Capripoxvirus tissue tropism and shedding: A quantitative study in experimentally infected sheep and goats

Abstract

Sheeppox virus and goatpox virus cause systemic disease in sheep and goats that is often associated with high morbidity and high mortality. To increase understanding of the pathogenesis of these diseases, we undertook quantitative time-course studies in sheep and goats following intradermal inoculation of Nigerian sheeppox virus or Indian goatpox virus in their respective homologous hosts. Viremia, determined by virus isolation and real-time PCR, cleared within 2 to 3 weeks post inoculation. Peak shedding of viral DNA and infectious virus in nasal, conjunctival and oral secretions occurred between 10 and 14 days post inoculation, and persisted at low levels for up to an additional 3 to 6 weeks. Although gross lesions developed in multiple organ systems, highest viral titers were detected in skin and in discrete sites within oronasal tissues and gastrointestinal tract. The temporal distribution of infectious virus and viral DNA in tissues suggests an underlying pathogenesis that is similar to smallpox and monkeypox where greatest viral replication occurs in the skin. Our data demonstrate that capripoxvirus infections in sheep and goats provide additional and convenient models which are suitable not only for evaluation of poxvirus-specific vaccine concepts and therapeutics, but also study of poxvirus-host interactions.

Fig. 1

Gross lesions following experimental capripoxvirus infection in sheep and goats. (A) Primary lesion on the lateral thorax of a sheep at DPI 4. The lesion was well-demarcated, thickened, red and 4 to 5 cm in diameter. The black mark encircling the lesion was applied to the skin at DPI 0 to identify the site of inoculation. (B) Multifocal papular exanthem on the lateral chest of a goat at DPI 8. Papules (arrowheads) were well-circumscribed, of variable shape, widely distributed and ranged from several millimeters to 1 to 2 cm in diameter. (C) Multifocal ulcerative enanthem in the soft palate of a goat at DPI 15. Ulcerated lesions (arrowheads) were well-circumscribed and ranged from several millimeters to 1 to 2 cm in diameter. Similar ulcerated lesions were present on the tongue, nasal mucosa or buccal mucosa in sheep or goats from DPI 10. (D) Multifocal nodular pneumonia in a goat at DPI 11. Nodules (arrowheads) were pale or red in color, distributed throughout the lung parenchyma and ranged from several millimeters to 4 to 5 cm in diameter. (E) Multifocal nodular enanthem in the rumen mucosa of a goat at DPI 13. Nodules (arrowheads) were pale in color, ranged from several millimeters to 1 to 2 cm in diameter and were readily apparent from the serosal surface of the organ. (F) Multifocal nodular lesions in the liver of a goat at DPI 13. Nodules (arrowheads) were well-circumscribed, pale in color and ranged from pinpoint to 5 mm in diameter.

Fig. 2

Daily rectal temperatures of sheep and goats infected with Nigerian SPPV or Indian GTPV, respectively. Mean temperatures for all surviving animals during the first three weeks post inoculation are shown, with error bars indicating standard deviations. The upper limit of the normal temperature range (40 °C) is indicated by dashed lines. Note that from DPI 15, data were generated from three sheep and one goat, respectively.

Fig. 3

Viral DNA concentrations in tissues collected from capripoxvirus-infected sheep and goats. Viral DNA concentrations were determined by qPCR and are expressed as log₁₀ viral genome copies per 100 ng of extracted tissue DNA; data are the mean of duplicate reactions with error bars indicating standard deviations. Tissues that were positive by virus isolation are indicated by a plus (+) symbol and an asterisk (^⁎) in x-axis labels denotes that a sample was not collected. Note that the goat DPI 4 inoculation site and nasal mucosa, the goat DPI 6 abomasum and the sheep DPI 8 and 15 contralateral lymph node samples were not assayed by qPCR. Note in addition that all qPCR positive samples with fewer than 10 viral genome copies per 100 ng of extracted DNA could not be accurately quantified and were arbitrarily assigned a value of 0.5 on the y-axis. Abbreviations: LN (lymph node); Normal Skin (skin with no apparent gross pathology); Kidney (kidney cortex).