Yellow fever

Victorian statutory requirement

Yellow fever virus (Group A disease) must be notified immediately by telephone or fax followed by written notification within five days.

Any health care provider who suspects yellow fever should immediately contact the Chief Quarantine Medical Officer.

School exclusion is not applicable.

Yellow fever is subject to Australian quarantine.

Infectious agent

Yellow fever virus (YFV) is a member of the flavivirus group.

Identification

Clinical features
Yellow fever is an acute viral disease of short duration. In mild cases, the only symptoms may be headache and fever or a ‘dengue-like’ illness with fever, chills and myalgia. Sometimes the infection may be inapparent.

More severe disease occurs in a small percentage of cases and is characterised by three stages:

stage one has fever, chills, backache, myalgia, nausea, vomiting and epistaxis. Faget’s sign of relative bradycardia with a high temperature with a slow pulse appears on the second day. On the third day the fever falls and the patient enters remission.
stage two is a remission that may last several hours to several days. Haemorrhages, anuria and delirium may occur without remission.
stage three shows development of the classic features of jaundice and haemorrhagic manifestations such as epistaxis, haematemesis, melaena and uterine bleeding followed by albuminuria, coma and death two to three days later.

Method of diagnosis
The diagnosis is based on the presence of laboratory, clinical and epidemiological evidence.
Laboratory evidence includes one of the following:

isolation of YFV from clinical material
detection of YF viral RNA in clinical material
seroconversion with a significant rise in IgG level to YFV
YFV specific IgM detected in the absence of IgM to other flaviviruses.

Yellow fever virus specific IgG on a single specimen confirmed by neutralisation and 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 (NHSQL) at Victorian Infectious Diseases Reference Laboratory (VIDRL) as per national quarantine guidelines. 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.

Clinical evidence
Clinical evidence includes acute onset with fever and jaundice and other possible manifestations of the disease.

Epidemiological evidence
Epidemiological evidence includes a history of travel to a yellow fever endemic country in the preceding six days and no history of vaccination with yellow fever vaccine in the preceding two months.

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

Incubation period

The incubation period is three to six days.

Public health significance & occurrence

Yellow fever is endemic in tropical areas of South America and Central Africa. Outbreaks may occur in unaffected areas if mosquitoes are infected by migrating humans or monkeys infected with the virus.

Ae. aegypti is widely distributed in 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

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

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

Mode of transmission

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

Human blood is infective for mosquitoes shortly before the onset of fever and for three to five days after. Mosquitoes require nine to 12 days after a blood meal to become infectious and remain so for life.

Susceptibility & resistance

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

Control measures

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 one year of age entering Australia within six days of leaving an infected country.

A yellow fever vaccination certificate is valid for ten years and begins ten days after vaccination. Vaccine providers in Victoria must be accredited with the Department of Human Services.

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 greater than five days after onset of infection. The case should be cared for in an isolation room or in a screened room with use of a mosquito net and a suitable knock down spray for mosquitoes if not in hospital.

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

Spray inside and around the home of the patient, and all houses within a half a 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.

Australian Quarantine and Inspection Service officers routinely place travel companions of the case under quarantine surveillance on entry into Australia for six days since last staying over night in a country where yellow fever may be present. During this period they are required to notify the Chief Quarantine Medical Officer if suffering from a febrile illness.

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

Outbreak measures

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

International measures

Yellow fever must be notified to the World Health Organization under the International Health Regulations (1969).

Additional sources of information

Australian Government Department of Health and Aged Care 1999, Guidelines for the management of human quarantine disease in Australia.
World Health Organization

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