Experimental orf in susceptible and previously infected sheep
Osman, Omar Abdel Hady
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The clinical responses of susceptible and previously infected sheep to orf infection were similar except in the speed of development. In both, the lesions progressed through the classical pox stages of papule, vesicle, pustule and scab. The duration of each stage was significantly shorter in previously infected sheep than in susceptible sheep. A few of the previously infected sheep (5.3 per cent) were refractory to reinfection.The histological changes associated with orf infection in susceptible and previously infected sheep were assessed. In both, there was a thickening of the epidermis at 30 hours post -scarification. Ballooning degeneration followed in the cells of the upper layers of the epidermis and progressed to form vesicles in four to six days in susceptible sheep and three to four days in previously infected sheep. Inclusion bodies were not observed. Resolution, as manifested by restitution of the normal histological structures, occurred in three to five weeks in susceptible sheep and in less than two weeks in previously infected sheep.Examination by electron microscopy of ultrathin sections of biopsy samples collected at intervals from susceptible and previously infected sheep confirmed the clinical observations and the observed histologi- cal changes revealing evidence of virus multiplication in previously infected sheep as well as in susceptible sheep. In susceptible sheep, the first evidence of virus multiplication was at 30 hours post- scarification whereas in previously infected sheep it was at three days. Virus persisted in skin tissue for 17 days in susceptible sheep and for three to seven days in previously infected sheep. Granular masses believed to be accumulations of new virus constituents were observed in the cytoplasms of the cells of the upper- most layers of the epidermis. They were diffusely distributed in the cytoplasms and were never observed in the nuclei. Immature and complete virus particles appeared in the cytoplasms and cytoplasmic vacuoles of the cells of the uppermost layers of the epidermis and in extracellular spaces. In one section only a few virus particles were observed over the nucleus.Electron microscopy did not permit differentiation between primary and secondary reactions when samples were collected within six days of scarification.Infectious virus was recovered from lesions in susceptible and previously infected sheep. Virus applied to scarified skin of both susceptible and previously infected sheep disappeared completely within 24 hours and samples collected at this time were not infective. Virus reappeared within 30 hours post - scarification in susceptible sheep and at two days in previously infected sheep. The increase in new virus was exponential, the highest titres of infectivity being detected in susceptible and previously infected sheep at four and five days post -scarification respec- tively. The high titres were maintained for more than two weeks in susceptible sheep but in previously infected sheep the plateau phase was short, namely, three days. Disappearance of virus from samples taken from previously infected sheep was very abrupt. The decline phase in susceptible sheep was spread over three weeks.Virus behaviour in cell cultures was also studied. Particles in different stages of development were seen in the cytoplasms of infected cells and were similar to those observed in skin sections. The rate of virus multiplication in cell cultures was similar to the rate of multiplication in susceptible and previously infected sheep but it occurred earlier.