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.³⁹

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), Kokabera, and Stratford virus.
• Malaria: Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale.

Ross River virus (RRV) and Barmah Forest virus (BFV) are endemic in Australia. Although RRV has been reported in all states of Australia, BFV has not been reported in Tasmania to date. Since the inception of the NNDSS in 1991, RRV has accounted for the largest number of notifications of arboviral disease. Dengue virus, though not endemic in Australia, has been associated with several outbreaks in far North Queensland since 1990. These outbreaks have occurred when travellers have imported dengue infection from abroad. The disease has then propagated via the mosquito vector Aedes aegypti, which is currently localised to North Queensland.

Murray Valley encephalitis is enzootic in parts of Northern Australia and activity of the virus is monitored throughout Australia with sentinel chicken flocks. Whereas significant activity has been detected in chicken flocks in Northern Western Australia and the Northern Territory since 1990, no activity has been evident in chicken flocks in New South Wales and Victoria over the same time period.³⁹ Kunjin virus disease, a milder form of encephalitis, is confined mainly to Queensland with occasional spread into areas of south-east Australia.

Barmah Forest virus infection

In 1997, 704 notifications of Barmah Forest virus were reported. In keeping with past years, no cases were reported from Tasmania and the lowest rates of disease were reported in Victoria, the Australian Capital Territory and South Australia (Map 10). The Northern Territory had the highest rate of 22.4 per 100,000, followed by Queensland with a rate of 10.6 per 100,000.

Males accounted for 402 (57%) of notifications and females for 300 (43%). Highest age-specific rates were in the 45 to 49 year age group at 7.7 per 100,000.

Peak activity was reported from February through to April (Figure 23).

Map 10. Notification rate of Barmah Forest virus infection, 1997, by Statistical Division of residence

Figure 23. Notifications of Barmah Forest virus infection, 1995-1997, by month of onset

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Dengue

There were 210 notifications of dengue virus for 1997, and 80 per cent of these were reported from Queensland. Notifications for the year peaked in January and then again in December (Figure 24).

More dengue cases were notified in 1997 than in all years since 1993. Sex was reported for all but one case, and the male to female ratio was 1.04:1. Peak age specific rates were in the 30 to 34 year age group at 1.75 per 100,000. Eight cases of dengue were reported in children less than 10 years of age.

Figure 24. Notifications of dengue, 1993-1997, by month of onset

Ross River virus infection

Of the 6,683 notifications in 1997, 2,382 (35%) were reported in the state of Queensland. The areas of Australia with the highest rates of disease included the Mallee region of New South Wales, the Northern region of South Australia, the Pilbara and Kimberley regions of Western Australia, and the Central West and South West regions of Queensland (Map 11). Of the 6,663 notifications for which sex was reported, 3,266 (49%) were female and 3,397 (51%) male. The highest age-specific rates were reported in the 35 to 39 year age group at 63.7 per 100,000. The lowest rates of disease were reported in children less than 10 years of age at 3.3 per 100,000 (Figure 25).

The most frequent months for disease onset were February, March and April. This is in keeping with seasonal trends observed in previous years (Figure 26).

Map 11. Notification rate of Ross River virus infection, 1997, by Statistical Division of residence

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Figure 25. Notification rate of Ross River virus infection, 1997, by age group and sex

Figure 26. Notifications of Ross River virus infection, 1993-1997, by month of onset

Arboviruses (Not elsewhere classified)

Altogether there were 18 notifications of arboviruses that were not elsewhere classified. No cases were reported from New South Wales. In the Northern Territory there were 2 notifications of Kunjin virus, and Western Australia reported 2 Kunjin and 2 cases of Murray Valley Encephalitis. The identification of the remaining 12 arboviruses was not reported.

Malaria

There were 746 notifications of malaria in Australia in 1997. In 571 (76%) the species was reported. P. vivax accounted for 344 (60.3%), P. falciparum 197 (34.5%), P. malariae 7 (1.2%) P. ovale 4 (0.7%) and P. falciparum/ P. vivax co-infection 19 (3.3%). Half of all notifications were in people aged between 20 and 40 years. Twenty-nine cases were reported in children aged between 0 and 4 years. Of these, 16 cases were P. vivax, 7 P. falciparum, 3 P. falciparum/vivax coinfections and 4 unknown.

Male to female notifications occurred at 2.3:1.

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Discussion

Ross River virus is the most common arboviral infection in Australia and has a wide geographic distribution. Barmah Forest virus, the other cause of epidemic polyarthritis, is less widely distributed and less frequent in occurrence. Both alphaviruses show the same seasonal pattern of occurrence. In 1997, BFV activity was down on what it had been in the previous two years. Ross River virus notification rates were lower than those reported in 1996, but still higher than all years dating back to 1993. Heightened awareness of epidemic polyarthritis has lead to increased testing and diagnosis over time.

In 1996-97 and in 1997-98 there were two outbreaks of dengue fever in Australia. Most laboratory confirmed dengue type 2 cases notified in early 1997 were from the Torres Strait.³⁹ In December of 1997 an outbreak of dengue type 3 occurred in Cairns.⁴⁰ The dengue strain in this latter outbreak was thought to have originated from South-East Asia. The outbreak continued into 1998.

Only 4 arboviruses not elsewhere classified were identified in the NNDSS. The 2 MVE and 2 Kunji were sporadic cases, and the former cases were notified from Western Australia.

The observation of increased malaria rates in young adults, who are predominantly male, is in keeping with observations over previous years. The occurrence of malaria in 29 children less than 4 years of age, including 3 mixed P. vivax/falciparum and 7 P. falciparum infections, highlights the vulnerability of younger age groups to this disease.

No notifications of Japanese encephalitis were reported in 1997. The first outbreak to have occurred was on the island of Badu in the Torres Strait in 1995. Seroconversion in sentinel pigs on the island of Saibai occurred in 1996 and 1997,³⁹ indicating that Japanese Encephalitis virus activity remains enzootic and the risk to human health therefore persists.

This article {extract} was published in Communicable Diseases Intelligence Vol 23 Number , 21 January 1999 and may be downloaded as a full version PDF from the Table of contents page. Volume 23 1999.