Yellow fever is a viral hemorrhagic disease that is caused by the yellow fever virus and transmitted by mosquitoes.
It is not just any mosquito that can spread the virus.
The mosquitoes that can spread the yellow fever virus belong to the Haemagogus species and the Aedes species. Some of these mosquitoes live and breed in the forests, some live and breed around houses, while others live and breed both in forests and around houses.
Mosquitoes breed in places that are around bodies of standing water, humid and tropical.
The Aedes and Haemagogus mosquitoes, and monkeys are reservoirs of the yellow fever virus.
There are different sequences through which the transmission of this virus can occur; these are called the transmission cycles. There are three types, namely:
- Jungle or Sylvatic yellow fever: This involves the transmission of the virus from yellow fever-infected non-human primates (like monkeys) to the mosquitoes and vice versa; as well as, transmission from the monkeys to humans through mosquitoes. The primary reservoir of yellow fever are monkeys. And so, in tropical rain forests, the infected monkeys are bitten by mosquitoes which spread the virus to uninfected monkeys. When humans travel through the forest or work there, the infected mosquitoes bite them as well, and transmit the virus to them – then they develop the yellow fever.
- Intermediate or Savannah yellow fever: This type of transmission involves the mosquitoes that live and breed both in the forests and around houses. And it is the type that occurs most commonly in Africa. The infected mosquitoes spread the virus to both human beings and monkeys. In other words, the mosquitoes spread the virus from human to human and from monkey to human. It usually occurs in areas that are close to forests.
- Urban yellow fever: This occurs when yellow fever-infected individuals to areas with high population of people who have not received the vaccine for yellow fever (and so have little or no immunity to the disease) and very many mosquitoes. And so, mosquitoes bite the yellow fever-infected person(s), become infected and spread the virus from person to person. Usually, the yellow fever-infected person who brings the virus to the urban areas was infected in the forest or Savannah.
These mosquitoes are active mostly during the daytime – this is when they are most likely to bite.
So, yellow fever spreads in the following ways:
- Through the jungle or sylvatic yellow fever transmission cycle.
- Through the intermediate or Savannah jungle fever transmission cycle.
- Through the urban yellow fever transmission cycle.
- It has been found that it can be transmitted directly into an uninfected person’s blood through needles that are contaminated with the yellow fever virus.
- A case of perinatal transmission of yellow fever has been recorded; three days before giving birth, the pregnant woman developed symptoms of yellow fever. When the baby was born, tests were carried out to check for the RNA of yellow fever in the new born baby, and it was positive. Twelve days after birth, the new born died from severe yellow fever.
Yellow fever-infected persons are infectious to the mosquitoes a short while before the fever starts and about 5 days after the fever starts. It is during this period that they transmit the virus to the mosquitoes that bite them.
Yellow fever is not transmitted by physical contact with infected persons – the virus cannot be spread from person to person through casual contact.