Psoroptic Mange (Sheep Scab) in Sheep and Goats

Psoroptic mange, commonly known as sheep scab, is a highly contagious, debilitating, and economically significant ectoparasitic disease of sheep and occasionally goats, caused by the non-burrowing mite Psoroptes ovis. The mite feeds on superficial skin layers, inducing an intense allergic and inflammatory response that leads to severe pruritus, wool loss, skin exudation, and secondary bacterial infections. In sheep, the disease is characterized by rapid spread through a flock, with lesions typically beginning on the back and shoulders, then extending over the entire body. In goats, the infestation is often less severe but can still cause significant morbidity. The disease is a major welfare concern and a notifiable disease in many countries, with strict movement controls and compulsory treatment programs. Economically, it results in substantial losses due to decreased wool quality and quantity, reduced weight gain, milk production losses in dairy goats, and increased susceptibility to secondary infections. In intensive production systems, the disease can cause mortality in severe cases, particularly in young or debilitated animals. The condition is managed through a combination of ectoparasiticides, biosecurity measures, and quarantine protocols.

The primary causative agent is Psoroptes ovis, a non-burrowing, obligate ectoparasitic mite of the family Psoroptidae. The mite is approximately 0.5-0.8 mm in length, with a rounded body and long, jointed pedicels ending in suckers. It lives on the skin surface, piercing the epidermis with its mouthparts to feed on tissue fluids and lymph. The mite's saliva and fecal material contain potent allergens that trigger a type I hypersensitivity reaction in the host. The life cycle is completed in about 10-12 days, with five stages: egg, larva, protonymph, tritonymph, and adult. Eggs are laid on the skin surface and hatch within 1-3 days. The mites are highly mobile and can survive off the host for up to 15-20 days, depending on environmental conditions, making fomite transmission possible. In goats, Psoroptes ovis can also infest, but the clinical signs are often less severe, and the mites may be found in the ears. Other Psoroptes species, such as P. cuniculi (ear mite of rabbits) and P. equi, are host-specific and do not typically cause disease in sheep. The mite is transmitted primarily by direct contact between animals, but also through contaminated bedding, shearing equipment, and handling facilities. The disease is more prevalent in cooler, humid climates, as the mites thrive in such conditions.

Psoroptic mange is a worldwide disease, with a higher prevalence in temperate regions with cool, moist climates. In sheep, the disease is most common in the winter months when wool is long and provides a favorable microclimate for mite survival. The disease affects all breeds, but wooled breeds, such as Merino and crossbreds, are more severely affected due to the dense fleece that facilitates mite spread. In goats, the disease is less common but can occur, especially in dairy goats housed in close confinement. The morbidity rate in an affected flock can be as high as 90-100%, with mortality rates ranging from 2-10% in severe cases, particularly in naive flocks. The economic impact is substantial, with losses from wool damage, reduced weight gain, and treatment costs. In the United Kingdom, sheep scab is a notifiable disease, and outbreaks lead to movement restrictions and compulsory treatment. The disease is also a significant welfare issue, as affected animals suffer from intense pruritus and skin lesions. The prevalence of the disease has increased in some regions due to the development of acaricide resistance, particularly to macrocyclic lactones. The disease is more common in extensively managed flocks where biosecurity is poor, and in areas with communal grazing or livestock markets. In goats, the disease is often subclinical, but can be a source of infestation for sheep.

The pathophysiology of psoroptic mange is primarily driven by the host's immune response to the mite's allergens. The mites feed on the skin surface, piercing the stratum corneum and epidermis with their mouthparts, causing mechanical damage and releasing salivary and fecal antigens. These antigens are recognized by antigen-presenting cells, leading to a type I hypersensitivity reaction with the release of histamine and other vasoactive mediators. This results in intense pruritus, erythema, and edema of the skin. The inflammatory response leads to the formation of papules, vesicles, and crusts, which are composed of serum, inflammatory cells, and mite debris. The constant scratching and rubbing by the animal cause further trauma, leading to wool loss and secondary bacterial infections. The skin lesions are characterized by hyperkeratosis, parakeratosis, and acanthosis. In chronic cases, the skin becomes thickened and lichenified. The systemic effects include weight loss, reduced feed intake, and decreased milk production, likely due to the stress and energy expenditure associated with the inflammatory response. In severe cases, the disease can lead to anemia and hypoproteinemia, particularly in young animals. The immune response is not protective, and animals can be reinfested. The mite's ability to survive off the host for several days contributes to the rapid spread within a flock.

Several factors predispose sheep and goats to psoroptic mange. Intrinsic factors include breed susceptibility, with wooled breeds being more severely affected. Age is also a factor, as young animals are more susceptible to severe disease. The immune status of the animal plays a role, as animals with compromised immune systems, such as those with concurrent diseases or poor nutrition, are more likely to develop severe infestations. Extrinsic factors include environmental conditions, with cool, humid climates favoring mite survival and transmission. Overcrowding and poor biosecurity practices, such as the introduction of new animals without quarantine, increase the risk of disease spread. Management practices, such as shearing, can also spread the mites through contaminated equipment. The use of shared grazing or livestock markets can facilitate transmission. The development of acaricide resistance, particularly to macrocyclic lactones, has been identified as a major risk factor for the persistence and spread of the disease. In goats, the disease is more common in dairy operations with intensive housing. The presence of other skin diseases, such as dermatophytosis, can also predispose to secondary mite infestations.

The clinical signs of psoroptic mange in sheep typically begin with intense pruritus, with affected animals rubbing against fences, posts, and other objects. The initial lesions appear on the back, shoulders, and sides, where the wool is dense. The skin becomes erythematous and edematous, and small papules and vesicles develop. As the disease progresses, the lesions coalesce, and the skin becomes covered with thick, yellowish crusts. The wool becomes matted and may fall out, leading to large areas of wool loss. The affected skin is often moist and exudative, and secondary bacterial infections can lead to a foul odor. In severe cases, the lesions can extend over the entire body, including the head and ears. Affected animals may show signs of systemic illness, including fever, anorexia, and weight loss. In goats, the clinical signs are often less severe, with lesions typically confined to the ears and neck. However, in some cases, the disease can spread to the rest of the body, causing pruritus and crusting. The intense pruritus can lead to self-trauma, with excoriations and secondary infections. In dairy goats, milk production may decrease. The disease can be fatal in young or debilitated animals, particularly if secondary infections become systemic.

Differential diagnoses for psoroptic mange include other ectoparasitic infestations, such as sarcoptic mange (Sarcoptes scabiei), chorioptic mange (Chorioptes bovis), and lice infestations (Bovicola ovis, Linognathus ovillus). Sarcoptic mange is characterized by burrowing mites, with lesions typically on the head and neck, and is less common in sheep. Chorioptic mange affects the lower legs and is less pruritic. Lice infestations cause pruritus and wool damage, but the lice are visible to the naked eye and can be distinguished from mites. Other skin diseases, such as dermatophytosis (ringworm), bacterial dermatitis, and photosensitization, can also cause similar clinical signs. Dermatophytosis is characterized by circular, crusty lesions, and the fungus can be identified by fungal culture. Bacterial dermatitis, such as dermatophilosis, is often associated with wet conditions and presents with scabby lesions. Photosensitization causes skin lesions on non-pigmented areas, and is associated with exposure to sunlight. Nutritional deficiencies, such as zinc deficiency, can also cause skin lesions. A definitive diagnosis is made by skin scrapings and microscopic identification of the mites.

The diagnostic algorithm for psoroptic mange begins with a thorough flock history and clinical examination. The presence of intense pruritus, wool loss, and crusty skin lesions, particularly on the back and shoulders, is highly suggestive. The diagnosis is confirmed by skin scrapings. The algorithm is as follows: 1) Perform a physical examination of affected animals, noting the distribution and appearance of lesions. 2) Collect skin scrapings from the edge of active lesions, using a scalpel blade and mineral oil. Scrape until capillary bleeding is observed. 3) Place the scrapings on a glass slide, add a drop of mineral oil or potassium hydroxide, and examine under a microscope at 10x or 40x magnification. Identify Psoroptes ovis mites, which are large, with long pedicels and suckers. 4) If mites are not found, repeat scrapings from multiple sites, as the mites may be sparse. 5) In cases where scrapings are negative, a therapeutic trial with an acaricide can be performed, and if the animal improves, the diagnosis is likely. 6) In goats, examine the ears for mites, as they may be present in the ear canal. 7) Rule out other differentials through appropriate tests, such as fungal culture for dermatophytosis or bacterial culture for dermatitis. 8) In a flock outbreak, treat all animals, as subclinical carriers may exist.

Laboratory findings in psoroptic mange are not specific, but may include eosinophilia and elevated serum immunoglobulin E (IgE) levels due to the hypersensitivity reaction. Skin scrapings are the primary diagnostic test, and the presence of Psoroptes ovis mites confirms the diagnosis. The mites can be identified by their morphology, with a rounded body and long pedicels. In chronic cases, secondary bacterial infections may lead to leukocytosis and elevated acute-phase proteins, such as fibrinogen. In severe cases, anemia and hypoproteinemia may be present due to blood loss and malnutrition. In goats, ear swabs may be taken to identify mites in the ear canal. There are no specific blood tests for the diagnosis of psoroptic mange. However, in research settings, serological tests, such as ELISA, have been developed to detect antibodies against Psoroptes ovis antigens, but these are not routinely used in clinical practice. Fecal egg counts are not relevant to this disease. The diagnosis is primarily based on clinical signs and microscopic identification of the mite.

Imaging is not typically used in the diagnosis of psoroptic mange. However, in cases of secondary bacterial infections, ultrasonography may be used to assess the extent of skin and subcutaneous tissue involvement. In severe cases, radiography may be performed to rule out underlying bone involvement, but this is rare. In goats, otoscopic examination of the ear canal may be performed to visualize mites in cases of ear infestation. Imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) are not indicated for this disease. The diagnosis is primarily based on clinical examination and skin scrapings. In research settings, high-frequency ultrasound has been used to measure skin thickness and assess the severity of lesions, but this is not a routine diagnostic tool.

Cytological examination of skin scrapings or impression smears from lesions may reveal mites, mite eggs, and inflammatory cells, including eosinophils and neutrophils. Histopathological examination of skin biopsies is not routinely performed but can be useful in chronic cases or when the diagnosis is uncertain. The histopathological findings include hyperkeratosis, parakeratosis, acanthosis, and spongiosis of the epidermis. The dermis shows a perivascular inflammatory infiltrate, predominantly composed of eosinophils, lymphocytes, and mast cells. In chronic cases, there is fibrosis and lichenification of the skin. The presence of mites in the stratum corneum or on the skin surface can be observed. Secondary bacterial infections may lead to suppurative dermatitis. In goats, histopathology of ear lesions may show similar changes. The histopathological features are consistent with a hypersensitivity reaction to the mite antigens.

The treatment of psoroptic mange involves the use of acaricides, with the goal of eliminating the mites from the affected animals and the environment. The choice of acaricide depends on the species, the production system, and the presence of resistance. In sheep, the most commonly used acaricides are macrocyclic lactones (e.g., ivermectin, doramectin, moxidectin), organophosphates (e.g., diazinon), and synthetic pyrethroids (e.g., deltamethrin, flumethrin). Ivermectin is administered subcutaneously at a dose of 0.2 mg/kg, with a repeat dose after 7-10 days to cover the mite life cycle. Doramectin is given at 0.3 mg/kg IM, and moxidectin at 0.2 mg/kg PO or SC. These drugs are effective against adult mites but may not kill eggs, so a second dose is necessary. Organophosphates and pyrethroids are applied as dips or pour-ons. Diazinon is used as a dip at a concentration of 0.05%, and deltamethrin as a pour-on at 0.5-1.0 mg/kg. These treatments are effective but have withdrawal times for meat and milk. In goats, ivermectin is used at 0.2 mg/kg SC, but the withdrawal times are longer. In addition to treating the affected animals, the environment should be cleaned and disinfected, as mites can survive off the host for up to 15-20 days. All in-contact animals should be treated, and new animals should be quarantined. In some countries, the disease is notifiable, and treatment may be compulsory. Supportive care, such as providing good nutrition and treating secondary bacterial infections with antibiotics, is also important. The use of anti-inflammatory drugs, such as flunixin meglumine, may be indicated to reduce pruritus and inflammation.

The prognosis for psoroptic mange is generally good with prompt and appropriate treatment. Most animals recover within 2-4 weeks after treatment, with resolution of pruritus and regrowth of wool. However, the prognosis is guarded in severe cases, particularly in young, debilitated, or immunosuppressed animals. Secondary bacterial infections can lead to complications, such as pneumonia or septicemia, which can be fatal. The prognosis is also affected by the presence of acaricide resistance, which may require the use of alternative treatments. In flocks with a history of the disease, the prognosis for eradication is good if strict biosecurity measures are implemented. In goats, the prognosis is generally good, but the disease can be more difficult to eradicate due to the presence of mites in the ears. The economic impact of the disease can be significant, but with proper treatment and management, the long-term prognosis for the flock is favorable.

Follow-up after treatment of psoroptic mange is essential to ensure complete eradication of the mites. Animals should be re-examined 7-14 days after treatment, and skin scrapings should be repeated to confirm the absence of mites. If mites are still present, a second treatment may be necessary. In flocks, all animals should be treated, and the environment should be cleaned and disinfected. Pastures should be rested for at least 2-3 weeks to allow any mites to die. New animals should be quarantined for at least 30 days and treated prophylactically before introduction to the flock. Regular monitoring for signs of pruritus and skin lesions should be conducted, especially during the high-risk seasons. In areas where the disease is notifiable, the veterinary authorities should be informed, and movement restrictions should be followed. The development of acaricide resistance should be monitored, and alternative treatments should be considered if resistance is suspected. In dairy goats, milk withdrawal times should be observed, and milk should be tested for drug residues before sale. A comprehensive flock health plan should be implemented to prevent future outbreaks.

Clinical pearls: 1) Psoroptic mange is highly contagious, so all in-contact animals must be treated, not just those showing clinical signs. 2) The mites are often found at the edge of active lesions, so skin scrapings should be taken from these areas. 3) In goats, always examine the ears for mites, as they may be the primary site of infestation. 4) The disease is more severe in wooled breeds, so early detection and treatment are crucial. 5) Macrocyclic lactones are effective, but a second dose is needed to kill newly hatched mites. 6) The use of anti-inflammatory drugs can provide rapid relief from pruritus. Pitfalls: 1) Failure to treat all animals in the flock can lead to rapid reinfestation. 2) Inadequate skin scraping technique can result in false-negative results. 3) The use of acaricides without considering resistance can lead to treatment failure. 4) Neglecting to clean the environment can result in reinfestation from surviving mites. 5) In goats, the disease may be subclinical, and carriers can introduce the disease to sheep. 6) Not observing withdrawal times can lead to drug residues in meat and milk.

The following are current drug protocols for the treatment of psoroptic mange in sheep and goats, based on Plumb's Veterinary Drug Handbook and AASRP guidelines. Ivermectin: Sheep and goats: 0.2 mg/kg SC, repeat in 7-10 days. Withdrawal times: meat 11 days (sheep), 14 days (goats); milk 4 days (sheep), 9 days (goats). Doramectin: Sheep: 0.3 mg/kg IM, repeat in 7-10 days. Withdrawal times: meat 35 days; milk not approved for use in lactating sheep. Moxidectin: Sheep: 0.2 mg/kg PO or SC, repeat in 7-10 days. Withdrawal times: meat 7 days (SC), 14 days (PO); milk 4 days (SC), 14 days (PO). Goats: 0.2 mg/kg SC, repeat in 7-10 days. Withdrawal times: meat 14 days; milk 4 days. Diazinon: Sheep: 0.05% dip, repeat in 7-10 days. Withdrawal times: meat 14 days; milk 7 days. Deltamethrin: Sheep: 0.5-1.0 mg/kg pour-on, repeat in 7-10 days. Withdrawal times: meat 35 days; milk 7 days. Flumethrin: Sheep: 1.0 mg/kg pour-on, repeat in 7-10 days. Withdrawal times: meat 35 days; milk 7 days. In addition, supportive care may include flunixin meglumine at 1.1-2.2 mg/kg IV or IM for inflammation and pain, and antibiotics for secondary infections, such as oxytetracycline at 10 mg/kg IM or penicillin G at 20,000-40,000 IU/kg IM. All treatments should be administered according to the label and veterinary supervision.

Evidence-based literature on psoroptic mange has focused on the efficacy of various acaricides, the development of resistance, and the epidemiology of the disease. A landmark study by Bates (2009) reviewed the control of sheep scab in the UK, highlighting the importance of biosecurity and the need for effective treatment strategies. Studies have shown that macrocyclic lactones, such as ivermectin and moxidectin, are highly effective against Psoroptes ovis, but resistance has been reported in some regions. A study by Sargison et al. (2002) reported the first case of ivermectin resistance in Psoroptes ovis in the UK, emphasizing the need for alternative treatments. The use of organophosphates and pyrethroids has been shown to be effective, but their use is limited by withdrawal times and environmental concerns. A study by O'Brien et al. (1994) evaluated the efficacy of a single injection of doramectin in the treatment of sheep scab, showing a 100% cure rate. The epidemiology of the disease has been studied in various countries, with risk factors including the introduction of new animals, communal grazing, and poor biosecurity. A study by Rose et al. (2009) used mathematical modeling to assess the impact of control strategies on the prevalence of sheep scab in the UK. The consensus among experts is that a combination of treatment, biosecurity, and surveillance is essential for the control and eradication of psoroptic mange. The AASRP and ECSRHM have published guidelines for the management of ectoparasitic diseases in small ruminants, emphasizing the importance of integrated pest management.