Swine Enteric and Systemic Diseases: Dysentery, Fever, and Bloody Diarrhea in Pigs

1. Introduction

Swine production faces significant economic losses from enteric and systemic diseases that manifest with diarrhea, fever, and dysentery. Two clinically important conditions that present with bloody diarrhea and systemic signs are swine dysentery (caused by Brachyspira hyodysenteriae) and classical swine fever (caused by classical swine fever virus, CSFV). While both can produce hemorrhagic feces, their etiologies, pathogenesis, and management differ fundamentally. This article provides a detailed reference for veterinary diagnosticians and clinicians on the etiology, clinical presentation, diagnostic differentiation, treatment, and prevention of these diseases.

2. Swine Dysentery: Etiology and Pathogenesis

Swine dysentery is a mucohemorrhagic colitis caused by the anaerobic spirochete Brachyspira hyodysenteriae. This Gram-negative, oxygen-tolerant anaerobe colonizes the large intestine, specifically the cecum and spiral colon. The bacterium produces hemolysins and lipooligosaccharides that damage colonic epithelial cells, leading to inflammation, necrosis, and hemorrhage.

2.1 Pathogenic Mechanisms

B. hyodysenteriae uses flagella-mediated motility to penetrate the mucus layer overlying colonic enterocytes. Once in contact with the epithelium, the spirochete induces an inflammatory response characterized by neutrophil infiltration, goblet cell hyperplasia, and mucosal edema. The hemolytic activity of the bacterium, mediated by a beta-hemolysin (hlyA), contributes to erythrocyte lysis and the characteristic bloody appearance of feces. The resulting colitis impairs water and electrolyte absorption, producing profuse diarrhea.

2.2 Clinical Signs

The incubation period ranges from 7 to 14 days. Affected pigs initially develop soft, yellow-to-gray feces that progress to watery, mucoid stools containing fresh blood and mucus. Clinical signs include:

• Profuse, watery diarrhea with visible blood and mucus
• Dehydration and weight loss
• Reduced feed intake
• Fever (mild to moderate, typically 40.0 to 41.0 degrees Celsius)
• Rough hair coat and sunken flanks
• Rectal prolapse in severe cases

Morbidity can reach 90 percent in naive herds, while mortality is typically low (5 to 10 percent) unless secondary infections or dehydration are severe.

3. Classical Swine Fever: Etiology and Pathogenesis

Classical swine fever (CSF), also known as hog cholera, is a highly contagious viral disease caused by classical swine fever virus (CSFV), a member of the genus Pestivirus within the family Flaviviridae. CSFV is an enveloped, single-stranded positive-sense RNA virus. The virus targets cells of the reticuloendothelial system, including macrophages, endothelial cells, and lymphocytes.

3.1 Pathogenic Mechanisms

CSFV enters the host via the oronasal route and replicates initially in tonsillar crypts. Viremia follows, with viral dissemination to lymphoid tissues, bone marrow, and vascular endothelium. The virus causes widespread vascular damage, thrombocytopenia, and immunosuppression. Hemorrhagic diathesis results from endothelial cell injury and platelet dysfunction, leading to petechial hemorrhages and bloody diarrhea. The severe leukopenia and lymphopenia predispose pigs to secondary bacterial infections.

3.2 Clinical Signs

Clinical presentation varies with viral strain and host immune status. Acute infections present with:

• High fever (41.0 to 42.0 degrees Celsius)
• Anorexia and lethargy
• Conjunctivitis and ocular discharge
• Respiratory signs (coughing, dyspnea)
• Constipation followed by profuse, watery diarrhea
• Bloody diarrhea (melena or frank blood) in severe cases
• Petechial hemorrhages on skin, ears, and abdomen
• Cyanosis of extremities
• Neurologic signs (tremors, incoordination, convulsions)

Chronic infections may show intermittent fever, wasting, and persistent diarrhea. Mortality in acute outbreaks approaches 100 percent.

4. Differential Diagnosis of Bloody Diarrhea in Pigs

Bloody diarrhea in pigs requires differentiation among several infectious and non-infectious causes. The following table summarizes key differentials.

5. Diagnostic Approaches

5.1 Laboratory Diagnosis of Swine Dysentery

Definitive diagnosis of swine dysentery relies on detection of B. hyodysenteriae in feces or colonic tissue.

• Fecal Culture: Selective media (e.g., trypticase soy agar with 5 percent bovine blood, spectinomycin, and colistin) incubated under anaerobic conditions at 37 degrees Celsius for 3 to 7 days. Colonies show strong beta-hemolysis.
• Polymerase Chain Reaction (PCR): Real-time PCR targeting the nox gene or 16S rRNA gene provides rapid, sensitive detection directly from feces. PCR is preferred over culture for routine diagnosis.
• Histopathology: Colonic sections show diffuse mucohemorrhagic colitis with goblet cell hyperplasia, crypt elongation, and spirochetes visible on silver stains (e.g., Warthin-Starry stain).

5.2 Laboratory Diagnosis of Classical Swine Fever

CSF diagnosis requires molecular or serological methods due to the virus's high pathogenicity and the need for rapid outbreak response.

• Reverse Transcription PCR (RT-PCR): Detects viral RNA in whole blood, serum, or tissue samples (tonsil, spleen, lymph node). Real-time RT-PCR is the gold standard for acute cases.
• Virus Isolation: Performed on porcine kidney cell lines (e.g., PK-15). Cytopathic effect is not always present; detection requires immunofluorescence or immunoperoxidase staining.
• Enzyme-Linked Immunosorbent Assay (ELISA): Antigen-capture ELISA detects CSFV antigen in tissue homogenates. Serological ELISA (antibody detection) is used for surveillance but is less useful in acute outbreaks.
• Immunohistochemistry: Detects viral antigen in formalin-fixed tissues, particularly tonsil and spleen.

5.3 Diagnostic Workflow

The following Mermaid diagram illustrates a decision tree for diagnosing bloody diarrhea in pigs.

flowchart TD
    A[Pig with bloody diarrhea and fever], > B{History and clinical exam}
    B, > C[High fever, hemorrhages, high mortality]
    B, > D[Mild fever, mucoid bloody diarrhea, high morbidity]
    C, > E[Suspect Classical Swine Fever]
    E, > F[RT-PCR on blood/tonsil]
    F, > G{Result}
    G, >|Positive| H[CSF confirmed: quarantine, culling]
    G, >|Negative| I[Consider other viral or bacterial causes]
    D, > J[Suspect Swine Dysentery]
    J, > K[Fecal PCR for Brachyspira hyodysenteriae]
    K, > L{Result}
    L, >|Positive| M[Swine dysentery confirmed: treat with antibiotics]
    L, >|Negative| N[Consider PPE, salmonellosis, trichuriasis]
    N, > O[Fecal PCR for Lawsonia, Salmonella culture, fecal flotation]

6. Treatment of Swine Dysentery

6.1 Antimicrobial Therapy

Treatment of swine dysentery relies on antimicrobials effective against B. hyodysenteriae. The spirochete has demonstrated resistance to several drug classes, making susceptibility testing important.

• Tiamulin: A pleuromutilin antibiotic administered in feed or water at 10 to 15 mg/kg body weight for 5 to 10 days. Tiamulin is highly effective but must not be used concurrently with ionophores (e.g., monensin) due to severe toxicity.
• Valnemulin: Another pleuromutilin with potent activity against Brachyspira spp. Used in feed at 75 to 100 ppm for 7 to 10 days.
• Lincomycin: A lincosamide given in feed at 100 to 200 ppm or by injection at 5 to 10 mg/kg for 3 to 5 days.
• Tylosin: A macrolide administered in feed at 100 to 200 ppm. Resistance is increasingly reported.
• Doxycycline: A tetracycline used in feed at 200 to 400 ppm for 7 to 10 days.

6.2 Supportive Care and Management

• Provide clean, fresh water with electrolyte solutions to correct dehydration.
• Isolate affected pigs to reduce transmission.
• Clean and disinfect pens thoroughly. B. hyodysenteriae survives in feces for up to 60 days; use phenolic or oxidizing disinfectants.
• Implement all-in/all-out management to break the infection cycle.

6.3 Antimicrobial Resistance Considerations

Resistance to tiamulin, lincomycin, and tylosin has been documented in B. hyodysenteriae isolates globally. Minimum inhibitory concentration (MIC) testing by broth dilution or agar dilution is recommended for herds with recurrent disease. Rotation of antimicrobial classes and adherence to withdrawal periods are essential.

7. Prevention and Control of Classical Swine Fever

7.1 Vaccination

Modified live virus (MLV) vaccines against CSFV are available and provide robust immunity. Vaccination is used in endemic regions as part of control programs. The vaccine is administered intramuscularly or subcutaneously, with immunity developing within 7 to 14 days. Vaccination strategies include:

• Routine vaccination of breeding sows and replacement gilts.
• Vaccination of piglets at 4 to 6 weeks of age in high-risk areas.
• Emergency vaccination during outbreaks to create a buffer zone.

Marker vaccines (E2 subunit vaccines) allow serological differentiation of infected from vaccinated animals (DIVA strategy) but are less commonly used in swine.

7.2 Biosecurity Measures

CSF is a notifiable disease in most countries. Control relies on strict biosecurity and stamping-out policies in disease-free regions.

• Quarantine new arrivals for 30 days.
• Prevent contact with feral pigs and contaminated fomites.
• Use dedicated farm clothing and footwear.
• Disinfect transport vehicles and equipment.
• Control wild boar populations in endemic areas.

7.3 Eradication Strategies

In CSF-free regions, eradication involves:

• Immediate notification of suspect cases.
• Stamping out (culling of infected and contact herds).
• Movement restrictions and surveillance zones.
• Tracing of animal movements and epidemiological investigation.

8. Integrated Disease Management

Effective control of swine enteric and systemic diseases requires a combination of diagnostic surveillance, antimicrobial stewardship, vaccination, and biosecurity. Herd health programs should include:

• Regular monitoring of fecal consistency and mortality records.
• Diagnostic testing of diarrheic pigs using PCR panels that include B. hyodysenteriae, Lawsonia intracellularis, and Salmonella spp.
• Serological surveillance for CSFV in endemic regions.
• Antimicrobial susceptibility testing for bacterial pathogens.
• Nutritional management to reduce gastric ulceration risk (e.g., finely ground feed, stress reduction).

9. Conclusion

Swine dysentery and classical swine fever represent two distinct but clinically overlapping causes of bloody diarrhea and fever in pigs. Accurate diagnosis requires integration of clinical signs, epidemiological context, and laboratory testing. Treatment of swine dysentery depends on effective antimicrobials and management changes, while CSF control relies on vaccination and biosecurity. Veterinary practitioners must maintain a high index of suspicion for both diseases and implement appropriate diagnostic and control measures to minimize economic losses and prevent disease spread.

References

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2. Moennig V, Becher P. Classical swine fever. In: Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, editors. Diseases of Swine. 11th ed. Wiley-Blackwell; 2019. p. 538-553.
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