• If turned out on green pastures, weanlings should be first treated with a macrocyclic lactone for strongyles at about 6–9 months of age. This treatment should include praziquantel for tapeworms. If FECs demonstrate ascarids still present, additional treatments with a benzimidazole should be considered for positive individuals.
• During the yearling year, the majority of horses should be expected to exceed 1,000 strongyle EPG, and a few may test positive for ascarids as well. Treatments should generally follow the protocol for high strongyle shedders. Based on this, yearlings will often receive anti-strongyle treatments in the spring, middle of the transmission season, and in the fall. Based on the duration of the strongylid transmission season (Figure 2), one or two additional treatments might be warranted. Praziquantel should be considered during the second half of the year.
• Two to 3-year-olds are expected to gradually decrease their strongyle shedding level, although a proportion will still be expected to be high strongyle shedders. In most scenarios, this age group will receive 3–4 annual treatments, with praziquantel included at least once in the second half of the year.
9.4. Parasite-specific treatment considerations
9.5. Strongyles Benzimidazole and pyrantel resistance should both be assumed until proven otherwise using a FECRT. This leaves the macrocyclic lactone class as the only option in many locations. However, efficacy should be routinely monitored regardless of the chosen class.
9.6. Ascarids Macrocyclic lactone resistance should be assumed until documented otherwise. Both benzimidazoles and pyrantel pamoate usually are valid treatment options, however surveillance of efficacy is still recommended. Theoretically, the benzimidazoles may be a safer choice because of their non-paralytic mode of action and slower onset of effect. However, this assumption is not supported by solid evidence. Fenbendazole when administered at 10 mg/kg for five consecutive days has demonstrated larvicidal efficacy against migrating ascarid larvae (Vandermyde et al., 1987). However, this would be an extra-label use of this product and the clinical benefits of such a treatment are unclear.
9.7. Tapeworms Drug choices for treatment of tapeworms include praziquantel (licensed in the U.S. for horses only in combination with ivermectin or moxidectin), or a cestocidal dose (double the nematode dose) of pyrantel pamoate. In most areas, this treatment should be given in the late fall after tapeworm transmission ends due to cold weather.
9.8. Pinworms Macrocyclic lactone resistance should be assumed until proven otherwise. Because the pruritus secondary to pinworm infections is caused by the material secreted by the female when depositing her eggs, washing the perineum and perianal region may help to relieve symptoms. After scrubbing, all materials should be discarded or washed in hot water with soap and/or
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