Infectious diseasesepidemiology and surveillance

Page content: Victorian statutory requirement | Infectious agent | Identification | Method of diagnosis | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures

Victorian statutory requirement

Dengue virus infection (Group B disease) requires notification within five days of diagnosis.

School exclusion: case should be isolated until the fever subsides to prevent further mosquito bites.

Infectious agent

Dengue virus (DENV) has four related but distinct serotypes: 1, 2, 3 and 4.

Dengue virus has been recognised since the latter part of the 18th century as causing epidemics in tropical and subtropical parts throughout the world. Dengue was first recognised in Townsville late in the 19th century and early in the 20th century. Outbreaks occurred in an area from the coast of Western Australia to the Northern Territory and down through high rainfall areas of Queensland and New South Wales. At that time Aedes aegypti mosquitoes were widely distributed in northern Australia and occurred as far south as the Victorian border in eastern Australia and south of Perth in Western Australia. By the 1970s Aedes aegypti were restricted to a small area of northern Queensland. Epidemic dengue returned to north Queensland in 1981–82. Other outbreaks occurred there in the 1990s, a time when Aedes aegypti mosquitoes were spreading westwards from Queensland to the Northern Territory border and towards the New South Wales border.

Identification

Clinical features
Dengue fever (break bone fever)
Dengue fever classically presents as an acute febrile illness of sudden onset. It is extremely debilitating with fever lasting three to five days, myalgia (particularly backache), arthralgia, retro-orbital pain, anorexia, gastrointestinal disturbance, rash and increased vascular permeability. There is a high subclinical rate of milder disease in children compared to adults and a low fatality rate. Recovery from infection with one serotype of the dengue virus results in homologous immunity but does not provide protection against infection with other serotypes.

Dengue haemorrhagic fever
Dengue haemorrhagic fever (DHF) is a severe complication of dengue virus infection. It occurs mainly in children and is characterised by abrupt onset of fever, haemorrhagic phenomena and thrombocytopaenia. In its severest form it may result in shock (dengue shock syndrome [DSS]), which has a high fatality rate. The rate of death from DHF without DSS is usually quoted at 1–5%. This is believed to be caused by immune enhancement when a person with dengue antibodies due to a previous infection is subsequently infected by a dengue virus of a different serotype.

Method of diagnosis

Dengue virus infection is diagnosed by a significant rise in antibodies to the dengue virus serotype.

Laboratory evidence requires one of the following:

• isolation of dengue virus from clinical material
• detection of dengue viral RNA in clinical material
• a significant rise in the level of dengue virus specific IgG proven by neutralisation or another specific test
• dengue virus specific IgM in the CSF in the absence of IgM to Murray Valley encephalitis, Kunjin or Japanese encephalitis viruses
• dengue virus specific IgM in serum, except in north Queensland. In north Queensland dengue virus specific IgM in serum is acceptable evidence only when this occurs during a proven outbreak.

Confirmation of laboratory results by a second arbovirus reference laboratory is required if the case occurs in previously unaffected areas of Australia. North Queensland is currently the only area with the potential for indigenous (epidemic) dengue fever in Australia.

Incubation period

The incubation period is usually short but varies from three to fourteen days.

Public health significance & occurrence

Dengue is not an endemic disease in Australia and the outbreaks which have occurred have been due to importations of the virus by a viraemic tourist or returning resident. It is important to rapidly diagnose the disease in returning residents and tourists to prevent local spread in receptive areas. Spread or introduction of Aedes aegypti from its present distribution in Queensland must be closely monitored.

Of great concern has been the repeated detection of imported Aedes albopictus mosquitoes into various parts of Australia dating from 1975 in Townsville. Since then it has been detected at various times and in various carriers on ships, in machinery and in car tyres in South Australia, Perth and Darwin. In 1998 it was trapped on a wharf in Cairns and similarly at West Melbourne in 2002. Preventing the introduction and establishment of Aedes albopictus remains a high priority because this mosquito has the potential to spread widely over Australia including southern areas. It can also transmit dengue and other arboviruses.

Reservoir

Humans are the only vertebrate hosts of the virus. There is a jungle cycle between monkeys and mosquitoes, but this plays no role in human disease.

Mode of transmission

Dengue is transmitted by the bite of an infected mosquito, particularly Aedes aegypti. This was first recognised by workers in Queensland early in the 20th century. Aedes aegypti breeds in fresh water and particularly in man made containers such as old tyres, pot plant holders, buckets and tree hollows in urban areas. Aedes albopictus is a mosquito common in South East Asia and Papua New Guinea and can also be an important vector. Other Aedes species are involved in the enzootic monkey cycle.

Period of communicability

There is no evidence of person to person transmission.

Susceptibility & resistance

Infection with a serotype of dengue virus does not necessarily confer immunity.

Control measures

Preventive measures
There are effective vaccines available against a number of the dengue virus serotypes.

Dengue fever can be prevented by:

• mosquito control measures
• personal protection measures such as long sleeves and mosquito repellents
• avoidance of mosquito-prone areas.

Control of case
Isolate the patient and prevent mosquito access until fever subsides.

Investigate the source of infection.

Control of contacts
Not applicable.

Control of environment

• Search for and eliminate breeding sites of Aedes aegypti in the urban area.
• Use mosquito repellents, mosquito nets and other methods of personal protection.
• Control Aedes aegypti near airports.
• Prevent importation of new vectors, for example Aedes albopictus.

Outbreak measures

Not applicable.