Results
Full-length CPV VP2 sequences were
amplified from 79 faecal samples collected at 20 veterinary practices in
5 Australian states (Figure 1).
The median age at diagnosis was 4
months (range 1 to 96 months). Of dogs for which complete vaccination
data were available, 49.1% (26/53) were incompletely vaccinated, 47.2%
(25/53) were unvaccinated, and 3.77% (2/53) were completely vaccinated.
The two dogs that developed parvoviral enteritis despite being
completely vaccinated were a 20-month-old female Cavoodle that developed
parvoviral enteritis 8 months after its first annual booster
vaccination, and a 3-year-old male Staffordshire Bull Terrier that
developed disease 5 months after receiving an annual booster vaccination
(Table 2).
There were 44 dogs for which outcomes were known, of which 52.3%
recovered (23/44), 16 (36.4%) died despite treatment and 5 (11.4%)
were euthanised without treatment. CPV VP2 sequence analysis detected
CPV-2b (426D) in 43/79 (54.4%) samples, CPV-2a (426N) in 34/79 (43.1%)
and CPV-2 in 2/79 (2.5%). The two isolates identified as CPV-2 were
from dogs with unknown vaccination histories.
On phylogenetic analysis the 79 CPV strains fell into six clades (Figure
2). The largest clade (Figure 2, Clade 1) comprised CPV-2b viruses. In
addition to 426-Asp, most viruses in Clade 1 contain one novel (5-Gly)
and two well-recognised (267-Tyr and 297-Ala) non-synonymous mutations
compared to “classical” strains of CPV-2 (e.g. strain “N” and strain
“CPV-b”), which are typically characterised by 267-Phe, 297-Phe and
5-Ala (Table 3). The majority of Clade 1 viruses originate from dogs in
adjacent rural regions in central west and far west NSW (Figure 1, Table
2). Clade 1 also contains several viruses from dogs from Western
Australia, all of which had a novel mutation 262-Thr compared to 262-Ala
in classical CPV-2.
Clade 2, the second largest clade is also the most heterogeneous,
comprising 16 CPV-2a viruses, and 5 CPV-2b viruses (Figure 1) and
contains viruses from all 5 States sampled in the study. In addition to
the 5-Gly, 267-Tyr and 297-Tyr aa residues described for Clade 1
viruses, unique aa mutations 564-Asn and 568-Ala were present all but
one of the viruses in this clade (Table 2).
Clade 3 contains most of the viruses sequenced from Victoria in 2015.
All of the viruses in this clade contain both a novel (21-Ala) and a
well-recognised (324-Ile) mutation compared to “classical” CPV-2
(324-Tyr, 21-Thr). With the exception of one CPV-2b (426-Asp) virus,
this clade comprises only CPV-2a viruses (426-Asn).
Viruses in Clade 4, all collected in 2019, are characterised by a novel
mutation (321-Lys) compared to “classical” CPV-2 (321-Asn). Three,
from dogs in Northwest NSW have identical VP2 sequences to an attenuated
CPV-2b vaccine strain (“SAH”), except for residue 570-Lys, which is
glutamate in the vaccine strain. The dogs were from a large breeding
establishment and two had been vaccinated with the same SAH strain,
while one, an unvaccinated puppy, died at 7 weeks of age after an 8-day
history of lethargy, dehydration, vomiting and haemorrhagic diarrhoea.
Similarly, one VP2 sequence from a dog in Western Australia differed by
only one aa (413-Asn) to the SAH vaccine strain (413-Asp) (Figure 1,
Table 2). The vaccination history of this dog was unknown.
Clade 5 contains CPV-2a viruses collected from dogs in metropolitan
Sydney between 2016 and 2018, which, with the exception of 297-Ala, did
not harbour the 5-Gly, 21-Ala and 267-Tyr mutations commonly present in
other Australian CPV-2a viruses. Clade 6 contained two CPV-2 VP2
sequences from dogs with unknown vaccination history.