Yellow fever

Notification requirement for yellow fever

Yellow fever virus is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days.

This is a Victorian statutory requirement.

Any healthcare provider who suspects yellow fever should also immediately contact the Chief Quarantine Medical Officer, as yellow fever is subject to Australian quarantine.

Yellow fever must be notified to the World Health Organization (WHO) under the International Health Regulations (2005).

Primary school and children’s services centres exclusion for yellow fever

Primary-school and children’s services centres exclusion is not applicable.

Infectious agent of yellow fever

Yellow fever virus is a member of the flavivirus group.

Identification of yellow fever

Clinical features

Yellow fever is difficult to diagnose, especially during the early stages. It can be confused with malaria, typhoid, dengue, hepatitis and other diseases, as well as poisoning.

Yellow fever is an acute viral disease of short duration. Once contracted, the virus incubates in the body for 3–6 days, followed by infection that can occur in one or two phases. The first, ‘acute’ phase usually causes fever, muscle pain with prominent backache, headache, shivers, loss of appetite, and nausea or vomiting. Most patients improve, and their symptoms disappear after 3–4 days.

However, 15 per cent of patients enter a second, more toxic phase within 24 hours of the initial remission. High fever returns, and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Bleeding can occur from the mouth, nose, eyes or stomach. Once this happens, blood appears in the vomit and faeces. Kidney function deteriorates. Half of the patients who enter the toxic phase die within 10–14 days; the rest recover without significant organ damage.

The infection is so named because of the yellow skin colour (known as jaundice) that is observed in people with a serious case of yellow fever.

Diagnosis

The diagnosis is based on the presence of laboratory, clinical and epidemiological evidence.

Laboratory evidence includes one of the following:

• isolation of yellow fever virus
• detection of yellow fever virus by nucleic acid testing
• seroconversion, or a fourfold or greater rise in yellow fever virus–specific serum IgM or IgG levels between acute and convalescent serum samples
• detection of yellow fever virus antigen in tissues by immunohistochemistry.

Yellow fever virus–specific IgG on a single specimen confirmed by neutralisation, where cross-reactions with other flaviviruses have been excluded, is suggestive of infection, and should be viewed in the context of clinical and epidemiological evidence.

All clinical specimens should be transferred immediately to the National High Security Quarantine Laboratory at the Victorian Infectious Diseases Reference Laboratory (VIDRL) as per national quarantine guidelines. The VIDRL should be contacted on (03) 9342 2600 to discuss requirements for confirmatory tests or for interpretation of laboratory results. Cross-reactivity with other flaviviruses can occur.

Laboratory suggestive evidence

Yellow fever virus–specific IgM detected in the absence of IgM to other relevant flaviviruses is suggestive of yellow fever.

Clinical evidence

A clinically compatible illness.with Yellow fever

Epidemiological evidence

History of travel to a yellow fever endemic country in the week preceding onset of illness.

A person with a febrile illness who has been in a yellow fever area within the previous 6 days is considered a suspected case and should be reported immediately.

Incubation period of yellow fever virus

The incubation period is 3–6 days.

Public health significance and occurrence of yellow fever

Yellow fever virus is endemic in tropical areas of South America and central Africa. WHO closely monitors reports of yellow fever. Yellow fever is considered to be endemic in 32 African and 13 Central and South American countries.

Outbreaks may occur in unaffected areas if mosquitoes are infected by migrating humans or monkeys infected with the virus.

The Aedes aegypti vector that can transmit the disease is common in coastal regions of north Queensland. The introduction of yellow fever virus to the Australian mosquito population could theoretically result in an urban outbreak of human disease. No other mosquito species in Australia are considered likely to be competent vectors.

Reservoir for yellow fever virus

In urban areas of endemic countries, the reservoirs are humans and Aedes mosquitoes. In forest areas, vertebrates other than humans (mainly monkeys and possibly marsupials) and forest mosquitoes are the reservoir. The viraemic period in monkeys and humans is too short for monkeys to act as a reservoir.

Humans have no essential role in transmission of forest or jungle yellow fever but are the primary amplifying host in the urban cycle.

Mode of transmission of yellow fever virus

Yellow fever is transmitted via infected mosquitoes. Mosquitoes become infectious 9–12 days after a blood meal from a viraemic host. Human-to-human transmission has not been documented.

Period of communicability of yellow fever

Human blood is infective for mosquitoes shortly before the onset of fever and for 3–5 days after. Mosquitoes require 9–12 days after a blood meal to become infectious, and remain so for life.

Susceptibility and resistance to yellow fever

Mild infections are common in endemic areas. Previous infection with dengue gives some degree of immunity. Passive immunity in infants born to immune mothers may last for 6 months. Infections confer lifelong immunity.

Control measures for yellow fever

Preventive measures

All travellers to endemic areas in Africa and South America should be immunised. Certification of yellow fever vaccination is required for travellers over 1 year of age entering Australia within 6 days of leaving an infected country. A yellow fever vaccination certificate is valid for 10 years and begins 10 days after vaccination. Vaccine providers in Victoria must be accredited with the department.

People arriving in Australia who are required to possess a yellow fever vaccination certificate but do not have one will be interviewed on arrival by Human Quarantine Officers (HQOs). HQOs will permit unvaccinated people to enter Australia provided they agree in writing to notify health authorities if they develop symptoms of yellow fever in the 6-day period following their departure from a declared yellow fever infected place. Anyone suspected or confirmed as having yellow fever may be placed under quarantine in a suitable medical facility.

Control of case

In Victoria, suspected or confirmed cases that require inpatient treatment should be referred to the Victorian Infectious Diseases Service at the Royal Melbourne Hospital, where adequate facilities for isolation are available if required. This is of particular concern in suspected cases where the differential diagnosis may include other viral or haemorrhagic fevers with greater potential for person-to-person spread.

There are no endemic foci of yellow fever vectors in Victoria; however, infection of Victorian mosquitoes is a theoretical risk. Therefore, the case should be protected from exposure to mosquitoes for more than 5 days after onset of infection. The case should be cared for in an isolation room, or in a screened room with use of an insecticide-treated mosquito net, if not in hospital.

Control of contacts

If a traveller to Australia is diagnosed with yellow fever and has been potentially exposed to Australian A. aegypti mosquitoes during the period of viraemia, or if the first recognised case is indigenous, the following measures should be considered:

• Spray inside and around the home of the patient, and all houses within a half-kilometre radius, with an effective insecticide to eliminate vectors. Potential vector breeding sites should be destroyed, emptied or sprayed within this area.
• Contacts of the patient who have not previously been immunised should be offered yellow fever vaccine. Other cases of mild febrile illness and any unexplained deaths possibly consistent with yellow fever should be investigated.

HQOs routinely place travel companions of the case under quarantine surveillance on entry into Australia for 6 days since they last stayed overnight in a country where yellow fever may be present. During this period, they are required to notify the Chief Human Quarantine Officer if suffering from a febrile illness.

Control of environment

A. aegypti mosquitoes should be eliminated near airports. Insect quarantine should be maintained to prevent the introduction of A. albopictus, a prevalent Asian species that is capable of transmitting yellow fever.

Outbreak measures for yellow fever

A single case of indigenous transmission constitutes an outbreak. In the event of an epidemic of yellow fever in an urban area, all people living in the area infested with A. aegypti should be offered yellow fever vaccine, and a wider program of mosquito spraying and breeding site elimination should be implemented.