Yellow fever is an acute viral hemorrhagic disease. It originated from the Central Africa,spread to West Africa and finally to America. It is transmitted by infected mosquitoes and ticks. has caused about 84,000-170,000 cases and 6,000 deaths. The first outbreak was recorded in Yucatan Penisula, Mexico in the year 1648.
MODE OF TRANSMISSION
Yellow fever virus is transmitted to people primarily through the bite of infected Aedes or Haemagogus species mosquitoes. Mosquitoes acquire the virus by feeding on infected non-humans (monkeys) and then can transmit the virus to other primates (human or non-human). People infected with yellow fever virus are infectious to mosquitoes shortly before the onset of fever and up to 5 days after onset.
Yellow fever virus has three transmission cycles: jungle (sylvatic), intermediate (savannah), and urban.
*The jungle (sylvatic) cycle involves transmission of the virus between monkeys and mosquito species found in the forest canopy. The virus is transmitted by mosquitoes from monkeys to humans when humans are visiting or working in the jungle.
*Intermediate (savannah) cycle exists that involves transmission of virus from mosquitoes to humans living or working in jungle border areas. In this cycle, the virus can be transmitted from monkey to human or from human to human via mosquitoes.
*The urban cycle involves transmission of the virus between humans and urban mosquitoes, primarily Aedes aegypti. The virus is usually brought to the urban setting by a human who was infected in the jungle or savannah.
The mosquito becomes contagious in 6-18 days after sucking blood of a human patient. The virus remains in the blood depending on the ambient temperature, usually 18days for ambient temperatures under 21°. Once infected,the vector remains infectious for it’s entire life (1-2 months).
As the virus is in the blood into the human, it multiplies in 3-6 days the lymphoid nodes and enters the blood to cause viraemia
SIGNS AND SYMPTOMS
The incubation period of yellow fever is 3 to 6 days. A person begins to manifest symptoms
after the incubation period
*Death may occur on day 7-10 of illness.
CLINICAL PRESENTATION OF YELLOW FEVER
Currently, no specific treatments for the disease exists. Efforts are being focused on managing symptoms and limiting complications. Prevention involves vaccination and mosquitoes control
The 17D vaccine is the only vaccine being employed for the prevention of yellow fever.
*The 17D vaccine : is a live attenuated vaccine developed from non-virulent 17D strain grown in chick embroyo and subsequently freezed dried. This vaccine is contraindicated in persons with severe allergies to protein and pregnant women.
It is required for every traveller to have the yellow fever card which signifies that a person has been vaccinated.