Host preferences of tabanid flies based on identification of blood meals by ELISA.

Tabanid flies in Australia are potential vectors of the parasite Trypanosoma evansi which causes the animal disease surra. It is endemic to most of south-east Asia and could enter Australia, but evaluation of the potential impact of a surra incursion requires identification of the major hosts of Australian tabanids. This study investigated the natural pattern of feeding and host preference by tabanid flies of Townsville, north Queensland by identification of ingested blood in trap-caught tabanids using ELISA. The assays were developed for identification of horse, cow, macropod and pig blood meals. Macropods were the most frequent food source for each of six major tabanid species in the area. This did not vary with location for one species, Tabanus pallipennis, despite macropod densities being lower than other hosts such as cattle and horses in some locations. Feeding patterns on other hosts generally depended on availability and density of animals. All tabanid species fed on at least three of the host species tested and mixed meals were also commonly encountered, suggesting a level of opportunistic feeding in addition to a preference for macropods. Some of the blood meals detected were possibly from previous gonotrophic cycles. The results indicate that all tabanid species examined could potentially transmit surra and all the host types investigated could be affected, but macropods face the highest transmission risk.

Feeding success and trappability of horse flies evaluated with electrocuting nets and odour-baited traps.

The relative capacity of different tabanid species to mechanically transmit infectious diseases was assessed by comparing their feeding success on a horse. Ten species were intercepted with electrocuting nets while approaching and leaving the horse: the most abundant were Tabanus pallipennis, Pseudotabanus silvester and Tabanus townsvilli. Of the tabanid species that managed to bite, the highest proportions of partially fed flies, which could potentially transmit pathogens to their next host, were for T. pallipennis (49%) and Dasybasis oculata (48%), while the lowest was in P. silvester (11%). T. townsvilli and T. dorsobimaculatus did not obtain blood meals, possibly because of their sensitivity to host defensive movements. The study also investigated whether catches from odour-baited traps could reliably predict tabanid activity around horses. The traps caught fewer flies, but captured the same species as the electrocuting nets surrounding a horse. A significant linear relationship between the two was observed for P. silvester, T. pallipennis and T. townsvilli but not for T. dorsobimaculatus. The results also demonstrate differences in trappability among tabanid species.

Alighting and feeding behaviour of tabanid flies on horses, kangaroos and pigs.

Successful mechanical transmission of surra between animals by tabanid flies (Diptera: Tabanidae) depends to a large extent on the blood-feeding behaviour of the tabanid species prevalent in the area. We studied tabanid-host interactions in Australia to better predict risk of surra transmission and design intervention strategies. At least six tabanid species were observed alighting on horses, pigs and kangaroos, but the most abundant were Tabanus pallipennis Macquart, Pseudotabanus silvester Bergroth and T. townsvilli Ricardo. The behaviour of tabanids in terms of landing location on the host body, duration of feeding and the proportion completing the blood-meal varied with fly species and host species. The findings predict that some species of tabanid such as T. pallipennis should be better vectors and some species of host such as pigs should be better reservoirs of surra based on the inability of flies to feed to repletion and longer feeding durations. This will result in multiple feeds and increased risk of exposure to the infectious agent, respectively, which increases the risk of transmission. Insecticide treatments should target preferred feeding sites on the host's body.