Diagnostic Techniques for porcine Circovirus Type 2 (PCV-2) by Optical and Transmission Electron Microscopy

The porcine circovirus type 2 was first described in Canada in 1991, associated with post-weaning multisystem syndrome of pigs, in animals without clinical signs of disease, and in animals with dermatitis and porcine kidney disease syndrome, reproductive disorders, porcine respiratory complex syndrome, proliferative and necrotizing pneumonia and congenital tremor (Allan and Ellis, 2000; Larochelle et al., 1994).

 But it was in 1974, that Tischer et al. first described the porcine circovirus (PCV- Porcine circovirus), using electron microscopy, a virus morphologically similar to a picornavirus, responsible for persistent infections in pig kidney cultures (PK-15) and without causing cytopathic changes. Subsequently it was demonstrated to be a single-stranded DNA virus, approximately 1.76 Kb, ambisense, circular, covalently closed (Tischer et al., 1982). It is the smaller animal virus described in both sizes in the genome, non-enveloped, with icosahedral symmetry and measuring 15 to 17 nm in diameter. Its replication capability is autonomous in mammalian cells, necessarily occurring during cell replication, since the virus is dependent on proteins produced in S phase of mitosis (Studdert, 1993). Faced with different genotypic and phenotypic characteristics, two types of porcine circovirus are being described, PCV-1, consisting of the contaminant sample cell cultures and considered apathogenic, and PCV-2, represented by the samples detected in sick animals (Allan et al., 1999). The seroconversion occurs in pigs between 3 and 4 weeks, under normal conditions, the antibodies being detected at different stages of breeding (Allan and Ellis, 2000). The viremia takes place between 7 and 16 weeks of age, with low prevalence in matrices. According to Segales and Domingos in 2002, the individual variation in the PCV-2 DNA detection takes place between 5 and 21 weeks, suggesting that some pigs have developed a persistent viremia. The pigs may also maintain PCV-2 in the nasal cavity, regardless of viremia or clinical signs of disease. This observation leads us to ask whether the PCV-2 of the nasal cavity is the result of viral replication in the nasal mucosa or the result of high levels of the pathogen in the environment. The viral presence in nasal swabs, tonsillar, fecal and urinary, bronchial, suggests that transmission can be oralnasal, fecal and urinary (Segales and Domingos, 2002). PCV-2 was also detected in aborted fetuses in late gestation and stillbirths (Lyo et al., 2001). The occurrence of type 2 circovirus was described in several European, Asian and North American countries (Allan and Ellis, 2000). In Brazil, PCV-2 was detected at the beginning of 2000, through viral isolation studies, detection by PCR and serological surveys in swine herds (Ciacci-Zanella et al., 2003). During a study conducted in the state of São Paulo, Bersano et al. (2003) used those same techniques and reported the occurrence of co-infection with PCV-2 and coronavirus. In 2004 Ruiz et al., while searching in 8 Brazilian states the occurrence of porcine circovirus and possible co-infections with PVS (porcine parvovirus), detected in the state of São Paulo, by PCR, a pig reagent for both 4 viruses (PCV-1 / PCV-2), 33 for PCV-1 and 2 for PCV-2, in 146 studied samples. The association between PVC-2 and other pathogens was described in the literature as a circovirosis, an immunosuppressive disease that made swine more vulnerable to other agents capable of causing respiratory and enteric diseases, leading to an increase of losses (Ristow et al., 2000; Moreno et al., 2003). These findings were also reported by Bersano et al., 2008, as related to the presence of PCV-2 in multiple bacterial infections. The circovirosis occurs endemically in technified farms, affecting mainly piglets at the end of the nursery phase and the first month of age. Mortality is usually between 3% and 10%, but it can reach values up to 35%. In Brazil, there are no wide studies to verify the frequency of the disease in many regions. Estimates based on unpublished data indicate a frequency of the disease in 62.05% of nursery and 66.75% of terminations in technified farms, with mortality rates ranging from 2% to 10%. Besides the impact on commercial pig farming, the circovirus is important in xenotransplantation. In several countries this technique has been the solution for the lack of organs (Boneva et al., 2001). The swine is one of the most studied animal species, due to the size of its organs, compatible with that of an adult human. Agents which do not normally infect humans can infect them after xenotransplantation. Despite the xenozoonosis by circovirus being low-risk, in cultures of human cell lines, this pathogen can be infectious (Tucker et al., 2003).

 This study was aimed at investigating the presence of porcine circovirus type 2 (PCV-2) in apparently healthy terminated pigs from slaughterhouses of São Paulo, SP, Brazil, using histopathology and transmission electron microscopy techniques, collaborating with the vision of the National Porcine Agribusiness.