Page 207 - Avian Virology: Current Research and Future Trends

200 | Kibenge et al.

fertilizer (Soos et al., 2008). Backyard chickens may pose a more orthoreoviruses (Slaght et al., 1978) or recombinant viral
significant and immediate threat of disease introduction to resi- proteins such as bacterially expressed σB (Shien et al., 2000),
dent avifauna than intensively managed broiler chickens because bacterially expressed σNS and P 17 (Xie et al., 2010), bacterially
of the high level of biosecurity associated with intensive poultry expressed σNS (Chen et al., 2004), bacterially expressed σB and
production (Soos et al., 2008). σC (Zhang et al., 2007), bacterially expressed σC and σB (Liu
The paucity of data on wild birds including pathogens, their et al., 2002), and σC expressed in methyltropic yeast (Yang et
distribution, and type of interactions with domestic poultry al., 2010) as coating antigens for detecting avian orthoreovirus
means that the role of wild birds and the epizootiology of avian antibody level in serum. ELISAs using recombinant viral proteins
reoviruses remain unknown. as coating antigens have lower non-specific binding reactions, a
higher correlation with virus neutralization tests, and a higher
ability to distinguish virus neutralization positive and negative
Diagnosis sera in aged birds compared with conventional whole virus
ELISA (Shien et al., 2000). Antigen capture ELISA uses coating
Laboratory diagnosis of avian orthoreovirus anti-orthoreovirus antibodies to detect avian orthoreoviruses.
Diagnosis of avian orthoreovirus diseases is difficult because they The antigen-capture ELISA using monoclonal antibodies against
are clinically indistinguishable from a number of other common a single protein of the orthoreovirus such as σC (Hsu et al., 2006)
disease conditions such as adenovirus infection and bacterial and σA (Pai et al., 2003) is usually less sensitive than others; how-
and mycoplasma synovitis (Stott, 1999). Therefore, laboratory ever, non-specific reactions are significantly reduced (Liu et al.,
diagnostic tests for rapid detection of avian orthoreoviruses are 2002).
needed to enable early diagnosis and prevent spread of the prob- The quantitation of avian orthoreovirus antibodies in chicken
lem to avoid economic losses. serum samples provides an important tool for diagnosis, permits
Virus isolation and identification in cell cultures, sero- the assessment of antibody status in a chicken flock including
logical methods and histopathology were the most common breeders, and predicts more precisely the proper time for vaccina-
traditional approaches for diagnosis of avian orthoreovirus dis- tion by measuring maternal antibody titres (Shien et al., 2000).
eases (Robertson and Wilcox, 1986). Although virus isolation However, avian orthoreoviruses exhibit considerable antigenic
and identification in cell culture was a reliable way for detection of or genetic variations (Rekik et al., 1990; Wu et al., 1994), and
an avian orthoreovirus infection, the procedure is laborious and considerable cross-reactions exist among heterologous types
time consuming (van der Heide et al., 1976; Wood et al., 1986; (Robertson and Wilcox, 1986), making diagnosis by serological
Meanger et al., 1995; Caterina et al., 2004), usually taking more means difficult. In addition, serology is often plagued by non-
than 7 days and possibly requiring SPF embryonated eggs to specific reactions and problems with reagent cross-reaction
prepare sensitive primary cell cultures (Zhang et al., 2006). Dif- (Caterina et al., 2004). When using commercial ELISA kits to
ferent culture systems available for orthoreovirus isolation have detect orthoreovirus antibodies, it is unknown whether or not
been discussed above. Samples intended for virus isolation can be that test reveals seroconversion against all variants and serotypes
stored at 4°C in transport medium for several days or at –20°C or of orthoreovirus. Therefore, a negative result obtained in the
–70°C for longer periods (Hollmén and Docherty, 2007). ELISA does not exclude the presence of antibodies to orthoreo-
virus creating uncertainty in the interpretation of the serological
Immunodiagnostic methods results. Similarly, a positive serology does not always indicate the
A variety of immunodiagnostic methods have been developed for presence of a pathogenic orthoreovirus in the sample as there are
the identification of avian orthoreoviruses or antibodies against many non-pathogenic orthoreovirus isolates. Lastly, there is a
them including the agar-gel precipitation test (Olson and Weiss, possibility that the serological tests which have been developed
1972), plaque neutralization test (Ide and Dewitt, 1979), direct for use in domestic poultry are not valid for diagnostic purposes
immunofluorescence staining technique (Jones and Onunkwo, in wild bird species.
1978), indirect immunofluorescence assay (Ide, 1982), micro-
titre serum neutralization test (Robertson and Wilcox, 1984), Molecular diagnostic methods
immunoperoxidase technique using avidin–biotin–peroxidase Molecular methods for detecting enteric viruses offer several
complex (ABC) (Tang and Fletcher, 1987), virus neutraliza- advantages over traditional methods (Pantin-Jackwood et al.,
tion test (Giambrone and Solano, 1988), Western blotting 2008). Thus, the detection of viral RNA from clinical samples
(Endo-Munoz, 1990), monoclonal antibody based indirect by conventional reverse transcriptase polymerase chain reaction
immunoperoxidase procedure (Li et al., 1996), immune-dot assay (RT-PCR) remains the first choice in early diagnosis (Zhang et
(Liu et al., 2000), dot immunobinding assay (Georgieva et al., al., 2006) and has been commonly used to detect avian orthoreo-
2002), and many enzyme-linked immunosorbent assay (ELISA) viruses in clinical samples. One main advantage of conventional
techniques (Slaght et al., 1978; Shien et al., 2000; Liu et al., 2002; RT-PCR over real-time RT-PCR is that the identity of the
Pai et al., 2003; Chen et al., 2004; Hsu et al., 2006; Zhang et al., amplicons (viruses) can be confirmed and further character-
2007; Xie et al., 2010; Yang et al., 2010). ized by sequencing them for accurate identification. Molecular
ELISA is a commercially available, sensitive, reproducible approaches to identification of avian orthoreoviruses in clinical
and efficient diagnostic tool (Slaght et al., 1978). It uses whole samples have been described by several authors. These include