Canine parvo virus (CPV) causes most dangerous and life threatening disease to puppies between weaning and six months of age. Parvoviruses are small, enveloped, single stranded DNA viruses that are known to cause disease in a variety of mammalian species (Carr et al., 1997).
The virus was first identified in 1978 in USA and was designated as CPV type 2 (CPV-2) for differentiating it from CPV-1, a previously recognized parvovirus of dogs known as minute virus of canines (Binn et al., 1970).
After its emergence, CPV-2 has spread globally and now it is endemic in most populations of domestic and wild canids (Parrish et al., 1988).
Analysis of CPV isolates by monoclonal antibodies and restriction enzymes have shown that a new antigenic strain, designated CPV type 2a (CPV-2a), became widespread around 1979 and it replaced the original strain during 1980 to 1981 in the USA (Parrish et al., 1985).
Later, examination of canine isolates identified another antigenic variant, designated CPV type 2b (CPV- 2b) that emerged around 1984 and after 1986 replaced CPV-2a in many parts of the USA (Parrish et al., 1991).
Currently, the antigenic variants have completely replaced the original type 2, which is still used in most commercial vaccines and are variously distributed in the canine population worldwide. The disease is endemic in India (Behera et al., 2015).
Retrospective analysis of canine parvoviral enteritis
Retrospective data on CPVE was collected from the VCC, VC&RI, Namakkal for the period from August, 2017 to July, 2019. Collected data had been analysed and graphical representation of gender, age and breed wise prevalence of CPVE was done.
1) Age-wise representation of canine parvoviral enteritis
Age-wise representation of CPVE revealed that higher incidence (72.66%) was recorded in less than six months age group (Fig 1). This finding is in corresponding with Hoskins, (1997)
, who reported that puppies between 6 weeks and 6 months of age appear to be more susceptible. For the first few weeks of life, puppies are protected against infection by maternally derived antibody (assuming the bitch has antibodies). However, maternal antibody to parvovirus has a half-life of approximately 10 days and as their maternal antibody titers decline, puppies become susceptible to infection (O’Brien, 1994)
. Declining of maternal antibody level after 3 months age in dogs might be one of the predisposing factors, which make them more prone to CPV infection (Parrish et al., 1988).
The higher incidence of CPVE below 6 months might be due to the affinity of the virus for rapidly multiplying intestinal crypt cells in weaning pups with higher mitotic index due to changes in bacterial flora as well as in the diet due to weaning (Prittie, 2004)
2) Sex-wise representation of canine parvoviral enteritis
Fig 1: Age-wise representation of canine parvoviral enteritis.
In this study, majority of the dogs represented with clinical signs of CPVE were of males (73.20%) rather than females (26.80%) which is in concurrence with the findings of Gombac et al., (2008)
and Thomas et al., (2014).
They reported a higher incidence of CPV in male dogs (78.26%) (Fig 2). The reason for high representation of males might be due to more chance of exposure of them to virus load due to behavioral characteristics mainly roaming and territorial behavior which also well explained by Deka et al., (2013).
Preference of rearing males than females might be another probable reason for high representation of CPVE.
3) Breed-wise representation of canine parvoviral enteritis
Fig 2: Sex-wise representation of canine parvoviral enteritis.
Highest representation (55.29%) of CPVE was noticed in non-descript dogs than the pure breed dogs including Germen Shepherds, Doberman, Spitz, etc
(Fig 3). Shukla et al., (2009)
and Joshi et al., (2000)
reported higher CPVE incidence of 56.9% and 27.23% in non-descript dogs than pure breeds, respectively. Higher prevalence of CPVE in non-descriptors might be due to population density making their close proximity to spread the infection or following poor vaccination schedule due to lack of awareness in public (Behera et al., 2015).
Fig 3: Breed-wise representation of canine parvoviral enteritis.
Incidence of CPVE in Spitz, Germen Shepherd, Labrador, Dobermann were of 9.3%, 7.3%, 6.8% and 4.4% respectively. Spitz is the most common toy breed reared in the reported geographical area next to non-descripts due to its availability and easy maintenance as pet or companion animal compared to other breeds. This might be the reason for high representation of CPVE in Spitz following non-descripts. Behera et al., (2015)
reported that German Shepherd and Labrador Retriever were found to be more susceptible with the incidences of 17.24% and 10.34%, respectively and same pattern of incidence was also noticed in this study. Houston et al., (1996)
reported that certain breeds have been shown to be at increased risk for severe CPV enteritis, including the Rottweilers, Doberman Pinscher, Labrador Retriever and German Shepherd and the reasons for breed susceptibility are unclear. There were significant alterations in the haemato-biochemical parameters in diseased vaccinated dogs (Hafid, 2019)
. Lower incidence of CPVE in pure bred in this region might be due to 1) only few pet owners are fascinated towards the rearing of pure bred dogs, 2) more maintenance cost and 3) less adoptability of pure bred dogs to the stressful tropical climatic conditions and high susceptibility to diseases.
Prevalence of CPVE in vaccinated and unvaccinated dogs
Vaccination of colostrum recipient puppies in early part of life preferably at 42 days of age and colostrum deprived puppies at 28 days of age with first dose of live attenuated canine parvoviral vaccine, boosters at 21 days interval up to 16 weeks of age, regular annual booster up to three years followed by boosters once in three years will protect them against CPVE (Greene and Decaro, 2012)
. Commercially available vaccines contain live attenuated canine parvovirus, canine distemper, parainfluenza virus and inactivated Leptospira antigens and canine adenovirus. Immunization of puppies usually started at 42 days of age. In this study, more than 95% of dogs did not receive complete recommended vaccination protocol (Fig 4). Probable reasons for the occurrence of CPVE in vaccinated dogs are, 1) administration of primary dose alone, 2) delayed booster vaccination, 3) lack of awareness on vaccination and 4) affordability of the pet owners.
Temporal distribution pattern of CPVE
Fig 4: Prevalence of CPVE in vaccinated and unvaccinated dogs.
Temporal analysis of CPVE (2017-19) in Namakkal, Tamil Nadu revealed the epidemic started at the last week of April, peaks in June and ends in August and second peak was noticed at November month (Fig 5). Kokilapriya et al., (2017)
also reported higher prevalence during South-West monsoon (July, 16.3% and August, 12.3%) and North-East monsoon (November, 12.7% and December, 16.3%) and lower during summer season (March, 4.1%, April, 4.7% and May, 4.8%). Dogs were three times more likely to be admitted with CPV enteritis in July, August and September months compared with the rest of the year (Houston et al., 1996).
Confirmation of CPV infection
Fig 5: Temporal distribution pattern of CPVE.
Collected faecal samples (123 numbers) were subjected to PCR for the confirmation using H primer as per the method described by Buonavoglia et al., (2001).
The “H” primer was selected to amplify a large fragment of the capsid protein encoding gene of CPV, encompassing at least six or seven informative amino acids responsible for important biological properties of the virus (Parker and Parrish, 1997)
. Hence, the same primer was used to identify the presence of CPV infection in the selected cases of CPVE. Out of 123, 87 animals (70.73%) were confirmed for CPVE by the detection PCR product at 630 bp on gel electrophoresis of amplified product (Plate 1).
Identification of risk factors associated with CPVE
Plate 1: Confirmation of CPV infection by PCR.
Cross-sectional study was used to investigate the relationship between disease and hypothesized causal factors. Relative Risk (RR) and Odds Ratio (OR) were used to determine the causal association as per the formula described by Martin et al., (1994).
Vaccination of puppies, maternal vaccination, roaming of dogs and time of weaning are considered as important risk factors and association in the occurrence of disease are calculated follows:
In this study, vaccination (RR - 2.42, OR - 11.85), roaming of dogs (RR - 2.28, OR -7.02), maternal vaccination (RR - 2.22, OR - 6.27) and early weaning (RR - 3.05, OR - 13.83) having positive statistical association with the occurrence of CPVE in dogs. Houston et al., (1996)
also reported that lack of vaccination in dogs is considered as an important risk factors associated with CPVE. Lack of maternal vaccination leads to increased susceptibility of puppies born to unimmunized bitches. This fact was also well explained by Carr et al., (1997),
who reported that the failure of passive transfer of antibodies via colostrum, incomplete or ineffective primary vaccination course, or failure of vaccination to induce immunity are the risk factors associated with CPV infection in puppies.
Weather based forecast model for prediction of association of environmental determinants with CPVE
Multiple linear regression developed for detection of association of weather parameters with CPVE:
Y = - 520.52 (-0.025) X1 + (-17.019) X2 + (34.998) X3 + (6.473) X4 + (-1.939) X5 + (- 10.037) X6