Clinical Syndrome

Inoculation most often occurs via the fecal-oral route; large numbers of viral particles are shed in the feces of infected animals. Because of the tremendous environmental hardiness of the virus, fomites and contaminated environmental surroundings are a major source of infection. After ingestion of viral particles by the host, the virus replicates in the lymphoid tissue of the tonsils, oropharynx, and intestinal tract and then progresses to viremia, occurring approximately 3 days after inoculation. During the viremic phase, signs of systemic illness such as fever, malaise, and lethargy may be present, along with fecal shedding. Progression of infection to rapidly dividing cells, such as those found in the intestinal crypts and bone marrow, occurs 5 to 7 days after inoculation. As a result, damage to the intestinal tract becomes severe enough to cause the hemorrhagic diarrhea and vomiting characteristic of CPV enteritis in the dog. Intestinal hypermotility is common, and animals should be monitored closely for development of intussusceptions, a potentially fatal complication.

Leukopenia is a consistent clinical finding in infected animals. Neutropenia and lymphopenia are a result of direct infection and destruction of white cell lines in the bone marrow; however, lymphocyte counts frequently rebound in the face of persistent neutropenia. This is explained in part by the continued demand for neutrophils in the gastrointestinal tract as well as direct viral effect on the bone marrow.

Less consistent secondary findings in the hematologic and biochemical picture are anemia and thrombocytopenia, often associated with loss and consumption in the gastrointestinal tract. In theory, direct infection of the bone marrow can contribute as well, but the impact is less severe than on the white blood cell line. Biochemical findings are also more commonly a consequence of the body’s reaction to infection rather than direct viral effect. Hypoglycemia can occur secondary to poor glycogen reserves of young animals with concurrent decreased oral intake of nutrients as well as sepsis secondary to bacterial translocation across the severely compromised gastrointestinal mucosa. Electrolyte abnormalities (i.e., hyponatremia, hypokalemia, and hypochloremia) are a result of the protracted vomiting and diarrhea and concurrent dehydration and metabolic acidosis.

Demonstration of Recent Parvoviral Infection

The most clinically useful method of detection of virus in the ill animal is via point-of-care enzyme-linked immunosorbent assays (ELISAs). These assays detect viral antigen in feces or rectal swabs and results are relatively sensitive and specific; however, false negatives and false positives can occur. False negatives can occur with infection of any CPV-2 variant secondary to relatively low concentrations of viral particles because of either decreased shedding in the later stages of the disease or dilutional effects from voluminous diarrhea. The impact of infection with CPV-2c on point-of-care ELISA results is controversial at this time. There are reports of CPV-2c causing clinical disease but negative point-of-care ELISA results. However, other studies have demonstrated similar results using such assays for CPV-2c as for CPV-2a and -2b (Decaro et al, 2010). Preliminary work in the author’s laboratory has identified three animals in a series of 49 dogs with a clinical diagnosis in which CPV was not detected in feces using point-of-care ELISAs, but CPV was detected using quantitative polymerase chain reaction (PCR). All three of these isolates were later determined to be the CPV-2c variant (Veir, unpublished data). However, because a complete history and clinical illness were not available for all of these animals, the same reasons for any false negative may be the cause. In another study, there was no statistical difference in the sensitivity of a point-of-care ELISA for detection of CPV-2b and CPV-2c (Markovich et al, 2012). False positives have been reported in association with recent vaccination with modified live parvoviral diseases. One manufacturer of point-of-care assays has an unpublished study demonstrating lack of reaction after vaccination in a group of 60 beagles vaccinated with modified live parvoviral vaccines (MLV). In a study performed in the author’s laboratory of 12 puppies vaccinated with a MLV parvoviral vaccine, feces from one puppy produced a positive result on a point-of-care ELISA 5 days after vaccination (Burton et al, 2008).

With the advent of PCR “panels” available from major diagnostic laboratories, PCR may be used more commonly in the diagnosis of CPV enteritis. Unfortunately, qualitative PCR using whole blood was frequently positive in the above study of 12 puppies as soon as 2 days after vaccination and continued until the study end point at 14 days after vaccination. The author’s laboratory has attempted to decrease the frequency of false positives caused by recent vaccination via PCR using quantitative, instead of qualitative, PCR assays. Using samples from the same 12 puppies and an additional 8 naturally infected puppies positive for CPV using a point-of-care ELISA, a significantly higher viral load was detected via quantitative PCR in the naturally infected group (Veir et al, 2009). Although not yet commercially available, qPCR may prove useful in differentiating naturally infected and recently vaccinated animals. Virus isolation, hemagglutination inhibition, and electron microscopy can be used to demonstrate recent infection; however, because of the turnaround time, these often are reserved for confirmation of disease or in group health situations.