A Stepwise Diagnostic Approach to Enteric Diseases in Pigs

Enteric diseases are among the most common causes of diarrhoea and production losses in pig herds. Because many bacterial, viral, and parasitic pathogens produce overlapping clinical signs, diagnosis should never rely on faecal appearance alone. A systematic diagnostic approach that combines herd history, clinical examination, necropsy findings, appropriate sample collection, and laboratory investigations provides the best opportunity to identify the underlying cause accurately and supports more informed clinical decisions¹. 

Start with Herd History and Clinical Examination 

The diagnostic process begins with gathering a comprehensive herd history. Information such as the age of affected pigs, duration of illness, morbidity and mortality rates, vaccination history, previous antimicrobial treatments, and recent management changes can provide valuable direction before laboratory investigations are initiated¹. 

During clinical examination, diarrhoea remains the most important clinical manifestation of enteric disease. Careful assessment of faecal characteristics, including colour (yellow, grey, bloody) and consistency (watery or creamy), can assist in developing an appropriate list of differential diagnoses. Vomiting should also be noted, as it is commonly associated with enteric viral infections¹. 

Evaluating disease distribution within the herd is equally informative. The number of affected litters, the number of piglets affected within each litter, and the parity of their dams may help distinguish endemic disease from the introduction of a new pathogen. Enteric diseases caused by endemic agents commonly affect litters from first-parity sows, whereas newly introduced pathogens are more likely to produce widespread herd outbreaks¹. 

Use Necropsy to Refine Differential Diagnoses 

Following clinical assessment, necropsy should be performed on three to five acutely affected pigs showing representative clinical signs that have not received antimicrobial treatment whenever possible¹. 

Although gross lesions rarely establish a definitive diagnosis on their own, they provide valuable information that guides further investigations. During post-mortem examination, veterinarians should assess: 

• The type of enteritis, such as catarrhal, fibrinous, or necrotic 

• Whether lesions involve the small intestine, large intestine, or both 

• The distribution of lesions, including focal, diffuse, or segmental involvement¹ 

These findings help narrow differential diagnoses and determine which laboratory tests should be prioritised. 

Proper Sampling Determines Diagnostic Quality 

Accurate laboratory results begin with correct sample collection. Selecting the appropriate animals, obtaining representative specimens, and preserving them properly are all essential steps in the diagnostic pathway ¹. 

Fresh, unopened sections of the jejunum, ileum, and large intestine should be collected separately for bacteriology and PCR to minimise contamination. Samples should be refrigerated at approximately 4 °C and ideally reach the laboratory within 24 hours. Tissue specimens intended for histopathology should be collected using standardised procedures and fixed promptly in 10% buffered formalin². 

Rapid post-mortem autolysis significantly reduces the diagnostic value of intestinal tissues. For this reason, tissues collected from euthanized pigs are generally preferred over those obtained from animals found dead, particularly when histopathological evaluation is planned¹. 

Combine Laboratory Tests for a Definitive Diagnosis 

No single laboratory test is sufficient for diagnosing most enteric diseases. Instead, complementary diagnostic techniques should be interpreted alongside clinical findings and pathological lesions¹. 

Quantitative bacteriology and quantitative real-time PCR help determine whether an organism is present in numbers compatible with disease. However, pathogen isolation alone may not confirm causation. Demonstrating virulence factors, including toxin genes, is often necessary to establish the pathogenic role of organisms such as enterotoxigenic Escherichia coli, Clostridium perfringens, and Clostridioides difficile¹. 

Histopathology remains a cornerstone of diagnosis by correlating microscopic lesions with pathogen detection. When indicated, immunohistochemistry further strengthens the diagnosis by demonstrating pathogen localisation within affected tissues. Conversely, serology offers limited value for most enteric diseases because suitable assays are often unavailable and the acute nature of outbreaks limits its practical usefulness¹. 

Practical Clinical Insights 

Successful diagnosis of enteric disease depends on integrating information from every stage of the investigation rather than relying on a single finding. Herd history provides context, clinical examination identifies patterns, necropsy defines lesion distribution, appropriate sampling preserves diagnostic quality, and laboratory testing confirms the aetiology. Importantly, avoiding diagnostic shortcuts is essential, as omitting bacteriology, PCR, or histopathology can result in overlooking the true cause of disease and delay appropriate herd-level interventions [7]. Following a structured diagnostic workflow allows veterinarians to approach enteric disease investigations with greater confidence while improving the accuracy and reliability of clinical decision-making [6]. 

References 

1. Luppi A, D’Annunzio G, Torreggiani C, Martelli P. Diagnostic approach to enteric disorders in pigs. Animals. 2023 Jan 18;13(3):338. https://www.mdpi.com/2076-2615/13/3/338 

2. Arruda PH, Gauger P. Optimizing sample selection, collection, and submission to optimize diagnostic value. Diseases of Swine. 2019 Jun 3:98-111. https://www.academia.edu/download/60674736/Diseases_of_Swine__11th_Edition_VetBooks.ir20190922-31639-a6z8g5.pdf#page=122