What you need to know about Yellow Fever

Kenya has confirmed Yellow Fever outbreak in Isiolo's Merti and Garbatulla sub-counties.

The first case was detected on January 12, 2022, according to the acting Director-General for Health, Dr Patrick Amoth.

In a statement, Dr Amoth said three deaths have been recorded, with 15 patients presenting fever, jaundice, muscle and joint pains.

“While the majority are young adults, males are most affected. Sadly, we have recorded three deaths so far. Out of the 6 samples analyzed at KEMRI, three turned positive through serology (Immunoglobulin M) and PCR. This is therefore to raise the alert in all 47 counties, more so the high-risk counties of Wajir, Garissa, Marsabit, Meru, Samburu, Baringo, Elgeyo Marakwet, West Pokot, and Turkana,” read the statement in part.

Amoth noted that the Ministry of Health (MoH) has put in place a national incident management structure to manage the outbreak, and is set to deploy a rapid response team to Isiolo and neighbouring counties to establish the extent of the outbreak.

“The Ministry plans to conduct Yellow Fever vaccination in Isiolo and other high-risk counties including Wajir, Garissa, Marsabit, Meru, Samburu, Baringo, Elgeyo Marakwet, West Pokot, Turkana, and possibly Tana River and Mandera,” stated Amoth.

So, What is Yellow Fever?

According to a fact sheet provided by the acting Director-General, Yellow fever is an acute viral hemorrhagic disease transmitted by infected Aedes mosquitoes. The "yellow" in the name refers to jaundice that affects some patients.

The virus is endemic in tropical areas of Africa and Central and South America and occur when infected people introduce the virus into heavily populated areas with high mosquito density and where most people have little or no immunity, due to lack of vaccination.

Signs and symptoms

Once contracted, the virus incubates in the body for 3 to 6 days. Many people do not experience symptoms, but when these do occur, the most common are fever, muscle pain with prominent backache, headache, loss of appetite, and nausea or vomiting. In most cases, symptoms disappear after 3 to 4 days.

A small percentage of patients, however, enter a second, more toxic phase within 24 hours of recovering from initial symptoms. High fever returns and several body systems are affected, usually the liver and the kidneys. In this phase, people are likely to develop jaundice (yellowing of the skin and eyes, hence the name "yellow fever'), dark urine and abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach.

Half of the patients who enter the toxic phase die within 7 - 10 days.

Diagnosis

Yellow fever is difficult to diagnose, especially during the early stages. A more severe case can 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.

Polymerase chain reaction (PCR) testing in blood and urine can sometimes detect the virus in the early stages of the disease. In later stages, testing to identify antibodies is needed (ELISA and PRNT).

Populations at risk

Forty-seven countries in Africa (34) and Central and South America (13) are either endemic for, or have regions that are endemic for, yellow fever.

In order to prevent the importation of the disease by travellers who visit yellow fever endemic countries, many countries require proof of vaccination against yellow fever before the issuance of a visa.

Transmission

The virus is an arbovirus of the flavivirus genus and is transmitted by mosquitoes, belonging to the Aedes and Haemogogus species. There are 3 types of transmission cycles:

Sylvatic (or jungle) yellow fever: In tropical rainforests, 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 travelling in the forest are bitten by infected mosquitoes and develop yellow fever.

Intermediate yellow fever: In this type of transmission, 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. This is the most common type of outbreak in Africa.

Urban yellow fever: 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 conditions, infected mosquitoes transmit the virus from person to person.

Treatment

Good and early supportive treatment in hospitals improves survival rates. There is currently no specific anti-viral drug for yellow fever but specific care to treat dehydration, liver and kidney failure, and fever improves outcomes. Associated bacterial infections can be treated with antibiotics.

Prevention

1. Vaccination

Vaccination is the most important means of preventing yellow fever.

The vaccine is safe, affordable and a single dose provides life-long protection against yellow fever disease. A booster dose of Yellow Fever vaccine is not needed.

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 travellers going to yellow fever endemic areas.

In high-risk areas where vaccination coverage is low, prompt recognition and control of outbreaks using mass immunization is critical.

People who are usually excluded from vaccination include infants aged less than 9 months, pregnant women - except during a yellow fever outbreak when the risk of infection is high; people with severe allergies to egg protein; and people with severe immunodeficiency due to symptomatic HIV/AIDS or other causes, or who have a thymus disorder.

2. Vector control

The risk of yellow fever transmission in urban areas can be reduced by eliminating potential mosquito breeding sites, including by applying larvicides to water storage containers and other places where standing water collects.

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

Source: Ministry of Health.