What is yellow fever? Beyond just a definition, I’m interested in understanding the specific causative agent, the etymology behind the name “yellow fever,” and how the disease impacts the body at a physiological level. How is yellow fever transmitted, detailing the specific vectors involved (e.g., mosquito species)? Are there other potential modes of transmission, even if rare? What are the incubation period details post transmission?
Answer
Yellow fever is an acute viral hemorrhagic disease transmitted by infected mosquitoes. The "yellow" in the name refers to the jaundice that affects some patients.
Causative Agent:
The disease is caused by the yellow fever virus, an RNA virus belonging to the Flaviviridae family.
Transmission:
Yellow fever is transmitted to humans primarily through the bite of infected Aedes and Haemagogus mosquitoes. The primary vectors are:
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Aedes aegypti: This mosquito is a highly adapted urban vector and is responsible for most urban yellow fever outbreaks. It thrives in close proximity to humans, breeding in artificial containers. It is most active during the day.
- Haemagogus species: These mosquitoes are primarily forest-dwelling mosquitoes and are involved in sylvatic (jungle) transmission. This typically involves transmission between mosquitoes and monkeys, with occasional spillover to humans who work or live in forested areas.
Transmission Cycle:
Yellow fever has three transmission cycles:
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Sylvatic (Jungle) Yellow Fever: This cycle involves transmission of the virus between forest-dwelling mosquitoes (Haemagogus and Sabethes species in South America, Aedes africanus in Africa) and non-human primates (monkeys). Humans working or traveling in the forest can be infected by mosquito bites in this cycle.
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Intermediate (Savannah) Yellow Fever: This cycle occurs in savannah areas in Africa. Semi-domestic mosquitoes (Aedes species) that breed around villages infect both monkeys and humans. Increased contact between humans and infected mosquitoes leads to outbreaks.
- Urban Yellow Fever: This cycle involves the transmission of the virus between humans and Aedes aegypti mosquitoes in urban areas. The virus is introduced into the urban setting by a viremic human (someone infected with the virus) who is bitten by an Aedes aegypti mosquito. The mosquito then becomes infected and transmits the virus to other humans. This cycle can lead to large and explosive outbreaks.
Mechanism of Transmission:
When a mosquito bites an infected human or animal, it ingests the virus. The virus then replicates within the mosquito’s midgut and subsequently disseminates to other tissues, including the salivary glands. After an extrinsic incubation period (typically 8-12 days, depending on temperature), the mosquito becomes capable of transmitting the virus when it bites another susceptible host. The virus is injected into the host’s bloodstream through the mosquito’s saliva.