Nature of the disease

• Sheeppox and goatpox are acute or subacute highly contagious serious diseases of sheep and goats caused by capripoxvirus characterized by fever, generalized pox lesions throughout the skin and mucous membranes, rhinitis, conjunctivitis, and respiratory distress. Mild and inapparent infections may occur.

• SP&GP are considered the most virulent pox diseases of domestic livestock, with outbreaks often causing death in 30-50% of affected animals and epidemics in a country for the first time may kill most affected animals.

• Sheeppox is a major threat to countries intensively rearing sheep due to losses in terms of mortalities, reduced productivity, lower quality of wool and leather, and the restriction in international trade.

• OIE classification: list "A."

History and Occurrence

SP&GP are ancient diseases, its contagious nature was first recognized in 1763, and its etiology and pathogenesis were first studied in 1868. SP was studied in depth in 1903 by Borrel. SP&GP are currently endemic in Africa north of the equator, the Middle East including Egypt , Turkey , Iraq , Iran , Afghanistan , and the Indian subcontinent. Sporadic outbreaks occur in South-Eastern Europe.

Etiology

• SP or GP virus is a Capripox virus of the family Poxviridae, one of the largest viruses, double stranded DNA genome, and closely related to Neethling lumpy skin disease virus.

• SP&GP viruses isolates vary in virulence and host specifity. In Middle East and India , sheep and goat pox viruses are apparently host specific, but in Kenya there are strains that affect both species.

• Serology, cross-infection, and cross-protection data indicate that the causal viruses are indistinguishable and can be considered as causing a single disease complex.

• SP&GP viruses replicate in a variety of tissue cultures such as lamb and calf kidney, testicle and fetal bovine lung cell cultures producing CPE vary with the virus strain and cell type. Some strains produce lesions on the chorio-allantoic membrane of embryonated hen's eggs.

• SP&GP viruses are very stable under normal environmental conditions and can survive for at least 3 months in scab material. In dark and cool places it remains viable for up to 2 years and on contaminated premises for periods as long as 6 months.

• SP&GP viruses are destroyed within a few minutes by direct sunlight and its infectivity is sensitive to ether and chloroform. common disinfectants are effective especially the iodofores and chlorine dioxide disinfectants

Hosts

• Sheep and goats are the natural hosts with variable host specifity. In enzootic areas, imported breeds are more susceptible than indigenous stock.

• Lambs and kids are more often and more severely affected than adults.

• Severe outbreaks usually occur during wet cold weather in animals weakened by parasites or other infections.

Transmission

• Skin lesions exudate and dried scabs are considered the main source of virus, however the virus is also present in saliva, ocular, and nasal discharges, milk, wool, hair, untreated hides, and respiratory aerosols.

• The disease is transmitted by direct contact of susceptible and sick or recovered animals through skin abrasions and indirectly through contaminated fomites and transport vehicles.

• Inhalation of aerosols generated from lung lesions or contaminated dust and blood sucking insects may play a minor role in transmission.

Sources of the virus

The virus is present in viremic blood "2-4 days", aborted fetuses, milk, and feces.

Key signs

•  Sheeppox

The IP is usually 7 days up to 2 weeks and the morbidity rate is up to 75% in susceptible non immune flocks. The mortality rate varies from 5-80% with a high rate and severe lesions in animals less than 4 months of age, the course of the disease is usually 1-2 months but in lambs it is usually shorter.

•  The malignant " acute" form

• The acute form is common in lambs and exotic sheep imported to endemic areas. It is characterized clinically by high fever, depression, anorexia, lacrimation, salivation, nasal discharge, rough hair coat, arched back, and the affected animals may die during this stage before development of typical pox lesions. The mortality rate in this form may reach 50%.

• 1-2 days later multiple lesions develop over the skin and mucous membranes particularly unwooled areas of the skin such as the perineum, inguinal area, scrotum, udder, ventral aspect of tail, axilla, and muzzle.

• They commence as macules, and then become popular, nodular, vesicular, pustlar, and finally dark scabs. The vesicular stage is rarely seen and some nodules may progress to larger masses resembles that of lumpy skin disease. The pocks in nostrils, mouth, and vulva erode and ulcerate.

• In many cases, respiratory distress and pneumonia may result from upper respiratory lesions complicated by secondary infections and/or enlargement of retropharyngeal lymph nodes.

• Pregnant sheep and goats commonly abort.

• Healing is very slow and the nodular lesions may be resorbed, indurate, or heal by sit-fast formation within 5-6 weeks.

•  The benign form

This form is more common in adults in native stock with low mortality rate 5-10 %. Systemic reactions are often missed and skin lesions are often restricted to the head and under the tail.

•  Goatpox in sheep is more severe than sheeppox.

•  Goatpox

Immunity

Recovery from infection results in solid immunity and resistance to reinfecion is attributed to cell-mediated immune mechanisms

lesions

• The typical skin lesions described above will be apparent at post-mortem.

• In malignant form, some or all pox lesions appear in the skin and may extend to the mouth, pharynx, larynx, and trachea. Gray-white nodules may occur in lungs, oesphagus and even further down the digestive tract.

• Hisopathologically, characteristic intracytoplasmic inclusion bodies and vaculated nucle "sheeppox cells" can be revealed with electron microscope in the infected skin cells.

Diagnosis

• A tentative diagnosis can be made on the bases of clinical, pathological, and epidemiological features of the disease.

• Samples and specimens :

• Heparinized blood during the febrile stage for virus isolation. Acute and convalescent sera for serological studies.

• Skin and lymph nodes biopsies of early lesions can be used for virus isolation, hisopathologic, and electron microscopic studies.

• Histopathology specimens such as skin, lung, and trachea lesions should be preseved in 10% buffered formalin without freezing whereas blood and sera should be submitted on dry ice.

• Rapid diagnosis can achieve by electron microscopic demonstration of virus in negatively-stained preparations of biopsy specimens.

• The virus can be isolated on lamb kidney cell cultures producing the pathognomoic " sheeppox cells" that can be revealed histopathologically with electron microscope .

• Direct IF on frozen tissues may be used to identifiy the specificity of CPE.

• The viral antigen can be identified by immunodiffusion tests.

• The viral antbodies can be identified by IFA, AGID and SNT.

• Differential diagnosis: P&GP may be confused with other skin infections such as contagious ecthyma "orf," dermatophilosis, bluetongue, mange, and peste des petits ruminants "PPR."

Treatment

The disease is notifiable and there is no specific treatment but broad-spectrum antibiotics may reduce losses from secondary bacterial infections. Supportive and symptomatic treatments are indicated in severly affected animals.

Prevention and control measures

• Prevention of SP or GP introduction is the usual goal in free countries through prohibition of the importation of sheep and goat fom endemic areas.

• Eradication is the usual goal in countries which SP or GP appears for the first time through immediate destruction of affected and exposed flocks, application of strict quarantine measures, and vaccination of the surrounding flocks.

• SP or GP management in endemic areas:

• Limiting the occurance and economic compact of the disease is the usual goal in endemic areas through the prophylactic immunization of all susceptible sheep and goat.

• Several modified live vaccines have been used for effective vaccination; the most widely employed cell-cultured attenuated vaccines are probaply the Romanian and Kenyan strains and these provide effective protection for 12 months or longer. Inactivated vaccines have not proven to be practical because they do not provide solid lasting immunity.

• Sheep and goat in high risk areas should be vaccinated annually.

• In Egypt , live attenuated freeze-dried vaccine is produced, prepared from the Romanian strain of sheep poxvirus. The vaccine is used to control sheeppox in sheep and LSD in cattle. 0.5ml of the vaccine is inoculated intradermally in the ventral aspect of the tail or the inner thigh. Local reaction at the site of injection after one week indicates successful vaccination.

References

El Sawalhy, A. A. (1999) : "Veterinary Infectious Diseases" 2 nd Edit. Ahram Disribution Agency,Egypt