Larvicidal activity of endectocides against pest flies in the dung of treated cattle

Cattle were treated with topical formulations of endectocides to assess the larvicidal activity of faecal residues against horn fly, Haematobia irritans (L.), house fly, Musca domestica L., and stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae). In laboratory bioassays, doramectin, eprinomectin and ivermectin suppressed horn fly in dung of cattle treated at least 4 weeks previously and suppressed house fly and stable fly in dung of cattle treated 1-5 weeks previously. Moxidectin suppressed horn fly in dung from cattle treated no more than one week previously and did not suppress house fly and stable fly. Results combined for the three species across two experiments suggested that, ranked in descending order of larvicidal activity, doramectin > ivermectin approximately = eprinomectin > moxidectin.     

Fly larvicidal activity in the faeces of cattle and pigs treated with endectocide products

Bioassays were conducted to study the effect of a single therapeutic dose of injectable ivermectin, doramectin or moxidectin given to cattle and pigs and excreted in their faeces, against larvae of the housefly, Musca domestica L. (Diptera: Muscidae). Five cattle were treated with each of the test products. Cattle faecal samples were collected before treatment and on days 1, 2, 3, 6, 10, 16, 20, 23 and 28 after treatment. Three groups of pigs, each comprising 12-14 pregnant sows and gilts, were used in the experiment. Pig faeces was collected from each group before treatment and on days 1, 3, 5, 7, 9, 11, 13, 15 and 20 after treatment. Thirty, first-stage larvae were placed into 100 g of faeces. Five replicates were examined for each time-point and for each endectocide group. Evaluation was based on the number of larvae surviving to adult emergence. Low numbers of adults emerged from samples taken from cattle 1 day after treatment, indicating that ivermectin and doramectin were rapidly excreted in the faeces and affected the development of the house fly. A larvicidal effect of both drugs in cattle faeces was present for a period of about 3-4 weeks and lasted a few days longer in cattle treated with doramectin than with ivermectin. In cattle, the larvicidal activity of moxidectin was first observed in faecal samples collected 2 days post-treatment; however, it killed fewer larvae than the other two drugs. The larvicidal effect of moxidectin subsequently decreased. Ivermectin and doramectin exhibited a pronounced larvicidal effect against the house fly in the faeces of pigs. The effect of doramectin was of longer duration. Moxidectin gave the weakest larvicidal effect in pig faeces. The main difference between the results obtained for the two livestock species is that peak toxicity occurred relatively later and for a shorter duration in pig than in cattle faeces.     

Mutations in the extracellular domains of glutamate-gated chloride channel alpha3 and beta subunits from ivermectin-resistant Cooperia oncophora affect agonist sensitivity

Two full-length glutamate-gated chloride channel (GluCl) cDNAs, encoding GluClalpha3 and GluClbeta subunits, were cloned from ivermectin-susceptible (IVS) and -resistant (IVR) Cooperia oncophora adult worms. The IVS and IVR GluClalpha3 subunits differ at three amino acid positions, while the IVS and IVR GluClbeta subunits differ at two amino acid positions. The aim of this study was to determine whether mutations in the IVR subunits affect agonist sensitivity. The subunits were expressed singly and in combination in Xenopus laevis oocytes. Electrophysiological whole-cell voltage-clamp recordings showed that mutations in the IVR GluClalpha3 caused a modest but significant threefold loss of sensitivity to glutamate, the natural ligand for GluCl receptors. As well, a significant decrease in sensitivity to the anthelmintics ivermectin and moxidectin was observed in the IVR GluClalpha3 receptor. Mutations in the IVR GluClbeta subunit abolished glutamate sensitivity. Co-expressing the IVS GluClalpha3 and GluClbeta subunits resulted in heteromeric channels that were more sensitive to glutamate than the respective homomeric channels, demonstrating co-assembly of the subunits. In contrast, the heteromeric IVR channels were less sensitive to glutamate than the homomeric IVR GluClalpha3 channels. The heteromeric IVS channels were significantly more sensitive to glutamate than the heteromeric IVR channels. Of the three amino acids distinguishing the IVS and IVR GluClalpha3 subunits, only one of them, L256F, accounted for the differences in response between the IVS and IVR GluClalpha3 homomeric channels.     

First Feline Case of Otodectosis in the Republic of Korea and Successful Treatment with Imidacloprid/Moxidectin Topical Solution

In April 2010, pruritic symptoms were recognized in 3 privately-owned Siamese cats raised in Gwangju, Korea. Examination of ear canals revealed dark brown, ceruminous otic exudates that contain numerous live mites at various developmental stages. Based on morphological characteristics of adult mites in which caruncles were present on legs 1 and 2 in adult females and on legs 1, 2, 3, and 4 in adult males while the tarsus of leg 3 in both sexes was equipped with 2 long setae, the mite was identified as Otodectes cynotis. Ten ear mite-free domestic shorthaired cats were experimentally infected with O. cynotis to evaluate the efficacy of 10% imidacloprid/1% moxidectin spot-on. Live mites were recovered from 1 of 10 treated cats on day 9 post-treatment (PT) while no live mites were observed from the ear canals of treated cats on days 16 and 30 PT. The efficacy of 10% imidacloprid/1% moxidectin spot-on on O. cynotis in cats was, therefore, 90% on day 9 and 100% on days 16 and 30 PT. This is the first report of otodectosis in 3 cats naturally infested with O. cynotis in Gwang-ju, Korea. Both natural and experimental infestations were successfully treated with 10% imidacloprid/1% moxidectin spot-on.     

Endectocide residues affect insect attraction to dung from treated cattle: implications for toxicity tests

A 3-year study was performed in southern Alberta, Canada to assess the effect of endectocide residues on the attractiveness of cattle dung to colonizing insects. In 2003 and 2004, insect captures were compared between pitfall traps baited with dung of untreated cattle and paired traps baited with dung of cattle that had been treated 7 days previously with topically applied doramectin, eprinomectin, ivermectin or moxidectin. Faecal residues associated with each compound affected insect captures in both spring and autumn of each year. Effects were detected (P < 0.05) for a total of 94 cases representing 27 insect taxa from 13 families in three orders (Coleoptera, Diptera, Hymenoptera). Two-fold differences in captures were common. Up to six-fold differences were observed. Eleven cases of attraction and 11 cases of repellency were associated with residues of doramectin. Eprinomectin tended to repel insects, with decreased captures for 19 of 29 cases of effect. Ivermectin showed a strong attractive effect, with increased captures for 17 of 25 cases. Moxidectin also showed a strong attractive effect, with increased captures for 17 of 18 cases. Comparisons between compounds suggested that results for doramectin best predicted results for eprinomectin and vice versa. In 2005, insect captures were compared between pitfall traps baited with dung of untreated cattle and traps baited with dung from cattle treated 3, 7 or 14 days previously with topically applied doramectin. Effects were detected in 14 cases plus one case of near significance (P= 0.053). Significant differences between control vs. days 3, 7 and/or 14 dung were detected in nine cases. Residues enhanced captures in seven of these cases. Day 14 dung affected captures in six of these cases. This study shows that endectocide residues can affect the number of insects attracted to colonize and oviposit in dung. Hence, the emergence of their offspring from field-colonized dung of untreated vs. endectocide-treated cattle should not be used as a measure of residue toxicity per se, but rather as a measure of 'insect activity'. Insect activity is a composite measure of residue toxicity, the number and species composition of insect colonists, and the mortality factors (e.g. predation, parasitism, competition) associated with the co-occurrence of these species in the dung pat.