**3. Canine parvovirus enteritis (CPVE)**

The CPV is of two types: CPV-1, commonly known as minute virus of canine and accountable for gastrointestinal and respiratory infection of dogs whereas, CPV-2,

most pathogenic type and is responsible for severe gastroenteritis/hemorrhagic gastroenteritis, in young puppies as well as adult dogs.

It is quite difficult to distinguish the clinical diseases caused by CPV-2 variants owing to its overlapping nature of signs and symptoms. These variants are believed to produce similar pathogenicity however; some studies showed that severity of clinical manifestations is influenced by variants of CPV-2 based on clinical, hematological, serological and histopathological examinations [36, 37].

Although puppies under 6 months of age are highly susceptible, adult dogs with insufficient immunity are also considered as high risk to the CPVE. CPV-2 can persist in the environment for more than a year, enabling susceptible dogs to pick up infection from CVP-2 contaminated feces, vomitus, or fomites. Although the feco-oral route is considered as primary path of disease transmission, infection through the oro-nasal route is also common in naive or under-immunized dogs due to ingestion of viruses shed in the vomitus or feces of CPV-2-infected animals [38]. However, direct contact or environmental contamination may also play a role [39]. Breed predisposition and seasonal prevalence of the disease are subject to considerable variations in wide geographical areas [40, 41].

Doberman, Rottweiler, and German shepherd (GS) dogs have been reported to be more susceptible to CPVE than other breeds [42]. Due to inherited immunodeficiency, the exotic breeds, German Sphered and Doberman, are more susceptible than the other breeds [43]. German shepherd has the highest CPV infection rate (70%) followed by the Doberman (55%) [44]. A cytokine bioassay revealed that the magnitude of TNF-α production by peripheral blood monocytes was greatest in dogs with a breedrelated risk for CPVE. When compared to mixed breeds, highly susceptible breeds such as Rottweiler and Doberman Pinscher produce more TNF-α in response to LPS stimulation [45]. Increased TNF activity is predictive of mortality in naturally occurring CPVE infection in veterinary medicine [46]. Therefore, it has been hypothesized that dogs with a breed-related risk of developing CPVE, a disease associated with sepsis, would have a greater pro-inflammatory cytokine response to endotoxin [45].

The incubation period of CPV-2 infection ranges from 4 to 14 days. The infected dogs start to shed virus few days prior to the visible clinical signs and shedding of virus gradually declines 3–4 weeks postexposure [47]. Following entry into the body, the CPV-2 rapidly multiply in oropharyngeal lymph node, thymus and mesenteric lymph node, resulting in viremia within one week of exposure. After that, the virus attacks rapidly multiplying cells of crypts of intestine, epithelium of the tongue, oral cavity, bone marrow, and cardiac myocytes, besides lung, spleen, liver, and kidneys [48]. The key pathogenic event in CPV-2 infection is the virus-induced destruction of enterocyte, leading to mucosal barrier disruption, and villous atrophy. This causes profuse vomiting and hemorrhagic diarrhea, nutrient malabsorption, dehydration/ hypovolemia, metabolic acidosis and/or alkalosis. The disruption of mucosal barrier allows bacterial translocation from intestinal compartment to systemic circulation, resulting in septicemia, endotoxemia, systemic inflammatory response syndrome as well as hypercoagulability [49]. The CPV-2 infection in the thymus and bone marrow precursor cells results in loss of thymic cortex and profound leucopenia, respectively [48]. Death may occur due to multi-organ failure when the affected dogs remain unattended [40, 49]. Previously, myocarditis was thought to be the acute cause of death in young puppies however, this form nowadays occurs rarely because of widespread CPV vaccination of dogs. The concurrent infections with parasitic, virus, or bacterial intestinal pathogens or stressors may aggravate the disease [50–52].

The degree of clinical manifestations may vary with age, breed, and immune status, duration of illness and virulence of virus. The clinical signs of dogs with

*Canine Parvovirus-2: An Emerging Threat to Young Pets DOI: http://dx.doi.org/10.5772/intechopen.104846*

CPV infection are nonspecific in nature and resembles to gastritis and enteritis. The most notable clinical signs of CPVE are lethargy, depression, weakness, lack of appetence, bouts of vomiting, and diarrhea. The diarrhea is characterized by foulsmell and mucoid to purely hemorrhagic because slugging of intestinal mucosa and bleeding. The excessive loss of fluid during vomiting and diarrhea causes marked dehydration that results in development of hypovolemic shock. Occasionally, intussusception occurs due to intestinal dysmotility. Neurologic signs in puppies with CPVE may result from hypoxia secondary to myocarditis, hypoglycemia, or intracranial thrombosis or hemorrhages [52]. The bacterial translocation from intestine to systemic circulation can cause fever, systemic inflammatory response syndrome and septic shock with hypotension and organ failure [40, 48]. Apart from diarrhea, respiratory distress, pulmonary congestion and edema, alveolar and bronchiolar hemorrhage and convulsions are also occasionally manifested due to hypovolemia, endotoxic and septicemic shock [8, 53]. The malabsorbtion of nutrients and inadequate storage of glycogen in muscle and liver result in hypoglycemic encephalopathy which leads to seizures. On hospital admission, the prognosis is poor in CPVE dogs with intussusception, systemic inflammatory response syndrome and severe leucopenia.
