Yellow fever is a disease that is caused by an arthropod borne virus from flavivirus genus of the family flaviviridae. Yellow fever remains a disease of significant public health importance despite the availability of a safe and efficacious vaccine, with an estimated 200,000 cases and 30,000 deaths annually globally.

The virus is found in tropical and subtropical region of Africa and Central/South America with the illness being endemic in 34 African countries (including Nigeria) and 13 Central and South American countries

What Causes Yellow Fever and How does it Spread?

aedes mosquito feeding on human skin

Yellow fever virus is an arbovirus of the flavivirus genus and is transmitted and spread through mosquitoes, which belongs to Aedes and Haemogogus species. The different mosquito species live in different habitats, some breed around houses, others in the jungle, and some in both habitats.  Yellow Fever virus has 3 types of transmission cycles:

  1. Sylvatic (or jungle) yellow fever

In tropical rain forests, monkeys, which are the primary reservoir of yellow fever, are bitten by wild mosquitoes of the Aedes and Haemogogus species, which pass the virus on to other monkeys. Occasionally humans working or traveling in the forest are bitten by infected mosquitoes and develop yellow fever.

2. Intermediate yellow fever

 In this transmission cycle, semi-domestic mosquitoes (those that breed both in the wild and around households) infect both monkeys and people. Increased contact between people and infected mosquitoes leads to increased transmission and many separate villages in an area can develop outbreaks at the same time. Intermediate yellow fever is the most common type of outbreak in Africa.

3. Urban yellow fever

 In this transmission Large epidemics occur when infected people introduce the virus into heavily populated areas with high density of Aedes aegypti mosquitoes and where most people have little or no immunity, due to lack of vaccination or prior exposure to yellow fever. In these kinds of conditions, infected mosquitoes transmit the virus from an individual to another person. 

What are the Symptoms of Yellow Fever?

Each person may have slightly different symptoms. The following are the most common symptoms:

Sudden onset of fever

Chills

Severe headache

Back pain

General body aches

Nausea

Vomiting

Fatigue (feeling tired)

Weakness

Bleeding nose/ear/mouth

How is Yellow Fever Diagnosed?

Yellow fever is hard to diagnose, especially in the course of the early stages. A more severe cases of yellow fever could  be confused with severe malaria, leptospirosis, viral hepatitis (especially fulminant forms), other haemorrhagic fevers, infection with other flaviviruses (such as dengue haemorrhagic fever), and poisoning.

Medical laboratory scientists can use Polymerase chain reaction (PCR) testing in blood and urine to check whether or detect the virus in early stages of the disease, testing to identify antibodies is needed (ELISA and PRNT) in later stages.

How is Yellow Fever Treated?

Currently there is no specific remedy for yellow fever. Good and early supportive treatment in hospitals including treating dehydration and under nutrition, management of metabolic acidosis, liver and kidney failure, correcting clotting abnormalities, and antibiotic treatment for secondary bacterial infections improves survival rates. Infection Prevention and Control (IPC)

Vaccination with the yellow fever vaccine offers life-long protection, and is the most essential and important means of preventing yellow fever.

How is Yellow Fever Prevented?

1.Vaccination

Vaccination is the most critical and important means of preventing yellow fever.

The yellow fever vaccine is safe, low-cost and an unmarried dose present life-long protection against yellow fever disease.

Several vaccination strategies are used to prevent yellow fever disease and transmission: routine infant immunization; mass vaccination campaigns designed to increase coverage in countries at risk; and vaccination of travelers going to yellow fever endemic areas.

In high-hazard region where vaccination coverage is low, prompt recognition and control of outbreaks using mass immunization is critical. It is critical or important to vaccinate most (80% or more) of the population at hazard to prevent transmission in a location with yellow fever outbreak.

2. Vector control

The risk of yellow fever transmission in urban region may be reduced by eliminating potential mosquito breeding sites, such as with the aid of applying larvicides to water storage containers and other places where standing water collects.

Vector surveillance and control are components used for the prevention and control of vector-borne diseases, especially for transmission control in epidemic situations. For yellow fever, vector surveillance focused on Aedes aegypti and other Aedes species will help inform wherein there is a risk of an urban outbreak.

Understanding the distribution of these mosquitoes within a country can permit a country to prioritize areas to strengthen their human disease surveillance and testing, and consider vector control activities. Presently there is a limited public health arsenal of safe, efficient and cost-effective insecticides that can be used against adult vectors. This is mainly due to the resistance of major vectors to common insecticides and the withdrawal or abandonment of certain pesticides for reasons of safety or the high cost of re-registration.

Personal preventive measures such as clothing minimizing skin exposure and repellents are recommended to avoid mosquito bites. The use of insecticide-treated bed nets is limited by the fact that Aedes mosquitos bite during the daytime.

3. Epidemic preparedness and response

Prompt detection of yellow fever and fast response via emergency vaccination campaigns are vital for controlling outbreaks.

World health organization (WHO) recommends that every country at risk have at least one national laboratory where basic yellow fever blood tests can be performed.  A confirmed case of yellow fever in an unvaccinated populace is taken into consideration and considered as an outbreak. A confirmed case in any context ought to be completely and fully investigated. Investigating teams need to assess, examine and respond to the outbreak with both emergency measures and longer-term immunization plans.

Reviewed on 10/4/2020

REFERENCES
ncdc.gov.ng(Nigeria center for disease controls

cdc.gov

hopkinsmedicine.org

https://www.who.int/news-room/fact-sheets/detail/yellow-fever.