Yellow Fever: Epidemiology and Virus Characteristics

Viral Haemorrhagic Fever

• Viral haemorrhagic fevers are a group of infectious diseases caused by arboviruses. These viruses are often transmitted by mosquitoes.
• A characteristic symptom of this group of diseases is bleeding from all natural body orifices.
• Flaviviruses (Group B) arboviruses cause a range of diseases, including yellow fever, dengue fever, Japanese B encephalitis, Rift Valley Fever, Kyasanur Forest disease and West Nile Fever.
• The high antigenic cross-reactivity of the viruses makes serological diagnosis difficult, especially in highly endemic areas.
• Virus isolation is the definitive diagnostic method for these diseases.

Yellow Fever

• Yellow fever is an acute infectious disease with a sudden onset and variable severity.
• The disease is caused by a virus transmitted by mosquitoes.
• The disease is characterized by fever, jaundice, haemorrhagic manifestations and albuminuria.
• The incubation period in humans is 3 to 6 days (maximum 10 days).
• Yellow fever is internationally notifiable.
• Fatality rates vary from 5 to 40%.
• An attack of yellow fever provides life-long immunity.
• Yellow fever mainly affects South America and tropical Africa (between 15° N and 100° S).
• The reservoir hosts for the virus are humans and monkeys.
• Yellow fever transmission occurs by the bite of Aedes spp mosquitoes.
• The control of yellow fever involves isolation of suspected cases, vector control to target breeding sites, using insecticides and vaccination.
• A vaccination certificate is valid for 10 years following vaccination.

Two epidemiological forms of Yellow Fever,

• Jungle fever (Sylvan): transmission between monkeys, and accidental human infection.
• Urban/Rural: transmission between humans (clinical or subclinical cases).

Laboratory Diagnosis of Yellow Fever

• Virus isolation from blood samples taken during the early stages (up to the 4th day) of the illness is used to diagnose yellow fever.

Dengue Fever in Sudan

• Sudan has seen a large outbreak (3,326 cases) of dengue fever in November 2022, with 23 deaths.
• North Kordofan is the most affected region, followed by West Kordofan.
• Other areas of Sudan are also reporting cases, such as North Darfur.
• Dengue fever and dengue hemorrhagic fever are prevalent in over 100 countries, especially in the tropics and subtropics. It affects over 2.5 billion people.

Dengue fever, Types and Symptoms

• Two types of dengue occur:
  • Dengue fever is non-fatal and characterized by fever, severe myalgia and joint pains. The illness is febrile.
  • Dengue Hemorrhagic fever (DHF) is commonly in children; it involves haemorrhagic phenomena and often results in significant mortality.
• Dengue fever is a significant illness creating discomfort, but it is usually not fatal.
• Symptoms usually occur in adults, and recovery might be prolonged even after the fever retreats in a week or so. Specific clinical presentations involve:
  • Severe muscle pains
  • Joint pain
  • Lymph node enlargement
  • Skin rash (resembling measles) on the 4th day
• DHF is typically observed in children but is sometimes observed in adults
• Features of DHF include:
  • Fever
  • Haemorrhagic manifestation
  • Liver enlargement

Dengue Fever - Key Facts

• Dengue fever has a wide distribution in the tropics, with significant hemorrhagic epidemics primarily in Southeast Asia
• The causative organism is Dengue virus (serotypes 1-4).
• Humans are the reservoir
• Transmission occurs through the bite of Aedes aegypti mosquitoes.
• Vector control is a key element in preventing the spread of dengue fever.
• The incubation period is 2 weeks.

Host Factors in Dengue

• Haemorrhagic manifestations are most frequent in children under 15 years of age.
• While peaks incidence are observed in the 3-6 age group, haemorrhagic dengue fever (DHF) can occur in all age groups.
• Infection confers immunity for approximately one year.

Laboratory Diagnosis of Dengue

• Virus isolation from the blood (collected in early stages of the illness is used): Serotypes can be identified.

Dengue Fever Control measures

• Vector control, active disease surveillance and community efforts for behavioural change, emergency preparedness combined with capacity building and training, are crucial for controlling its spread.

Kyasanur Forest Fever

• Kyasanur Forest Fever emerged after people cleared forests for grazing lands.
• Disease was first observed in monkeys in 1955.
• Humans started to die from haemorrhagic fever in 1957
• The case fatality rate was 20%.
• The virus that causes this fever is transmitted by ticks from cattle but the source of the virus still remains unknown, although monkeys were also victims.

Lassa Fever

• Lassa Fever was first identified in Nigeria in 1969 but may have been present for longer, possibly confused with other diseases like malaria or yellow fever.
• It is now endemic in parts of West Africa.
• Lassa Fever has a 10% fatality rate, and the reservoir is the brown rat ( Mastomys natalensis).

Ebola

• Ebola first emerged in southern Sudan and northeastern Zaire (now the Democratic Republic of Congo) in 1978.
• Different strains cause the disease, and it is highly infectious and lethal (approximately 90% fatality rate).
• In both Cameroon and Central African Republic, pygmies carry antibodies for the disease but there is no observed cases of the disease.
• The source of the Ebola virus remains unknown.
• Monkeys in the Philippines carry a third strain of the virus(Sudan strain of the virus is closer to Philippine strain).

Further Reading

• Clinical Problems in Tropical Medicine (J. R. Harries, A.D. Harries & G. C. Cook)
• Park's Textbook of Preventive and Social Medicine.