Overview

Yellow fever is an epidemic disease transmitted by mosquitoes and preventable through vaccination, which infects humans through the bites of infected mosquitoes.¹ The yellow fever mosquito is the Aedes aegypti. Its worldwide distribution has yielded substantial interest due to its pivotal function as a biological transmitter of several arboviruses, namely dengue, chikungunya, yellow fever, and Zika viruses.²

Mosquitoes become infected with the virus when they feed on primates (either human or non-human) that are already infected. They can then pass on the virus to other primates (either human or non-human).³ The viremic phase (when infected individuals are infectious to mosquitoes) is during the initial four days of illness until the fever and other symptoms subside.⁴

The widespread distribution of Aedes aegypti can be mostly attributed to global travel and commerce. Throughout history, the mosquito has been transported between continents by ships and was formerly present in southern Europe from the late 18th to the mid-20th century.^5,6 Aedes aegypti flourishes in highly populated regions lacking appropriate water sources, waste disposal systems, and sanitation facilities.⁷

Taxonomy and classification

• KINGDOM: Animalia
• PHYLUM: Arthropoda
• CLASS: Insecta
• ORDER: Diptera
• FAMILY: Culicidae
• GENUS: Aedes
• SUBGENUS: Stegomyia

Species facts

Aedes aegypti as Disease Vector

Aedes aegypti mosquitoes are carriers of arboviruses responsible for widespread diseases like Zika, dengue fever, yellow fever, and chikungunya, causing millions of cases yearly.

Feeding behaviour and transmission

Only female Aedes aegypti mosquitoes feed on blood, essential for their egg development. When a mosquito bites a person infected with an arbovirus, the virus can replicate within the mosquito and then be transmitted to other people through subsequent bites.⁸

Host preference and sensory cues

Most Aedes aegypti mosquitoes prefer to bite humans (anthropophilic behaviour).⁹ They use sensory cues like heat, carbon dioxide, and body odour to locate human hosts.¹⁰

Life cycle and breeding habitats

Aedes aegypti mosquitoes undergo larval and pupal stages in aquatic environments. Females lay eggs above the waterline in containers of fresh water, where they can remain dormant for months before hatching.¹¹

Fig. 1: Aedes aegypti life cycle. Created with BioRender.com.

Reproductive Behavior

After laying eggs, female Aedes aegypti mosquitoes resume seeking hosts in a repeating cycle known as the 'gonotrophic cycle.' The gonotrophic cycle of Aedes aegypti encompasses several stages:

1. Host seeking: Females search for a host to feed on
2. Blood meal ingestion: Upon finding a host, they feed on blood, which is necessary for egg development
3. Blood digestion: The ingested blood is digested to extract nutrients for egg maturation
4. Ovary maturation: As the eggs develop, the ovaries mature in preparation for egg-laying
5. Egg laying: Once females locate a suitable oviposition site, they lay their eggs, completing the cycle

Transmission cycles

Yellow fever virus has three transmission cycles: jungle (sylvatic), inter­mediate (savannah), and urban.

Fig. 2: Yellow fever transmission cycles. Created with BioRender.com.

Factors that play a part in transmission cycles

The global spread of Aedes aegypti has facilitated the transmission of diseases it carries over the past 25 years.¹² Although currently limited by its sensitivity to cold winters, climate change could expand its range.¹³

As human populations grow, more habitats suitable for this mosquito vector will emerge, increasing the risk of disease transmission due to its preference for multiple hosts in a single reproductive cycle.^12,14,15 Movement of infected individuals can lead to outbreaks of arboviruses in new areas. Global expansion and redistribution of Aedes aegypti has caused disease transmission in some regions of the world.¹⁶ 

Geographic distribution of Aedes aegypti

Aedes aegypti has a widespread geographical distribution, primarily in tropical and subtropical regions worldwide. Historically, it has been reported as established in:

• All Mediterranean countries (Europe, Middle East and North Africa)
• In the Caucasus (southern Russia, Georgia, Azerbaijan)
• Continental Portugal
• Both the Atlantic archipelagos (Canaries and Azores)^17,18

It is currently distributed throughout the tropics, including Africa (from where it originates) and a number of sub-tropical regions such as:

• South-eastern United States
• The Middle East
• South-East Asia
• The Pacific and Indian Islands
• Northern Australia¹⁹

Fig 3. Current known distribution of Aedes aegypti in Europe as of October 2023.²⁰

Vector control

To reduce the risk of yellow fever transmission in urban areas, several strategies can be employed:²¹

Eliminating mosquito breeding sites

• Targeted efforts to eliminate standing water sources where mosquitoes breed, such as water storage containers, gutters, and discarded containers
• Application of larvicides to stagnant water bodies prevents mosquito larvae from developing into adults

Personal protection measures

• Encouraging the use of protective clothing to minimise skin exposure to mosquito bites, especially during peak biting times
• Promotion of mosquito repellents to deter mosquitoes from biting

Utilising insecticide-treated bed nets

While bed nets are effective against night-biting mosquitoes like Anopheles species (malaria vectors), their effectiveness against Aedes mosquitoes, which bite during the daytime, is limited.

Vector surveillance and control

• Monitoring mosquito populations through vector surveillance programs to identify high-risk areas and potential outbreaks
• Implementing targeted control measures, such as insecticide spraying, in areas where Aedes aegypti and other Aedes species are prevalent

FAQs

What is yellow fever? 

Yellow fever is a viral disease caused by a virus transmitted to humans through the bite of an infected mosquito.

Which vector transmits yellow fever?

The primary vector responsible for transmitting yellow fever is the Aedes aegypti mosquito, particularly in urban environments. This mosquito species is known to bite during the daytime and can transmit the yellow fever virus from infected individuals to uninfected ones through its bites.

Where does Aedes aegypti breed?

Aedes aegypti mosquitoes breed in various habitats where clean still water is present. These include:

• Water-storage tanks
• Plastic bottles
• Barrels
• Vases
• Flower pot saucers

How to identify Aedes aegypti mosquito?

• Size: Typically ranging from 4 to 7 millimetres, they are relatively small compared to other mosquito species
• Distinctive traits: Easily recognisable by their black and white stripes on the thorax, abdomen, and legs, setting them apart from other common mosquito types
• Lyre-shaped pattern: Adult Aedes aegypti mosquitoes exhibit white scale bands in a lyre-shaped pattern on the dorsal part of the thorax, giving them a distinctive appearance

Where does the yellow fever virus occur? 

The yellow fever virus is predominantly found in tropical and subtropical regions of South America and Africa. It is present in 47 countries across Africa, Central, and South America. Approximately 90% of reported cases annually are concentrated in Sub-Saharan Africa.

How can yellow fever infection be prevented?

Vaccination is the main prevention strategy against yellow fever. The vaccine is safe and cost-effective, providing lifelong immunity with a single dose. Vaccination rates must reach a minimum of 80% among at-risk populations to prevent outbreaks.

Mosquito control becomes crucial in regions where vaccination coverage is low or the vaccine is unavailable. This involves eradicating mosquito breeding sites and employing insecticides to kill adult mosquitoes and larvae in high-density areas. Community participation, such as cleaning drains and covering water containers to prevent mosquito breeding, plays a vital role in mosquito control efforts.

Summary

Yellow fever is a viral disease spread by Aedes aegypti mosquitoes that can be prevented with immunisation. This mosquito species, which can be found worldwide, is known to transmit a variety of arboviruses, including Dengue, Zika, and Dhikungunya. Aedes aegypti breeds in standing water sources and prefers to feed on humans.

The disease transmission cycle involves infected mosquitos biting humans, which can lead to epidemics. Vector control, which includes removing breeding grounds and employing insecticides, is critical for limiting yellow fever spread, particularly in urban settings. Furthermore, at least 80% vaccination coverage among at-risk populations is required to prevent outbreaks.

Yellow fever is found primarily in tropical and subtropical regions of South America and Africa. Aedes aegypti mosquitoes can be identified by their size, striking black and white patterns, and lyre-shaped pattern on the thorax. Overall, controlling yellow fever requires an understanding of transmission dynamics as well as the implementation of efficient prevention and control strategies.