This article {extract} was published in Communicable Diseases Intelligence Volume 23 Number 11 - 28 October 1999 and may be downloaded as a full version PDF from the Table of contents page.

Results continued

Vectorborne diseases

Arthropod borne viruses which are able to replicate in arthropod vectors and in vertebrate hosts are collectively referred to as arboviruses. The nationally notifiable vectorborne diseases include several arboviruses and malaria. Although there are over 70 types of arboviruses in Australia only a small proportion of these are capable of causing disease in humans.³⁷

The NNDSS collects information on:

• Alphaviruses: Barmah Forest virus and Ross River virus.
• Flaviviruses: Dengue virus, and Arboviruses (not elsewhere classified) which include Murray Valley Encephalitis (MVE), Kunjin virus, Japanese Encephalitis (JE), Kokobera, and Stratford virus.
• Malaria: Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale.

Barmah Forest virus infection

In 1998, 558 notifications of Barmah Forest virus were reported. Lower rates were reported in 1998 compared to previous years. In keeping with past years, no cases were reported from Tasmania and low rates of disease were reported in Victoria, the Australian Capital Territory and South Australia (Map 9). The Northern Territory had the highest rate of 11.1 per 100,000, followed by Queensland with a rate of 10.2 per 100,000.

Males accounted for 56% of notifications. The male to female ratio was 1.3:1. The highest age-specific rate was in the 55-59 year age group. Peak activity followed previous seasonal trends and was reported from January through to April (Figure 30).

Figure 30. Notifications of Barmah Forest virus infection, 1995-1998, by month of onset



Map 9. Notification rate of Barmah Forest virus infection, 1998, by Statistical Division of residence


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Dengue

There were 557 notifications of dengue in 1998, more than double the number notified in 1997 (210). Over 80% of these were reported from Queensland with a rate of 3.0 per 100,000. Notifications for the year peaked in February and then again in September and December (Figure 31). More dengue cases were notified in 1998 than in any year since 1993. The male to female ratio was 1.5:1. Peak age specific rates were in the 25-29 year age group. Eighteen cases of dengue were reported in children less than 10 years of age.

Figure 31. Notifications of dengue, 1991-1998, by month of onset

Ross River virus infection

A total of 3,094 Ross River virus infection notifications were received in 1998. The Australian rate (16.5 per 100,000) was the lowest recorded since 1995. One-thousand nine-hundred and fifty notifications (63%) were reported in the State of Queensland. The areas of Australia with the highest rates of disease were the Northern and Fitzroy regions in Queensland (Map 10). The male to female ratio was 1.0:1. The highest age-specific rates were reported in the 35-39 years age group. The lowest rates of disease were reported in children less than 10 years of age (Figure 32).

The most frequent months for disease onset were February, March and April. This is in keeping with seasonal trends observed in previous years (Figure 33).

Figure 32. Notification rate of Ross River virus infection, 1998, by age group and sex



Figure 33. Notifications of Ross River virus infection, 1992-1998, by month of onset


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Map 10. Notification rate of Ross River virus infection, 1998, by Statistical Division of residence

Arboviruses (not elsewhere classified)

Altogether there were 81 notifications of arboviruses that were not elsewhere classified. These were predominantly reported from Queensland and Victoria.

A case of Japanese encephalitis (JE) was reported in Queensland in March 1998. This was the first case of JE to be diagnosed on the Australian mainland. It was believed that the infection was acquired while the man was working on a boat on the west coast of the Cape York Peninsula. A further case was reported in the Torres Strait in March 1998.³⁸

Malaria

There were 705 notifications of malaria in Australia in 1998. The highest rates per 100,000 population were notified from Northern Territory (14.2), Queensland (9.7) and the Australian Capital Territory (7.8). Notification onset dates peaked in February which is characteristic of other years. Overall rates have remained fairly constant in Australia since 1993. Species notified were Plasmodium vivax (P. vivax) 326 (46%), P. falciparum 175 (25%), P. malariae 7 (1.0%), P. ovale 11 (1.6%) and P. falciparum/P. vivax co-infection 14 (2.0%) and other 1 (0.1%). Speciation was unknown for 171 notifications (24%). More notifications were from males, the male to female ratio being 2.3:1. Forty-seven per cent of all notifications were in people aged between 20 and 40 years. Twenty-eight cases were reported in children aged between 0 and 4 years.

Discussion

1998 was a quiet year for notifications of Ross River and Barmah Forest viruses. However, dengue and Japanese Encephalitis (JE) were of concern in 1998.

The rate of notifications of dengue (3.0 per 10,000) was three times higher than the previous year, although still lower than the peak of 4.5 per 100,000 in 1993. Epidemic activity of dengue was restricted to far north Queensland. An outbreak of dengue type 3 occurred in December 1997 in Cairns and spread to the Port Douglas and Mosman areas, and continued throughout 1998. The dengue strain identified in this outbreak was thought to have originated from South-East Asia.³⁹ To date dengue has not caused outbreaks in southern areas of Australia because of the absence of a suitable vector. The importance of ensuring that vectors that are tolerant to cold climates do not become established in Australia is highlighted by the dengue outbreak in 1998.⁴⁰

The first case of JE in the Australian mainland was identified in March 1998 in a fisherman from Mitchell River, Far North Queensland. Testing of sentinel pigs on the mainland showed seroconversion to the disease over the same time period.³⁷ Although JE infections have occurred in recent years in the Torres Strait islands, until 1998 none had been identified on mainland Australia, raising the possibility that JE virus may have become epizootic, or even enzootic in northern Australia.³⁷ Monitoring of human and animal populations to determine the extent of JE virus activity in Queensland is continuing.

Australia remains free from endemic malaria but hundreds of imported cases occur each year. Prompt public health action is important to prevent local transmission of the disease. Malarial surveillance is particularly important in northern Australia where environmental conditions favour transmission of malaria, were it to be reintroduced.