Occurrence of CPV-2, CAV-1 and co-infection in dogs with enteritis
During the study period, a total of 216 faecal samples were collected from the dogs with gastroenteritis. The diagnosis of CPV-2 and CAV-1 infections was carried out by PCR as evident by the presence of expected 631 bp and 508 bp product in agarose gel under UV transillumination, respectively (Fig 1 and Fig 2). Out of 216 samples, 90 (41.67%), 12 (5.56%) and 26 (12.04%) samples were found positive for CPV-2, CAV-1 and co-infection (both CPV-2 and CAV-1), respectively. It is very difficult to precisely diagnose the CPV-2 and CAV-1 infections solely based on their clinical signs as most of the infectious and non-infectious causes of gastroenteritis produce clinical signs that are overlapping in nature. The present study findings are in accordance with earlier studies as reported from different parts of India
(Phukan et al., 2010; Srinivas et al., 2013; Thomas et al., 2014; Behera et al., 2015). The high occurrence of CPV-2 infection might be due to easy transmission of infection by faeco-oral route, resistant nature of the virus to commonly available disinfectants and persistence of virus in the environment for longer times in subtropical humid climatic condition. Relatively lower occurrence of CAV-1 in the present study might be either due to less incidence and low endemicity of the diseases or low rate of faecal secretion as the CAV-1 virus prefers to reside in the kidneys and is excreted in the urine (
Greene, 2012).
Age wise occurrence of CPV-2, CAV-1 and co-infection
The occurrence of CPV-2 infection was found highest in the dogs belonging to the age group 0-3 months (44.45%) and samples were found negative from the faecal samples collected from dogs of the age group >1 year. Similarly, the occurrence of CAV-1 infection was highest in the pups of 0-3 months of age and faecal samples collected from 6 months to one year old dogs were found negative. In contrary to the above, the occurrence of co-infection with CPV-2 and CAV-1 was found to be highest in the age group 3-6 months and found nil in the age group 6 months-1 year (Table 2). The CPV-2 and CAV-1 infections are most frequently seen in young animals less than one year old, but, unvaccinated dogs of all ages can be affected (
Greene, 2012;
Thomas et al., 2014; Duque-Garcia et al., 2017). The CPV-2 replicate in cells (myocardiocytes, small intestinal crypt epithelial cells, lymphopoietic tissue and bone marrow) which are under rapidly dividing stage and young animals will be having more number of such type of cells in comparison to adult animals
(Mylonakis et al., 2016). The CAV-1 replicates in hepatic parenchymal cells and vascular endothelial cells
(Decaro et al., 2007). The presence of maternal antibodies plays a significant role to protect dogs from infection. The young puppies are generally protected from CPV-2 and CAV-1 infection by maternally-derived antibodies but susceptibility increases as maternally acquired antibodies starts waning (
Greene, 2012;
Decaro et al., 2004; Thomas et al., 2017b). It has been reported that dogs under six months of age have higher risk of CPV-2 and CAV-1 infections and the adult dogs are more resistant to their infection due to age related reduction in susceptibility and the presence of specific immunity derived from vaccination or previous infections (
Decaro and Buonavoglia, 2012;
Greene, 2012;
Duque-Garcia et al., 2017).
Sex wise occurrence of CPV-2, CAV-1 and co-infection
The occurrence of CPV-2 infection was found to be slightly more in female dogs (51.11%) as compared to male dogs (48.89%) but no such difference was noticed with respect to CAV-1 infection. The occurrence of co-infection with CPV-2 and CAV-1 was found more in male (69.23%) than female dogs (30.77%). There is no sex predilection for CPV-2 and CAV-1 infection in dogs (
Greene, 2012;
Thomas et al., 2014). It is speculated that male animals usually travel more than female and thus they are more susceptible to field infections
(Khan et al., 2006).
Breed wise occurrence of CPV-2, CAV-1 and co-infection
The occurrence of CPV-2 infection was found highest in Labrador Retriever breed (35.56%) and no faecal samples were found positive in Cocker Spaniel breed. The occurrence of CAV-1 infection was also found highest in Labrador Retriever breed (50.0%) whereas, no faecal samples were found positive in Spitz, German Shepherd (GSD), Dobermann, American Bully, crossbred and Cocker Spaniel breeds. The occurrence of co-infection with CPV-2 and CAV-1 was found to be highest in non-descript breed (30.77%) (Table 3). Labrador is the most reared breed in this region and this may be the reason for relatively high percentage of positive cases observed in Labrador dogs. Similar findings have been reported by previous studies on epidemiological aspects of CPV-2 infection in dogs
(Thomas et al., 2014; Behera et al., 2015; Roy et al., 2018). Earlier workers observed no breed or sex susceptibility to CPV-2 and CAV-1 infections. CPV-2 and CAV-1 infections can be seen in dogs of any breed, age, or sex, but puppies below 6 months of age are highly susceptible (
Prittie, 2004;
Greene, 2012). The maximum occurrence of co-infection with CPV-2 and CAV-1 in non-descript dogs may be because of lack of vaccination awareness among the pet owners of the region.
Occurrence of CPV-2, CAV-1 and co-infection based on vaccination status
The occurrence of CPV-2, CAV-1 and co-infection with both the viruses was found to be highest in unvaccinated dogs (68.88%, 83.33% and 53.84%, respectively) and was least in the dogs that received three doses of vaccine (Table 4). Occurrence of infection in immunized dogs is probably due to generation of inadequate antibody response in presence of high level of resident maternal antibody or differences in genetic make-up of viruses between vaccine strain and field strain from natural disease
(Maya et al., 2013; Mittal et al., 2014; Miranda and Thompson, 2016). Such differences need to be investigated through molecular epidemiology and extensive characterization of both field and vaccine strains of viruses in order to reveal finer details.