Salmonella Gallinarum and Salmonella Pullorum in Poultry: Fowl Typhoid and Pullorum Disease

Introduction

Fowl typhoid and pullorum disease are two of the most economically significant bacterial infections affecting poultry worldwide [127, 136]. Both diseases are caused by host-adapted members of Salmonella enterica subsp. enterica serovar Gallinarum (S. Gallinarum) [104, 127]. The etiologic agents are classified as biovars: S. Gallinarum biovar Gallinarum (bv. Gallinarum) causes fowl typhoid, while S. Gallinarum biovar Pullorum (bv. Pullorum) causes pullorum disease [80, 104]. These biovars are non-motile, host-restricted to avian species, and do not typically cause disease in mammals [122, 127]. The distinction between the two diseases was recognized as early as the early 20th century, and they remain notifiable to veterinary authorities in many jurisdictions [116, 136]. Despite decades of control efforts, outbreaks continue to occur in commercial and backyard flocks, often linked to persistent infections in breeder stock or environmental contamination [1, 45, 107]. This article provides an exhaustive review of the bacteriology, pathogenesis, clinical presentation, diagnostic methodologies, antimicrobial resistance patterns, and control strategies for these two ancient but still relevant poultry pathogens.

Etiology and Taxonomy

Salmonella enterica serovar Gallinarum is a Gram-negative, facultative anaerobic rod belonging to the family Enterobacteriaceae [104, 127]. The serovar is divided into two biovars based on biochemical and genetic differences: bv. Pullorum and bv. Gallinarum [69, 109]. Both biovars lack flagella (non-motile) due to mutations in the fliC flagellin gene, and they share the somatic O-antigen group O9,12 [80, 137]. Key biochemical distinctions include differences in dulcitol fermentation, ornithine decarboxylase activity, and mucate utilization [80, 144]. Genomically, bv. Pullorum and bv. Gallinarum are closely related but contain region of difference (ROD) sequences that enable molecular differentiation [109, 120]. Comparative pan-genomics has revealed that bv. Gallinarum strains possess a larger genome than bv. Pullorum, with specific virulence-associated genes and pseudogenes that contribute to host adaptation [69, 109, 120]. The live attenuated vaccine strain SG9R is derived from bv. Gallinarum and has been used extensively for fowl typhoid control [93, 118]. The evolutionary divergence of the two biovars is thought to have occurred via genomic decay and pseudogene formation, particularly in metabolic and virulence loci [69, 114, 120].

Pathogenesis and Virulence Factors

Both biovars are highly invasive and cause systemic infection in poultry, but they differ in their preferred host age and disease progression [127, 136]. S. Pullorum primarily affects young chicks, often causing high mortality within the first weeks of life, while S. Gallinarum affects birds of all ages, particularly adults, leading to a more protracted disease course [80, 127]. The infection route is typically fecal-oral, with bacteria penetrating the intestinal epithelium and disseminating via the bloodstream to internal organs such as the liver, spleen, heart, and reproductive tract [72, 130].

Key virulence factors include the Salmonella pathogenicity islands (SPIs). SPI-1 and SPI-2 encode type III secretion systems (T3SS) essential for invasion and intracellular survival [72, 125]. SPI-14 has been identified as a critical virulence determinant for systemic infection in chickens caused by S. Gallinarum [72]. The thioredoxin reductase gene (trxB) and the mgtC gene are also involved in virulence and intracellular replication within macrophages [71, 88]. Type 1 fimbriae play a role in adhesion to host cells, and their expression differs between S. Enteritidis and S. Gallinarum, correlating with tissue tropism [91, 98]. The lipopolysaccharide (LPS) structure, including O-antigen and lipid A, contributes to endotoxicity and resistance to host antimicrobial peptides [48, 52]. Deletion of genes involved in LPS modification (e.g., pagL, arnT) reduces endotoxicity and can be exploited for vaccine development [48, 52, 74]. The purB gene, involved in purine biosynthesis, is essential for virulence; its deletion attenuates the bacterium and provides protective immunity in vaccinated chickens [2, 56].

Comparative transcriptomics of S. Gallinarum, S. Dublin, and S. Enteritidis in the avian host have revealed host-specific expression patterns of metabolic and stress-response genes [96]. S. Gallinarum carries the T6SS encoded by SPI-19, which can complement colonization defects in other serovars [99]. The anti-inflammatory proteins encoded by certain loci in S. Pullorum contribute to persistence in the host [39]. A novel virulence plasmid-cured derivative has been shown to induce immunity without causing disease [142].

Clinical Signs and Pathology

Pullorum disease is primarily a septicemic disease of chicks. Infected chicks exhibit lethargy, anorexia, huddling, white diarrhea, and high mortality [80, 127, 136]. Postmortem lesions include unabsorbed yolk sac, caseous cecal cores, nodular lesions in the liver, spleen, and lungs, and occasionally arthritis [80, 83, 136]. In adult carriers, infection is often subclinical but can lead to vertical transmission via eggs [80, 127].

Fowl typhoid presents with acute or subacute septicemia in older birds. Clinical signs include depression, ruffled feathers, anemia, and greenish-yellow diarrhea [3, 90, 136]. Mortality can be high in acute outbreaks [43, 79]. Necropsy findings include hepatomegaly, splenomegaly, bronze discoloration of the liver, hemorrhages on the heart and serosal surfaces, and fibrinous pericarditis [3, 79, 136]. In laying hens, oophoritis and peritonitis are common, and the infection can persist in the reproductive tract, leading to egg transmission [59, 90]. Outbreaks of fowl typhoid in turkeys have also been reported [79].

Diagnostic Approaches

Diagnosis relies on a combination of bacterial isolation, serological testing, and molecular methods [80, 127].

Culture and isolation: Samples from liver, spleen, yolk sac, or feces are plated on selective media such as MacConkey agar, brilliant green agar, or XLD agar [80, 127]. Presumptive colonies are identified biochemically [144]. Biovars are differentiated by dulcitol and ornithine decarboxylase tests [80, 129].

Serological testing: Pullorum and fowl typhoid are detected using rapid whole blood agglutination tests and ELISA kits that detect antibodies against O9,12 antigens [141, 145, 147, 149, 150]. ELISA tests are useful for flock screening [90, 141]. However, cross-reactivity with other group D salmonellae can occur [80].

Molecular diagnostics: Numerous PCR-based methods have been developed to differentiate the two biovars. Early assays targeted the rfbS gene [133] and fliC [137]. More recently, multiplex PCR assays based on torT and I137_14430 [94], ratA [121], glgC and speC [128], and fimH with high-resolution melting (HRM) [51] have been validated. A one-step multiplex PCR for accurate detection and differentiation of both biovars is now available [4, 113]. Quantitative PCR (qPCR) and multiplex qPCR can simultaneously detect and quantify S. Gallinarum and S. Pullorum [24, 117]. A dual-gene colorimetric LAMP assay allows genus-level detection of Salmonella and specific identification of bv. Gallinarum [23]. Enzyme-activated loop primer probe LAMP based on single-nucleotide polymorphisms (SNPs) in the group_17537 gene has been developed for rapid on-site detection of S. Pullorum [50]. A closed-tube SNP-based assay provides similar specificity [110, 111]. FRET-based PCR discriminates the two biovars using dual-emission fluorescence [64]. Whole-genome sequencing (WGS) is increasingly used for epidemiological tracing, virulence profiling, and antimicrobial resistance gene detection [1, 28, 44, 49, 57, 62, 68, 69, 75, 83, 109]. A sequencing-based method for quantifying intracellular gene-deletion mutants enables functional genomic studies [46].

The following table summarizes key diagnostic techniques:

The diagnostic decision tree for differentiating S. Pullorum and S. Gallinarum using molecular methods is illustrated below:

flowchart TD
    A["Clinical sample: liver, spleen, yolk sac, feces"] --> B{Isolation on selective agar}
    B --> C["Biochemical screening: TSI, LIA, urease"]
    C --> D{Non-lactose fermenter?}
    D -->|Yes| E[Serogroup O9,12 agglutination]
    D -->|No| F[Reject / other Enterobacteriaceae]
    E --> G["Confirmatory PCR: invA / 16S rRNA"]
    G --> H{Dulcitol fermentation?}
    H -->|Negative| I[S. Pullorum]
    H -->|Positive| J[S. Gallinarum]
    I --> K["Molecular confirmation: SNP-LAMP or fimH-HRM or torT/I137_14430 PCR"]
    J --> K
    K --> L[Biovar-specific result]
    L --> M["Report: Pullorum disease or Fowl typhoid"]

Epidemiology and Global Distribution

S. Gallinarum and S. Pullorum are distributed worldwide, with prevalence varying by region and husbandry practices [76, 80, 107]. A global dataset of prevalence from 1945 to 2021 showed persistent infection in many countries [107]. Recent genomic surveillance has revealed the emergence and dissemination of distinct lineages. In Brazil, multiple lineages of bv. Gallinarum have been described based on WGS [49, 68, 75, 115]. An emerging pathogenic strain of S. Gallinarum has been identified in broiler chicks in Brazil, characterized by high virulence and multidrug resistance [3, 43]. In China, evolutionary shifts in epidemiology and genomic lineages have been documented over 50 years, with increasing antimicrobial resistance and clonal spread of multidrug-resistant bv. Pullorum [1, 57]. A study from Morocco provided the first comprehensive genomic characterization of S. Gallinarum in that region, highlighting the presence of resistance genes [28]. In Bangladesh, a combined phenotypic and molecular survey documented high levels of antimicrobial resistance in both biovars [5]. In Southern Africa, field strains closely related to the SG9R vaccine have been isolated, raising concerns about reversion or vaccine-associated outbreaks [93]. In Croatia, an outbreak of fowl typhoid in commercial turkeys was reported [79]. In Nigeria, misidentification of S. Typhi as S. Gallinarum has occurred due to serological cross-reactivity [35]. A systematic review and meta-analysis of poultry salmonellosis in Africa confirmed that S. Gallinarum and S. Pullorum remain prevalent [76].

Transmission: Both biovars are transmitted horizontally via the fecal-oral route and vertically through contaminated eggs [80, 127]. Carrier hens can intermittently shed the bacteria in feces and eggs, perpetuating infection in subsequent generations [126, 127]. The poultry red mite (Ectoparasites of Poultry: Dermanyssus gallinae, Ornithonyssus sylviarum, Knemidocoptes mutans, Knemidocoptes gallinae, and Argas persicus – Identification, Life Cycles, and Control) has been shown to harbor S. Gallinarum and may act as a vector [70].

Antimicrobial Resistance

Antimicrobial resistance (AMR) is a growing concern in fowl typhoid and pullorum disease management [5, 1, 28, 42, 44, 57, 85]. Resistance to sulfonamides, tetracyclines, and aminoglycosides is frequently reported [5, 57, 85]. A study on random mutations in dihydropteroate synthase (folP) elucidated the molecular basis of sulfonamide resistance in S. Gallinarum [86]. Biofilm formation is common among resistant isolates and contributes to persistence in the environment [42, 85]. Whole-genome sequencing has identified a variety of acquired resistance genes, including those encoding β-lactamases and aminoglycoside-modifying enzymes [28, 44, 57]. The global resistome of S. Gallinarum is shaped by mobile genetic elements (plasmids, transposons) that interact with the host genome [45, 57]. In Brazil, genomic characterization of AMR in serogroup B and D1 strains showed distinct resistance profiles [44]. A study from Pakistan reported high resistance rates in field isolates [102]. Monitoring of AMR using both phenotypic and genotypic methods is essential for rational therapy [5, 28].

Control and Prevention

Control strategies include biosecurity, eradication of carrier birds, vaccination, and the use of alternatives to antibiotics such as probiotics, prebiotics, bacteriophages, and phytochemicals [6, 80, 108].

Vaccination: The live attenuated SG9R vaccine (derived from bv. Gallinarum) has been used for decades and provides protection against fowl typhoid [7, 61, 118]. However, field strains closely related to SG9R have been isolated, highlighting the need for improved vaccine safety and differentiation [93]. Newer vaccine candidates include genetically defined mutants such as purB deletion strains [2, 56], gamma-irradiated whole-cell vaccines [30], and LPS-modified strains with reduced endotoxicity [48, 52]. Bivalent oral vaccines expressing viral antigens (e.g., H9N2 avian influenza HA and NA-M2e) from an attenuated S. Gallinarum backbone have shown dual protection [31, 89]. Outer membrane vesicles (OMVs) derived from S. Typhimurium have demonstrated cross-protective efficacy against S. Gallinarum colonization [8].

Probiotics and prebiotics: Bacillus subtilis supplementation reduces S. Pullorum infection in broilers [9]. Lacticaseibacillus rhamnosus and other lactobacilli inhibit S. Pullorum colonization via barrier enhancement and competitive exclusion [10, 26, 36]. Lactobacillus helveticus postbiotics mitigate S. Gallinarum colonization [87]. Synbiotics (probiotics plus prebiotics) have shown preventive effects against S. Enteritidis infection and may be extrapolated to S. Gallinarum [11]. Fermented feed additives can enhance immune responses [95]. Yeast fractions have also been evaluated [112].

Phage therapy: Bacteriophages targeting S. Gallinarum (e.g., phage SGP007, SGP009, SGP004) have been isolated and characterized for their lytic activity, genomic features, and potential for biocontrol [12, 13, 25, 29, 131]. Phage cocktails have been assessed for safety and efficacy in laying hens [32, 54, 63]. Phage therapy can improve growth performance and modulate the microbiome in chickens [14]. However, phage-specific immune responses and practical delivery remain challenges [14, 103].

Essential oils and plant extracts: Oregano essential oil, cinnamon oil, eucalyptol, and citrus oils have demonstrated antimicrobial activity against both biovars [15, 81, 105]. Banana peel extract powder and other plant-derived compounds have been investigated as alternatives to antibiotics [27, 67]. Quercetin-conjugated nanoparticles also show antibacterial activity [33, 60]. The probiotic potential of LAB from various sources has also been explored for controlling Salmonella [16, 22, 38, 77]. However, large-scale field efficacy data remain limited.

Biosecurity: Cleaning and disinfection of poultry houses, all-in/all-out management, and control of rodents and mites are essential [80, 134]. In many countries, eradication programs based on testing and slaughter of positive flocks have successfully reduced prevalence [116, 127].

Conclusion

Salmonella Gallinarum biovars Pullorum and Gallinarum remain significant pathogens for the global poultry industry. Advances in molecular diagnostics and genomics have greatly improved the speed and accuracy of their detection and differentiation. The emergence of multidrug-resistant clones and the close relationship between field isolates and vaccine strains underscore the need for continuous surveillance and the development of next-generation vaccines. Non-antibiotic interventions such as probiotics, phage therapy, and phytochemicals offer promising adjuncts to conventional control, but their integration into comprehensive flock health programs requires further validation. Ongoing genomic epidemiology will be critical to track the evolution and spread of these host-adapted pathogens.

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.

References

[1] Liu F, Zhang L, Shen Q et al. Fifty years of chicken-source Salmonella in China: evolutionary shifts in epidemiology, antimicrobial resistance, and genomic lineages. Poult Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41621339/

[2] Farooq MU, Mukhtar M, Suleman M et al. Evaluation of immunogenicity and protective efficacy of Salmonella Gallinarum knockout purB mutant strain in chicken. Vet Res Commun. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41557085/

[3] Oliveira ES, Coelho Lopes M, Dos Santos Carneiro Lacerda M et al. Experimentally induced lesions of an emerging pathogenic strain of Salmonella Gallinarum in broiler chicks. Avian Pathol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41568410/

[4] Xiong D, Yuan L, Fei X et al. Establishment and application of a new one-step multiplex PCR for the accurate detection and differentiation of Salmonella Gallinarum biovars Pullorum and Gallinarum. BMC Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41957795/

[5] Kingshuk MMR, Alam SB, Rahman MS et al. The landscape of Salmonella enterica serovar Gallinarum-Pullorum antimicrobial resistance in Bangladesh's poultry industry: a combined phenotypic and molecular study. Microbiologyopen. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42271175/

[6] El-Shall NA, Adiguzel MC, Abd El-Ghany WA et al. Salmonella infection in chickens: pathogen, pathogenesis, and dietary non-drug feed additives as alternatives to antibiotics - a comprehensive review. Folia Microbiol (Praha). 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41762430/

[7] Filho RCP, Sarabia J, Sesti L et al. Evaluation of the efficacy of different vaccination programs using two live Salmonella vaccines against Salmonella Enteritidis, Salmonella Typhimurium, and Salmonella Gallinarum in brown layer hens. Avian Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41973010/

[8] Shang K, Choi YR, Son JE et al. Cross-protective efficacy of outer membrane vesicles (OMVs) derived from Salmonella enterica serovar Typhimurium against Salmonella enterica serovars colonization in SPF chicken. Biology (Basel). 2025. URL: https://pubmed

[9] Chen Y, Li H, Zhang X et al. Dietary Bacillus subtilis Group reduces the general infection of Salmonella Pullorum in broiler chicken. Antibiotics (Basel). 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42041352/

[10] Chuwatthanakhajorn S, Wedley A, Watts A et al. Lactobacilli isolated from the caecum of healthy broilers with antimicrobial activity are probiotic candidates for controlling Salmonella. Probiotics Antimicrob Proteins. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41855006/

[11] Babot J, Hidalgo V, Fernández M et al. Preventive effects of synbiotic supplementation against Salmonella Enteritidis infection in broiler chickens. Can J Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41855562/

[12] Muntaha ST, Khan GN, Khan I et al. Isolation, characterization and antibacterial potential of bacteriophages SGP009, SGP004, and SGP007 targeting Salmonella enterica serovar Gallinarum. Int Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41961216/

[13] Muntaha ST, Khan GN, Khan I et al. Genomic insights into phage SGP007 reveal a potent therapeutic candidate for targeted control of salmonellosis in poultry. Eur J Clin Microbiol Infect Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41604108/

[14] Nazir I, Perez D, Vargas SJR et al. Assessing the impact of phage therapy on growth performance, microbiome and phage specific immune response in chickens. Sci Rep. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41946778/

[15] Yasmin S, Taimoor M, Noor H et al. Evaluation of the effect of essential oils of Cinnamomum zeylanicum and Eucalyptus globulus against Salmonella Enteritidis and Salmonella Gallinarum in broilers. J Adv Vet Anim Res. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41923850/

[16] Mafe AN, Makut MD, Owuna JE et al. Development, functional profiling, and probiotic evaluation of indigenous LAB from Nigerian fermented foods. World J Microbiol Biotechnol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42217062/

[17] Duysak T. Engineering AraC L9K for inducer-independent pBAD activation in Salmonella Gallinarum. J Biotechnol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42061758/

[18] Talie MD, Ahmad N, Malik MA et al. Antimicrobial potential of silver nanoparticles synthesized from mushroom extracts against drug-resistant pathogens. New Microbes New Infect. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41696520/

[19] Kim JH, Park JM, Choi KS et al. Direct rescue and rapid confirmation of antigen-binding of dominant V(H) and V(L) of chickens. Prep Biochem Biotechnol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41688866/

[20] Fei X, Moussa J, Guerra PR et al. Comparative pan-genomics and in vivo validation identify genetic factors important for virulence of Salmonella enterica serovar Gallinarum and serovar Enteritidis in the avian host. Microbiol Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41570667/

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.