Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 30, 2003
Publication Date: October 20, 2003
Citation: Pruett Jr., J.H., Steelman, C.D., Miller, J.A., Pound, J.M., George, J.E. 2003. Distribution of horn flies on individual cows as a percentage of the total horn fly population. Veterinary Parasitology. 116:251-258. Interpretive Summary: The horn fly is a serious economic pest of cattle. Their blood-feeding, in numbers up to several thousand on a single animal, can result in a loss of weight gain and milk production. Horn flies are controlled with insecticides. However, there is considerable interest in reducing the amount of insecticide used in the environment. Fewer insecticides are being developed for livestock use, and horn flies are becoming increasingly resistant to those that are available. Development of novel non-chemical control technologies is needed. It has been previously noted that certain cows within the herd carry relatively few horn flies while others are heavily infested. This trait has also been shown to be heritable. Our work focuses on defining the genetic basis of this trait. In the current study we examined, for 18 consecutive weeks, 23 mixed-breed herd cows for horn fly carrying capacity. We found that we could classify 4 cows of the herd as low carriers and 5 cows as high carriers. We found that low carriers could carry economic levels of flies when the population of flies was extremely large. However, regardless of the population size they carried a consistent low percentage of the total population. We proposed an hypothesis based upon successful blood-feeding to explain this fly distribution. Basically, newly emerged flies would attempt feeding on the low carrier. If unsuccessful they would search for a more suitable host. Thus, flies would aggregate on a suitable host to a limit, while the number of flies on the low carrier, as a percentage of the population, would remain stable. As this hypothesis is further tested, and the mechanisms defined, it may be possible to select for cattle that are poor hosts for horn flies. These "resistant" cattle may then be used by producers for the control of horn flies without the use of insecticide.
Technical Abstract: Twenty-three mixed-breed herd cows were phenotyped for their ability to serve as a suitable host for Haematobia irritans, the horn fly. Based upon consistent observations within the lower quartile or upper quartile of individual fly counts, four cows were phenotyped as low carriers and five cows were phenotyped as high carriers of horn flies. The cows designated as low carriers consistently carried levels of flies below the economic threshold. However, during a period of fly population explosion, low carriers harbored flies well above the economic threshold. Although the number of flies counted on these low carrying cattle increased as the population increased, the relative percentage of the population that they carried changed very little. An hypothesis is proposed to explain this observation, and future studies are suggested.