Megge Miller, Paul Roche, Keflemariam Yohannes, Jenean Spencer, Mark Bartlett, Julia Brotherton, Jenny Hutchinson, Martyn Kirk, Ann McDonald, Claire Vadjic

Results - Vectorborne diseases

A total of 6,780 notifications of mosquito-borne disease and malaria were reported to NNDSS during 2003 (6.5% of all notifications to NNDSS). The viral diseases notified include those caused by alphaviruses (Barmah Forest and Ross River virus) and flaviviruses (the viruses causing dengue, Murray Valley encephalitis, Kunjin and Japanese encephalitis). Aspects of the ecology of these viruses and the clinical features of the disease they cause have previously been described.⁹ This section also reports on malaria notifications.

Alphaviruses

Alphaviruses are RNA viruses which cause disease epidemics characterised by fever, rash and arthropathy. In Australia, Barmah Forest virus and Ross River virus are the alphaviruses of major public health significance.

Barmah Forest virus infection

There were 1,370 cases of Barmah Forest virus (BFV) infection notified to NNDSS in 2003. Ninety-seven per cent of these were reported from Queensland (872 cases) and New South Wales (451 cases). The highest rate of notification occurred in Queensland (23 cases per 100,000 population). The national notification rate was 6.9 cases per 100,000 population, which is the highest since reporting began in 1995. There was a peak in notifications of BFV infection in Queensland in April 2003 (78.7 cases per 100,000 population) and in New South Wales (24.9 cases per 100,000 population, Figure 48 ) wh ich were the highest recorded in these jurisdiction in the last five years.

Figure 48. Notification rates of Barmah Forest virus infections, Queensland, the Northern Territory and Australia, January 1998 to December 2003

The highest rate of BFV infection in 2003, was in the Richmond Tweed area of New South Wales (106.1 cases per 100,000 population, Map 6). Rates of BFV infection were also high (>15.9 cases per 100,000 population) in most of Queensland.

Map 6. Notification rates of Barmah Forest virus infection, Australia, 2003, by Statistical Division of residence

The age and sex distribution of BFV notifications are shown in Figure 49. The notification rate was highest in the 45–49 year age group (14.1 cases per 100,000 population) and the male to female ratio was 1:1.

Figure 49. Notification rates of Barmah Forest virus infections, Australia, 2003, by age group and sex

Ross River virus infection

A total of 3,841 cases of Ross River virus (RRV) infection were notified to the NNDSS in 2003. There were 2,517 cases reported in Queensland and 661 cases reported in Western Australia. The highest rates were reported in Queensland (66.3 cases per 100,000 population) and the Northern Territory (60.5 cases per 100,000 population). The national rate for RRV notifications was higher than in 2002 but within the range of rates in previous years.

RRV infection notifications in Queensland peaked in April 2003 at 30 cases per 100,000 population (Figure 50). This was the highest rate since 1999. During the last quarter of 2003, the number of notifications of RRV infection increased largely due to an outbreak in the south-west of Western Australia (Map 7). During the summer of 1988–89 and again in 1995–96, the same region in Western Australia experienced a large outbreak of RRV infections. 19,20 The latest outbreak commenced in September 2003 and tapered off in April 2004, with a total of some 1,570 cases from throughout Western Australia (805 from the south-west and great southern, 485 from Perth, the remainder from elsewhere in the State). This was the largest ever outbreak of RRV in Western Australia.

Map 7. Notification rates of Ross River virus infection, Australia, 2003, by Statistical Division of residence

Figure 50. Notification rates of Ross River virus infection, Queensland, Western Australia and Australia, January 1998 to July 2004

The age and sex distribution of RRV notifications are shown in Figure 51. The notification rates were highest in the 45–49 year age group (37.1 cases per 100,000 population) and the female to male ratio was 1.1:1.

Figure 51. Notification rates of Ross River virus infection, Australia, 2003, by age group and sex

Flaviviruses

Flaviviruses are single-stranded RNA viruses, some of which are associated with epidemic encephalitis in various regions of the world. In Australia, flaviviruses of public health importance are Murray Valley encephalitis (MVEV), Kunjin (KUNV), Japanese encephalitis and dengue viruses.

Early warning of increased MVEV and KUNV activity in Australia is provided by the Sentinel Chicken Surveillance Program. Antibodies to MVEV and KUNV are detected in flocks located in four Australian States. Reports of the 2002/3 and 2003/4 seasons have been published, 21 with the most recent report by Broom and Whelan, 2004, included in this issue.

Murray Valley encephalitis virus and Kunjin virus

MVEV and KUNV activity is normally restricted to northern Australia. Incursions of MVEV into south-eastern Australia, under appropriate weather conditions, are rare but have in the past resulted in epidemics of Murray Valley encephalitis virus.

During 2003 no cases of MVEV infection were notified.

There were 19 cases of Kunjin virus infection in 2003, all of which were reported from Queensland. The cases of KUNV infection were symptomatic cases with mild febrile disease and without encephalitis, detected in the enhanced surveillance in Queensland during the outbreak of dengue in 2003. There are no sentinel chicken sites located in Queensland, thus making it difficult to determine if there was elevated circulation of KUNV at the same time as the dengue outbreak.

Dengue

Dengue viruses (DENV) were transmitted within Australia only in northern Queensland, where the vector mosquito Aedes aegypti is endemic. Cases notified in other parts of Australia and not acquiring their infection in Queensland were therefore all acquired overseas.

There were 868 cases of DENV infection notified to NNDSS during 2003. Most cases were reported from Queensland (727 cases, 19.1 cases per 100,000 population) where there was an outbreak of DENV serotype 2.

The age and sex distribution of DENV notifications is shown in Figure 52. The female to male ratio was 0.9:1. Most cases in males occurred in the 25–29 year age group (8.6 cases per 100,000 population), and in females in the 20–24 and 25–29 year age groups (8.1 cases per 100,000 population).

Figure 52. Notification rates of dengue, Australia, 2003, by age group and sex

An outbreak of DENV serotype 2 began in Cairns in January 2003. The index case was a woman who had been in Papua New Guinea and became ill with symptoms in Cairns on 22 January. Two smaller outbreaks of DENV serotype 2 were also identified in Townsville and Mareeba. In October, an outbreak of DENV type 2 that affected 98 persons, occurred on Yam Island. By November the outbreak had spread to Thursday Island where 71 cases were reported in the latter months of 2003 (Table 16, Figure 53). Mosquito control and community education were conducted continuously until the outbreak ended in 2004.

Table 16. Outbreaks and locally acquired cases of dengue, Queensland, 2003 to 2004

Period	Location	Total number of cases	Dengue serotype	Comments
Jan–July 2003	Cairns area	451	Type 2	First case was imported from Papua New Guinea
	Cairns area	3	Type 1	Unknown source
March 2003	Mareeba	1	Type 1	No link to Cairns
March–July 2003	Townsville area	20	Type 2	Source likely to be Cairns
Sept–Nov 2003	Yam Island	98	Type 2	Unknown source
Nov 2003–April 2004	Thursday Island	71 in 2003 (171 cases total outbreak)	Type 2	Imported from Yam Island
Nov 2003–May 2004	Townsville area	14 in 2003 (55 cases total outbreak)	Type 2	Carry-over from previous outbreak

Source: Queensland Health

Figure 53. Notification rate of dengue by month, January 1991 to December 2003, the Northern Territory, Queensland and Australia

Japanese encephalitis virus

Incursions of Japanese encephalitis virus (JEV) into the Torres Strait Islands in 1995 and mainland Australia in 1998 have earlier been described.²² Since 1998 no further infections in mainland Australia have been identified, and there were no cases reported in 2003. A number of sentinel pig herds in northern Queensland and the Northern Territory are serologically tested at regular intervals to identify any new incursion of JEV into mainland Australia.

Seroconversions in the sentinel pig herds on the Torres Strait islands have detected the presence of JEV each year from 1995 to 2003, with the exception of 1999. Evidence for the presence of the virus from sentinel pigs on the mainland has only occurred in 1998, the same year in which the human infections occurred.

Outside Australia, there is a strong likelihood that JEV is now endemic on the island of Papua New Guinea. Ritchie and Rochester²³ have found that the incursions of JEV into Australia in 1995 and 1998 were associated with low pressure systems that led to strong northerly winds blowing from New Guinea to Cape York Peninsula. A review of the emergence of JEV in the Australasian region describes the potential for JEV to be introduced to Australia and how any incursion should be controlled.²⁴

Flavivirus (NEC)

There were 81 notifications of 'Flavivirus – not elsewhere classified' in 2003. These include flavivirus infections (e.g. MVEV and KUNV), where serology was unable to differentiate the different viruses.

Sixty-eight of these notifications were from Queensland, where serological evidence of previous infection with flaviviruses was detected in cases under investigation in the dengue outbreaks in Queensland during 2003.

Malaria

In 2003 there were 601 notifications of malaria, which is comparable with 699 cases in 2001, but higher than the number of malaria cases notified in 2002 (increase of 32 per cent). All notified cases were acquired overseas. The majority of notifications were from Queensland (n=253) and New South Wales (n=120).

Tasmania reported 27 cases of malaria, 19 of which were found during screening at a refugee clinic. Of the 19 cases, 18 had Plasmodium falciparum and one had P. malariae. None of the cases had symptomatic disease but met the national case definition. Most of the cases (17/19) were paediatric with ages ranging from 3 to 13 years (David Coleman, Tasmanian Department of Health and Human Services, personal communication).

The largest number of notifications of malaria was amongst males in the 20–24 year age group, and in females in the 25–29 year age group (Figure 54). The male to female ratio was 2:1.

Figure 54. Notifications of malaria, Australia, 2003, by age group and sex

The infecting Plasmodium species were reported in 567 of 601 (94%) notifications (Table 17).

Table 17. Infecting Plasmodium species reported in notified cases of malaria, Australia, 2003, by state or territory

Malaria species	State or territory								Australia
	ACT	NSW	NT	Qld	SA	Tas	Vic	WA
P. falciparum	7	41	19	60	15	19	20	38	219
P. vivax	11	71	19	136	11	7	36	7	298
P. ovale	0	2	0	12	0	0	1	2	17
P. malariae	0	3	0	3	1	1	0	1	9
P. falciparum and P. vivax	0	0	1	20	0	0	2	0	23
P. malariae and P. vivax	0	1	0	0	0	0	0	0	1
Unknown	0	2	1	22	1	0	0	8	34
Total number of cases	18	120	40	253	28	27	59	56	601

 

This article {extract} was published in Communicable Diseases Intelligence Vol 29 No 1 March 2005 and may be downloaded as a full version PDF from the Table of contents page.