The risk of an explosion of cases of yellow fever in African cities is real. As you know, the yellow fever virus infects humans and several species of nonhuman primates and is transmitted by the bite of insect vectors. The transmission to humans may occur through three cycles:
- the jungle or forest, in the tropical forests of Africa and South America: the virus circulates between apes and is occasionally transmitted to humans who, for whatever reason, he ventures into the rainforest;
- Intermediate: occurs in the villages of the African savannah, where mosquitoes semidomestiche bite either apes and humans;
- urban: the virus is introduced in areas of high population density and the carrier transmits the infection from person to person, causing large outbreaks. In Africa
carriers are more frequently involved
- Aedes africanus in the cycle of the jungle is a nocturnal predator and prefers to obtain its blood meal from monkeys rarely bites humans, are so rare in the real jungle yellow fever in Africa;
- Aedes Simpson and other species of Aedes in the intermediate cycle, in which case we speak of "bridging vector (carrier deck) for the propensity to bite humans and monkeys either: the transmission of virus to humans occurs in villages near the jungle
- Aedes aegypti in the urban cycle: a person who was infected in the jungle, when he returned to town can give rise to an initial outbreak that spreads rapidly through the urban vector. Other times it is the carrier that is infected accidentally introduced in the city (for es. through used tires).
Experts fear an explosion epidemic in African cities including area considered at risk according to the possible simultaneous presence of the virus and the vector (indicated in the map of CDC - Yellow Book 2010).
Two factors combine to increase the risk of epidemic:
- rapid urbanization: every year in Africa's urban population increases by about 4%. It is susceptible to the disease that are concentrated in areas characterized by poor housing, inadequate hygienic conditions, poor access to clean water causes residents to stock up water in open containers, the ideal site for the proliferation of Aedes aegypti;
- the poor vaccination coverage of the population in the 40s of last century the universal vaccination with the vaccine FNV (French Neurotropic Vaccine) had significantly reduced the incidence of the disease. Vaccination was discontinued in the '80s because it had been found an increased risk of encephalitis in children: it is a vaccine prepared in cell cultures of rat brain. The current vaccine (referred to as 17D), despite having a very high safety profile, has never reached a satisfactory vaccine coverage in the population.
These two factors are behind the increase in cases of yellow fever in Africa during the last decade. It should however be noted that outbreaks were concentrated in West Africa, while in East Africa the situation is less dramatic. What impact does this different epidemiological situation in the assessment of risk for the traveler? And what are the implications for our business counseling, particularly with respect to a traveler who goes to Kenya or Tanzania? This will be the topic for a future post.
For those who wish to deepen: it signals the valuable reviews on the yellow fever Christina Gardner and Kate Ryman recently appeared in The Journal of Laboratory and Clinical Medicine, and from which I suddenly a lot of information contained in this post.
Christina L. Gardner, Kate D. Ryman. Yellow fever: a reemerging threat. Clin Lab Med 30 (2010) 237-260 doi: 10.1016/j.cll.2010.01.001
http://www.labmed.theclinics.com/
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