Arthropod-borne viruses (arboviruses) are responsible for many important vector-borne
diseases of man and animals including dengue, yellow fever, Japanese encephalitis,
tick-borne encephalitis, Rift Valley fever, West Nile fever, chikungunya, Ross River
disease, and bluetongue. Detection or accurate prediction of virus activity in vector
populations and specific diagnosis of infection in the human or animal host are crucial
components of effective control and treatment strategies and facilitate early warning
of potential or existing outbreaks and initiation of vector management and/or vaccination
programs.
One of the key elements of the control of arbovirus transmission is early detection
of virus activity or increased virus activity in vector populations. Surveillance
programs designed to monitor these parameters provide an early warning system of increased
risk of transmission and disease outbreak. One approach routinely used by groups to
monitor virus activity is sentinel animal surveillance. This is particularly useful
for veterinary or zoonotic arboviruses where domestic animals (e.g., cattle, pigs
or chickens) can be effectively employed. While serological monitoring of strategically
positioned flocks or herds can provide early warning of virus transmission in a specific
region, there are many difficulties with this system: the costs of maintaining the
animals (particularly in remote areas), lack of specificity of serological assays,
and the ability to only target viruses that infect the selected sentinel animal or
those that are transmitted by vectors that feed on the sentinel host. To address these
problems, a new approach (reviewed by A. F. van den Hurk and colleagues in this issue)
has recently been applied to the specific detection of virus activity in mosquito
populations, which is particularly useful for remote locations. The specific detection
of viral RNA expectorated by infected mosquitoes feeding on a sugar-coated matrix
has enabled these investigators to collect samples over prolonged periods in remote
locations and specifically identify viruses carried by any mosquito feeding on the
sugar bait. The potential application of this system to a wide variety of arbovirus
surveillance scenarios is very promising.
Another important component of arbovirus surveillance is the detection of virus in
arthropod vector populations. Traditional methods include trapping vectors such as
mosquitoes, ticks, and midges, identifying them to species level and analysing vector
pools for known viruses of interest. Virus isolation by inoculating mice, embryonated
eggs, or colonized vector species has largely been replaced by in vitro methods (e.g.,
inoculation of arthropod and vertebrate cell lines), but is still the most effective
means for monitoring virus activity for some viruses. However, the enhanced technologies
for the specific detection of viral nucleic acid, such as multiplexed real-time PCR
protocols, provide more rapid, sensitive, and specific approaches for detecting virus
activity in vector populations. Furthermore, the recent application of next-generation
sequencing technologies to rapidly analyze nucleic acid of unidentified viral isolates
provides a revolutionary approach for the discovery and genetic characterization of
new vector-borne viruses.
Monitoring arthropod vector populations is also an important component of arbovirus
surveillance, particularly for detecting an increase in known vectors or the introduction
of a new species into an area (e.g., recent incursions of Aedes albopictus into Europe
and Australia). In addition to the labor-intensive methods of trapping and identifying
vectors by morphology or genetic analysis, new strategies such as satellite-based
remote sensing of vector breeding sites and assessment of the risk of virus transmission
based on proximity to human or animal habitation (see the paper by Susan N. Rossmann
and colleagues in this issue) can rapidly provide highly useful data on a very large
scale.
Accurate and timely diagnosis of arbovirus infections is also crucial to ensure appropriate
patient management, for the reporting of virus activity in a region and to allow instigation
of control strategies such as vector management, vaccination, and public awareness
campaigns. Serological assays are predominantly used for this purpose; however, many
problems exist with traditional methods including the use of live virus for antigen
production and plaque reduction neutralization tests, lack of specificity due to cross-reactivity
between related viruses, and the costs of running individual assays for each viral
antigen used. However the use of recombinant viral antigens (whole proteins, domains
or peptides) in multiplexed formats such as microsphere immunoassays provides rapid,
sensitive, and specific analyses that can be coupled to large-scale antigen production
methods (see the paper by J. He and colleagues in this issue) and high-throughput
robotic systems in the diagnostic laboratory. For some viral infections, enhanced
real-time PCR protocols provide a more specific alternative with the detection of
viral nucleic acid in human or animal samples (see the review by N. Johnson and colleagues
in this issue). The availability of several commercial point-of-care assays, particularly
for dengue and West Nile viruses, also provides a useful tool for the clinician or
veterinarian treating the patient on presentation at the clinic (see the reviews by
S. D. Blacksell and J. M. Hobson-Peters in this issue).
The new technologies and novel approaches referred to above and elaborated on in the
papers in this special issue provide an excellent platform for the advancement of
arbovirus surveillance and diagnosis. Thorough evaluation of their effectiveness against
traditional methods in the field and clinic and their application to different arboviral
diseases will allow their routine implementation and unleash the potential to vastly
improve our ability to manage these diseases in the future.
Roy A. Hall
Bradley J. Blitvich
Cheryl A. Johansen
Stuart D. Blacksell