A print friendly PDF version is available from this Communicable Diseases Intelligence issue's table of contents.

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 number cause disease in humans (Mackenzie, 1998).

The NNDSS collects information on 2 alpha viruses, Barmah Forest (BF) and Ross River (RR) viruses, and one flavivirus, dengue, as well as malaria and arboviruses (not elsewhere classified). This category includes infections with the flaviviruses Murray Valley encephalitis (MVE) virus, Kunjin virus, Japanese encephalitis (JE) virus, Kokobera virus and Stratford virus, as well as the alphavirus Sindbis.

In the States and Territories, data on human cases are supplemented by sentinel chicken surveillance (seroconversions to MVE and Kunjin viruses), animal surveillance (seroconversions to JE in pigs), vector data, virus isolations and meteorological data.

Alphavirus infections

Barmah Forest virus infection

Barmah Forest virus was first isolated from mosquitoes trapped in the Barmah Forest in Victoria in 1974. The first association with disease in humans was described in 1988. Subsequently epidemics of BF virus disease were reported from the Northern Territory (1992), Western Australia (1992-1993) and New South Wales (1995).⁵³ BF virus infection is characterised by polyarthritis, myalgia, rash, fever, lethargy and malaise and may cause a chronic disease in some patients.⁵⁴Aedes and Culex mosquitoes spread the disease and marsupials are a suspected host. The Southern Oscillation Index, which is closely related to temperature and rainfall patterns in eastern Australia also appears to be linked to levels of BF virus disease.⁵⁵

In 1999, 639 notifications of BF virus infection were reported, representing a slight increase above the number of cases reported in 1998. The highest rates were reported in the Northern Territory (9.3/100,000 population) and Queensland (8.8/100,000 population). Rates were very low in southern states; no cases were reported from South Australia (Map 9). The male to female ratio was 1.5:1. The highest rates of infection were in those aged 45-49 years (Figure 34). Peak notifications were in the period January to April and followed previously observed seasonal trends (Figure 35). The first reports of BF virus disease were reported from Tasmania in 1999.

Map 9. Barmah Forest virus infection rate by Statistical Division of residence, 1999



Top of pageFigure 34. Notification rate for Barmah Forest virus infections, Australia, 1999, by age and sex



Figure 35. Notifications of Barmah Forest virus infections, Australia, 1995 to 1999 by month of onset

Ross River virus infection

Ross River virus is the most common arbovirus reported in Australia and the cause of the most arboviral disease. Sporadic cases occur throughout Australia. Epidemics, associated with heavy rainfall occur in temperate regions while transmission in tropical north-eastern Australia occurs throughout the year. Major outbreaks have been recorded in Western Australia (1991-1992 and 1995-1996), Victoria and South Australia (1993 and 1997), New South Wales (1996 and 1997) and Queensland (1996). Since 1991, more than half of all reports of RR virus have originated in Queensland. Recent evidence indicates that the virus may persist in desiccation-resistant eggs of the Aedes spp mosquito, which would explain the rapid onset of cases after heavy rain and flooding. Marsupials and horses have been implicated as hosts for the virus and flying foxes may be responsible for the wide spread dispersal of different genetic types of the virus.⁵³ Clinical RR virus disease occurs most commonly in adults, marked by arthralgia and myalgia (joint and muscle pain). True arthritis occurs in over 40 per cent of patients, while about 50 per cent of patients have a fever or rash.⁵⁷

There were 4,416 notifications of RR virus infections in 1999, an increase from 17.0 to 23.3 cases per 100,000 population since 1998. Rates were highest in the Northern Territory (81.4/100,000 population), Queensland (65.7 per 100,000 population) and Western Australia (33.6/100,000 population) (Map 10). The male to female ratio was 1.1:1. The highest rates were in the 35-39 year age group (Figure 36). Peak reporting was in the first and second quarters of the year (Figure 37).

Map 10. Ross River virus infection notification rate by Statistical Division of residence, 1999



Figure 36. Notification rate for Ross River virus infections, Australia, 1999, by age and sex



Figure 37. Notifications of Ross River virus infections, Australia, 1991 to 1999, by month of onset



Top of page

Flavivirus infections

Dengue fever

Historical trends of dengue in Australia

Despite periodic epidemics of dengue fever since the 1980s, dengue virus is not endemic in Australia. The spread of dengue in Australia is limited to the range of the mosquito vector Aedes aegypti, which is limited to the Torres Strait Islands and north Queensland.⁵³

Outbreaks of dengue in Australia have included a few cases of dengue type 1 in Cairns and the Torres Strait. An outbreak of more than 900 confirmed cases in Townsville and Charters Towers in 1992-1993 was caused by dengue type 2. In 1996-1997 another outbreak of dengue type 2 occurred in the Torres Strait. In 1997-1998 165 cases of dengue type 3 and 12 of dengue type 2 were reported from Cairns.⁵³

Dengue haemorrhagic fever (DHF), first described in Australia in 1897⁵⁶ is a major complication arising from secondary infection with heterologous serotypes of the dengue virus which enhances viral uptake and replication. This complication, which primarily affects children in endemic countries, has a high fatality rate and is common in countries such as Thailand. Two cases of DHF have been reported in Australia, one in 1992 and another in 1997.⁵³ There is a concern that introduction of other dengue serotypes into northern Australia could increase the risk of dengue haemorrhagic fever.

Dengue occurrence in 1999

There were 131 notifications of dengue in 1999, a rate of 0.7 per 100,000 population. This was a significant reduction on the 1998 rate of 3.1 per 100,000 population. The highest rates were found in the Northern Territory (16.1 per 100,000 population) and Queensland (1.8 per 100,000 population). The male to female ratio was 1.2:1. The highest rate among men was in the 45-49 year age group and in the 25-29 year group for women. Notifications for the year peaked in the summer (first and fourth quarters of the year, Figure 38). While all cases of dengue reported from the Northern Territory were infected overseas, an outbreak of dengue in north Queensland in 1999 accounted for 45 cases or 73 per cent of the State's total.

Figure 38. Notifications of dengue fever, Australia, 1991 to 1999, by month of onset

Arbovirus infections not elsewhere classified (NEC)

In 1999 there were 62 cases of infections with arboviruses 'not elsewhere classified' reported (a rate of 0.3 per 100,000 population). This rate was similar to that found in 1998 (0.5 per 100,000 population). The cases reported in 1999 were predominantly from Victoria and the Northern Territory. The male to female ratio was 0.9:1. The highest rate for women was in the 50-54 year age group and for men in the 45-49 year age group. While not specifically identified in NNDSS, reports from individual States and Territories indicate that there were no reports of infection with Murray Valley encephalitis or Japanese encephalitis from any jurisdiction in 1999.

A report of the finding of the mosquito species Culex gelidus from the Northern Territory following the first detection of this species in Brisbane in 1999 has potentially important implications.⁵⁸ The report noted prolific breeding of this species in wastewater around dairies, sewerage treatment facilities, abattoirs and piggeries. The mosquito is an important vector and amplification host for the virus causing Japanese encephalitis. Larval surveys are required to delineate the range of this mosquito before control programs are implemented.

Top of page

Malaria

Australia has been free of endemic malaria since 1983. Sporadic cases are reported primarily among returning travellers in malaria endemic countries such as Indonesia and Papua New Guinea. The three requirements for malaria transmission exist in Australia: infected humans, mosquito vectors and suitable climate. Surveillance of malaria and the rapid entomological response to prevent infection of local Anopheles mosquitoes are important public health activities in northern Australia.⁵⁹

In 1999 there were 724 cases of malaria reported to the NNDSS. Overall, the national rates remained stable compared with those reported in 1998 (3.8 per 100,000 population compared with 3.6 per 100,000 population). Among the jurisdictions, the highest rates were reported from the Northern Territory (34.7 per 100,000 population; an increase from 14.2 per 100,000 population in 1998), Queensland (8.7 per 100,000 population) and the Australian Capital Territory (7.0 per 100,000 population). The male to female ratio was 2.6:1. The peak rates were in the 25-29 year age group for men and in the 15-19 age group for women.

Malarial parasites were identified and reported in 588 (81%) of the cases. Plasmodium vivax was the most common isolate (358 cases, 61% of the total), followed by P. falciparum (193 cases, 26% of cases).

Information on overseas travel in cases of malaria was available from Victoria and the Northern Territory only. In Victoria, 72 per cent of the 81 cases reported had a history of recent travel in Papua New Guinea and/or Indonesia.³⁵ All Northern Territory malaria notifications were in people who had travelled to malaria endemic countries. In the Northern Territory cases the history of anti-malarial prophylaxis was also recorded. Among the 63 cases reported in 1999, 44 (70%) had not taken any prophylaxis, 4 (6%) had taken some and 15 (24%) had taken full courses of anti-malarial medications (data summarised from reports in NT Communicable Diseases Bulletin Vol 6).

Other vectorborne disease surveillance

AQIS exotic mosquito interceptions in 1999

In 1999, the Australian Quarantine Inspection Service (AQIS) reported 30 interceptions of mosquitoes on various imported goods. Seventeen species of mosquitoes were identified, of which 11 species were considered unknown to Australia. These were 8 interceptions of Culex spp, one of Aedes spp, one of Coquillettidia spp and one of Toxorhynchites spp. These figures indicate a constant threat of importation of exotic mosquito species, some of which may be vectors for disease.

Sentinel Chicken Surveillance Programme

Sentinel chicken flocks are used to monitor flavivirus activity in Australia. The main viruses of concern are Murray Valley encephalitis (MVE) and Kunjin. In 1999, 26 flocks were maintained in the north of Western Australia, seven in the Northern Territory, 9 in New South Wales and 10 in Victoria. Flocks in Western Australia and the Northern Territory were tested year round and those in New South Wales and Victoria were tested only from November to March during the main risk season. Maps identifying the location of these flocks were published in Commun Dis Intell 1999;23:55 and bimonthly reports on seroconversions in sentinel chickens were published in CDI throughout 1999.

A summary of seroconversion to MVE virus in Western Australia and the Northern Territory sentinel chicken flocks in 1999 is shown in Figure 39. The peak months of seroconversion are between February and August. There were only small numbers of seroconversions to Kunjin virus in the same flocks. There were no seroconversions to any flavivirus among sentinel chicken flocks in New South Wales or Victoria in 1999.

Figure 39. Seroconversions to Murray Valley encephalitis virus in sentinel chickens, Western Australia, and the Northern Territory, 1999




This article was published in Communicable Diseases Intelligence Volume 25, No 4, November 2001.