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Vectorborne diseases

Vectorborne diseases under surveillance in Australia in 2000 included arboviruses (arthropod borne viruses) and malaria. In this year the NNDSS collected information on 2 alpha viruses (Barmah Forest virus and Ross River virus), and one flavivirus (dengue) as well as malaria. Other arboviruses not including Barmah Forest, Ross River and dengue viruses were designated 'arbovirus not elsewhere classified (NEC)'. This category included 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 2000, there were 6,069 notifications of vectorborne diseases to the NNDSS (6.8% of total notifications).

Surveillance of human infection with MVE and Kunjin viruses is supplemented by sentinel chicken surveillance. Animal surveillance measuring seroconversions to JE in pigs is also used to complement surveillance in humans. Vector data, virus isolations and meteorological data are used to predict conditions suitable for an outbreak of arbovirus disease, and complement animal and human surveillance mechanisms.

Trends in the reporting of arboviruses over the period 1991 to 2000 are shown in Tables 21 and 22. The number of notifications classified as arbovirus (NEC) has decreased since 1995, when Barmah Forest virus became notified separately. Since then, notification rates for Barmah Forrest virus have remained stable. In comparison, dengue and Ross River virus notification rates have showed periods of increased disease activity over this time frame. The number of notifications of malaria have remained consistent over the decade. The notification rate of vectorborne disease depends on annual rainfall patterns, the mosquito populations and the exposure of humans to mosquitoes.

Control of mosquito populations and interception of exotic mosquito species, which may be disease vectors are important control strategies for vectorborne disease. Media warnings to residents during times of increased risk emphasise personal protection and risk reduction by reducing potential mosquito breeding sites.⁵⁹

Table 21. Trends in number of notifications of arboviral infections, Australia, 1991 to 2000*

Disease	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Arbovirus infection NEC	196	64	173	31	43	12	19	88	62	69
Barmah Forest virus infection	-	-	-	-	762	876	691	529	638	634
Dengue	18	373	681	17	39	123	174	579	132	215
Malaria	787	731	669	706	618	853	749	660	732	951
Ross River virus infection	-	5,701	5,254	3,828	2,644	7,783	6,596	3,151	4,416	4,200

* All jurisdictions reported for all years with the following exception
Dengue not reported from Australian Capital Territory (1991 to 1992).

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Table 22. Trends in notification rates of arboviral infections, Australia, 1991 to 2000* (rate per 100,000 population)

Disease	1991	1992	1993	1994	1995	1996	1997	1998	1999	2000
Arbovirus infection NEC	1.1	0.4	1.0	0.2	0.2	0.1	0.1	0.5	0.3	0.4
Barmah Forest virus infection	-	-	-	-	4.2	4.8	3.7	2.8	3.4	3.3
Dengue	0.1	2.2	3.9	0.1	0.2	0.7	0.9	3.1	0.7	1.1
Malaria	4.6	4.2	3.8	4.0	3.4	4.7	4.0	3.5	3.9	5.0
Ross River virus infection	-	32.6	29.7	21.4	14.6	42.5	35.6	16.8	23.3	21.9

* All jurisdictions reported for all years with the following exception
Dengue not reported from Australian Capital Territory (1991 to 1992).

Alphavirus infections

Barmah Forest virus infection

This virus was first isolated from mosquitoes trapped in the Barmah Forest in Victoria in 1974. Outbreaks of Barmah Forest disease have been described since the virus was first shown to cause human disease in 1988. Barmah Forest virus infection is characterised by polyarthritis, myalgia, rash, fever, lethargy and malaise and may cause a chronic disease in some patients.⁶⁰Aedes and Culex mosquitoes are the major mosquito vectors, while marsupials are suspected vertebrate hosts.

In 2000, 634 notifications of Barmah Forest virus infection were reported, similar to the 638 cases reported in 1999. The highest rates were reported in the Northern Territory (4.6 cases per 100,000 population) and Queensland (9.7 cases per 100,000 population). Rates were very low in southern states; no cases were reported from the Australian Capital Territory and Tasmania (Map 7). The male to female ratio was 1.4:1. The highest rate of infection (6.6. cases per 100,000 population) was in those aged 50-54 years, although the notification rates across the 35-69 age range were similar (Figure 40). Peak notifications were in the period January to April and followed previously observed seasonal trends (Figure 41).

Map 7. Notification rates of Barmah Forest virus infection, Australia, 2000, by Statistical Division of residence



Figure 40. Notification rates of Barmah Forest virus infection, Australia, 2000, by age and sex


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Figure 41. Trends in notification rates of Barmah Forest virus infection, Australia, 1995 to 2000, by month of onset

Ross River virus

Ross River virus is the most common cause of arbovirus disease notified in Australia. While sporadic cases occur throughout Australia, epidemics occur in temperate regions and in tropical north-eastern Australia throughout the year. Epidemics in temperate regions are associated with heavy rainfall. 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.⁶¹

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). Queensland has had the largest number of cases of Ross River virus infection for the past 3 years (1998 to 2000).

Clinical Ross River 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.⁶²

There were 4,200 notifications of Ross River virus infections in 2000, giving a rate of 21.9 cases per 100,000 population, a slight decrease from the 23.3 cases per 100,000 population observed in 1999. Rates were highest in the Northern Territory (65.5 cases per 100,000 population), Western Australia (57.5 cases per 100,000 population) and Queensland (41.4 cases per 100,000 population) (Map 8). The male to female ratio was 1:1. The highest notification rate for females (40.6 cases per 100,000 population) was in the 35-39 year age group. The highest rate for men (38.5 cases per 100,000 population) was in the 40-44 year age group (Figure 42). Peak reporting was in the first and second quarters of the year (Figure 43).

Map 8. Notification rates of Ross River virus infection, Australia, 2000, by Statistical Division of residence



Figure 42. Notification rates of Ross River virus infection, Australia, 2000, by age and sex


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Figure 43. Trends in notification rates of Ross River virus infection, Australia, 1991 to 2000 by month of onset

Flavivirus infections

Dengue fever

Historical trends of dengue in Australia

Dengue fever is an acute febrile illness characterised by sudden onset, fever, headache and rash. Dengue haemorrhagic fever⁶³ is a major complication arising from secondary infection with heterologous serotypes of the dengue virus. This complication has a high fatality rate. Two cases of dengue haemorrhagic fever have been reported in Australia, one in 1992 and another in 1997.⁶² There is a concern that introduction of other dengue serotypes into northern Australia could increase the risk of dengue haemorrhagic fever.

Dengue virus is not endemic in Australia and the spread of dengue in Australia is limited to the range of the mosquito vector Aedes aegypti which spans the Torres Strait Islands and north Queensland.⁶² An outbreak caused by dengue type 2 of more than 900 confirmed cases occurred in Townsville and Charters Towers in 1992 to 1993. 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.⁶²

Dengue occurrence in 2000

There were 215 notifications of dengue in 2000, a rate of 1.1 cases per 100,000 population, an increase on the 1999 rate of 0.7 cases per 100,000 population. The highest rates were found in the Northern Territory (47.6 cases per 100,000 population) and Queensland (2.4 cases per 100,000 population). The male to female ratio was 1.6:1. The highest notification rates among men were in the 35-39 and 50-54 year age groups (3.2 cases per 100,000 population). The highest notification rate for women, at 2.4 cases per 100,000 population, was in the 30-34 year age group. Notifications of dengue for 2000 peaked in Summer (first and fourth quarters of the year, Figure 44).

Figure 44. Trends in notification rates of dengue fever, Australia, 1991 to 2000, by month of onset



In all jurisdictions except Queensland, all dengue cases were acquired overseas (n=131). In Queensland, 11 (13%) cases were identified as acquired within Australia, 22 (26%) acquired overseas and the source of infection was unknown in the remaining 51 cases.

The Western Pacific Region, which includes countries in East Asia and the Pacific, reported 45,603 cases of dengue in 2000. The number of cases in the region has decreased since 1998, when there was a pandemic of dengue across the region. In 2000, cases increased on 1999 figures in only 2 countries - Cambodia and Palau - and in both countries increases were seen in the numbers of all serotypes (WPR/WHO. Summary of the dengue situation in the Western Pacific Region - an update. 2001. www.wpro.who.int/document/DENGUE_SITUATION_IN_WPR_Aug01.doc)

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Arbovirus: not elsewhere classified

In 2000, there were 69 notifications of arboviruses 'not elsewhere classified' reported to the NNDSS, giving a rate of 0.4 cases per 100,000 population. This rate was similar to that in 1999 (0.3 cases per 100,000 population). The jurisdiction reporting the largest number of cases of arbovirus infections NEC in 2000 was Victoria, notifying 26 (38%) of the 69 cases in that year. The male to female ratio was 1.4:1. The highest rate for women (0.8 cases per 100,000 population) was in the 50-54 year age group and for men the highest rate (1.2 cases per 100,000 population) was in the 65-69 year age group.

While cases of infection with Murray Valley encephalitis virus were only separately reported by Western Australia in 2000, information provided by the individual jurisdictions indicated there were 16 cases in the year (Table 23). Exceptional weather conditions in 2000 provided ideal conditions for mosquito breeding and MVE virus transmission. The activity in Western Australia was unusual as there was a new southerly extension into the Mid-west region.⁶⁴

Table 23. Confirmed cases of Murray Valley encephalitis virus infection, Australia, 2000

Jurisdiction where diagnosis was made	Likely place of disease acquisition	Cases (Deaths)
Northern Territory	Darwin	1
	Alice Springs	3
	WA*	2(1)
	SA	1
Western Australia	Pilbara	2
	Mid-west	3
	Gascoyne	1
	Murchison	1
Total		16

* includes one case acquired in the Kimberley and a second case acquired in Mid-west/Kimberley region.

Data from sentinel chicken surveillance (Figure 45) provided an early warning of disease activity in 2000. In Western Australia, seroconversions in sentinel chickens preceded likely dates of exposure in human cases by 4-18 weeks in all but one case.⁶⁵

Figure 45. Seroconversions to Murray Valley encephalitis virus in sentinel chickens, Western Australia and Northern Territory, 1999 to 2000



While not specifically identified in the NNDSS, there were 4 cases of Kunjin virus infection identified in 2000. No cases of Japanese encephalitis were reported in 2000. The last case of Japanese encephalitis in Australia was reported in 1998.

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Malaria

While Australia has been free of endemic malaria since 1983, sporadic cases are reported among travellers returning from malaria endemic countries. The three requirements for malaria transmission exist in Australia: infected humans carrying gametocytes in their blood, mosquito vectors and suitable climate. Thus, surveillance of human cases of malaria and the rapid entomological response to prevent infection of local Anopheles mosquitoes are important public health activities in northern Australia.⁶⁶

In 2000, there were 951 cases of malaria reported to the NNDSS, giving a rate of 5.0 cases per 100,000 population. This represented an increase in the notification rate, compared with the 3.9 cases per 100,000 population reported in 1999. Among the jurisdictions, the highest rates were reported from the Northern Territory (38.9 cases per 100,000 population), Queensland (11.5 cases per 100,000 population) and the Australian Capital Territory (5.7 cases per 100,000 population). The male to female ratio was 3.7:1, an increase on the ratio for 1999 (2.6:1). The peak notification rates for men (29.2 cases per 100,000 population) and for women (4.5 cases per 100,000 population) were in the 20-24 year age group.

Malarial parasites were identified and reported in 943 (99%) of the 951 cases reported to the NNDSS. Plasmodium vivax was the most common isolate (717 cases, 75% of the total), followed by P. falciparum (194 cases, 20%).

Travel to malaria endemic countries was documented in all cases in New South Wales, the Northern Territory, South Australia, Tasmania, Victoria and Western Australia. The travel data were also recorded for 96 of 409 notifications in Queensland. The data were not collected in the Australian Capital Territory. Data on the use of anti-malaria prophylaxis were available from Victoria and the Northern Territory. In Victoria 50 per cent of cases had not taken prophylaxis and a majority of these were either newly arrived migrants or Australian residents visiting relatives in their country of birth. In the Northern Territory, 53 (70%) had received prophylaxis, 21 (28%) had not, while the status of the remaining 2 patients was unknown.

Malaria in the Australian Defence Forces returning to Australia from duty in East Timor in 2000 accounted for 267 cases. While all of the 5,500 troops were given prophylaxis with doxycycline, 64 developed symptoms of malaria during their 4-5 months in East Timor and a further 212 soldiers developed symptoms on return to Australia. Of soldiers developing malaria while in East Timor, two-thirds were infected with P. falciparum, all of which were successfully treated with mefloquine and doxycycline. By contrast all but two of the soldiers who developed malaria on return to Australia were infected with P. vivax. When these soldiers were treated with primaquine, 44 soldiers had relapses, which suggested that P. vivax in East Timor was primaquine tolerant.⁶⁷

Other vectorborne disease surveillance

AQIS exotic mosquito interceptions in 2000

In 2000, the Australian Quarantine and Inspection Service reported 41 interceptions of mosquitoes on various imported goods. Of the 41 interceptions, 22 species were considered unknown to Australia, or of limited distribution, including 15 interceptions of Aedes aegypti, six of Aedes albopictus and one Culex spathifurca. Thus, in 2000 there remained a constant threat of importation of exotic mosquito species, some of which may be vectors of human disease.


This article was published in Communicable Diseases Intelligence Volume 26, No 2, June 2002