LABORATORY CONFIRMATION OF AFRICAN SWINE FEVER
Laboratory tests need to be carried out to confirm the diagnosis of ASF, because the drastic control measures are expensive and cause hardship to owners and government alike. These measures should not be put in place unnecessarily. The tests that exist are used to detect the virus itself by growing it, evidence that the virus was present (virus antigen, genetic material) or the reaction of the animal to the virus (antibodies in blood serum). In acute outbreaks of ASF, it may not be possible to detect antibodies, as the pigs die before they have time to produce them. The standard tests therefore involve detection of the virus. Tests to detect antibodies are useful for identifying pigs that have survived infection and for carrying out surveys to determine whether the disease is endemic in a country or area.
Detection of the virus in cell culture
F virus, the presence of virus in cell cultures can be demonstrated by adding red blood cells to the culture. These are attracted to the surface of infected cells, to which they cling and form “rosettes”, a phenomenon known as haemadsorption. The virus may be injected into pigs to demonstrate that it is capable of infecting pigs and causing disease. Some strains of virus do not cause red blood cells to absorb to the surface of cells that they have infected, but dead cells in the culture will become obvious after a few days.
The advantage of culturing the virus is that it can then be characterized to determine the strain.
Detection of virus antigens by immunofluorescence
Impression smears of lymph nodes and spleen on glass slides are treated with antibodies labelled with a dye that will fluoresce when examined under a special microscope (Figure 17). Positive and negative controls are used to ensure that the slides are interpreted correctly. This test can be carried out fairly rapidly and is used in most African laboratories that have the capacity to diagnose ASF.
Detection of virus antigens by polymerase chain reaction (PCR)
This test requires specialized facilities. It is used most frequently in reference laboratories to obtain a rapid diagnosis, as isolation in cell culture and demonstration of the virus by adsorption of red blood cells or cell damage (cytolysis) usually takes several days. Results are obtainable within 24 hours, and rely less on personal interpretation than immunofluorescence. The test can be carried out on a variety of tissues, but for practical purposes lymph nodes and spleen on ice or in glycerosaline are the samples of choice.
Detection of viral antigen by immunoperoxidase staining
Viral antigen may be detected in cells in histopathological preparations from formalin-fixed material by the immunoperoxidase staining technique. This method, which usually takes 5–7 days, is slower than PCR or immunofluorescence, but is useful if the only tissues available have been preserved in formalin. It is useful as a research tool to determine the distribution of viral antigen.