Canine parvovirosis: A new test to improve immunity against parvovirus

Despite the availability of an effective vaccine, canine parvovirus continues to spread. There is, in fact, a critical period in a puppy’s life during which it is susceptible to infection but does not respond to vaccination. To improve the vaccination schedule, serological tests are performed, but they are not perfect. Veterinarians have sought to refine the hemagglutination inhibition test protocol to avoid misinterpretation of results and ineffective vaccination.   Canine parvovirus type 2 (CPV-2) is the primary infectious agent causing hemorrhagic gastroenteritis in puppies and young dogs. This parvovirus emerged in the 1970s and has since continued to evolve, generating different antigenic variants (known as CPV-2a, 2b, and 2c) that have gradually replaced the original strain. Despite the widespread adoption of a vaccine against the virus and advances in our understanding of its pathobiology resulting from 50 years of research, CPV-2 epizootics continue to occur within the canine population. In fact, maternal antibodies in young puppies can interfere with the effectiveness of vaccination. The persistence of maternal antibodies interferes with the development of an active immune response to administered vaccines and limits the immunization of dogs against CPV-2. Maternal antibodies are passed on to puppies when they ingest colostrum and can therefore vary between litters and among puppies within the same litter. Above a certain antibody threshold (1:80), puppies are protected against infection and disease, whereas below another threshold (1:20), the animals are susceptible to both infection and disease. Between these two thresholds, antibodies do not provide protection against CPV-2 infection but may interfere with active immunization. There is therefore a period, lasting approximately two to five weeks, during which the puppies’ antibodies fall between these two thresholds: they are susceptible to infection but do not respond to vaccination. In recent years, veterinarians have recommended measuring maternal antibody levels before the first vaccination to optimize vaccine protection and avoid unnecessary injections. To this end, they use serological tests to determine the concentration of maternal antibodies, predict the optimal timing for vaccination, and confirm post-vaccination immunity. Viral seroneutralization and hemagglutination inhibition tests have become the gold standard for detecting and measuring CPV-2-specific antibodies. Viral neutralization assays, which identify protective immunity, are reliable and sensitive but technically demanding and may require additional steps to overcome the cytopathic effect of CPV-2. Consequently, the hemagglutination inhibition test has been widely adopted in laboratories worldwide for measuring CPV-2-specific antibodies. However, this test is not without its limitations either. The presence in canine serum of nonspecific agglutinins against porcine red blood cells—which are commonly used in the test—can mask the presence of CPV-2-specific antibodies at lower serum dilutions, particularly at the titers relevant for vaccination. This can lead to false-negative results. Furthermore, artifactual precipitation of excess red blood cells—in the absence of CPV-2-specific antibodies—can also result in false-positive results in dogs susceptible to infection. Veterinarians therefore sought to improve the hemagglutination inhibition test protocol, in order to minimize the interference of nonspecific agglutinins and artifactual red blood cell precipitation on the measurement of antibody titers, and thereby improve test interpretation. They proposed a protocol in which canine serum is preincubated with porcine red blood cells for 12 hours to eliminate nonspecific agglutinins. A lower concentration (0.1% versus 0.8%) of porcine red blood cell suspensions is also used to limit artifactual precipitation. A panel of canine sera, collected from 80 dogs of all ages and with varying titers of neutralizing antibodies, was analyzed using the new protocol to test its effectiveness. The researchers thus demonstrated that pretreating the sera with red blood cells prevents nonspecific hemagglutination and that lowering the concentration of red blood cells prevents artifactual precipitation. This new protocol therefore increases the accuracy of test interpretation and reduces interference from nonspecific agglutinins, which are primarily observed in puppies. This reduces the likelihood of an incorrect assessment of passive or active immunity in puppies when deciding whether to administer or postpone vaccination. This protocol helps identify an optimal vaccination window, which could be sufficient to curb canine parvovirus infections.