CONTAGIOUS ECTHYMA

Contagious ecthyma (contagious pustular dermatitis or orf) is a highly infectious disease of goats and sheep characterised by pustular and scabby lesions on the muzzle, commissures of the lips and nostrils. The disease is caused by a contagious ecthyma virus of the genus Parapoxvirus and family Poxviridae and, it also affects man.

Epidemiology

Contagious ecthyma is endemic in most African countries. The sero-prevalence of the disease in goats has been reported to be 3.0-19.0 % in the humid zone of Nigeria, 1.6 % in Mali, 14.0 % in Kenya and 14.0% in Tanzania. The disease has also been encountered in small ruminants in Sudan, Ethiopia, Uganda and Malawi.

All breeds of goats and sheep are affected with contagious ecthyma but the disease is chiefly confined to kids and lambs of 3-6 months old. The source of infection is the affected animal or contaminated inanimate objects and, transmission occurs by contact. The contagious ecthyma virus cannot penetrate the intact skin, hence abrasions of the skin caused by spiky plants, hypodermic needles and surgical operations facilitate the penetration of the virus. The contagious ecthyma virus is highly resistant to desiccation and can survive in dry scabs for years.

Pathogenesis

Initial multiplication of the virus occurs at the site of primary infection. Viraemia develops and is followed by subsequent localisation of the virus in the epithelial cells of the Malphigian layer of epidermis of the target organs especially the head, extremities and udder. Cells of the genital tract, lungs and liver can also be infected. The cytopathic effects of the virus in the infected cells include the development of papules, vesicles, pustules and scabs.

Clinical features

Contagious ecthyma is characterised initially by appearance of erythema which later develop into papules and pustules. When the pustules rupture, the pus forms a thick layer of grey crust and later on result in discrete and thick scabs which are crumbly but adherent to the underlying tissues. Lesions usually begin at the oral commissures and then spread to the lips, muzzle, nostrils, ears and sometimes to the buccal and nasal mucosae. Lesions may also occur on the coronet, interdigital cleft, skin of the udder and teats, vulva, preputial orifice, perineal area, thighs and axillae. Adjoining scabs coalesce and form continuous plaques. Fissures which occur between scabs cause soreness. The lesions may become malignant. Invasion of the virus into respiratory and alimentary tracts may occur leading to pneumonia and gastroenteritis. Morbidity may reach 70-90 % but mortality is rare although it may be considerable if secondary bacterial infection occurs. Spontaneous recovery occurs in 2-3 weeks. Lesions along the alimentary tract interfere with feeding and result in considerable weight loss and emaciation.

Pathological features

At gross pathology, contagious ecthyma is characterised scabby lesions on the affected areas and in malignant cases there are ulcerative lesions in the nasal cavity, trachea, oesophagus, abomasum and small intestine. Inflammation and oedema of the affected dermis is evident in histopathological sections. Parakeratosis, acanthosis, and ballooning degeneration occur in keratinocytes. Nuclear pyknosis and oesinophilic intracytoplasmic inclusion bodies in the affected cells are common features. The virus can be demonstrated in ultra-thin sections of the affected tissue by examination under electron microscope.

Diagnosis

Clinical signs and lesions of contagious ecthyma are quite characteristic. Confirmation of the disease can be achieved by electron microscopy, tissue culture and transmission experiments. Complement fixation, virus neutralisation and gel diffusion tests are the common serological methods used in the diagnosis of the disease.

Contagious ecthyma has to be differentiated from dermatophilosis, bluetongue, goat/sheep pox and ulcerative dermatosis. Lesions caused by dermatophilosis are small, light and yellowish in colour and can be easily removed unlike the thick greyish black tenacious scabs of contagious ecthyma and, distortion of the lips and muzzle is not observed in dermatophilosis. Dermatophilosis can be confirmed by demonstration of Gram-positive mycelial-like D. congolensis in impression smears made from the under surface of the lesions. Bluetongue is commonly associated with the presence of vector midges (Culicoides spp) and is characterised by a systemic reaction; excoriation, ulceration and necrosis of the lips and buccal mucosa; lesions in the coronet and high mortality which are not features of contagious ecthyma. The presence of distinctive pox lesions, a febrile reaction and high mortality are features that can be used to distinguish goat/sheeppox from contagious ecthyma. Ulcerative dermatosis is manifested by pyo-ulcerative lesions with thin, brown and bloody scabs. However, isolation and characterisation of the causative viruses is important in order to confirm the above diseases.

Control

Antiseptic ointments such as Lugol's iodine are used to protect secondary bacterial infection on lesions. Parenteral antibiotics are useful in the treatment of systemic secondary bacterial infections. Good nursing such as provision of soft diet to severely affected cases may enhance recovery. Local autogenous live-virus vaccines prepared from vesiculo-pustular material are effective. Attenuated virus vaccines are commercially available. In endemic areas, annual vaccination of 6-8 week kids or lambs is recommended.