Sheeppox and goatpox

GENERAL INTRODUCTION: POXVIRIDAE

Introduction

Sheep- and goatpox are malignant systemic pox diseases of sheep and goats characterized by fever, macules developing into papules and necrotic lesions in the skin and nodular lesions in internal organs, secondary infections and death in susceptible stock. The diseases in sheep and goats are caused by strains of capripoxvirus which are indistinguishable serologically, and although strains derived from sheep may show genomic differences from those derived from goats, there are isolates with characteristics of both sheep and goat isolates.⁹ Similarly, while most isolates cause disease in either sheep or goats, there are isolates equally pathogenic in both species.³⁵ Therefore, the classification used so far, based on the animal species from which the capripox virus is isolated, i.e. goatpox and sheeppox for viruses isolated from goat and sheep respectively, is inappropriate. To be informative nomenclature needs to take other criteria such as molecular-based data that enable clear differentiation between these two viruses into consideration.^{43, 44, 45} Experimentally, all sheep isolates will replicate in goats, and goat isolates will replicate in sheep, providing effective cross-immunity. Because sheep- and goatpox are serious epidemic diseases that may cause important losses in affected herds and represent also a major constraint to international trade for affected countries, they are classified in the list of animal diseases to be notified to the World Organization for Animal Health, the OIE: outbreaks should be reported immediately to the OIE. A third disease caused by a capripoxvirus, lumpy skin disease of cattle, can also elicit protective immunity against sheep- and goatpox in sheep and goats, as illustrated by the Kenyan sheep- and goatpox vaccine which is derived from a lumpy skin disease virus that was isolated from sheep.⁵⁵ The difference in the geographical distribution of lumpy skin disease from that of sheep- and goatpox, as well as the fact that no sheep- or goatpox viruses have been isolated from cattle, suggests that sheep- and goatpox viruses do not cross from small ruminants to cattle.¹²

Aetiology

Capripoxviruses are large brick-shaped, double stranded DNA viruses, morphologically indistinguishable from orthopoxviruses, measuring 295 by 265 nm.³⁴ The virion is covered in short tubular elements which give it a different appearance from orf virus, which is more oval in shape and covered in a continuous filament. Poxviruses have a very stable genome which is demonstrated by DNA restriction patterns of isolates collected in 1959 appearing the same as 1986 isolates, indicating very little change in the genome over time.³⁸  However, sheep-and goatpox viruses will have subtle genetic changes to their genomes over time which can be identified using genome sequencing. In addition, there is evidence for recombination events occurring between strains of capripoxvirus in the field, as has also been seen in vitro, and this could result in changes in host range or virulence.²⁷

As with other poxviruses, sheep- and goatpox are susceptible to sunlight and detergents containing lipid solvents, but in dark environmental conditions, such as contaminated animal sheds, it can persist for many months.^{24, 56} The source of environmental contamination occurs from infected animals shedding the virus. In addition, scabs shed from recovered animals contain large amounts of virus in association with antibody, but it is not known whether these scabs could remain a source of infection; certainly, it is difficult to recover live virus on tissue culture from scab material, but the presence of the virus within type A inclusion protein could protect the virus in the environment although this has not yet been proven.

Epidemiology

Sheep- and goatpox occur in Africa north of the equator, Turkey, the Middle East, many parts of Asia, including Pakistan, Afghanistan, Kazakhstan, Kyrgyzstan, India, Nepal and parts of China and Russia. In Morocco, sheeppox only occurs. New introductions of disease into a country are generally only identified in one of the two host species, i.e. depending on the isolate introduced. For example, goatpox was introduced into Bangladesh from India in 1984, and sheeppox has caused occasional outbreaks in Italy (1983), Greece (1988, 1995, 1996, 1997, 1998, 2000, 2006, 2007, 2013-2017) and Bulgaria (1995, 1996, 2013), having spread from Turkey, probably in illegally imported animals. In 2005 and 2008 outbreaks of goatpox occurred in Vietnam, likely originating from China.¹ In Mongolia, there was an outbreak of sheeppox in 2006-2007 and then goatpox in 2008⁶ and sheeppox and goatpox outbreaks were still occurring between 2013 and 2017. In Russia there have been outbreaks in 2008, 2010, 2011, and 2012 along the regions bordering China.⁴⁶