Dysentery on swine: what is it, causes, symptoms, prevention

Swine Dysentery (SD) stands as a formidable challenge in pig production worldwide, a severe infectious disease primarily characterized by mucohemorrhagic diarrhea. Caused predominantly by the bacterium Brachyspira hyodysenteriae, and increasingly by other strongly beta-hemolytic Brachyspira species like B. hampsonii and B. suanatina, this condition continues to inflict significant economic losses on swine operations. Understanding its insidious nature, transmission pathways, and the latest control strategies is crucial for maintaining herd health and profitability.

The etiology of the scourge

B.rachyspira hyodysenteriae is a Gram-negative, anaerobic spirochete with a distinctive spiral shape. Its pathogenicity lies in its ability to colonize the large intestine, leading to inflammation, excessive mucus production, and hemorrhage. While B. hyodysenteriae has historically been the primary culprit, the emergence of other virulent Brachyspira species with similar clinical presentations necessitates precise laboratory diagnosis to differentiate between them. These bacteria are robust, capable of surviving for extended periods in moist feces and lagoon water, making environmental control a significant hurdle.

Recognizing the clinical signs

The clinical presentation of swine dysentery can vary in severity, but typically follows an incubation period of 5 to 21 days, though it can extend up to 60 days. While all ages of swine can be affected, it is most commonly observed in growing and finishing pigs (12 to 75 kg). Initial signs are often subtle, including dullness, depression, and reduced appetite. As the disease progresses, the hallmark mucohemorrhagic diarrhea emerges:

• Initial stages: soft, gray to yellow, mucoid feces.
• Progression: diarrhea becomes increasingly severe, containing copious amounts of mucus and fresh blood, often appearing dark or tar-like due to digested blood.
• Systemic effects: pigs rapidly lose body condition, exhibiting sunken eyes, dehydration, and hollow flanks. Despite a fluctuating appetite, affected animals often continue to drink.
• Mortality: while mortality can be variable, it can reach up to 50% in untreated groups, primarily due to dehydration and electrolyte imbalances.

The disease spreads steadily within and between pens, rather than explosively, as the microorganism accumulates in the environment. In chronically infected herds, the disease may become restricted to the growing pig population, while in naïve populations, it can affect all age groups.

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The pathways of transmission

Transmission of Brachyspira is primarily fecal-oral. Infected pigs, even clinically normal carriers, shed the bacteria in their feces, contaminating the environment. This makes biosecurity a cornerstone of prevention. Common transmission routes include:

• Direct contact: ingestion of fecal material from infected pigs.
• Contaminated environment: survival of the bacteria in lagoon water, moist feces, and soil.
• Fomites: contaminated equipment, feed distribution trucks, boots, and clothing can serve as vectors.
• Pest vectors: rodents, birds, and flies can carry the organism, spreading it between and within farms. Mice, for instance, have been reported to shed B. hyodysenteriae for extended periods.
• Carrier animals: purchased breeding stock or replacement animals, seemingly healthy, can introduce the pathogen into a naive herd. Carrier sows can also transmit the agent to their piglets.

Stressors such as diet changes, poor hygiene, high humidity, and crowded conditions can exacerbate the disease and trigger clinical outbreaks in subclinically infected pigs.

The economic toll of swine dysentery

The economic impact of swine dysentery on pig production is substantial and multifaceted. Beyond direct mortality, the disease leads to significant production losses:

• Reduced growth performance: affected pigs exhibit decreased average daily gain (ADG) and poor feed conversion ratios (FCR) due to appetite loss and impaired nutrient absorption.
• Treatment costs: the reliance on antimicrobials for treatment incurs considerable expense, further complicatedounded by potential drug resistance.
• Labor and management: increased labor is required for monitoring sick animals, administering treatments, and implementing rigorous cleaning and disinfection protocols.
• Depopulation and repopulation: in severe or persistent outbreaks, complete depopulation and repopulation with disease-free stock, while effective, represents a massive financial outlay.
• Market value reduction: chronically affected pigs may be stunted or fail to thrive, reducing their market value.

Estimates suggest that annual losses due to clinical swine dysentery can be significant, potentially reaching hundreds of millions of dollars in major swine-producing regions. This underscores the critical need for effective control and prevention strategies.

Strategies for control and prevention

Controlling swine dysentery requires a comprehensive approach encompassing biosecurity, sanitation, judicious antimicrobial use, and management practices.

Biosecurity

• Source stock: procure replacement animals only from farms certified free of swine dysentery.
• Quarantine: implement strict quarantine protocols for all incoming animals.
• Traffic control: limit and control vehicle and visitor movement onto the farm, ensuring thorough cleaning and disinfection of anything entering the premises that could carry feces.
• Vermin control: implement robust rodent and bird control programs, as they are known reservoirs and vectors.

Management and sanitation

• All-in-all-out production: implement all-in-all-out pig flow by age group or facility, allowing for thorough cleaning and disinfection between batches.
• Hygiene: maintain high standards of hygiene, particularly on concrete floors, which should be washed down daily.
• Disinfection: utilize effective disinfectants such as chlorhexidine, formaldehyde, cresols, and phenols, applied after thorough cleaning.
• Dietary management: diet can significantly influence disease severity. Diets that promote small intestinal digestion, such as meal over pellets, may reduce the substrate available for Brachyspira in the large intestine. Some studies even suggest that pigs fed cooked white rice show fewer clinical signs despite exposure.

Antimicrobial use

• Early intervention: antimicrobials are effective if administered early in the disease course. Water medication (e.g., with lincomycin, tiamulin, tylosin) is often preferred for rapid group treatment.
• Individual treatment: severely affected pigs may require injectable antimicrobials.
• Sensitivity testing: due to increasing antimicrobial resistance, sensitivity testing of Brachyspira isolates is crucial to guide treatment selection.
• Withdrawal periods: strict adherence to withdrawal times for all medications is essential.

Vaccination

While commercial vaccines for swine dysentery are available in some regions, their effectiveness can be variable and often serotype-specific. Research into more broadly effective vaccines, including those targeting mucosal immunity, is ongoing.