Patogênese experimental de parapoxvírus bovino 2 em bezerros, e poxvírus suíno como modelo para inativação viral em carcaças

Abstract

Animal poxviruses present sanitary and economic importance as disease agents, but they have also been used as vaccine vectors, immunostimulants and as models for other viruses. This Thesis is composed of two studies involving Poxviridae viruses, with different approaches. In the first study, the pathogenesis of bovine Parapoxvirus 2 (Pseudocowpox virus, PCPV) infection was studied in calves. In the second study, swine poxvirus (Swine poxvirus, SPV) was used as a model for viral inactivation in carcasses. In study 1, 3-4 month old calves inoculated intranasally (n=6) or in the nasal plane (n=2) with a PCPV isolate replicated and shed virus until day 13 post-inoculation (pi) , but did not develop local or systemic signals until day 20pi. On days 28-34pi, however, seven (7/8) inoculated calves developed an asynchronous clinical course, with development of few to multiple papulopustular, erosive-fibrinous, and scaly lesions on the snout, in some cases extending to the lips and gum. In some animals, the lesions coalesced, forming extensive fibrinotic/necrotic and scaly plaques covering almost the entire snout. The clinical course lasted from 8 to 15 days and progressively decreased after day 42pi. Viral DNA was detected by PCR in swabs collected from lesions of six animals between days 34 and 42pi. Histological examination of fragments collected from the snout lesions of two affected calves (day 36pi) revealed marked epidermal hyperplasia and severe orthokeratotic and parakeratotic hyperkeratosis covered by thick crusts. The epidermis showed multifocal areas of coalescing necrosis of keratinocytes and mild multifocal vacuolar degeneration. Occasional micro-abscesses were also observed in the epidermis. Sera from calves inoculated at 50pi showed partial neutralization of the virus at low dilutions (1:5, 1:10 and 1:20), indicating seroconversion. In study 2, the viability of SPV in swine carcasses was investigated by the AGB (above ground burial) method, as a model for the African swine fever virus (ASFV). For this, SPV was inoculated intrafemorally in 90 carcasses of adult pigs. Bone marrow samples were collected and tested periodically over 12 months. In the in vitro study, inoculate of Senecavirus A (SVA), bovine viral diarrhea virus (BVDV) and SPV were mixed with bone marrow material and kept at 21-23°C for 30 days. In both studies, virus viability was assessed by viral isolation, while the presence of viral nucleic acid was assessed by qPCR. In the field study, SPV remained viable in 11 (55%) bone marrow samples collected on day 7 alone; later only viral DNA was detected. Complete viral inactivation is estimated to have occurred around day 11pi. The in vitro tests revealed a variable resistance of the studied viruses. Viability was estimated at 80, 118 and 28 days for SPV, SVA and BVDV, respectively. Thus, in the first study, a late and severe clinical course associated with PCPV inoculation in calves was observed, uncommon in experimental infections by bovine parapoxviruses. In the second study, the AGB technique was shown to be effective in the short-term inactivation of SPV. Thus, considering that the rate of inactivation of SPV is comparable to that of ASFV in field studies, this technique can be explored for future studies with ASFV. Furthermore, this study contributes to the understanding of the kinetics of virus inactivation under specific conditions, which is critical to design and apply countermeasures in case of biosecurity breach in animal mortality management sites.