Hornflies Haematobia irritans

Haematobia irritans, the hornfly, is one of the most obvious blood-sucking parasites of cattle. It occurs almost worldwide and induces damage due to blood loss, annoyance and disease transmission. Accepted estimates of the economic threshold of hornflies on cattle range from 50 to 200 flies per animal, depending on geographic location (Andress et al., 2000). Horn-fly control involves mainly killing adult flies sucking blood on cattle, but inhibiting hornfly development in the manure may also be an action (Miller et al, 1981; Lysyk and Colwell, 1996). Marley et al. (1993) showed that the subsequent increase in hornfly numbers on an ivermectin-treated herd was predicted by a developmental rate equation interpreted by the time of cessation of larval fly inhibition in field dung. Estimating efficacy against hornflies is not easy, and large differences, with the same ML drug, between studies may be found. A likely cause of variation among study sites may be to the number of hornflies, the size of herd and pastures, and differences in herd management factors that affect the number of immigrating flies.

Treatment with doramectin pour-on (Farkas et al., 2000) resulted in a persistent efficacy of 98.9, 95.9 and 93.4% at 21, 28 and 35 days post-treatment, respectively. At the end of the study, on day 49, efficacy was still 75.1%. Acceptable hornfly control was obtained for 6-7 weeks with one treatment and 13 weeks by the use of two treatments of doramectin pour-on (Andress et al., 2000).

Several studies evaluating the persistent efficacy of ivermectin pour-on against naturally occurring hornflies on cattle were reported. Lysyk and Colwell (1996) observed that ivermectin pour-on reduced adult populations of H. irritans by 90% for 8-16 days and by 50% for 18-26 days. Larval survival in manure was reduced by 90% for 8 and 15 days and by 50% for 19 and 24 days, consistent with the results presented for reduction in adult numbers. Marley et al. (1993) reported that late spring treatment of cattle with a single dose of pour-on ivermectin resulted in reduced hornfly populations for approximately 6 weeks, with percentage efficacy exceeding 80% for at least 26 days post-treatment. Other studies generally demonstrated longer duration effects on adult populations. Lancaster et al. (1991) reported a greater than 50% adult reduction for 35-42 days following spring application of ivermectin, and also reported increased levels of control with repeated applications of ivermectin in isolated treatment herds. Finally, Uzuka et al. (1999) found that topically applied ivermectin was 100% efficacious against hornflies for up to 35 days.

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