Chapter 17

Chapter 17. Diseases of the skin and eye

Contagious pustular dermatitis (CPD, Contagious ecthyma, Scabby mouth) | Actinobacillosis | Squamous cell carcinoma | Photosensitization | Burns | Other skin diseases | Contagious ophthalmia, Ovine infectious keratoconjunctivitis, (Pinkeye) | Exotic diseases | Recommended reading Return to Sheep Health & Production Index

Contagious pustular dermatitis (CPD, Contagious ecthyma, Scabby mouth) The disease is caused by a parapoxvirus and is mainly seen in lambs and weaners. Infection occurs classically on the lips but also occurs on the lower legs and on the oral mucosa. Lesions in other sites of the body are usually trivial and of no consequence. The lesions commence with one or more small focal areas of inflammation which develop into vesicles and pustules then rupture, forming scabs at sites other than the oral mucosa. The disease normally runs a course of 2 to 3 weeks before resolving completely. Infection with the virus of CPD requires a break in the epithelium. Trauma to the skin can be caused by rough feed, thistles or grass seeds. On the legs, lesions occur above the coronary band, behind the pastern or on the interdigital skin. Infections are often preceded by persistent wetting that comes from walking through very wet pasture or in flooded paddocks. The resultant maceration of the skin provides the opportunity for the virus to enter the skin. Infection of the site of application of ear tags can occur, leading to an enlargement of the hole and a loss of the eartag[1]. In lambs, lip lesions can infect ewes' teats; the ewes then refuse to allow the lambs to suck. Mastitis and necrosis of the quarter is an occasional consequence. CPD can also occur on the scrotum. Rarely (but not in Australia), the virus has caused severe systemic infection with significant mortality. Occasionally severe forms, but not systemic forms, are seen in Australia. CPD is seen most commonly in summer months. The virus does not persist on sheep but can survive for years in the environment. Scabs are a rich source of the virus.

Economic consequences of CPD The results of infection are generally trivial but at times can be serious. The presence of CPD lesions on sheep at particular times may lead to management complications such as delays in shearing or sale of rams. Limb lesions can confuse a diagnosis of benign footrot as well as causing some lameness. Weaners with lip and mouth lesions will graze less and lose weight which, at particular times of the year, may have serious consequences on their survival rates and/or their need for later supplementation.

The infection has been transmitted rapidly between sheep in 'Sharlea' sheep sheds and between sheep on ships en route to the Middle-East. The resultant inappetence can lead to increased mortalities but, more importantly, the presence of lesions on arrival has led to the refusal of importing countries to accept delivery. Humans are readily infected but the disease is rarely important.

Vaccination A commercial vaccine prepared from scab material is available. It is applied with a small, double-pointed fork which holds a small volume of vaccine suspension and simultaneously scratches the skin and dispenses the fluid. The usual site for vaccination is the inside of the thigh and the operation can be done conveniently at lamb marking. It is advisable to examine the site of inoculation on a number of sheep 3 days after treatment to ensure a successful 'take'. Pustules should be evident along the site. Immunity after one vaccination or natural infection is strong and remains effective for 2 to 3 years. The vaccine is prepared from fully virulent virus so is capable of causing the disease. Consequently, any animals who miss vaccination are likely to develop the disease naturally from the contamination of the environment caused by infected vaccinates.

Diagnosis The diagnosis is generally made on clinical grounds but can be confirmed by the demonstration of virus in lesion material by electron microscopy. Limb lesions can be misdiagnosed as strawberry footrot. Severe cases must be differentiated from bluetongue and sheep pox, both of which have high mortalities, unlike CPD. Dermatophilosis occurs in woolled skin only; photosensitisation is diffuse through the face and exposed skin. Sheep and goats rarely contract vesicular stomatitis when exposed.

Treatment There is no effective treatment. The disease resolves spontaneously after 2 to 3 weeks. Secondary infection of the limbs of valuable animals may warrant antibiotic treatment.

Actinobacillosis Actinobacillus lignierisi in sheep affects the skin of the face, lips, nose, lower jaw and lower part of the neck, rather than the tongue, as it does commonly in cattle[2]. The disease, when it affects the lips and muzzle is called 'leather lips'[3].

Squamous cell carcinoma The increased exposure to sunlight of the vulva and bare perineal skin of ewes that have had a radical Mules operation with a short-docked tail leads to an increased incidence of

carcinoma[4]. The carcinomas develop in ewes as young as 2 years of age and the incidence increases with age. Affected ewes are susceptible to fly strike and are usually culled or killed when the lesion is detected. Many affected ewes are in poor condition. Preventive treatment involves using a modified mules technique, which leaves a V-shaped piece of wool-growing skin on the proximal third of the dorsum of the tail and amputating the tail at the third coccygeal joint when tailing.

Photosensitization Hepatogenous photosensitization follows the accumulation of phylloerythrin that occurs in animals suffering liver damage caused by toxins associated with plants or plant material. Facial eczema and tribulosis, both associated with ingestion of sporidesmin, are typical examples of hepatogenous photosensitization in sheep. Liver damage can also result from ingestion of Japanese millet, lantana, ragwort (Senecio jacobaeum) and crowfoot (Erodium cicutarium, E crinitum). An inherited congenital photosensitivity of Corriedale and Southdown sheep has also been described in which phylloerythrin accumulation follows a defect in bile excretion. Primary photosensitization occurs when the sensitising photodynamic agent is ingested directly. Plants containing such agents include St John's wort (Hypericum perforatum) and perennial ryegrass (Lolium perenne). The pathogenesis of photosensitization associated with lucerne and the Brassica spp canola (previously called rape) and kale is unknown. The areas affected on sheep are the non-woolled areas, particularly the face, ears and tail. Affected sheep become restless, shake their heads and seek shade. Oedema of the face is often marked, and in rams, must be differentiated from Clostridium novyi infection (bighead). Eventually, the skin sloughs leaving a raw area which is slow to heal. The tips of the ears may also slough.

Burns Sheep are common victims of bushfires in Australia and often large numbers are killed when they are trapped in corners of paddocks and are unable to escape downwind. Veterinarians are often involved in the assessment and management of sheep which survive the immediate effects of fire. It is recommended[5] that burnt sheep be individually examined as soon as possible after fires and allocated to groups according to their prognosis for recovery without extensive treatment. All sheep should be tipped up and their feet, legs, belly and udder inspected. Severe burns to the lower leg, including the knee, hock and hooves are the most significant lesions. Severe burns may not be obvious at first but after two days the skin appears dry, scorched and leathery. Sheep with respiratory distress usually have a poor prognosis because they are likely to develop lung abscessation. Sheep classed as 'likely to survive' should placed in a paddock with soft soil (such as sand), good feed and easy access to water and shade. Many will be inappetent for five days and lose condition for two weeks before starting to recover. Burns to the prepuce, scrotum and

teats recover well provided they are not too severe and the passage of urine is unaffected[6]. Semen quality may be affected for up to 6 months following scrotal burns.

Other skin diseases Distal dry gangrene caused by tall fescue (Festuca arundinacae) occurs but is rare in Australia.

Contagious ophthalmia, Ovine infectious keratoconjunctivitis, (Pinkeye) In sheep, pinkeye is associated with infection with Mycoplasma conjunctivae[7]. There are possibly other infectious agents involved. Spread occurs from infected and carrier sheep by flies and dust. Weaners are most commonly affected. Infection is characterised by swelling of the conjunctivae, lacrimation and staining of the face and, in some cases, corneal ulceration. Once it is established that the signs are not caused by grass seeds, affected mobs of sheep should be left at pasture without treatment. Yarding exacerbates the condition and increases transmission. When treatment of individual sheep is desired, antibiotic ointments are appropriate; macrolide antibiotics, tetracyclines and chloramphenicol all have activity against mycoplasmas.

Exotic diseases Capripox infection (Sheep pox and Goat pox) These two pox viruses are generally host specific although goat pox can infect sheep and cause severe disease. The viruses do not occur in Australia but occur in North Africa, Middle East, southern Europe, China and India. Merino and European breeds are very susceptible to infection and much more so than native African and Middle Eastern breeds.

Epidemiology Sheep pox is highly contagious. It is spread usually during close contact with infected animals, through abrasions, inhalation and possibly by arthropod bites. Fomites and areas contaminated by virus can also provide a source of infection because the virus is very resistant in the environment. It can survive months in dry scab material and on hair and wool out of sunlight. Skin lesions are the main source of virus. In endemic areas, the prevalence of infection is often low for some time with periodic epidemics.

Pathogenesis Following infection there is a viraemia after about 7 days, peaking at day 10 to 14 and persisting for a week or two. During this time the virus is distributed widely throughout the body, including to the skin. The skin lesions are characteristic pox lesions.

Clinical signs The severe, acute form of the disease occurs in lambs and fully susceptible animals. There is marked depression and prostration, high fever and ocular and nasal discharges. Affected animals may die at this time or develop skin lesions on the non-woolled (hairy) areas of skin, on the nares, in the mouth and on the vulval mucosa within 1 or 2 days. In susceptible animals the disease is very severe, with a mortality rate of 50% to 100%. In adult animals of resistant breeds, or partially immune animals, the disease is milder with no systemic reaction. Skin lesions occur and are often concentrated under the tail. Severe losses can occur in ewes from acute secondary mastitis if the virus invades the udder. Healing of skin lesions is slow.

Diagnosis Bluetongue and CPD also have buccal, nasal or skin lesions but clinical signs differ from sheep pox. CPD lesions are usually more proliferative in type. Bluetongue lesions also differ and have a different distribution. Clinical pathology aids diagnosis; using tests for both virus detection and serology.

Treatment and control There is no effective treatment. Control in endemic areas is achieved by vaccination.

Psoroptic mange (Sheep scab) Sheep scab, caused by the mite Psoroptes ovis, was present in australia for over 100 years until its eradication in 1896. It has been eradicated from a number of other countries but is still present in the UK, continental Europe, Africa, Middle East, Asia, Central and South America. The disease induces severe wool damage and causes sheep to lose much condition and some sheep to die as a consequence of infestation.

Epidemiology The mites complete all stages of their life cycle on the sheep although adult mites can survive off sheep for 2 to 3 weeks. They have a short life-cycle (11 - 12 days) and can increase in population very rapidly. They increase in autumn and winter and tend to regress in summer to sub-clinical infections in protected body areas such as the groin, scrotum and interdigital fossa. Spread occurs by close contact between sheep but fomites and infested premises can be responsible for new infestations.

Clinical signs The skin-puncturing habits of the mites cause intense irritation of the skin. Initially the lesions are small papules, oozing serum, often noticed first on the sides of the sheep. The wool is

pulled and chewed by the sheep in response to the irritation. The lesions increase in size and coalesce and become covered in a thin yellow scab, often bleeding from the sheep's rubbing and biting. The wool over infected areas may contain large amounts of this scab material and is severely matted. Parts of the fleece are shed completely and severely affected sheep become very thin.

Diagnosis Severe cases resemble dermatophilosis but the pruritus is more marked. Scrapie and infestations with lice or itchmite cause pruritus but have no visible skin lesions. Definitive diagnosis is made by demonstrating the mites in scrapings from the edge of lesions and from scabs collected from the base of wool fibres.

Treatment and control Treatment is carried out by prolonged dipping (2 minutes or more) in acaricide solution, repeated after 10 to 12 days if necessary, depending on the chemical used. Regional control programs require the planned treatment of all flocks in one area.

Recommended reading Radostits OM, Blood DC and Gay CC (1994) Contagious ecthyma In Veterinary Medicine VIII edition, Bailliere Tindall, p 1125 Beveridge WIB (1981) Contagious pustular dermatitis In Viral Diseases of Farm Livestock, Australian Agricultural Health and Quarantine Service/Australian Government Publishing Service, Canberra (Animal Health in Australia vol 1) p 52 [1] Allworth MB, Hughes KL and Studdert MJ (1987) Contagious pustular dermatitis (orf) of sheep affecting the ear following ear tagging Aust Vet J 64 p 61 [2] Beveridge WIB (1983) Infection with Actinobacillus lignierisi In Bacterial Diseases of Cattle, Sheep and Goats, Australian Bureau of Animal Health/Australian Government Publishing Service, Canberra (Animal Health in Australia vol 4) p 4 [3] Brightling A (1988) Sheep diseases, Inkata Press Pty Ltd, Melbourne & Sydney, p 75 [4] Vandergraaff R (1976) Squamous-cell carcinoma of the vulva in Merino sheep Aust Vet J 52 p 21 [5] Coghill K Saving burnt livestock Department of Agriculture, Victoria, Agdex 400/29 [6] Brightling A (1988) Burns In Sheep Diseases Inkata Press, Melbourne & Sydney [7] Surman PG (1973) Mycoplasma aetiology of keratoconjunctivitis ("Pink-eye") in domestic ruminants Aust J Expl Biol & Med Sci 51 p 589