Yellow fever is a haemorrhagic disease, usually transmitted by mosquitoes. Still prevalent in countries where vaccination coverage is insufficient, this viral disease can be fatal. In France, yellow fever is subject to mandatory reporting.

1. Definition of yellow fever

Yellow fever is a viral disease caused by the amaril virus. This virus is transmitted by several types of mosquitoes, including those of the Aedes species. Monkeys act as intermediate hosts. Mosquitoes act as vectors and reservoirs, transmitting the virus to their offspring. Today, the virus is endemic in parts of the world such as South and Central America and tropical Africa. The World Health Organisation (WHO) lists the presence of the virus in 47 countries. According to Vidal, vaccination against yellow fever is compulsory for people travelling to the following countries: Angola, Burundi, Central African Republic, Chad, Congo, Côte d'Ivoire, Democratic Republic of Congo, Gabon, Ghana, Guinea-Bissau, Mali, Niger, Sierra Leone, South Sudan, Togo and Uganda. In addition to these countries, yellow fever occurs in parts of Argentina, Benin, Bolivia, Brazil, Burkina Faso, Cameroon, Colombia, Ecuador, Ethiopia, Gambia, Guinea, Equatorial Guinea, Guyana, Kenya, Liberia, Nigeria, Panama, Paraguay, Peru, Senegal, Sudan, Suriname, Trinidad and Tobago and Venezuela. Since 2017, an epidemic of yellow fever has affected parts of Brazil. Therefore, vaccination is recommended for people travelling to Espírito Santo, Rio de Janeiro, São Paulo (except the city of São Paulo) as well as regions in the state of Bahia.

The disease is called yellow fever because of jaundice it causes in some people. The severity of the disease varies, but the virus can be fatal. As a result, yellow fever is notifiable in France, but also to the WHO, as the disease is covered by international health regulations. According to the World Health Organisation, 200,000 people contract yellow fever worldwide each year, and the disease causes 30,000 deaths. In contrast, all curable forms of the disease result in lifelong immunity. Apart from prevention and vaccination, there is no treatment for the disease.

2. Symptoms of yellow fever

According to Santé Publique France, yellow fever is asymptomatic in 50 to 85% of cases. The first symptoms appear within 3 to 6 days of the bite that caused the infection. When symptoms appear, the first symptoms are flu-like: fever, chills, muscle or joint pain and headache. These clinical signs may be suggestive of influenza, dengue or malaria. Minimal bleeding may also occur. In most cases, these symptoms disappear spontaneously.

Generally, an improvement is observed after 3 to 4 days. However, in some patients, this remission is short-lived and transient, followed by a transition to the second phase of the disease. This is followed by a recurrence of fever, accompanied by a deterioration in the patient's general condition, prostration, and a significant drop in pulse and blood pressure. The patient also presents a haemorrhagic syndrome with bleeding from the mouth, nose or eyes, bleeding from the stomach accompanied by vomiting of blood, jaundice, and liver disorders. According to data from the Pasteur Institute, death occurs in 20 to 60% of cases of severe forms. Death is often preceded by a coma, convulsions and a phase of delirium.

3. Causes of yellow fever

Humans can be infected in different ways:

Through bites from wild mosquitoes, infected while in the forest. This is called the selvatic cycle and is prevalent in Latin America and in forested areas of Africa;
Through the bites of Aedes aegypti mosquitoes, when the virus is introduced into urban areas from rural areas. This so-called urban cycle is responsible for major yellow fever epidemics;
Through the bites of 'semi-domestic' mosquitoes that infect both monkeys and humans. This "intermediate" cycle remains the most common in Africa. It is responsible for small epidemics affecting rural villages.

4. Yellow fever: when to consult?

There are no specific clinical manifestations of yellow fever. It is recommended to consult a doctor as soon as flu-like symptoms and fever appear following a trip to a high-risk country.

5. Examinations and diagnosis of yellow fever

In the early stages of the disease, yellow fever can be difficult to identify. Indeed, the symptoms can be confused with those of other tropical diseases such as dengue, and malaria, but also leptospirosis, measles, hepatitis or viral haemorrhagic fevers. Blood tests can sometimes detect the virus early. A definitive diagnosis can be made by carrying out a serology test, looking for specific antibodies (ELISA and PRNT) in the blood.

6. Treatments for yellow fever

There is no specific treatment for yellow fever. Only symptomatic treatment can be considered. Combating dehydration, and reducing fever and vomiting are essential measures to limit the symptoms of the disease. To reduce pain, painkillers are recommended. In the case of superinfection, antibiotic therapy may be necessary. Avoiding kidney and liver complications can improve the patient's vital prognosis. Depending on the severity of the symptoms, hospitalisation in an intensive care unit is sometimes necessary.

7. How to prevent yellow fever

Prevention of yellow fever is based on: increased disinfection of aircraft, vector control in areas where the virus is circulating, and vaccination. There are several vaccination strategies, depending on the endemic areas:

vaccination of travellers in high-risk countries;
systematic vaccination of infants;
mass vaccination, with the aim of increasing vaccination coverage in a country at risk.

The yellow fever vaccine provides effective protection for life, 30 days after the injection in 99% of cases. Vaccination is compulsory for people travelling to French Guiana and recommended for any stay in an endemic area such as Africa or Latin America. Vaccination is required from the age of 1 year but is possible from 6 months. Moreover, no booster is necessary, except in a few cases, for example:

for children vaccinated before the age of 2, a booster at the age of 6 is recommended in case of travel to an endemic area;
In the event of a confirmed epidemic in the country of destination, a second injection is recommended for people whose vaccination is more than 10 years old.

The yellow fever vaccine is a live, inactivated vaccine prepared in an embryonated chicken egg. As such, it is contraindicated in immunocompromised individuals, those with progressive disease, those with an allergy to egg proteins, or pregnant women. In the vast majority of cases, this vaccine does not cause any adverse effects. However, a mild flu-like illness with fever may occur in the days following vaccination. There may also be a reaction at the injection site in the form of redness or pain. Paracetamol can be taken to relieve these symptoms. Please note: that the yellow fever vaccine is not covered by the health insurance system, and its price may vary depending on the vaccination centre.

In France, only centres approved by the Ministry of Health can give yellow fever injection. The doctor records, and proof of vaccination in an international vaccination booklet, must be presented by all travellers arriving in an endemic risk area. In case of non-vaccination, the doctor at the centre must complete a certificate of contraindication in French, English and also in the language of the destination country. On-site, the authorities may then impose a period of isolation of 5 to 7 days. Although vaccination remains the most effective preventive measure, the use of insecticide repellent to combat mosquito bites is strongly recommended. Controlling the vectors of the disease helps to limit the spread of yellow fever. In this sense, collective measures against mosquitoes can be taken by the authorities in each country.

Individually, the transmission of the disease can be prevented by eliminating places where water stagnates and by using larvicides. In addition, since it is impossible to eradicate mosquitoes, the wearing of protective clothing is essential. Other methods are effective in preventing the risk of contamination: insecticide-treated mosquito nets, and skin and textile repellents.