Angola reported its first cases of yellow fever (YF) late last December. Since then, it has spread rapidly to the Democratic Republic of Congo, Mauritania, Kenya and China.

According to the World Health Organization (WHO), Angola is grappling to control YF, which has infected 2,954 people and caused 328 deaths by June 8.

There are now more travellers infected with YF than in the last 50 years, each one with the potential to spread it globally.

All about the fever

The YF virus is transmitted by the Aedes and Haemogogus mosquitoes.

The former is found in Asia, Africa, North and South America, and Australia; and the latter in Central and South America.

The mosquitoes live in different habitats; some around houses (domestic), some in the jungle (wild), and some in both habitats (semi-domestic).

YF is endemic in 47 countries – 34 in Africa and 13 in Central and South America.

The transmission cycle types are jungle, intermediate and urban.

In jungle YF, bites of infected monkeys by wild mosquitoes transmit YF to other monkeys.

Sometimes, people who work or travel in the jungle can get YF after bites from infected mosquitoes.

In intermediate YF, monkeys and people are affected by bites from infected mosquitoes that breed in the wild and around households. This is the most common type of transmission in Africa.

Urban YF occurs when infected people introduce the virus to populated areas with high mosquito density and where people have little or no immunity. Person-to-person spread occurs from the bites of infected mosquitoes.

The incubation period is three to six days. Many of the infected have no symptoms.

A mother holds her son suffering from yellow fever as she waits for a prescription at a hospital in Luanda​, Angola. From its first reported cases last December, yellow fever in Angola has now spread as far as China. — Reuters

A mother holds her son suffering from yellow fever as she waits for a prescription at a hospital in Luanda​, Angola. From its first reported cases last December, yellow fever in Angola has now spread as far as China. — Reuters

Where there are symptoms, the common ones are fever, muscle pain, severe backache, headache, poor appetite, and nausea or vomiting. The symptoms disappear in most people after three to four days.

However, a small percentage of patients enter a toxic phase within 24 hours of recovery from the initial symptoms.

There is high fever and affliction of several body systems, usually the liver and kidneys.

The skin and eyes become yellow (jaundice), which gave rise to the name, yellow fever.

There is also darkening of the urine, abdominal pain, vomiting and bleeding, which can occur from the mouth, nose, eyes or stomach.

About half of those who enter the toxic phase die within seven to ten days.

The diagnosis of YF is difficult, especially in the initial phase. It can be confused with dengue, Zika infection, Chikungunya infection, malaria, leptospirosis, viral hepatitis, Ebola and other haemorrhagic fevers, and poisoning.

There is no specific treatment for YF.

Supportive treatment involves treating dehydration, fever, liver and kidney failure, bleeding, and associated bacterial infections.

Good and early supportive treatment improves survival rates.

Prevention methods

There are two methods of preventing yellow fever, i.e. vaccination and mosquito control.

A single dose of the YF vaccine, which has been used for decades, provides effective immunity within 10 days for more than 90% of those vaccinated and within 30 days for 99% of those vaccinated.

Its side effects are usually mild and may include headaches, muscle aches and low-grade fevers. There have been rare reports of serious side effects, which occur in 0.4 to 0.8 per 100,000 people vaccinated.

When there is an outbreak of YF, it is important that 80% or more of the population at risk is vaccinated to prevent transmission. Mosquito control involves eliminating potential breeding sites in urban areas and killing adult mosquitoes and larvae.

Community involvement is critical. The increase in the number of dengue cases in Malaysia in the past two years is an indicator of the challenges encountered in, and the effectiveness of, mosquito control.

The Aedes mosquito was eliminated from most of the Americas in the middle of the 20th century, but has now recolonised urban areas there. However, YF continues to place the heaviest burden on health systems in Africa.

A recent modelling study reported that YF may infect up to 1.8 million individuals in Africa annually, resulting in 180,000 cases and 78,000 deaths.

In Asia

There are several factors that are favourable to the introduction of YF into Asia.

They include about two billion people living in Aedes-infested areas, almost all of whom have no immunity to YF; large numbers of travellers from active outbreak areas in Africa; high volumes of air traffic between active epidemic areas in Africa and Asia, and within Asia; importation of cases during the Aedes breeding season; YF imported into dengue-endemic areas; difficulty in the diagnosis of YF; and inadequate vaccine stocks and inadequate surveillance structures in Asian countries.

The growing number of imported cases in China in a short time is worrying.

Any one of these cases, particularly if undiagnosed, can be the source of YF in another Asian country because of the high volumes of Asian air travel.

The supplies of YF vaccine is running out. New vaccine supplies cannot be boosted rapidly because the production process is slow, thereby limiting capabilities to produce large quantities of vaccine.

It is therefore highly unlikely that sufficient vaccines would be available for an emergency response to a YF outbreak in Asia. These conditions raise the alarming possibility of a YF epidemic, with a case fatality of up to 50%, and the vulnerability of the situation.

The WHO has rung the alarm bells. How prepared is Malaysia?

What are the processes to deal with suspected cases of YF, an old disease that has acquired renewed vigour recently?

Which and where are the facilities that medical practitioners can utilise for diagnostic and therapeutic purposes?

What are the stock levels of YF vaccine like?

What is the response to the threat from the triumvirate of dengue, Zika and YF, all transmitted by the Aedes mosquito, a permanent resident in Malaysia?

That there is a need for an adequate pre-emptive response from policymakers to avert a potential catastrophe is not an understatement.

Dr Milton Lum is a past president of the Malaysian Medical Association. The views expressed do not represent that of organisations that the writer is associated with. The information provided is for educational and communication purposes only and it should not be construed as personal medical advice. Information published in this article is not intended to replace, supplant or augment a consultation with a health professional regarding the reader’s own medical care. The Star disclaims all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.