmission.

Source: WHO

http://www.who.int/mediacentre/factsheets/fs100/en/