57       Poxviridae

 

The family Poxviridae contains the largest viruses which cause disease in domestic animals. The family is divided into two subfamilies, Chordopoxvirinae, the poxviruses of vertebrates, and Entomopoxvirinae, the poxviruses of insects. Genetic recombination within genera results in extensive serological cross-reactions and cross-protection. These double-stranded DNA viruses replicate in the cytoplasm and are stable in the environment under dry conditions.

Infections with poxviruses usually result in vesicular skin lesions (Table 57.1). Smallpox, caused by variola virus, was formerly a human disease of major international significance. The use of vaccinia virus for the prevention of smallpox, first introduced by Jenner in the late eighteenth century, eventually led to the global eradication of this highly contagious disease at the close of the twentieth century.

Table 57.1 Members of the Poxviridae of veterinary significance.

Virus	Genus	Host species	Significance of infection
Vaccinia virus	Orthopoxvirus	Wide host range	Infections in sheep, water buffaloes, rabbits, cattle, horses and humans. Used as a recombinant virus vector for rabies vaccine
Cowpox virus	Orthopoxvirus	Rodents, cats, cattle	Species of small rodents are the likely reservoir hosts. Cats are the principal incidental hosts; infection results in skin lesions. Rare cause of teat lesions in cattle. Transmissible to humans
Uasin gishu virus	Orthopoxvirus	Unknown wildlife reservoir, horses	Rare disease, reported in Kenya and neighbouring African countries. Causes papilloma-like skin lesions in horses
Camelpox virus	Orthopoxvirus	Camel	Widely distributed in Asia and Africa. Causes systemic infection with typical pox lesions; severe infection in young camels
Pseudocowpox virus	Parapoxvirus	Cattle	Common cause of teat lesions in milking cows; causes milker's nodule in humans
Bovine papular stomatitis virus	Parapoxvirus	Cattle	Produces papular lesions on the muzzle and in the oral cavity of young cattle. Transmissible to humans
Orf virus	Parapoxvirus	Sheep, goats	Primarily affects young lambs; causes proliferative lesions on the muzzle and lips. Transmissible to humans
Sheeppox/goatpox virus	Capripoxvirus	Sheep, goats	Endemic in Africa, Middle East and India. Causes generalized infection with characteristic skin lesions and variable mortality
Lumpy skin disease virus	Capripoxvirus	Cattle	Endemic in Africa. Causes generalized infection with severe lesions and variable mortality
Swinepox virus	Suipoxvirus	Pigs	Causes mild skin disease. Occurs worldwide. Transmitted by the pig louse (Haematopinus suis)
Fowlpox virus	Avipoxvirus	Chickens, turkeys	Causes lesions on the head and on the oral mucous membrane. Occurs worldwide. Transmitted by biting arthropods
Myxoma virus	Leporipoxvirus	Rabbits	Causes mild disease in cottontail rabbits, the natural host, and severe disease in European rabbits (myxomatosis). Introduced into Europe, Australia and Chile as a biological control measure
Squirrelpox virus	Unassigned	Red and grey squirrels	Important factor in decline of native red squirrels (Sciurus vulgaris) in Great Britain; carried by grey squirrel (Sciurus carolinensis) introduced from North America
Nile crocodilepox virus	Crocodylidpoxvirus	Nile crocodile	Cause of skin lesions in wild and farmed crocodiles

Clinical infections

Transmission of poxviruses can occur by aerosols, by direct contact, by mechanical transmission through arthropods and through fomites. Skin lesions are the principal feature of these infections. Several virus-encoded proteins are released from infected cells, including a homologue of epidermal growth factor which stimulates cell proliferation. Typically, pox lesions begin as macules and progress through papules, vesicles and pustules to scabs which detach, leaving a scar. In generalized infections there is a cell-associated viraemia and recovered animals have solid immunity. Some localized pox infections may induce transient immunity and reinfection can occur.

Three closely related parapoxviruses, namely pseudocowpox virus, bovine papular stomatitis virus and orf virus, infect ruminants. These viruses are transmissible to humans, producing lesions which are clinically similar. Moreover, the three viruses are morphologically indistinguishable and identification of the causal agent relies on nucleic acid analysis.

Capripoxviruses are economically important viruses producing generalized infections with significant mortality in domestic ruminants. Sheeppox virus, goatpox virus and lumpy skin disease virus are closely related and share a group-specific structural protein (p32), which allows the same vaccine to be used against each virus.

Many avian species are susceptible to infection with members of the genus Avipoxvirus. Although antigenic relationships exist among avian poxviruses, this relatedness is variable. Virus species within the genus, named in accordance with their affinity for particular host species, include fowlpox virus, canarypox virus, pigeonpox virus and turkeypox virus. The type species of the genus is fowlpox virus.

Diagnosis

Diagnosis can often be made solely on clinical grounds. Skin biopsies or postmortem specimens may be used for laboratory confirmation. Eosinophilic intracytoplasmic inclusions may be demonstrable histologically in epidermal cells. Electron microscopy can be used for the rapid identification of poxvirus particles in material from lesions. Parapoxviruses can be readily distinguished from members of the other genera. For some species, virus may be isolated in testis or kidney cell monolayers. An antigen-trapping ELISA has been developed for the detection of capripoxvirus antigen. Protocols for PCR assays for the detection of viral DNA are also available.

Control

Vaccines are available for a number of poxviruses and control is based on annual vaccination. Inactivated vaccines are less effective than modified live vaccines because cell-mediated immunity is the predominant protective response. A recombinant vaccine providing protection against lumpy skin disease and peste des petits ruminants has been developed. In flocks endemically infected with orf virus, control is based on the use of a fully virulent live vaccine derived from scab material or cell culture. Ewes should be vaccinated by scarification in the axilla at least eight weeks before lambing. Close to lambing, ewes must be moved to a new grazing area in order to minimize exposure of lambs to infectious vaccinal scab material.