External parasites in sheep and goats remain a major constraint for small ruminant production, affecting animal welfare, growth, milk yield and quality, and sometimes causing mortality. The most relevant ectoparasites in these animals include lice (Bovicola ovis, Linognathus spp.), mites (Psoroptes ovis, Sarcoptes scabiei, Chorioptes spp.), ticks (Rhipicephalus microplus, Ixodes ricinus), and myiasis-causing flies. A comprehensive approach to controlling ectoparasites must incorporate various measures, including herd management, hygiene, and the judicious use of antiparasitic treatments.
Control programs work best when built on correct parasite identification and epidemiology. Lice are usually winter problems in temperate regions and spread mainly by direct contact; mange mites can be present at high quantities in crowded housing; ticks are present in vegetation and pose a higher risk for animals with access to pasture; wildlife animals acting as hosts can transmit parasites to domestic species; and microclimate may also play a role in external parasite epidemiology.
Because resistance is increasingly reported across ectoparasite groups, treating "blindly" without diagnosis increases cost and failure risk.
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Good flock management helps reduce parasite transmission through quarantine, appropriate housing hygiene, reduced animal-to-animal contact, and careful management of grazing areas. These non-drug measures can also improve the effectiveness of treatments and help slow the development of resistance.
Many outbreaks begin with purchased animals. To prevent this, a robust quarantine protocol should be implemented, including physical isolation for ideally 2–3 weeks, thorough inspection for signs like pruritus, wool loss, crusting, and lice eggs, and delayed mixing of new animals until they are clinically clean. This is particularly important for scab mites and lice, which spread rapidly via close contact.
While most ectoparasites live on the host, the environment still matters:
Ticks are strongly linked to pasture edges, shrubs, and humid vegetation. To reduce their impact, it is best to avoid high-risk paddocks (dense vegetation, humidity, presence of trees, and possible contact with wild animals) during peak tick activity, mow the edges of fields, and clear back scrub when possible. Installing fences can also help by limiting contact with wild animals—such as deer and wild boars—that contribute to tick population growth.
Targeting treatments based on monitoring aligns with integrated parasite management principles and reduces selection pressure for resistance (Wall, 2012; Bowman, 2020).
A low-tech but powerful habit in managing livestock health is routine scoring. This practice involves regularly assessing pruritus and mapping lesions to track skin issues, as well as conducting lice counts or parting fleece checks to monitor infestations. Additionally, it includes counting ticks on sentinel animals to gauge parasite loads. Finally, evaluating treatment outcomes at 2–4 weeks ensures interventions are effective and adjustments can be made if needed.
Treatment selection depends on the parasite involved, production system, and resistance status. Synthetic pyrethroids, macrocyclic lactones, and newer compounds such as fluralaner all have advantages and limitations that should be considered before treatment.
Synthetic pyrethroids such as cypermethrin and deltamethrin have been widely used in many countries as pour-on, dip, or spray formulations. However, resistance to pyrethroids has been extensively documented. Organophosphates such as diazinon, while still available in some South American countries, are increasingly restricted in Europe due to operator safety concerns and environmental toxicity.
Macrocyclic lactones (MLs) represent a key pharmacological advance in ectoparasite control in small ruminants over the past three decades. They act by binding selectively to glutamate-gated chloride channels in invertebrate nerve and muscle cells, causing irreversible hyperpolarization and paralysis of the parasite.
A critical limitation of most MLs is their contraindication in dairy animals during lactation. Eprinomectin overcomes this limitation. Its unique physicochemical properties result in a very low milk-to-plasma partition coefficient, producing negligible residue levels in milk (Alvinerie et al., 1999). Consequently, eprinomectin is the only macrocyclic lactone authorized for use in small ruminants without any milk withdrawal period.
Fluralaner, a member of the isoxazoline class, represents one of the most significant recent innovations in ectoparasite pharmacology. Originally developed for companion animals, its use in small ruminants has attracted growing scientific interest. Fluralaner acts by blocking GABA-gated and glutamate-gated chloride channels in arthropods, causing uncontrolled neuronal excitation and death. Crucially, its selectivity for invertebrate channels confers a high safety margin in mammals.
A practical integrated plan often looks like this:
Alvinerie, M., Sutra, J. F., Galtier, P., & Mage, C. (1999). Pharmacokinetics of eprinomectin in plasma and milk following topical administration to lactating dairy cattle. Research in Veterinary Science, 67(3), 229-232.
Bowman, D. D. (2020). Georgis’ Parasitology for Veterinarians (11th ed.). Elsevier.
Taylor, M. A., Coop, R. L., & Wall, R. L. (2016). Veterinary Parasitology (4th ed.). Wiley-Blackwell.
Wall, R. (2007). Ectoparasites: future challenges in a changing world. Veterinary parasitology, 148(1), 62-74.