Mosquito-borne diseases are a growing concern in Australia, and an understanding of the spatial distribution of infection is required to refine surveillance strategies and public health interventions. Barmah Forest virus (BFV) disease is an arboviral disease endemic to Australia.1
The virus was isolated from mosquitoes in 19742
and the first human cases were reported in 1986.3
The incubation period of BFV is probably seven to 10 days and symptoms of disease include rash, fever, arthralgia, myalgia and lethargy.4,5
BFV is a common cause of epidemic polyarthritis in Australia and carries important morbidity and economic impacts. It affects both genders and people of all ages. There is no specific treatment or vaccine available.6
New South Wales (NSW) uses a notifiable diseases register to record data on 57 communicable diseases and medical conditions. Case demographics are entered into an electronic database at sites across the state when standardized clinical and laboratory case definitions are met. For the majority of notifiable conditions, including BFV disease,7
the case definition is based on a suggestive clinical picture and confirmatory laboratory findings. It is mandatory for laboratories to report notifiable disease detections and convey available patient information to public health authorities. Routinely collected data include the patient’s name, date of birth, sex, residential address and suspected date of disease onset. Owing to the large number of disease notifications and the nature of collection, data quality and completeness is adequate for monitoring disease trends but may be insufficient to allow detailed analysis of risk and exposure. To obtain this information for BFV disease it was necessary to conduct case follow-up interviews.
Seasonal BFV notifications in the Hunter New England Local Health District (HNELHD) of NSW
() ranged from 60 to 173 between 2001 and 2011, with an average of 112 reports each season (incidence rate: 13.2 per 100 000).8,9
Location of the Hunter New England local health district within Australia
There is some uncertainty regarding the natural reservoirs of BFV in Australia. Although low levels of neutralizing antibodies have been detected in kangaroos, wallabies, possums, horses, cats and dogs after experimental infection, the detected viraemia is considered too low for an insect vector to acquire the virus.10,11
The genetic similarity of BFV strains across Australia, as well as the pace in which they spread, suggests an avian or bat host.12
There is a diverse range of mosquito species confirmed as vectors of BFV. The majority of important vector species, such as Aedes vigilax, Aedes procax, Aedes camptorhynchus
and Verrallina funerea,
are associated with either coastal estuarine wetlands (i.e. saltmarsh and mangrove habitats) or brackish water environments (i.e. tea-tree and paperbark swamps).13–15
However, some species associated with freshwater habitats and urban environments, such as Aedes notoscriptus
, Culex annulirostris
and Coquillettidia linealis,
may also be involved.13,16,17
Given the diversity of potential vector species and differences in the environmental drivers of mosquito population abundance within the different habitats, it can be difficult to assess the regional health risks posed by BFV.
Although BFV disease has been documented in every state and territory of Australia, notifications occur predominantly on the Australian east coast, with sporadic inland cases.18,19
However, due to the limited knowledge of BFV’s natural reservoir and a geographic distribution determined solely by the residential addresses of confirmed cases, the true distribution of the virus and thus the areas of risk are not definitively understood.
In this paper we aim to determine the likely risk exposure areas for BFV infection in the HNELHD of NSW, Australia by conducting structured interviews with all notified cases from June 2010 to May 2011 and to compare these identified risk exposure areas with those identified as having elevated standardized incidence rates when using routinely collected notification data from 2001 to 2010.