Diagnosis
PPV should be considered in a differential diagnosis of reproductive failure of swine whenever there is evidence of embryonic or fetal death or both. The pathologic sequelae of maternal infection during gestation have been described (see the section on clinical signs). If gilts but not sows are affected, maternal illness is not seen during gestation, there are few or no abortions or fetal developmental anomalies, and other evidence suggests an infectious disease, then a tentative diagnosis of PPV-induced reproductive failure can be made. The relative lack of maternal illness, abortions, and fetal developmental anomalies differentiates PPV from most other infectious causes of reproductive failure. However, a definitive diagnosis requires laboratory support.
Several mummified fetuses (<16 cm in length) or lungs from such fetuses, if sufficiently developed, should be submitted to the diagnostic laboratory. Larger mummified fetuses (i.e., more than about 70 days of gestational age), stillborn pigs, and neonatal pigs are not recommended for submission unless they are the only samples available. If infected, their tissues will usually contain antibody that interferes with laboratory tests for either virus or viral antigen.
If females fail to farrow despite being anestrus and are sent to an abattoir, their uteruses should be collected and examined for affected fetuses. Sometimes only remnants of fetal tissues remain when fetuses die early in the middle third of gestation. Nevertheless, these are adequate samples if tested for viral antigen by IF microscopy. The absence of affected fetuses or fetal remnants does not exclude PPV-induced reproductive failure. When all embryos of a litter die and are completely resorbed after the first few weeks of gestation, the dam may remain endocrinologically pregnant and not return to estrus until after the expected time of farrowing.
Identification of viral antigen by IF microscopy is a reliable and sensitive diagnostic procedure. Sections of fetal tissues are prepared with a cryostat microtome and are then reacted with standardized reagents. The test can be completed within a few hours. In the absence of a fetal antibody response, antigen is seen throughout fetal tissues (Fig. 8A, B); even when antibody is present, infected cells usually can be detected in fetal lung (Fig. 8C).
Detection of viral hemagglutinin also has been recommended as a diagnostic technique. Tissues are triturated in diluent and then sedimented by centrifugation. The supernatant fluid is tested for agglutinating activity for guinea pig erythrocytes. This test requires a minimum of laboratory equipment and is effective in the absence of antibody.
Virus isolation is less suitable as a routine diagnostic procedure than either of the aforementioned tests. Infectivity is slowly but progressively lost after fetal death; as a result, isolation of virus from mummified fetuses that have died as a result of infection is sometimes unsuccessful. Moreover, the procedure is time-consuming, and contamination is a constant threat because of the stability of PPV in the laboratory and because cell cultures are sometimes unknowingly prepared from infected tissues. IF microscopy is often used to determine whether PPV has been isolated in cell culture.
In general, serologic procedures are recommended for diagnosis only when tissues from mummified fetuses are not available for testing as previously described. Results with maternal sera are of value if antibody is not detected, thus excluding PPV as a cause, and if samples collected at intervals reveal seroconversion for PPV coincident with reproductive failure. Because PPV is ubiquitous, the presence of antibody in a single sample is otherwise meaningless. However, a determination of relative amounts of antibody present as immunoglobulin M and G can indicate the recency of infection. Detection of antibody in sera of fetuses and stillborn pigs and in sera collected from neonatal pigs before they nurse is evidence of in utero infection, since maternal antibody does not cross the maternal-fetal junction. When serum is not available, body fluids collected from fetuses or their viscera that have been kept in a plastic bag overnight at 4?C have been used successfully to demonstrate antibody.
Read more about this topic: Porcine Parvovirus