Natural Infection of Goats with Orf (Contagious ecthyma) and Its Diagnosis

Abstract

Background: The Study was intended to evaluate some common diagnostics that could supplement the clinical and histological identification of orf in goats.Methods: Samples from suspected clinical cases of orf (contagious ecthyma) were collected from various organized and unorganized goat herds around Guwahati, Assam. Presumptive diagnosis was based on the typical signs and lesions. For confirmatory diagnosis, various molecular and immunoassays were employed for the detection of orf virus or circulating antibodies.Result: Cutaneous lesions observed included solitary to multifocal erythema, papules, vesicles, pustules and scab stages on the lips, ears, gums, tongue, udder and perineal region. No mortality was observed and the morbidity rate varied between 35-60%. Microscopic lesions in skin biopsies were typically marked by epidermal hyperplasia and ballooning degeneration of the keratinocytes along with other changes. Eosinophilic intracytoplasmic inclusion bodies were demonstrable within keratinocytes only in early papulo-vesicular stages. Dermis was infiltrated with polymorphonuclear and mononuclear cells in varying proportions. Initial screening of samples was done with PCR systems to specifically detect parapoxvirus DNA employing a semi-nested PCR targeted against the partial B2L gene (with reported primers yielding 594 bp and 235 bp amplified products) and an uniplex PCR targeting the whole B2L gene (with reported primer set to yield an amplified product of 1206 bp). Agar gel immuno-diffusion (AGID) failed to develop positive immunoprecipitation with hyperimmune serum against scab lysates; however, counter-immuno-electrophoresis (CIE) showed positive immunoprecipitation in the same samples. For rapid and in situ detection of ORFV antigen in tissues, immunofluorescent and immunoperoxidase techniques were successfully employed both on cryosections and formalin fixed skin biopsies. Immunofluorescent technique on cryosections was found to be easy, rapid and more specific. A dot-ELISA was also developed for successful confirmation of orf virus from clinical samples. For antibody detection in convalescing goats, an indirect-ELISA and a dot-ELISA was also successfully tested to demonstrate for antibody detection in convalescing goats, but protective titres later after infection was not addressed. Monoclonal antibody employed in the assays was found to be specific, sensitive and versatile for virus detection from direct clinical samples. It is contemplated that assays employing hyperimmune sera or detection of circulating antibody against orf virus may have limited diagnostic applications owing to its partial and transient humoral immunity and the inherent property of the virus to modulate and interfere with the host response and evade immune mechanisms.