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place these three viruses in the proposed new virus family Cir- or plasma samples from a patient with human immunodeficiency
coviridae which was formalized by the International Committee virus and 3 patients with kidney transplants. Phan et al. (2012)
on Taxonomy of Viruses (ICTV) in 1995 (Lukert et al., 1995). detected a third gyrovirus (HGyV3) in a child with diarrhoea and
This classification was not without controversies. Noteborn and in chicken meat in Chile, while Chu et al. (2012) found HGyV4
Koch (1995) argued that CAV belonged in a different virus family in human stool samples and chicken meat. Since then, (H)GyV5,
than PCV and PBFDV based on the lack of sequence similarities -6 (Gia Phan et al., 2013), GyV7 (Zhang, W. et al., 2014), and
and antigenic determinants as well as structural and transcrip- GyV9 (Phan et al., 2015) were reported in Tunisia, the USA, and
tional differences between CAV, on the one hand, and PCV and France, respectively. GyV8 was isolated from a northern fulmar
PBFDV, on the other hand. Todd et al. (1991) had noted similar (Fulmarus glacialis) and has thus far not been found in humans
differences between these two groups of viruses, suggesting that or chickens (Li et al., 2015). Smuts (2014) reported the presence
it would be inadvisable to place the 3 viruses in the same family. of CAV, AGV2/HGyV and HGyV3 in stool samples from healthy
The isolation and subsequent sequencing of torque teno children and children with diarrhoea. Although some of these
viruses (TTV), torque teno mini virus (TTMV) and torque teno viruses were found in stool samples from patients with diarrhoea,
midi virus (TTMDV) from humans pointed to similarities in there is no evidence that the HGyVs are actually infecting humans.
genomic organization between CAV and TTV (Davidson and It is certainly possible that these ‘passenger’ viruses are ingested
Shulman, 2008; Biagini, 2009; Hino and Prasetyo, 2009; Rosa- with meat products from different species including chickens.
rio et al., 2012). Biagini (2015) proposed including TTV and A simplified phylogenetic tree shows the genetic relatedness of
similar viruses, including CAV, in the new family of Anelloviridae. CAV, AGV2 and HGyV (Fig. 9.2).
This was officially approved by the ICTV in 2015 (Adams et al., Several chicken cyclovirus and chicken circovirus sequences
2016) with a total of 12 genera including Gyrovirus, with CAV as were found in chicken meat obtained from a market in Maiduguri
the only recognized species. ICTV also recognized two genera (Nigeria) (Li et al., 2010). Subsequent full-length sequencing
within the Circoviridae: Circovirus and Cyclovirus (ictvonline. showed that the two cycloviruses were basically identical, while
org/virusTaxonomy; accessed 9 November 2016). Fig. 9.1 shows the circoviruses were closely related to pigeon circoviruses (Li et
the main differences in genomic structure between circovirus, al., 2011). The relevance of these findings is not clear. Likewise,
cyclovirus, gyrovirus, tornovirus, alphatorquevirus (TTV), TTV-like viruses have been occasionally identified in chicken
betatorqueviruses (TTMV) and gammatorqueviruses (TTMDV) sera. Leary et al. (1999), using primers for a highly conserved 242
(Rosario et al., 2012). Table 9.1 summarizes the current status of base sequence in the untranslated region found 4/21 sera samples
these single-stranded, circular DNA viruses and the presence of from chickens positive for this fragment. A search of DNA data
these viruses in chickens. bases by Casey and Schat (unpublished data, 2016) showed that
The main emphasis of this chapter will be on CAV, but I will this fragment did not show any homology with CAV or torque
briefly discuss the other single-stranded, circular DNA viruses teno sus virus (TTSuV). The latter was included to ensure that
detected in chickens. When appropriate I will compare character- the presence could not be related to vaccination of chickens
istics of these viruses with CAV in other sections of this chapter. with MD vaccines using trypsin that may be contaminated with
A CAV-like virus, avian gyrovirus 2 (AGV2), has been described TTSuV (Teixeira et al., 2011). Kulcsar et al. (2010) found 5/13
in Brazil, South Africa, The Netherlands, China, Hong Kong and vaccines for Newcastle disease contaminated with TTV. Sequence
the USA (Rijsewijk et al., 2011; Chu et al., 2012; dos Santos et analysis suggested that the contaminants belonged to the swine
al., 2012; Abolnik and Wandrag, 2014; Zhang, W. et al., 2014; TTV genogroup2, but the primers were located in a highly con-
Ye et al., 2015; Yao et al., 2016). Based on sequence information served genomic region. TTV-like viruses have also been reported
there may be several genotypes of AGV2 (dos Santos et al., 2012) in backyard chicken flocks in Iran (Bouzari and Salmanizadeh,
and perhaps more than one species (Zhang, W. et al., 2014). 2015; Bouzari and Shaykh Baygloo, 2013) but the importance of
The importance of AGV2 for the poultry industry is not clear. these findings is not clear at this time. A study in Brazil failed to
In some instances, AGV2 sequences were detected in brains of find evidence of TTV in chicken plasma samples (Catroxo et al.,
chickens with neurological signs (dos Santos et al., 2012; Abolnik 2008).
and Wandrag, 2014), but experimental infections have not been
conducted. A concern is that several vaccines produced in SPF Morphology
chicken embryos or chicken embryo fibroblasts (CEF) were con- CAV virions consist of non-enveloped, icosahedral particles vary-
taminated with AGV2 (Varela et al., 2014). ing in size from 19 to 26.5 nm. Depending on the publication, the
Interestingly, a gyrovirus with similarities to CAV and AGV2 mean diameter of the particles has been reported as 19–21 nm
was isolated from skin swabs from healthy humans in France (Sau- (Goryo et al., 1987b; Imai et al., 1991), 23.5 nm (Todd et al.,
vage et al., 2011). This virus, named human gyrovirus (HGyV) is 1990), 25 nm (Gelderblom et al., 1989), and 26.5 nm (McNulty
closely related to AGV2. HGyV differs from AGV2 only by 3–7% et al., 1990a; Todd et al., 1991). This variation could be a result
in the amino acid sequence of VP1, 2, and 3. Shortly afterwards, of differences in calibration of the equipment and/or staining
the presence of HGyV was demonstrated in human stool samples techniques. For example, Todd et al. (1991) compared negative
and in chicken meat in Hong Kong (Chu et al., 2012) and in blood staining with 2% uranyl acetate, pH 4.1 (UA) and 2% phospho-
samples from healthy donors in France (Biagini et al., 2013). In tungstic acid, pH 7.6 (PTA). The size of the virus particles was
Italy, Maggi et al. (2012) reported the presence of HGyV in blood estimated to be 26.5 ± 1.2 nm using UA versus 21.7 ± 1.4 nm with