Introduction to Salmonella in Poultry

 

Dr. Majed Hamed Al Saegh / poultry pathologist / Australia

 

Salmonellosis in poultry is a significant bacterial disease that poses major concerns for both animal health and public health. It is caused by various Salmonella serotypes, which can lead to systemic infections in birds and potential zoonotic transmission to humans through contaminated poultry products. Salmonella infections are responsible for economic losses in the poultry industry due to reduced productivity, increased mortality, and strict regulatory measures aimed at food safety.

 

Salmonella bacteria are Gram-negative, facultative anaerobic rods belonging to the Enterobacteriaceae family. They are broadly categorized into two groups based on host specificity: host-adapted serotypes, such as Salmonella Pullorum and Salmonella Gallinarum, which primarily affect poultry, and non-host-adapted serotypes, such as Salmonella Enteritidis and Salmonella Typhimurium, which have a wider host range and are commonly associated with foodborne illnesses in humans.

 

The disease manifests in different forms depending on the serotype involved. Salmonella Pullorum causes Pullorum disease, an acute septicemic condition in young chicks, while Salmonella Gallinarum is responsible for Fowl Typhoid, which affects older birds. Non-host-specific paratyphoid Salmonella infections, caused by S. Enteritidis and S. Typhimurium, often remain asymptomatic in poultry but are a leading cause of human salmonellosis through contaminated eggs and meat.

 

Transmission of Salmonella occurs through vertical transmission (from infected hens to eggs and offspring) and horizontal transmission via contaminated feed, water, feces, equipment, and contact with infected birds or rodents. The bacteria can persist in poultry houses and hatcheries, making effective biosecurity and hygiene measures crucial in controlling its spread.

 

Diagnosis of Salmonella infections in poultry is conducted through bacterial culture, serological tests, and molecular techniques such as PCR. Effective management strategies include vaccination, antimicrobial interventions, and strict biosecurity measures to reduce the risk of infection in flocks.

 

Due to its zoonotic nature, Salmonella control in poultry is a critical component of public health, requiring integrated efforts from veterinarians, poultry producers, and food safety authorities.

 

Diagnosis of Salmonella in Poultry

Accurate diagnosis of Salmonella infections in poultry is crucial for disease management, prevention, and control. Several diagnostic methods are available, ranging from traditional bacterial culture techniques to modern molecular approaches. The choice of diagnostic method depends on factors such as the stage of infection, the need for rapid detection, and the intended application (field diagnosis, research, or regulatory compliance).

 

1. Clinical Diagnosis

While clinical signs provide an initial indication of Salmonella infection, they are often nonspecific and overlap with other bacterial diseases in poultry. Common symptoms include: (Depression, lethargy, and reduced activity, White, pasty diarrhea (Pullorum disease) or yellowish-green diarrhea (Fowl typhoid), Poor growth and weight loss, Reduced egg production in laying birds, Respiratory distress in severe systemic infections,

Since clinical signs alone are not definitive, laboratory confirmation is necessary.

 

2. Post-Mortem (PM) Diagnosis

Gross pathological lesions observed during necropsy can provide useful clues, particularly in systemic infections:

 

Pullorum Disease (Salmonella Pullorum): ( Enlarged liver with multiple necrotic foci, Peritonitis and congested lungs, Unabsorbed yolk sac in young birds, Necrotic, caseous cecal cores

 

Fowl Typhoid (Salmonella Gallinarum): (Enlarged, friable liver with bronze discoloration, Enlarged spleen and kidneys, Catarrhal enteritis with bile-stained intestines, Yellowish exudate in the pericardium.

 

Paratyphoid Infections (e.g., S. Enteritidis, S. Typhimurium): (Focal hepatic necrosis, Enteritis with mucosal erosion, Cecal cores with fibrinous exudate, While post-mortem findings are valuable, confirmation requires laboratory testing.

 

3. Bacteriological Culture (Gold Standard Diagnosis)

Isolation and identification of Salmonella from poultry tissues, feces, or environmental samples is the most reliable diagnostic method. The process involves:

 

Sample Collection:  Cloacal swabs, feces, or organ tissues (liver, spleen, intestines), Environmental samples (water, feed, litter).

 

Selective Enrichment & Plating: Pre-enrichment in buffered peptone water (BPW),  Selective enrichment in Rappaport-Vassiliadis (RV) broth or Selenite broth, Plating on Xylose Lysine Deoxycholate (XLD) agar, Brilliant Green Agar (BGA), or Hektoen Enteric (HE) agar

 

Biochemical Identification:

Triple Sugar Iron (TSI) test: Salmonella produces an alkaline (red) slant with a yellow butt and hydrogen sulfide (H₂S) production

Urease and citrate utilization tests

Agglutination using Salmonella O- and H-antigen-specific antisera

While culture is definitive, it requires 3–5 days, making it less practical for rapid screening.

 

4. Serological Tests

Serological assays detect antibodies against Salmonella in poultry, mainly for flock monitoring:

Rapid Serum Agglutination Test (SAT):  Detects antibodies to S. Pullorum and S. Gallinarum, Useful for screening breeder and layer flocks, May give false positives due to cross-reactivity with other bacteria

Enzyme-Linked Immunosorbent Assay (ELISA): Detects specific Salmonella antibodies in serum or egg yolk, More sensitive than SAT and useful for large-scale screening

Although serology is useful for surveillance, it does not distinguish between active infection and past exposure.

 

5. Molecular Diagnosis (PCR & RT-PCR)

Polymerase Chain Reaction (PCR) has revolutionized Salmonella detection by providing rapid and specific results:

Conventional PCR:  Detects Salmonella DNA in clinical samples

Real-Time PCR (qPCR):  Quantifies Salmonella loads with high sensitivity and specificity

Multiplex PCR:  Identifies multiple Salmonella serovars in a single reaction

 

Common target genes include:

invA (invasion-associated gene; highly conserved in Salmonella)

hilA (regulator of virulence genes)

stn (Salmonella enterotoxin gene)

 

PCR-based methods offer fast results within hours, making them ideal for surveillance and outbreak investigations.

 

6. Whole Genome Sequencing (WGS) & Next-Generation Sequencing (NGS)

Advanced sequencing techniques are increasingly used to:

• Identify Salmonella serovars and antimicrobial resistance genes.
• Trace outbreaks and track the epidemiology of Salmonella in poultry and food chains.
• Differentiate field strains from vaccine strains.
• While WGS provides comprehensive data, its routine use is limited due to high costs and technical expertise requirements.

 

7. Rapid Diagnostic Tests (RDTs)

Lateral Flow Assays (LFAs) and Immunochromatographic Strips provide on-site detection of Salmonella antigens in poultry farms, offering quick screening results within minutes. However, they lack sensitivity compared to PCR or culture methods.

 

Therapy and Control of Salmonella in Poultry: The treatment and control of Salmonella infections in poultry require a multifaceted approach, including antibiotic therapy, biosecurity measures, vaccination, and probiotic interventions. Due to the zoonotic nature of Salmonella, treatment strategies must also consider antimicrobial resistance (AMR) and food safety concerns.

 

1. Antimicrobial Therapy

Antibiotic treatment is commonly used to manage Salmonella infections in poultry; however, it should be employed with caution due to increasing AMR and regulatory restrictions on antimicrobial use in food-producing animals.

1. Commonly Used Antibiotics

 

• Fluoroquinolones (e.g., Enrofloxacin, Ciprofloxacin): Effective against Salmonella but increasingly restricted due to AMR concerns.
• Macrolides (e.g., Tylosin, Erythromycin): Less effective against systemic infections but may reduce bacterial shedding.
• Aminoglycosides (e.g., Gentamicin, Neomycin): Often used in poultry but require careful dosage due to potential nephrotoxicity.
• Beta-lactams (e.g., Amoxicillin, Ampicillin): Historically used but now less effective due to resistance.
• Sulfonamides and Trimethoprim Combinations (e.g., Sulfamethoxazole-Trimethoprim): Sometimes used, but resistance is increasing.
• Tetracyclines (e.g., Doxycycline, Oxytetracycline): Resistance is common, reducing their effectiveness.

 

1. Route of Administration

Oral Administration: Antibiotics are often given in drinking water or feed for flock-wide treatment.

Injection (Intramuscular or Subcutaneous): Used for individual birds with severe infections (rare in commercial flocks).

 

1. Concerns with Antimicrobial Use

– Development of antimicrobial-resistant Salmonella strains that can be transmitted to humans.

– Regulatory bans on the use of certain critically important antibiotics in poultry production.

– Risk of antibiotic residues in meat and eggs, requiring withdrawal periods.

 

1. Alternatives to Antibiotics: Given the risks associated with antibiotic therapy, alternative strategies are being explored, including phage therapy, essential oils, and metal-based antimicrobials (e.g., copper and silver nanoparticles).

 

2. Vaccination Strategies

Vaccination plays a crucial role in controlling Salmonella in poultry, reducing bacterial colonization and shedding.

 

1. Types of Salmonella Vaccines

1- Live Attenuated Vaccines, Examples: Salmonella Enteritidis (SE) vaccine, Salmonella Typhimurium (ST) vaccine. Administered via drinking water, spray, or eye drops. Induce cell-mediated immunity and mucosal immunity.

2- Inactivated (Killed) Vaccines, Require booster doses for full efficacy. Often combined with adjuvants for longer-lasting immunity.

3- Autogenous Vaccines (Farm-Specific Strains), Custom-made for specific outbreaks or endemic strains.Useful in regions with emerging or antibiotic-resistant strains.

 

1. Vaccination Schedule

Day-old chicks: Live attenuated vaccines in drinking water.

6–8 weeks of age: Booster vaccination.

Laying hens: Revaccination to reduce vertical transmission.

 

3. Biosecurity Measures

Prevention is the best strategy to control Salmonella in poultry. Strict biosecurity can significantly reduce the risk of infection.

 

1. Farm Hygiene and Management

– Disinfection: Regular cleaning of poultry houses with formalin, quaternary ammonium compounds, or peracetic acid.

– Litter Management: Dry and clean litter reduces bacterial survival.

– Water Sanitation: Acidification of drinking water (using organic acids like citric or formic acid) helps control Salmonella.

 

1. Rodent and Pest Control

– Rodents are major reservoirs of Salmonella; controlling mice and rats is essential.

– Insect control (flies and beetles) also reduces transmission risks.

 

1. Movement Control

– Limiting human and vehicle movement between farms minimizes cross-contamination.

– Footbaths and clean clothing for workers help prevent bacterial spread.

 

4. Probiotics, Prebiotics, and Competitive Exclusion

 

1. Probiotics (Live Beneficial Bacteria)

Competitive Exclusion: Good bacteria outcompete Salmonella in the gut.

Common probiotic strains: Lactobacillus spp., Bifidobacterium spp., and Bacillus spp.

Administered via feed or water.

 

1. Prebiotics (Non-Digestible Fibers That Promote Gut Health), Examples: Fructooligosaccharides (FOS), Mannanoligosaccharides (MOS), Help reduce Salmonella colonization in the intestines.

 

1. Synbiotics (Probiotics + Prebiotics), Combining both improves gut health and reduces Salmonella shedding.

 

5. Bacteriophage Therapy

– Bacteriophages (viruses that infect bacteria) are a promising alternative to antibiotics for controlling Salmonella infections.

– Phage-based feed additives can reduce Salmonella colonization in poultry.

– Targeted phage therapy minimizes disruption to the gut microbiome.

– Already approved for food safety applications in some countries.

 

6. Organic Acids and Essential Oils

– Organic acids and phytogenic feed additives help control Salmonella in poultry: Organic Acids (e.g., Formic acid, Propionic acid, Lactic acid): Lower gut pH and inhibit Salmonella growth.

– Essential Oils (e.g., Oregano oil, Thyme oil, Garlic extract): Have antimicrobial and anti-inflammatory properties.

 

7. Metal-Based Antimicrobials

Recent studies have explored the use of copper, silver, and zinc nanoparticles as antimicrobial agents against Salmonella. These metals:

• Disrupt bacterial cell walls.
• Inhibit Salmonella colonization in the gut.
• Offer an alternative to antibiotics.

 

8. Heat Treatment and Feed Additives

Pelleting of Feed:  High temperatures during feed processing reduce Salmonella contamination.

Feed Additives: Beta-glucans and yeast extracts: Boost immune responses. Medium-chain fatty acids (MCFAs): Inhibit bacterial growth.

 

9. Eradication Programs

Many countries implement Salmonella control programs, including:

• Test-and-Slaughter Policies: Removing infected breeder flocks.
• Certification Programs: Ensuring Salmonella-free poultry products.

For example, the EU Salmonella Control Program mandates routine testing and control measures to minimize Salmonella in poultry.

 

 

 

The Zoonotic Impact of Salmonella Infections in Poultry

Introduction

Salmonella is one of the most significant zoonotic pathogens associated with poultry, posing serious risks to human health worldwide. Poultry, including chickens, turkeys, and ducks, are major reservoirs of Salmonella, which can be transmitted to humans through direct or indirect contact. The consumption of contaminated poultry meat, eggs, or products is the primary route of infection.

 

Human Salmonella infections, collectively known as salmonellosis, lead to substantial public health and economic burdens, particularly in developing countries where food safety regulations may be inadequate. This review outlines the zoonotic impact of Salmonella, its transmission routes, clinical effects in humans, risk factors, and control strategies.

 

1. Transmission of Salmonella from Poultry to Humans

The primary routes of Salmonella transmission include:

• Foodborne Transmission: Contaminated Poultry Meat: Salmonella colonizes the intestines of birds without causing clinical disease, leading to contamination during slaughter and processing.
• Raw or Undercooked Eggs: Some Salmonella serotypes, especially S. Enteritidis, can infect developing eggs in laying hens, contaminating eggs before shell formation.
• Cross-Contamination: Kitchen surfaces, utensils, and food handlers can spread Salmonella from raw poultry to other foods.
• Direct Contact with Infected Birds or Their Environment:
• Farm Workers and Poultry Handlers: Individuals working with infected poultry may acquire Salmonella through fecal-oral transmission.
• Backyard Poultry Owners: Pet chickens and ducks have become popular, increasing the risk of household infections, especially in children.
• Water and Feed Contamination: Salmonella can persist in contaminated water supplies and poultry feed, contributing to widespread infections in both animals and humans.
• Fomites and Vectors: Rodents, flies, and insects act as mechanical carriers, spreading Salmonella from poultry farms to human settlements.

 

2. Clinical Effects of Salmonella in Humans

Human Salmonella infections vary in severity depending on the serotype, host immunity, and infectious dose. The main forms of salmonellosis include:

• Gastroenteritis (Non-Typhoidal Salmonellosis – NTS): Causative Serotypes: S. Enteritidis, S. Typhimurium, S. Heidelberg . Incubation Period: 6–72 hours after ingestion. Symptoms: Nausea, vomiting, abdominal cramps, watery or bloody diarrhea, fever and chills, dehydration (in severe cases). Duration: Typically 4–7 days, but can be longer in immunocompromised individuals.

 

• Invasive Salmonellosis (Bacteremia and Septicemia): Affects elderly, infants, and immunosuppressed individuals. May cause sepsis, meningitis, endocarditis, and multi-organ failure.

 

• Reactive Arthritis (Reiter’s Syndrome): Post-infectious complication following Salmonella gastroenteritis. Symptoms include joint pain, inflammation, and eye irritation. Can last weeks to months and may become chronic.

 

• Typhoid-Like Illness: Some poultry-associated Salmonella strains (e.g., S. Gallinarum) can cause prolonged fever and systemic disease in immunocompromised individuals.

 

3. Risk Factors for Human Salmonella Infection

Several factors increase the likelihood of Salmonella transmission from poultry to humans:

• Poor Food Handling and Hygiene: Inadequate cooking of poultry products
• Cross-contamination of raw poultry with ready-to-eat foods. Lack of handwashing after handling raw meat
• Consumption of High-Risk Foods: Raw or undercooked eggs (e.g., homemade mayonnaise, eggnog), Poultry products from non-regulated sources (e.g., backyard poultry, live markets)
• Poultry Farming and Processing Practices:
• Intensive poultry production increases bacterial load in flocks.
• Overcrowding and poor biosecurity facilitate rapid Salmonella spread.

 

Antimicrobial Resistance (AMR): Widespread antibiotic use in poultry has led to the emergence of multidrug-resistant (MDR) Salmonella strains. MDR S. Typhimurium DT104 is a major public health concern.

Immunocompromised Populations: Elderly individuals, young children, pregnant women, and people with weakened immune systems are at higher risk of severe infections.

 

4. Control and Prevention Strategies

Poultry Industry Measures:

 

Biosecurity Programs: Prevent Salmonella entry into poultry farms.

Vaccination: Live and inactivated Salmonella vaccines reduce bacterial shedding in poultry.

Probiotics and Prebiotics: Competitive exclusion of Salmonella in the gut.

Phage Therapy: Targeted bacteriophage treatments against Salmonella.

Antibiotic Stewardship: Restrict unnecessary antibiotic use in poultry farming.

 

Food Safety and Consumer Education:

Proper Cooking: Poultry should be cooked to ≥75°C (165°F) to kill Salmonella.

Avoid Raw Eggs: Use pasteurized eggs in recipes requiring raw eggs.

Prevent Cross-Contamination: Separate raw poultry from other foods; clean utensils and surfaces after handling raw poultry.

 

Regulatory and Surveillance Programs:

Pathogen Reduction Programs in poultry production.

Routine Salmonella testing in food-processing plants.

Public health monitoring of outbreaks and antimicrobial resistance trends.

 

Personal Hygiene and Backyard Poultry Safety:

Wash hands thoroughly after handling poultry or cleaning coops.

Keep backyard poultry away from food preparation areas.

Do not allow small children to handle live birds without supervision.

 

5. Global Burden of Zoonotic Salmonella Infections. World Health Organization (WHO): Estimates 93.8 million cases of foodborne Salmonella infections annually, resulting in 155,000 deaths.