Yellow fever is a hemorrhagic fever caused by the yellow fever virus, belonging to the Flaviviridae family (the name derives from the Latin flavus, meaning yellow). The virus is transmitted to humans by a mosquito of the genus Aedes spp.
The disease is transmitted by infected mosquitoes. The disease has two main cycles: urban and sylvatic.
In the urban cycle, humans are reservoirs of the virus and the infection occurs via the domestic mosquito, the Aedes aegypti. The mosquito transmits the virus from infected individuals to susceptible individuals.
In the sylvatic cycle, mosquitoes are infected by monkeys, which act as a reservoir for the virus. Mosquitoes then bite and infect humans who for any reason venture into forests (it is the predominant cycle in African and South American equatorial forests, ‘jungle’ cycle). Other cycles have been hypothesised where other species of mosquitoes, humans and non-human primates could be involved (one example is the Savannah cycle).
The virus, originating in Africa, has successively spread throughout the world, establishing areas of endemicity (between the 15th parallel north and the 10th parallel south in South America and Africa). The frequency of transmission varies based on the density of the vector and the presence of susceptible hosts.
Symptoms and complicationsThe disease manifests in most cases as asymptomatic (abortive form) or with flu-like symptoms. In a small percentage of the cases, the disease presents with more severe symptoms (classical form) and is potentially lethal.The classical form presents 3 phases:
- violent onset with fever, headache, muscle pain, nausea;
- after 3-4 days there is a period of remission with reduced fever and symptoms; the abortive form of yellow fever heals at this point;
- 15-25% of infected people enter a phase of "intoxication" with moderate-to-severe illness, return of fever, jaundice, both cutaneous and internal bleeding. Deaths occur in 20% of serious cases.
Impact on the population
According to World Health Organization (WHO) estimates, each year there are numerous epidemics totalling about 200,000 cases. Among the states of Central Africa and South America those most affected are Nigeria and Peru. Ninety-five percent of all cases of yellow fever occur in Africa (mainly in states that have never been French colonies, given the presence of vaccination in the latter). Of the 11,297 African cases reported in 1990-99, 2,648 were fatal (most of these were in Nigeria).
The risk of illness and death from yellow fever in the unvaccinated traveller who visits African endemic areas is respectively 1 in 1,000 and 1 in 5,000; in South America the risk is 10 times lower. According to the American Center for Disease Control and Prevention (CDC) the risk of yellow fever is 0.4-4.3 cases per 1,000,000 travellers.
In Italy, in recent years, there have been no reported cases and the possibility of getting sick is a consequence of travel to endemic areas (the absence of the vector means it cannot be spread).
The main preventive measures are pest extermination, protection from mosquito bites and vaccination. Sylvatic (jungle) yellow fever is preventable only by vaccination.
Sources / Bibliography
- Modern Infectious Disease Epidemiology - (A. Krämer, M. Kretzschmar, K. Krickeberg;)
- Manuale per il Controllo delle Malattie Trasmissibili - (Rapporto ufficiale dell'APHA - D. L. Heymann)
- Encyclopedia of Virology - 3rd Ed. - (B.W.J. Mahy, M.H.V. Van Regenmortel et al.)
- Virology: Principles and Practice - (J. Carter, V. Saunders et al.)
- Foundations in Microbiology - 8th Ed. - (K. P. Talaro, B. Chess et al.)
- Vaccini e vaccinazioni - 3rd Ed. - (G. Bartolozzi et al.)
- Harrison's Principles of Internal Medicine - 16th Ed. - (Kasper, Braunwald, Fauci. Longo et al.)