Salmonella in Chickens: Clinical Signs, Zoonotic Risks, and Diagnostic Differentiation from Other Enteric Pathogens

Introduction

Salmonellosis in poultry represents a complex disease spectrum caused by multiple serovars of Salmonella enterica subsp. enterica. These infections range from acute systemic disease with high mortality to asymptomatic carrier states that perpetuate environmental contamination and foodborne zoonotic transmission [1, 2, 3]. The genus Salmonella comprises over 2,600 serovars, but only a limited subset is epidemiologically relevant to chickens. These are broadly divided into host-restricted serovars (S. Gallinarum, S. Pullorum) that cause severe systemic disease in poultry, and broad-host-range serovars (S. Enteritidis, S. Typhimurium, S. Infantis, S. Kentucky) that are frequently carried subclinically but pose significant zoonotic hazards through the food chain [4, 5, 68, 96]. Understanding the clinical presentation, zoonotic implications, and accurate laboratory differentiation from other enteric pathogens is critical for effective flock management and public health protection.

Clinical Signs of Salmonella Infections in Chickens

The clinical expression of salmonellosis in chickens depends on the infecting serovar, bird age, immune status, route of exposure, and concurrent infections. Two major clinical syndromes are recognized: the acute septicemic forms caused by host-adapted serovars (fowl typhoid and pullorum disease) and the enteric/subclinical carrier states typically induced by paratyphoid serovars [47, 149].

Fowl Typhoid (Salmonella Gallinarum)

Fowl typhoid, caused by Salmonella enterica serovar Gallinarum biovar Gallinarum, is a septicemic disease affecting chickens of all ages, although mortality is often highest in adult layers and growers [91, 115, 149]. The incubation period following oral inoculation is 3 to 5 days. Clinical signs include acute onset of somnolence, drooping wings, ruffled feathers, anorexia, pyrexia, and a characteristic greenish-yellow diarrhea [91, 101, 117]. Morbidity and mortality can reach 100% in naive flocks [71, 101]. Postmortem findings typically reveal hepatomegaly with a bronze discoloration, splenomegaly, hemorrhagic enteritis, and fibrinous pericarditis [91, 145]. In chronic cases, ovarian regression and peritonitis are observed in laying hens [75, 141].

Pullorum Disease (Salmonella Pullorum)

Pullorum disease, caused by Salmonella enterica serovar Gallinarum biovar Pullorum, primarily affects young chicks under 3 weeks of age [106, 149]. Clinical signs appear within 48 to 72 hours post-hatch and include profound weakness, huddling, anorexia, pasty white diarrhea (chalky vent), labored breathing, and ataxia [99, 106, 118]. Mortality can exceed 80% in the first week [100, 106]. In older birds, subclinical ovarian localization leads to vertical transmission with infected eggs and hatchery outbreaks [6, 106, 149]. S. Pullorum is host-restricted and non-zoonotic, but its control is essential for trade and flock health [7, 46, 77].

Paratyphoid Salmonellosis (S. Enteritidis, S. Typhimurium, S. Infantis, and Others)

Paratyphoid Salmonella serovars are characterized by their broad host range and ability to colonize the chicken intestine without inducing overt clinical signs in many cases [47, 87, 96]. However, under stress or in young birds, clinical outbreaks can occur. Experimental infection with S. Enteritidis in broilers produces lethargy, reduced feed intake, and watery diarrhea, with mortality ranging from 5% to 27% depending on strain and dose [98, 109, 140]. S. Typhimurium infection similarly induces enteritis, typhlitis, and occasional septicemia in chicks, with yellow mucoid droppings [87, 99, 125, 138]. S. Infantis has emerged as a major serovar in many regions, frequently harboring multidrug resistance plasmids (pESI) but causing minimal clinical disease in chickens [8, 55, 65]. S. Kentucky is often isolated from asymptomatic chickens but can cause catarrhal enteritis and weight loss under experimental conditions [48, 72, 125]. Co-infection with immunosuppressive agents such as avian influenza virus (H9N2) markedly exacerbates clinical severity and pathogen shedding [9, 92].

Clinical Signs by Serovar: Comparative Overview

Zoonotic Risks of Chicken-Associated Salmonella

The zoonotic potential of Salmonella from chickens is a major One Health concern. Human salmonellosis is predominantly caused by the broad-host-range serovars, especially S. Enteritidis and S. Typhimurium, which are transmitted through contaminated poultry meat and eggs [2, 10, 3, 65, 80]. S. Infantis has recently emerged as a significant foodborne pathogen globally, often carrying extended-spectrum beta-lactamase and colistin resistance genes [8, 11, 55]. S. Kentucky, while less frequently implicated in human illness, has been associated with multidrug-resistant infections and may carry zoonotic potential [48, 72].

The primary route of human exposure is the consumption of undercooked eggs or meat from infected birds. Additionally, direct contact with infected flocks or contaminated environments can lead to human infection, particularly among poultry workers [12, 13, 64, 90]. The presence of carrier birds that shed Salmonella without clinical signs is a critical obstacle to risk mitigation [47, 86, 96]. Surveillance studies from multiple countries have demonstrated high prevalence of multidrug-resistant Salmonella isolates in chicken carcasses, retail meat, and slaughterhouse environments, underscoring the need for effective on-farm control programs [1, 5, 14, 42, 69, 76, 82, 84]. The host-restricted serovars Gallinarum and Pullorum are not considered zoonotic, but their control is essential for maintaining flock health and minimizing economic losses [7, 46, 89].

Diagnostic Differentiation from Other Enteric Pathogens

Accurate diagnosis of salmonellosis in chickens requires integration of clinical observation, postmortem examination, bacteriological culture, and molecular methods. The clinical signs of Salmonella infection overlap significantly with those of other common enteric and septicemic pathogens, including colibacillosis (Escherichia coli), necrotic enteritis (Clostridium perfringens), coccidiosis (Eimeria spp.), histomoniasis (Histomonas meleagridis), and Necrotic Enteritis in Broiler Chickens: Clostridium perfringens Virulence Factors, Gut Microbiome, and Probiotic Control Strategies [15, 16, 49, 88, 147].

Differential Diagnosis Table: Key Enteric Pathogens in Chickens

Laboratory Diagnostic Approaches

Bacteriological Culture and Serotyping

Traditional isolation of Salmonella from cloacal swabs, feces, or organ samples remains the gold standard [15, 12, 90, 96]. Samples are pre-enriched in buffered peptone water, followed by selective enrichment (e.g., Rappaport-Vassiliadis broth) and plating onto xylose-lysine-deoxycholate (XLD) agar. Suspect colonies are confirmed biochemically and serogrouped using polyvalent O and H antisera [96, 123]. Final serovar determination requires agglutination with specific factor sera or genomic analysis [21, 7, 72].

Molecular Detection and Differentiation

Polymerase chain reaction (PCR)-based methods offer higher sensitivity and faster turnaround than culture. The invA gene is a widely used target for genus-level detection, but false negatives may occur due to sequence variation [86, 121]. Serovar-specific PCR assays have been developed for simultaneous detection of S. Enteritidis, S. Pullorum, S. Typhimurium, and S. Infantis using multiplex PCR [19]. For differentiation of S. Pullorum from S. Gallinarum, a fimH-based high-resolution melting (HRM) PCR or a multiplex assay targeting torT and I137_14430 genes provides reliable discrimination [22, 52]. Recombinase polymerase amplification (RPA) combined with lateral flow dipsticks enables rapid on-site detection of S. Pullorum and S. Enteritidis [17]. Loop-mediated isothermal amplification (LAMP) targeting single-nucleotide polymorphisms in the group_17537 gene further enhances field diagnostics [18].

Quantitative PCR (qPCR) assays with TaqMan probes allow high-throughput quantification of S. Pullorum in chicken samples [23]. For differentiation of S. enterica subspecies arizonae (subsp. IIIa) from other salmonellae, a subspecies-specific qPCR has been validated. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) provides a reliable proteomic identification of Salmonella isolates, though it cannot distinguish between closely related serovars in all cases.

Serological Methods

Indirect enzyme-linked immunosorbent assays (ELISAs) based on recombinant proteins (e.g., Sptp, SifA) are available for flock-level screening [24, 58]. These assays detect antibodies against Salmonella but cannot differentiate active infection from vaccination or past exposure. The plate agglutination test (PAT) using whole-cell antigens is commonly used for pullorum disease and fowl typhoid surveillance in breeding flocks [118, 145].

Diagnostic Decision Algorithm

The following Mermaid flowchart illustrates a recommended diagnostic workflow for suspected Salmonella infection in chickens:

flowchart TD
 A["Clinical signs: depression, diarrhea, mortality"] --> B{Necropsy}
 B --> C[Bronze liver, splenomegaly, pericarditis?]
 C -->|Yes| D[Collect liver, spleen, cecal contents]
 C -->|No| E[Consider other enteric pathogens]
 E --> F[Fecal oocyst exam for coccidiosis]
 E --> G[Anaerobic culture for Clostridium]
 D --> H[Selective enrichment + XLD culture]
 H --> I[Biochemical confirmation + MALDI-TOF]
 I --> J{Serovar identification}
 J --> K[Serogrouping with O/H antisera]
 J --> L["Multiplex PCR: invA, sdfI, fliC, etc."]
 L --> M[Differentiate Pullorum vs Gallinarum via fimH-HRM or torT PCR]
 H --> N[TaqMan qPCR for rapid quantification]
 N --> O["'Confirm with serology (ELISA') at flock level"]
 K --> P[Report serovar + antimicrobial susceptibility]

Conclusion

Salmonellosis in chickens encompasses a spectrum of diseases from acute septicemic fowl typhoid to subclinical carrier states of paratyphoid serovars. The zoonotic significance of S. Enteritidis, S. Typhimurium, S. Infantis, and S. Kentucky demands rigorous on-farm biosecurity and surveillance programs. Clinical differentiation from other enteric pathogens requires a systematic approach incorporating postmortem examination, bacterial culture, molecular serotyping, and serology. Advances in multiplex PCR, HRM analysis, RPA-LFD, and proteomic identification have improved diagnostic precision and speed. Management strategies must integrate antimicrobial stewardship given the high prevalence of multidrug resistance, and should consider alternatives such as phage therapy, probiotics, and vaccines to reduce Salmonella carriage [4, 25, 26, 9, 27, 28, 29, 30, 53, 57, 63, 66, 70, 75, 88, 104, 120, 139, 148]. Continued genomic surveillance of circulating serovars and resistance determinants is essential for informed control policies [2, 5, 31, 21, 7, 42, 59, 69, 78].

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.