Tabanid flies and potential transmission of Trypanosoma evansi in Queensland / by Mutizwa Odwell Muzari Muzari, Mutizwa Odwell

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Muzari, Mutizwa Odwell
Horseflies - Ecology - Queensland, Northern.; Townsville -- Queensland -- Insect vectors -- Animal diseases -- Quarantine -- Biosecurity -- Trypanosoma evansi -- Tabanids -- Surra -- Tabanid flies -- Horseflies -- Disease control -- Disease transmission dynamics; Trypanosoma evansi - Epidemiology.
Australia has quarantine policies and surveillance activities considered to be among the best in the world with regards to many exotic pests and diseases, but it has very low levels of preparedness against surra, a deadly infectious disease of animals caused by the blood-borne protozoan parasite Trypanosoma evansi. The country is currently free of surra, but the disease is endemic to many countries in Southeast Asia, some of whose geographical proximity to northern Australia is a cause for concern. The parasite has a very wide host range and is transmitted mechanically through the bite of blood-feeding tabanid flies, particularly those species which have a tendency to switch hosts during a blood meal. Because Australia has many tabanid species and many potential host animals, surra is recognized as a major threat to local livestock industries and wildlife. Previous experimental studies have indicated that infection with T. evansi can cause acute disease and high mortality in marsupials. Consequently, surra is one of the priority exotic diseases listed on the North Australia Quarantine Strategy (NAQS) program. However, the major limiting factor for Australia's biosecurity plans and disease preparedness with regards to surra is the lack of information on the transmission dynamics and relative impact among the various animal species in northern Australia because little is known about the ecology of available tabanid species and nothing is known about their vectorial capacity, host preferences and host range. The major aims of this research project were therefore to evaluate and compare the vectorial potential of the various tabanid species among some of the major animal species in north Queensland, as well as evaluate the flies' ecological preferences and responses to current surveillance techniques. Successful mechanical transmission of surra depends to a large extent on the blood feeding behaviour of the tabanid species attracted to particular host animals and the responses of the animals to attack by tabanid flies. Because the pathogen survives for a relatively short period on tabanid mouthparts, interrupted feeding is considered to be the most important single factor responsible for the role of tabanids in mechanical transmission. Therefore the relative vectorial potential of the different tabanid species was estimated by assessing the feeding success, feeding duration and feeding frequency of the most abundant tabanid species in the study area, using direct field observations on horses, pigs and kangaroos as well as using electrocuting nets around horses. Tabanid host preferences were evaluated by serological analysis of trap-caught tabanids using ELISA to identify blood meals originating from cattle, horses, pigs and macropods, which are among the most abundant potential hosts in north Queensland. Twelve tabanid species were identified during the studies, and the most abundant were Pseudotabanus silvester Bergroth, Tabanus pallipennis Macquart, T. townsvilli Ricardo, T. dorsobimaculatus Ricardo and T. strangmannii Ricardo. Their feeding behaviour varied with fly species and host species, and the findings predict that some species such as T. pallipennis will be better vectors, while others such as P. silvester are unlikely vectors. Surra could infect all host types studied and pigs are likely to be potential reservoirs of the infection, while macropods are a key host type for most tabanid species and probably face the highest risk in the event of surra incursion. The highest frequency of host blood identified in tabanids was from macropods (61-80% in five tabanid species) even in locations where estimated densities of macropods were relatively lower than other potential hosts. Therefore the prevalence of large populations of wallabies and feral pigs in northern Australia is an important risk factor for the rapid spread of surra and would present a major challenge for effective control of the disease. Assessment of current surveillance techniques showed that octenol-baited Nzi traps performed well at indicating the presence of tabanid species in the area, could reliably predict the activity of some tabanid species on animals, but were not reliable for other tabanid species. Trapping tabanids in different types of habitat for two consecutive wet seasons showed that most tabanid species preferred savannah woodland to open grassland, although T. pallipennis maintained relatively high populations in both types of habitat while P. silvester was only found in woodland areas. Woodland areas proved to be the better habitat type for routine surveillance. The findings from these studies facilitate much better prediction of the risk of surra incursion and form a basis for design of rational surveillance, quarantine and intervention strategies. Predictive risk models for surra can now be based not only on general abundance of tabanids, but also on the relative risk to specific hosts and relative vectorial capacity of different tabanid species, hence making the models more reliable. Available options to minimise attack by tabanids and transmission of surra among domestic animals are discussed, and the urgent need to develop strategies to protect the marsupials of Australia is highlighted.
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