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Results, cont'd

Vectorborne diseases

A disease that is transmitted to humans or other animals by an insect or other arthropod is known as a vectorborne disease. Vectors of most concern in Australia are typically mosquitoes that are able to transmit viruses or parasites to humans.

There were 8,244 notified cases of mosquito-borne diseases (4% of total notifications), which was similar to the number of cases in 2009 (n = 8,232). Notifiable mosquito-borne diseases include those caused by the alphaviruses (Barmah Forest virus and Ross River virus), flaviviruses (dengue, Murray Valley encephalitis, Kunjin, Japanese encephalitis and yellow fever) and malaria (a parasitic disease caused by Plasmodium spp). Yellow fever is reported under quarantinable diseases.

Rates of infection for a geographical location for vectorborne disease notifications represent the place of residence rather than the place of acquisition of infection, although in many instances this may be the same. Further information about these vectorborne diseases can be found in the National Arbovirus and Malaria Advisory Committee (NAMAC) annual report 2009–10.⁶⁰

Alphaviruses

Alphaviruses are single-stranded RNA viruses that cause disease epidemics characterised by fever, rash and polyarthritis. There are a variety of mosquito vectors for Barmah Forest virus (BFV) and Ross River virus (RRV), which facilitate the transmission of these viruses in diverse environments (freshwater habitats, coastal regions, salt marshes, floodwaters, established wetlands and urban areas).⁶¹ The reservoirs of these viruses are mammals, particularly macropod marsupials. In Australia, BFV and RRV are the alphaviruses of major public health significance, accounting for 80% (n = 6,618) of vectorborne disease notifications.

Figure 57: Notified cases of Barmah Forest and Ross River virus infections, Australia, 2005 to 2010, by month and year

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Barmah Forest virus infection

There were 1,471 notified cases of BFV in Australia in 2010; a rate of 6.6 per 100,000 population compared with the 5-year mean of 8.3 per 100,000 population (Figure 57). The highest rates of BFV notifications were reported by the Northern Territory (35.7 per 100,000), and Queensland (20.1 per 100,000). Cases were reported in all jurisdictions. The median age of cases was 47 and 51% of cases were in males.

Ross River virus infection

There were 5,147 notifications of RRV infection in Australia in 2010, a rate of 23 notifications per 100,000 population compared with a 5-year mean of 21.4 per 100,000 population (Figure 57). Nearly half of all cases were from Queensland (46%, n = 2,383), but the highest rate was in the Northern Territory (146.3). The median age of cases was 43 and 45% of notifications were in males.

Flaviviruses

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

The Sentinel Chicken Program is a surveillance scheme involving New South Wales, the Northern Territory, Victoria and Western Australia. Chicken flocks are located in strategic locations and are regularly tested for antibodies to MVEV and KUNV. This program is designed to provide early warning of flavivirus activity (excluding DENV and JEV).⁶² A sentinel chicken surveillance report was published as part of the NAMAC annual report 2009–10.⁶⁰

Dengue virus infection

There were 1,201 notified cases of DENV infection reported in 2010, which was a 14% decrease from the 1,402 cases reported in 2009 (Figure 58). In 2010, the median age of dengue cases was 39 years and 52% of cases were in males.

Figure 58: Notified cases of dengue virus infection, Australia, 2005 to 2010, by month and year and place of acquisition

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Whilst the number of dengue cases overall decreased compared with previous years, the number of overseas-acquired cases was the highest on record (Figure 58) and most cases of dengue in 2010 were acquired overseas (93%, n = 1,119). Where the country of acquisition of overseas cases was provided (85%, n = 1,019), the most frequently reported country was Indonesia (66%, n = 673), followed by Thailand (11%; n = 109).

The increasing number of overseas-acquired dengue cases in Australia is likely to be due in part to the increasing frequency of travel to countries such as Indonesia, but also to the increasing incidence of dengue in the South East Asian region. In 2010, over 2.2 million cases of dengue were reported to the WHO from the Americas, South East Asia and Western Pacific.⁶³ Dengue is now endemic in more than 100 countries, compared with 40 years earlier, when only 9 countries had experienced severe epidemics.

Overseas acquired cases were most frequently reported amongst younger and middle-aged adults (Figure 59) particularly females in the 20–24 year age group (n = 75), reflecting the frequency of overseas travel for these age groups. Amongst older adults (aged 50+ years), overseas-acquired dengue was more common amongst males (62% of cases were male).

Figure 59: Notified cases of overseas-acquired dengue, Australia, 2010, by age group and sex

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Local transmission of dengue virus in Australia is restricted to areas of northern Queensland where the key mosquito vector, Aedes aegypti, is present. Dengue is not endemic to Queensland, but outbreaks occur when the virus is imported via international travellers or residents returning home from overseas. In 2010, the proportion of cases that were locally acquired was much lower than in 2009 (66%), and there were no significant local outbreaks. The majority of locally-acquired cases were reported from Queensland, with a small number from other states (n = 9), but all were acquired in North Queensland. In 2010, majority of locally-acquired cases that were typed, were due to serotype 2 (91%, 52/57).

No cases in 2010 were reported to have been fatal. Dengue-related deaths have been very rare in Australia. A death reported in March 2009 and 2 deaths in early 2004 were the first deaths attributed to dengue in over 100 years.⁶⁴

Japanese encephalitis virus infections

Japanese encephalitis is the leading cause of childhood encephalitis in Asia.⁶⁵ The usual host of the virus is birds or pigs, and it is transmitted to humans through the bite of infected mosquitoes of the genus Culex. There were no notified cases of JEV infection reported to the NNDSS in 2010. JEV infection is rare in Australia with only 3 cases reported in the past 10 years, all acquired overseas.

JEV emerged in Australia in 1995 with an outbreak in the Torres Strait. In an outbreak in 1998, a further 2 cases were reported, one of them acquired on the mainland (Cape York Peninsula).⁶⁶ Seasonal incursions of JEV have been detected (usually in sentinel pigs) in the Torres Strait every year except 1999.⁶⁵ Targeted vaccination programs of residents of the Torres Strait commenced in 1995.

Kunjin virus infection

There were 2 notified cases of KUNV infection reported in 2010; one from Queensland and one from the Northern Territory. Between 2005 and 2009, there were 8 notifications of KUNV infection.

Murray Valley encephalitis virus infection

During 2010, there were no notifications of MVEV infection reported. There were 9 notifications between 2005 and 2009 (Table 6).

Arbovirus infections (NEC)

In 2010, there were 24 notified cases of arbovirus infection not elsewhere classified (NEC). There was 1 notification each from New South Wales and Queensland, 10 from the Northern Territory and 12 from Victoria. The median age of cases was 36 years and 33% of cases were male.

Ten cases were chikungunya virus infection, reported by the Northern Territory, one was unspecified, and the remainder were flavivirus infections (Kokobera [1], not further specified [13]).

Malaria

Malaria is a serious acute febrile illness which can be transmitted to humans through the bite of an infected mosquito. It is caused by parasites of the genus Plasmodium that includes 5 species that cause disease in humans – vivax, falciparum, malariae,knowlesi and ovale.¹⁶ There were 399 notified cases of malaria in Australia in 2010, which was down from 526 in 2009.⁶⁷

All cases in 2010 were acquired overseas. Australia was declared malaria free in 1981, and since then, there have been 2 reported outbreaks of locally-acquired malaria; in 1986 and 2002 with a total of 15 cases. Where the country of acquisition was available (84%, n = 337 notifications), the most frequently reported country was Papua New Guinea (28%, n = 94, with 53 of these reported from Queensland), followed by India (16%, n = 53).

Malaria was most frequently reported amongst males aged 20–29 years with 69% of all malaria cases being males (Figure 60).

Figure 60: Notified cases of malaria, Australia, 2010, by age group and sex

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The infecting Plasmodium species was reported for 97% of malaria notifications in 2010 (Table 16). The predominant infecting species were P. falciparum (45%) and P. vivax (43%).

Table 16: Notified cases of malaria, Australia, 2009, by parasite type and state or territory

Plasmodium species	State or territory								Aust
	ACT	NSW	NT	Qld	SA	Tas	Vic	WA
									n	%
P. falciparum	1	58	3	53	3	2	26	35	181	45
P. vivax	0	44	8	64	4	1	37	15	173	43
P. ovale	0	12	0	3	0	1	2	1	19	5
P. malariae	0	5	0	1	0	0	1	3	10	3
P. falciparum and P. vivax*	0	0	0	0	0	0	1	1	2	1
P. falciparum and P. malariae*	0	0	0	0	0	0	0	1	1	0
Plasmodium species	1	5	0	5	1	1	0	0	13	3
Total	2	124	11	126	8	5	67	56	399	100

* New South Wales, South Australia, Tasmania, Victoria, Western Australia and the Northern Territory report mixed species infections per notified case. Queensland and the Australian Capital Territory report 1 notification for each species in a mixed infection.