Psoroptes ovis (Sheep Scab): Etiology, Pathogenesis, Diagnostics, and Control

Introduction

Psoroptes ovis is an astigmatid mite of the family Psoroptidae that causes a highly contagious, exudative dermatitis known as psoroptic mange or sheep scab [1]. The parasite infests sheep, cattle, rabbits, horses, and other ungulates, leading to substantial economic losses through reduced weight gain, wool damage, and compromised animal welfare [2, 3, 1]. Sheep scab is endemic in many regions, including the United Kingdom, where it remains a major welfare and production concern [4, 5]. The disease is characterized by intense pruritus, wool loss, and the formation of thick, yellowish crusts on the skin [6]. Early and subclinical infestations are difficult to detect by traditional microscopy, prompting the development of molecular and serological diagnostic tools [7, 4, 8, 9, 10, 11]. The emergence of resistance to macrocyclic lactone (ML) acaricides has further complicated control efforts [12, 13, 14, 15, 16].

Etiology and Life Cycle

Psoroptes ovis is an obligate, non-burrowing ectoparasite that feeds on skin exudates and lymph [1]. The mite completes its entire life cycle on the host, typically within 10 to 14 days [1]. All motile stages (larva, nymph, adult) are found on the skin surface, where they pierce the epidermis to feed, causing an inflammatory serous exudate that dries to form crusts [1]. Transmission occurs through direct contact between infested and naive animals, or via contaminated fomites such as shearing equipment and bedding [5, 1]. The mite can survive off the host for several days under favorable conditions, facilitating indirect spread [1].

Clinical Signs and Pathogenesis

The hallmark of sheep scab is intense pruritus, leading to rubbing, biting, and wool loss [6]. Affected sheep exhibit restlessness, poor nutritional status, and yellowish crusted skin lesions with edge exudate and erythema [6]. In a natural outbreak, clinical signs may be absent in a substantial proportion of infested animals; serological testing has revealed that many clinically normal sheep are seropositive, indicating subclinical infestation [4]. The disease has a significant impact on ewe body condition and lamb birthweight when infestation occurs during mid-pregnancy [2]. Organ weight analysis in experimentally infested sheep demonstrates reduced omental and perirenal fat, lower thymus weight, and increased liver, adrenal, and peripheral lymph node weights, reflecting heightened metabolic demand, chronic stress, and immune activation [3]. Histopathological examination reveals eosinophilic pustular dermatitis with marked eosinophilia in peripheral blood [6]. The immune response is skewed toward a Th2-type profile, with elevated eosinophils and IL-4 levels, particularly in susceptible breeds such as Belgian Blue cattle [17]. Anti-inflammatory treatment with dexamethasone suppresses lesion development and eosinophil infiltration, confirming the role of host inflammatory responses in pathology [17].

Diagnosis

Microscopic Examination

The traditional gold standard for diagnosis is microscopic detection of mites or eggs in skin scrapings [7, 9, 10]. However, this method has poor sensitivity for low-level or early infestations, and false negatives are common [7, 4, 9, 10]. In a comparative study, microscopy detected only 33.3% of infections at 7 days post-infection (dpi), whereas PCR and ELISA achieved 88.9% and 77.7%, respectively [7].

Molecular Diagnostics

A PCR assay targeting the internal transcribed spacer 2 (ITS2) region of ribosomal DNA has been developed for P. ovis detection [7]. The ITS2-PCR has a detection limit of 40.3 pg/μL DNA and shows high specificity [7]. In experimentally infested rabbits, ITS2-PCR detected infection earlier than microscopy or serology, and its positivity rate dropped to 0% by 7 days post-treatment, making it useful for monitoring treatment success [7]. Field studies confirmed a higher detection rate with PCR (19.4%) compared to microscopy (11.1%) [7].

Serological Assays

Several enzyme-linked immunosorbent assays (ELISAs) have been developed using recombinant antigens. An ELISA based on recombinant PsoSP3 (rPsoSP3-iELISA) demonstrated 77.7% sensitivity at 7 dpi and remained 100% positive at 7 days post-treatment, indicating its utility for detecting past exposure [7]. A more recent indirect ELISA using recombinant cystatin (rPsoCys-iELISA) achieved 95.4% sensitivity and 95.7% specificity, with an area under the ROC curve of 0.991 [10]. This assay outperformed microscopy in detecting low-level and early infestations (90% vs. 40% at 3 weeks post-infestation) [10]. Another ELISA based on recombinant arginine kinase (rPoc-AK) showed 94.4% sensitivity and 88.2% specificity, detecting infection as early as 1 week post-infection [11]. Immunoproteomic approaches using two-dimensional gel electrophoresis coupled with LC-MS/MS have identified additional early diagnostic antigens, including PsoSP3, Pso14-3-3(1), and Pso14-3-3(2) [9]. An improved ELISA technique for cattle has also been described [8, 18]. Serological testing is particularly valuable for identifying subclinically infested animals that may act as reservoirs for disease spread [4].

Differential Diagnosis

Sheep scab must be differentiated from other causes of pruritus and dermatitis, including lice infestations (e.g., Damalinia ovis, Linognathus ovillus), sheep ked (Melophagus ovinus), and fungal infections [19, 1]. The presence of mites in skin scrapings confirms the diagnosis [19].

Treatment and Resistance

Macrocyclic Lactones

Macrocyclic lactones (MLs), including ivermectin, doramectin, and moxidectin, are widely used for treatment and control of sheep scab [5, 13, 14, 15, 20, 21, 22, 23]. However, resistance has emerged in both sheep and cattle populations [12, 13, 14, 15, 16]. In the UK, resistance to moxidectin, ivermectin, and doramectin has been documented in P. ovis from outbreak populations [14, 16]. In Belgian Blue cattle, unambiguous treatment failure was detected on 12 of 16 farms, with resistance confirmed by in vivo efficacy parameters [15]. Pharmacokinetic-pharmacodynamic analysis indicates that both peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) relative to minimal inhibitory concentration (MIC) are critical for efficacy against resistant mites [13]. Doramectin 1% and moxidectin 1% achieved 100% efficacy in one study, while long-acting formulations showed delayed responses [13]. Oral doramectin (200 μg/kg) has been effective in rabbits [24]. Ivermectin administered as two subcutaneous injections (1 mL/33 kg) at a 10-day interval resolved an outbreak in Brazil [6]. Controlled-release ivermectin boluses provide prolonged prophylaxis [25, 23].

Alternative Acaricides

Permethrin-based products, including combinations with dinotefuran and pyriproxyfen, have shown efficacy against P. ovis in rabbits [26, 27]. Lotilaner, an isoxazoline, achieved 100% parasitological efficacy by day 7 in naturally infested rabbits [28]. Oral ivermectin (400 μg/kg) was 100% effective against P. ovis in rabbits by day 7 [29]. Plant-derived essential oil components, particularly carvacrol, demonstrate strong acaricidal activity in vitro and in vivo, reducing mite counts by 98.5% in cattle after three weekly topical applications [30]. The entomopathogenic fungus Metarhizium pingshaense combined with peppermint essential oil or its constituents (menthol, menthone) also shows acaricidal potential [31]. A deltamethrin-based product (Delcid 7.5) provided persistent activity for 21 days after a single treatment in sheep [32].

Resistance Mechanisms

Genomic analysis of ML-resistant P. ovis from cattle has identified constitutive overexpression of a cytochrome P450 monooxygenase (CYP) gene and two tandemly located UDP-glucuronosyltransferase (UGT) genes [12]. Copy number variation at these loci is associated with resistance: the CYP gene is amplified in resistant populations, while the UGT genes are massively amplified in all populations, with a weaker selection signal [12]. These findings suggest distinct mechanisms of amplification and gene regulation underlying ML resistance [12].

Integrated Control

Effective control of sheep scab requires a combination of biosecurity, quarantine, strategic treatment, and monitoring [5]. All incoming sheep should be inspected and, ideally, treated with an effective acaricide before introduction to the flock [5]. Dipping with organophosphate or synthetic pyrethroid formulations remains an option in some regions [5]. The use of serological testing can identify subclinically infested animals that might otherwise perpetuate the disease [4]. Resistance management should include rotation of acaricide classes, targeted treatment based on diagnosis, and avoidance of underdosing [5, 14]. The development of molecular markers for resistance, such as CYP and UGT copy number variants, may facilitate surveillance and guide treatment choices [12].

Frequently Asked Questions

What is the causative agent of sheep scab?

Sheep scab is caused by the astigmatid mite Psoroptes ovis, which feeds on skin exudates and causes a severe exudative dermatitis [1].

How is sheep scab transmitted?

Transmission occurs primarily through direct contact between infested and naive animals, but indirect spread via contaminated fomites (e.g., shearing equipment, bedding) is also possible [5, 1].

What are the clinical signs of sheep scab?

Intense pruritus, wool loss, yellowish crusted skin lesions, erythema, and restlessness are typical [6]. Subclinical infestations are common and can only be detected by serological or molecular methods [4].

How is sheep scab diagnosed?

Diagnosis is confirmed by microscopic identification of mites in skin scrapings, but PCR (ITS2-PCR) and ELISA (e.g., rPsoSP3, rPsoCys, rPoc-AK) offer higher sensitivity, especially for early or low-level infestations [7, 9, 10, 11].

What treatments are available for sheep scab?

Macrocyclic lactones (ivermectin, doramectin, moxidectin) are commonly used, but resistance is widespread [14, 15, 16]. Alternative acaricides include permethrin, lotilaner, and plant-derived compounds such as carvacrol [28, 27, 30].

Is resistance to macrocyclic lactones a problem?

Yes, resistance to ivermectin, doramectin, and moxidectin has been documented in sheep and cattle populations in the UK and Europe [12, 13, 14, 15, 16]. Resistance is associated with overexpression and copy number amplification of CYP and UGT genes [12].

Can sheep scab be prevented?

Prevention relies on biosecurity, quarantine of new animals, strategic treatment, and monitoring using sensitive diagnostic tests [5]. Serological screening can identify subclinical carriers that might introduce the disease into a naive flock [4].

Mermaid Diagram: Diagnostic Workflow for Sheep Scab

flowchart TD
    A[Sheep with pruritus, wool loss, crusts], > B{Clinical suspicion of sheep scab}
    B, > C[Perform skin scraping]
    C, > D{Microscopy}
    D, >|Mites/eggs detected| E[Confirmed diagnosis]
    D, >|Negative| F[Consider PCR or serology]
    F, > G[ITS2-PCR on skin scraping or exudate]
    G, >|Positive| E
    G, >|Negative| H[ELISA (rPsoSP3, rPsoCys, rPoc-AK)]
    H, >|Positive| I[Subclinical or recent exposure]
    H, >|Negative| J[Consider alternative diagnoses]
    E, > K[Treatment based on susceptibility]
    K, > L[Monitor post-treatment with PCR or microscopy]
    L, >|Negative| M[Clearance confirmed]
    L, >|Positive| N[Re-treat or switch acaricide class]

References

[1] CABI Compendium. Psoroptes ovis. 2022. https://www.semanticscholar.org/paper/616765dc295a91216e3db283d207fe2480ab5246

[2] Sargison N, Scott P, Penny C, et al. Effect of an outbreak of sheep scab (Psoroptes ovis infestation) during mid-pregnancy on ewe body condition and lamb birthweight. The Veterinary Record. 1995. https://www.semanticscholar.org/paper/508e92886f29af10bc672258962ec31b18c500e5

[3] Rehbein S, Hamel D, Visser M, et al. Psoroptes ovis mange in sheep: Organ and tissue weights as indicators of the impact on organismal metabolism, lymphatic system and body composition. Veterinary Parasitology. 2025. https://www.semanticscholar.org/paper/e330978cdb2a9a7e870cb34a255740d990f907fb

[4] Burgess S, Innocent G, Nunn F, et al. The use of a Psoroptes ovis serodiagnostic test for the analysis of a natural outbreak of sheep scab. Parasites & Vectors. 2012. https://www.semanticscholar.org/paper/17dbda98df31d232f6fb40913e915f293d4ce91c

[5] Burgess STG, Stubbings L. Diagnosis and treatment of sheep scab. A & A Practice. 2023. https://www.semanticscholar.org/paper/4f67bd68caf6153d833caaadc2275528375b64d9

[6] Jardim Filho JO, Almeida GL, Piazer J, et al. Psoroptes ovis infestation of sheep in São Vicente do Sul, Rio Grande do Sul, Brazil. 2020. https://www.semanticscholar.org/paper/bb4c291f65b98e91f875678fa86488909692f44a

[7] Gu X, Zhou Y, Hao G, et al. Development of the first PCR for detection of Psoroptes ovis var. cuniculi infection and its comparison to microscopic examination and serological assay in rabbits. Veterinary Parasitology. 2023. https://www.semanticscholar.org/paper/912cebb14939e58d456673c89ae94f79d16ce188

[8] Ochs H, Lonneux J, Losson B, et al. Diagnosis of psoroptic sheep scab with an improved enzyme-linked immunosorbent assay. Veterinary Parasitology. 2001. https://www.semanticscholar.org/paper/723ee59d444866dbbcadc8e1c37a0880b2706539

[9] Gu X, Tian Y, Zhang CY, et al. Application two-dimensional gel electrophoresis coupled with LC-MS/MS to identify candidate serodiagnostic antigens for early detection Psoroptes ovis var. cuniculi infection. Veterinary Parasitology. 2024. https://www.semanticscholar.org/paper/366f2fc80343bcd4265f14836585cbf6e8454914

[10] Gu X, Yang F, Wang C, et al. A novel cystatin in Psoroptes ovis var. cuniculi: molecular characterization, serodiagnostic potential, and its anti-inflammatory property on rabbit peripheral blood mononuclear cells. Parasites & Vectors. 2024. https://www.semanticscholar.org/paper/b900df6e93c8e6eac246881b8401605b8ed9b287

[11] Gu X, Gu J, Ren Y, et al. Evaluation of an Indirect ELISA Using Recombinant Arginine Kinase for Serodiagnosis of Psoroptes ovis var. cuniculi Infestation in Rabbits. Frontiers in Veterinary Science. 2019. https://www.semanticscholar.org/paper/4af61504810649104f2feabd877e05345a9e03ee

[12] Hearn J, van Mol W, Meyermans R, et al. Over-expression and increased copy numbers of a cytochrome P450 and two UDP-glucuronosyltransferase genes in macrocyclic lactone resistant Psoroptes ovis of cattle. bioRxiv. 2025. https://www.semanticscholar.org/paper/2e96b86eed7a1c849ae301250ee5b42c9e364243

[13] Herrera R, Miró V, Lifschitz A, et al. Integrated pharmacokinetic-pharmacodynamic analysis of macrocyclic lactones in sheep infested with resistant Psoroptes ovis. Veterinary Parasitology. 2025. https://www.semanticscholar.org/paper/eddfd488f07c11d6c5c532f7c3f031b631a18df0

[14] Sturgess-Osborne CM, Burgess STG, Mitchell SE, et al. Multiple resistance to macrocyclic lactones in the sheep scab mite Psoroptes ovis. Veterinary Parasitology. 2019. https://www.semanticscholar.org/paper/e367005c1235ba9ba2c914d711b31e93c925e663

[15] van Mol W, De Wilde N, Casaert S, et al. Resistance against macrocyclic lactones in Psoroptes ovis in cattle. Parasites & Vectors. 2019. https://www.semanticscholar.org/paper/424d4256fb4063dda837cf2d6af5389cd1243458

[16] Doherty E, Burgess S, Mitchell S, et al. First evidence of resistance to macrocyclic lactones in Psoroptes ovis sheep scab mites in the UK. The Veterinary Record. 2018. https://www.semanticscholar.org/paper/dfee12d0bde4f82fad7693d8e0c1fbbaf75c81a4

[17] Chen Z, Claerebout E, Chiers K, et al. Dermal immune responses against Psoroptes ovis in two cattle breeds and effects of anti-inflammatory dexamethasone treatment on the development of psoroptic mange. Veterinary Research. 2021. https://www.semanticscholar.org/paper/fca811591d2442ba9807fd418bd6521b2cc56aa6

[18] Lonneux J, Mignon B, Bossaert K, et al. An improved ELISA technique for the diagnosis of Psoroptes ovis infestations in cattle. 2005. https://www.semanticscholar.org/paper/0b761d21c58a074713e187dfeb4b25be294daebc

[19] Sargison N. Differential diagnosis and treatment of sheep scab. A & A Practice. 1995. https://www.semanticscholar.org/paper/b493a28dea68c286051c86b314669b96163df866

[20] Larroza M, Soler P, Robles C, et al. Doramectin efficacy against Psoroptes ovis in sheep: Evaluation of pharmacological strategies. Experimental Parasitology. 2020. https://www.semanticscholar.org/paper/788345b389bf76ebafce59b7cb819abb0105b0df

[21] Bates P, Groves B. Failure of a single treatment with ivermectin to control sheep scab (Psoroptes ovis) on artificially infested sheep. The Veterinary Record. 1991. https://www.semanticscholar.org/paper/c632b34add0867b87b6c1a7cc1030f73d94c2a0c

[22] Soll M, Carmichael I, Swan G, et al. Treatment and control of sheep scab (Psoroptes ovis) with ivermectin under field conditions in South Africa. The Veterinary Record. 1992. https://www.semanticscholar.org/paper/b8a81c04fec155562e9c1cf441c2e59952d6d130

[23] Forbes A, Pitt S, Baggott D, et al. A review of the use of a controlled-release formulation of ivermectin in the treatment and prophylaxis of Psoroptes ovis infestations in sheep. Veterinary Parasitology. 1999. https://www.semanticscholar.org/paper/c8ad82e8c816f819624f3fdad0a366bb4cfece04

[24] Santos RRD, Coelho CN, Nunes T, et al. Efficacy of oral doramectin as treatment for Psoroptes ovis and Leporacarus gibbus in naturally infested rabbits. 2017. https://www.semanticscholar.org/paper/12ecda7d900c1cd1b1453aa49d95329f8ae99aa8

[25] Rehbein S, Pitt S, Langholff W, et al. Therapeutic and prophylactic efficacy of the Ivomec SR Bolus against nematodes and Psoroptes ovis in cattle weighing more than 300 kg at the time of treatment. Parasitology Research. 1997. https://www.semanticscholar.org/paper/70f972c973c650b9194f4bdd1b971078d6617685

[26] Melo R, Fernandes JI, Vieira VP, et al. Efficacy of the pyrethroid permethrin on the control of Psoroptes ovis in naturally infested rabbits. Revista Brasileira de Parasitologia Veterinária. 2008. https://www.semanticscholar.org/paper/ea35a20bd35baf138e6235d6528848e5a92466ee

[27] Borges DA, Oliveira PC, Dias J, et al. Efficacy of a dinotefuran, permethrin and pyriproxyfen combination product in the treatment of rabbits naturally infested by Psoroptes ovis. Veterinary Parasitology. 2022. https://www.semanticscholar.org/paper/edd3746d06b9c720c5a4f009566f878bea8e1b44

[28] Machado M, Borges DA, Campos DR, et al. Efficacy of lotilaner for treating rabbits naturally infested by Psoroptes ovis and Leporacarus gibbus, in Rio de Janeiro, Brazil. Experimental Parasitology. 2025. https://www.semanticscholar.org/paper/237e8922ba7e78f99cbcba0935a92800970752aa

[29] Coelho CN, Nunes TAP, Batista LO, et al. Eficácia da ivermectina oral no controle de Psoroptes ovis e Leporacarus gibbus em coelhos naturalmente infestados. 2014. https://www.semanticscholar.org/paper/9466657bac68f281603ed124e0adcb426ab34c78

[30] Chen Z, van Mol W, Vanhecke M, et al. Acaricidal activity of plant-derived essential oil components against Psoroptes ovis in vitro and in vivo. Parasites & Vectors. 2019. https://www.semanticscholar.org/paper/1d56380ab26d465ff3cb99ba358884321bb486d2

[31] Carneiro AS, Bório VS, Meirelles LN, et al. Morphological and infection responses of Psoroptes ovis var. cuniculi to Metarhizium pingshaense and peppermint-based compounds. Experimental Parasitology. 2026. https://www.semanticscholar.org/paper/daad1f789c4cc0b5f43b293083da64fadf7eded1

[32] Engashev S, Engasheva E, Kolesnikov V, et al. Persistent activity and therapeutic efficacy of Delcid 7.5 against scab mites (Psoroptes ovis) in sheep. Russian Journal of Parasitology. 2023. https://www.semanticscholar.org/paper/e7da9a92bc5f10167334687f63f8afaf12fed9a9

[33] Oliveira PG, Ferreira LBS, Martins NA, et al. Psoroptes ovis mange in a dog in Brazil. Annals of Parasitology. 2021. https://www.semanticscholar.org/paper/4586ee8386a9b288ea580458a8f94db9b6c48f6c

[34] van Mol W, De Wilde N, Casaert S, et al. In vivo and in vitro parameters for acaricide efficacy of macrocyclic lactones against Psoroptes ovis in cattle. 2020. https://www.semanticscholar.org/paper/ff2cb8d541251731a6406c705d8da9ccea2bfef3

[35] Lonneux J, Nguyen TQ, Delhez M, et al. Antibody response after treatment in cattle infected with Psoroptes ovis. Research in Veterinary Science. 1997. https://www.semanticscholar.org/paper/7af434faa