Close relationship between equine and human molluscum contagiosum virus demonstrated by in situ hybridisation

To determine whether the virus responsible for human molluscum contagiosum (MCV) is the causal agent of a similar disease in horses, in situ hybridisations using cloned fragments of human MCV DNA labelled with digoxigenin were carried out on formalin-fixed biopsy sections of lesions from two horses with molluscum contagiosum-like skin lesions. In both instances there was evidence of specific hybridisation of the labelled probe to target DNA in the sections under high stringency conditions, identified by the development of a deep blue-purple stain in the cytoplasm of cells in the stratum spinosum and stratum granulosum of the lesions and the absence of non-specific hybridisation in adjacent non-lesional areas of the epidermis. These results indicate that on the basis of very close homology of their viral DNA sequences, the causative virus of equine molluscum contagiosum is either identical with, or very closely related to, its human equivalent.