Just how significant is that pathogen?
Josepha DeLay
Animal Health Laboratory, University of Guelph, Guelph, ON
AHL Newsletter 2023; 27: (4):13.
The use of new techniques such as high throughput sequencing (HTS) in disease investigation can lead to identification of many potential pathogens in a given clinical scenario. Meeting the criteria of a modified version of Koch’s postulates establishes causation between a pathogen and a disease condition. However, this is complicated by subclinical infections and multifactorial disease conditions which do not necessarily follow direct rules of cause and effect.
Disease causation is straightforward for some swine pathogens, such as influenza A virus and transmissible gastroenteritis virus (TGEV). Detection of these pathogens in pigs is associated with specific disease conditions, and these pathogens aren’t normally detected in healthy animals. Other pathogens may not have such a clear cut, consistent role in disease causation in every clinical situation, and infection may not always result in disease. PCV-2 is an example of this more complex scenario.
Correlating PCR or culture results with results of other test methods, especially histopathology and related direct detection tests such as immunohistochemistry (IHC) and in situ hybridization (ISH), is very helpful in assessing a pathogen’s role in individual clinical cases. PCR and culture are important to assess the presence and quantity of a pathogen. Histopathology and direct detection techniques (IHC, ISH) allow visualization of lesions potentially caused by the pathogen, and determine if the pathogen is present in association with the lesion of interest. Including formalin-fixed histopathology samples with a case can dramatically strengthen the certainty of a diagnosis. In diagnostic pathology, seeing really is believing. Tissues are assessed for presence of lesions indicative of infection with various pathogens. Where available, direct detection tests are used to confirm presence of the pathogen in association with the characteristic microscopic lesion. For some disease conditions such as PCV2 and PCV3-associated disease, confirmation of the diagnosis is dependent on identification of characteristic histologic lesions and co-localization of virus in lesions.
As an example, interpreting the clinical significance of a moderately high Ct value for porcine rotavirus can be problematic in young pigs with mild diarrhea. In some pigs, rotavirus may be present in the intestine with a low to moderate viral load, but may not necessarily be causing disease. If histologic sections from intestine of these same PCR-positive pigs have evidence of intestinal villus atrophy with attenuated enterocytes (i.e., lesions typical of viral enteritis), presence of these compatible lesions adds support to the clinical significance of the positive PCR result. If rotavirus antigen is also detected in these lesions using IHC, then we can be confident that rotavirus is contributing to diarrhea in the group. Conversely, if rotavirus is detected by PCR but these pigs lack histologic lesions of atrophic enteritis, the contribution of rotavirus to clinical enteritis is less likely. As always, selecting the ‘right’ pigs to test is an important factor in correct interpretation of test results, and should focus on acutely affected, untreated animals.
The same approach is also very useful in the investigation of emerging pathogens, where we might not yet have a complete understanding of the prevalence and significance of a pathogen in various populations and geographic areas. Relying on a single test method can lead to misinterpretation of results, and the use of multiple test methods in conjunction with histopathology is encouraged. Omitting histopathology is a missed opportunity. Finally, circling back to correlate test results with clinical history and gross postmortem lesions is vital to the understanding and final interpretation of diagnostic testing.