Efficacy of orally administered invermectin against larval stages of Oestrus ovis in sheep

Antiparasitic activity of ivermectin: Four decades of research into a "wonder drug"

Parasitic diseases still pose a serious threat to human and animal health, particularly for millions of people and their livelihoods in low-income countries. Therefore, research into the development of effective antiparasitic drugs remains a priority. Ivermectin, a sixteen-membered macrocyclic lactone, exhibits a broad spectrum of antiparasitic activities, which, combined with its low toxicity, has allowed the drug to be widely used in the treatment of parasitic diseases affecting humans and animals. In addition to its licensed use against river blindness and strongyloidiasis in humans, and against roundworm and arthropod infestations in animals, ivermectin is also used "off-label" to treat many other worm-related parasitic diseases, particularly in domestic animals. In addition, several experimental studies indicate that ivermectin displays also potent activity against viruses, bacteria, protozoans, trematodes, and insects. This review article summarizes the last 40 years of research on the antiparasitic effects of ivermectin, and the use of the drug in the treatment of parasitic diseases in humans and animals.

Prophylactic Effects of Ivermectin and Closantel Treatment in the Control of Oestrus ovis Infestation in Sheep

The sheep nasal bots Oestrus ovis is parasite of the nasal cavities and sinuses of small ruminants causing oestrosis, one of the most frequent parasitic diseases in sheep and goats. The widely use of ivermectin and closantel by the sheep breeders in the treatment and prophylaxis of gastrointestinal nematodes resulted in widespread cases of anthelmintic resistance. However, there is no report about cases of O. ovis with drug-resistance. In this study, we evaluated the prophylactics and therapeutic effects of both antiparasitics in sheep with O. ovis natural infestation. The trial was carried out from early December 2019 to March 2020, with 30 crossbred males lambs allocated into three groups of 10 animals each: control (without treatment), treated with ivermectin (0.2 mg/kg subcutaneously) and treated with closantel (10 mg/kg orally). The animals were kept together grazing the same pasture area. The treatment groups were drenched in two occasions 70 days apart: on 5th December 2019 and on 13th February 2020. On 19th March 2020, all lambs were slaughtered. The lamb heads were removed and sectioned along their longitudinal and sagittal axis to search for larvae. Recovered O. ovis larvae were counted and identified according to their developmental stage (L1, L2, and L3). Seven of the control lambs were infested with O. ovis larvae ranging from six to 17 larvae (11.6 mean infestation intensity). All recovered larvae from control group were intact and active. Three animals treated with ivermectin had O. ovis larvae (1-3 larvae), however they were dead and in degeneration. The animals treated with closantel did not have any larvae. The clinical suggestive signs of oestrosis were scarce over the experimental period. The averages of daily weight gain were similar (p > 0.05) among groups. Closantel and ivermectin had high efficacy against oestrosis and O. ovis parasitism did not hinder the performance of lambs.

Prevalence of Oestrus ovis (Diptera: Oestridae) in sheep from the São Paulo Central region, Brazil

The heads of 139 slaughtered sheep were examined with the aim of determining Oestrus ovis prevalence and infestation intensity in the central region of the State of São Paulo, Brazil. Heads from slaughtered sheep were examined and the first (L1), second (L2) and third (L3) O. ovis larval instars were recovered from the nasal and sinus cavities. O. ovis larvae were detected in 13.7% of the sheep analyzed. The monthly mean intensity of infestation ranged from 1 to 10.2 larvae per infested head, with general mean intensity of 4.5 larvae/infested head. Of the total of 85 larvae, 21.2% were L1, 37.6% L2 and 41.2% L3. In conclusion, all different O. ovis larvae instars were recovered from slaughtered animals, which demonstrates the existence of favorable climatic conditions for the fly activity and larval development of O. ovis in sheep raised in the Central region of the State of São Paulo.

Efficacy of moxidectin long-acting injectable formulation (1 mg/kg bodyweight) against first instar larvae of Oestrus ovis in naturally infected sheep

The objective of the current study was to evaluate the efficacy of a single treatment with a long-acting injectable formulation of moxidectin (MOX) at 1.0 mg/kg bodyweight (b.w.) against natural infection by nasal bots (Oestrus ovis) in sheep with special attention to first instar larvae (L1). Firstly, a local farm with clinical history of oestrosis was chosen to conduct the assay. A total of 49 sheep were pre-selected at the end of the summer according to the presence of evident clinical signs of infection and confirmed later by means of an indirect ELISA against excretory-secretory products from L1 to detect IgG antibodies. After that, 24 sheep were chosen to carry out the study on the basis of positive serology and age since the oldest ones were selected. The day 0 of the assay, the treatment group was administered with the MOX formulation by subcutaneous injection at the base of the left ear and the control group was administered with a saline solution in the same way. All sheep were slaughtered on day 28 post-treatment (pt). At the necropsy, the head of all sheep were cut off and split into two sagital sections and all larvae from nasal passages, septum, middle meatus, conchae and sinuses were recovered. After the necropsy, a significant number of L1 was only found in the control group and therefore the efficacy of the MOX formulation was only calculated against this stage. As a result, the formulation was 90.2% effective against L1 for sheep slaughtered at day 28 pt.

Persistent efficacy of a long acting injectable formulation of moxidectin against natural infestations of the sheep nasal bot (Oestrus ovis) in Spain

Cydectin(®) 2% LA Solution for Injection for Sheep (Pfizer Animal Health) is a long-acting (LA) formulation of moxidectin for the treatment and prevention of mixed infections of gastro-intestinal nematodes, respiratory nematodes and certain arthropod parasites in sheep. To evaluate the duration of persistent efficacy against nasal bots (Oestrus ovis), a natural exposure study was conducted in Spain during the summer of 2011. One hundred and twenty nasal bot-free, Rasa Aragonesa sheep were randomly allocated to eight groups of 15 animals each. On Day 0, four groups were treated at the recommended dose rate of 1 mg moxidectin/kg bodyweight. Four groups remained untreated as negative controls. All animals were held in nasal bot-proof housing except for exposure to natural challenge when one group of treated sheep and one of group of control animals were transferred to a local pasture at either 0-20, 20-40, 40-60, or 60-80 days after treatment. Following challenge, sheep were scored for clinical signs of bot infestation, necropsied and the heads sectioned for larval recovery. Nasal bot larvae were retrieved from 7 to 11 control sheep following each exposure period indicating that adult bots were active throughout the study. In the first challenge up to 20 days after treatment, when sheep were slaughtered immediately after exposure, the majority of larvae were first instar (L1) and only 3 of the 15 control sheep were infested with second instars (L2). There was 100% efficacy against L2 and 38.1% reduction in the number of live L1 in the treated sheep but mean counts were not significantly different between treatment and control groups (P ≥ 0.05). For the subsequent exposure periods 20-80 days after treatment (necropsies 7-9 days after challenge), 6-10 sheep were infested with L1 and 9-11 control sheep were infested with L2 and third instars (L3). There was negligible efficacy against L1, but treatment with moxidectin resulted in 100% control of L2 and L3. These results are consistent with the biology of nasal bots and control with a systemic agent, as the slower growing L1 have limited feeding and are therefore less susceptible to systemic parasiticides. The study demonstrated that the persistent efficacy of this long-acting injectable formulation of moxidectin protects against the development of active O. ovis infestations for at least 80 days after treatment.

Ectoparasite control in small ruminants

Ectoparasites are a common problem in small ruminants of North America. Management of ectoparasites in small ruminants can be challenging for producers and veterinarians. It is important for the veterinarian to make an accurate diagnosis of the type of ectoparasite that is infesting the animal, then to develop a plan that most effectively and economically controls the ectoparasite. Effective and economic control of an ectoparasite infestation begins with an understanding of the ectoparasite's life cycle and how that life cycle affects the animal. It should be noted that climate and geographical area can affect the life cycle of specific ectoparasites, so it is important for veterinarians to educate themselves about their specific environment. Once the life cycle has been addressed, then the veterinarian should decide which intervention will provide the best control. Intervention possibilities may range from insecticides to environmental management or a combination of several methods. The veterinarian should provide the producer with realistic goals that define specific limitations of ectoparasite control.