Towards effective African swine fever detection and diagnosis

1. Polymerase chain reaction (PCR)

PCR is used to detect the ASF virus genome in porcine samples like blood, organs and ticks. It enables the diagnosis of ASF to be made within hours of the sample’s arrival to the laboratory and provides a sensitive, specific and rapid alternative to virus isolation.

This method provides higher sensitivity and specificity than alternative methods such as the antigen enzyme-linked immunosorbent assay (ELISA) and the direct fluorescent antibody test (FAT). However, its extreme sensitivity makes it susceptible to cross-contamination and proper precautionary measures should be taken to minimize and control this risk.

PCR is the tool of choice in the case of peracute, acute or subacute ASF infections. Since it detects the viral genome, it may be positive even when no infectious virus is detected by virus isolation, making it a very useful tool for the detection of ASF DNA in pigs infected with low or moderately virulent strains.

2. ASF virus isolation

Virus isolation is based on the inoculation of sample material onto susceptible primary cell cultures of porcine origin, monocytes and macrophages. If the ASF virus is present in the sample, it will replicate in the susceptible cells, producing a cytopathic effect in the infected cells.

Virus isolation is recommended as a reference test for the confirmation of positive results of a prior antigen-positive test like ELISA, PCR or FAT. They are also recommended when ASF has already been confirmed by other methods, particularly in the case of the first outbreak in an area. Virus isolation is also essential if the objective is to obtain virus stocks for future molecular and biological characterization studies.

3. Fluorescent antibody test (FAT)

The FAT can be used to detect ASF antigen in pig tissues. The principle of the test is the microscopic detection of viral antigens on impression smears or thin cryosections of organ material. This is a highly sensitive test for cases of peracute and acute ASF and can be carried out rapidly.

It is a robust test but has been largely replaced by PCR and reagents are no longer widely available. However, it is important to note that in subacute and chronic disease, the FAT has a significantly decreased sensitivity of around 40 percent.

4. ELISA test

Viral antigens can also be detected using ELISA, which is cheaper to set up than PCR methods and allows large-scale testing of samples in a short time without special laboratory equipment. However, as in the case of the FAT, the antigen ELISA has a significantly decreased sensitivity in subacute and chronic diseases.

Field samples are often in poor condition and therefore also decrease the sensitivity of the test. It is thus recommended to use the antigen ELISA only as a herd test and in conjunction with other virological and serological tests.

Below is a summary of the laboratory diagnostic techniques used on ASF:

Source: Food and Agriculture Organization of the United Nations. African swine fever. Detection and diagnosis (2017)

About the author

Marco Farina joined DKSH in February 2016 as General Manager, Business Line Scientific Instrumentation, Business Unit Technology. He oversees global business development and has spent the last ten years developing and growing business in different emerging markets in Europe, Middle East, Africa and Asia. He now lives in Bangkok with his wife and two kids.