Canine parvovirus: An update on variants - DVM
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Canine parvovirus: An update on variants
A review of what's happening with canine parvovirus in the United States.


DVM360 MAGAZINE


Diagnosis

Canine parvovirus is the most common cause of enteric disease in puppies. After oral infection followed by systemic infection, canine parvovirus is shed for five to seven days. However, it has been shown that dogs infected with CPV-2c may shed the virus for up to 51 days.

There are a variety of tests available for laboratory-based diagnosis of CPV-2 infection. For ante-mortem diagnosis, fecal samples (2-5 grams) are preferred. For post-mortem diagnosis, pieces of unfixed, small intestine and tongue are preferred.

Canine parvovirus can cause focal infection of the intestines. Three discontinuous loops of the small intestine should be submitted for direct fluorescent antibody testing. Anti-CPV-2 monoclonal antibody 3B10 from VMRD Inc. has been found to react with all circulating genotypes of CPV-2 in the United States. This antibody is suitable for immunohistochemistry on formalin-fixed tissues.

For genotyping, nucleic acid-based techniques are needed. CPV-PCR can be used and followed by sequencing and analysis of the sequence. Amino-acid position 426 determines the genotype of the CPV-2. For CPV-2 and CPV-2a, the amino acid at position 426 is asparagine (Asn); for CPV-2b, the 426 is aspartic acid (Asp); and for CPV-2c the amino acid at position 426 is glutamine (Glu).

In the clinic, ante-mortem diagnosis can be performed using ELISA and SNAP tests. A variety of factors, including the time of collection of the samples after infection, can affect the outcome of these diagnostic tests.

All CPV-2 viruses hemagglutinate erythrocytes. This universal property of the virus can be used to develop an instant, easy, robust animal-side diagnostic test for CPV-2 antigen (SAT) and antibody diagnosis (SIT).

These instant tests do not require any instrumentation and are suitable for application in kennels by owners because they require minimal training. The procedure requires mixing a drop of fecal sample and erythrocytes (porcine erythrocytes or canine and feline erythrocytes). The agglutination reaction is quick/instant. The test performs well at room temperature, and with ll circulating genotypes of CPV-2 in the United States.

Vaccines

Current CPV-2 vaccines can be classified according to the amount of CPV-2 antigen: high- and low-antigen-dose CPV-2 vaccines. High-antigen-dose vaccines claim to immunize the puppies in the presence of maternal antibodies. Higher amounts of CPV-2 vaccine will lead to higher and longer shedding by the puppies, leading to second-hand exposure to the virus. Most major commercial vaccines contain modified-live virus (MLV) of CPV-2 or CPV-2b genotypes. There is no commercial vaccine that contains CPV-2a. Moreover, CPV-2a has not been found in nature in the United States.

Commercial vaccines containing CPV-2 isolates may not provide robust serum antibody responses against CPV-2b and CPV-2c isolates. It was recently published that all commercial vaccines provide protection against current circulating genotypes of CPV-2 based on experimental studies just presented recently. However, other published studies contradict claims of CPV-2 vaccine efficacy under field conditions.

Clearly, the observations made by diagnostic virologists/pathologists, canine breeders, field veterinarians and shelters do not agree with the universal efficacy of CPV-2 vaccines worldwide. After the introduction of commercial CPV-2 vaccines, the problem is significantly reduced but CPV-2 continues to be the No. 1 infection in puppies, worldwide.

CPV-2b vaccines have been found to have higher efficacy compared to some available CPV-2, based on the published literature. Currently, there is no commercial vaccine that contains CPV-2c antigen. Commercial companies are evaluating the efficacy of MLV CPV-2c vaccines.

Antigenic cross-reactivity

Serum antibody titers are a good measure of immune status of the dog against CPV-2.

Antibodies against one genotype will cross-react with other genotypes of CPV-2. Traditionally, hemagglutination-inhibition tests using porcine erythrocytes is used to assess the level of antibodies in serum samples. A titer of > 1:80 is considered to be protective. Most commercial vaccines induce this titer after a single vaccination in puppies that are serologically negative.

Serum neutralization testing is slightly cumbersome, but it is the preferred method for determination of the animal's immune status. Differences in SN titers among CPV-2 isolates are considered insignificant if it is < 4-fold SN titers. CPV-2 isolates that differ by 4-fold or more are considered significant. Anti-CPV-2c antiserum reacts better with CPV-2c and CPV-2b isolates compared with CPV-2b antiserum.


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Source: DVM360 MAGAZINE,
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