Burkholderia pseudomallei and Melioidosis in Equines: Diagnosis in Tropical Regions

Introduction

Melioidosis is a severe infectious disease caused by the soil saprophyte Burkholderia pseudomallei, a Gram-negative facultative intracellular bacillus. The global burden of melioidosis is estimated at 165,000 human cases annually, with 89,000 deaths, and the pathogen is now recognized as endemic in at least 45 countries, predominantly in tropical and subtropical regions. While the majority of reported melioidosis literature focuses on human medicine, the disease affects many mammals, including horses, and is increasingly documented in veterinary settings [43, 95]. A systematic review and meta-analysis of global animal melioidosis prevalence identified seroprevalence rates exceeding 20% in some livestock populations in endemic areas, though equine-specific data remain sparse. The expanding geographic footprint of B. pseudomallei driven by climatic events, anthropogenic movement, and improved surveillance underscores the need for robust veterinary diagnostic capacity, particularly in tropical regions where horses are used for work, sport, and subsistence [98, 1].

In equids, melioidosis may manifest as acute septicemia, chronic abscessation, pneumonia, or osteomyelitis, mimicking other endemic diseases such as glanders (caused by Burkholderia mallei), strangles (Streptococcus equi subsp. equi), and tuberculosis [111, 2]. Definitive diagnosis is essential for appropriate antimicrobial therapy and infection control, yet diagnostic challenges are amplified in resource-limited tropical settings. This article provides a comprehensive, publication-grade review of the pathophysiology, clinical presentation, and diagnostic approaches for B. pseudomallei infection in horses, with emphasis on culture-based, molecular, serological, and emerging point-of-care methods applicable to tropical veterinary practice.

Equine Melioidosis Epidemiology in Tropical Regions

Burkholderia pseudomallei is endemic in soils and surface waters of Southeast Asia, northern Australia, South Asia, parts of Africa, and the Americas [94, 98]. Environmental factors favoring its persistence include high rainfall, soil temperatures above 18°C, low organic matter, and neutral to slightly acidic pH [3, 105]. Horses acquire infection through percutaneous inoculation, inhalation of contaminated dust or aerosols, ingestion of contaminated water or feed, and occasionally through arthropod vectors [126, 95]. In endemic regions, equine melioidosis cases have been documented in Australia, Thailand, Malaysia, and Brazil, with sporadic reports from other tropical areas [43, 98]. A recent Australian study identified 45 melioidosis cases in companion animals (24 dogs and 21 cats) between 1997 and 2025, but equine cases are less frequently published, likely due to underdiagnosis and low clinical suspicion [2]. The use of horses for agricultural work in monsoon-affected regions increases exposure risk, especially during rainy seasons when B. pseudomallei is mobilized from soil to surface waters [39, 105]. Serological surveys in endemic areas have reported seroprevalence rates of 10-30% in apparently healthy horses, suggesting subclinical infection is common [76, 43]. The potential for equine-to-equine transmission is low, but horses may serve as sentinel animals for environmental contamination and human risk [35, 64].

Pathogenesis and Virulence Factors

B. pseudomallei possesses an arsenal of virulence determinants that facilitate intracellular survival and dissemination. Following entry through breaks in the skin or via the respiratory or gastrointestinal epithelia, the bacterium adheres to host cells using type 1 fimbriae and other adhesins. It then invades non-phagocytic cells and survives within phagosomes by evading oxidative bursts and autophagy [117, 128]. The type III secretion system (T3SS-3) mediates escape from the endocytic vacuole into the cytoplasm, where the bacterium polymerizes host actin via the BimA protein to propel itself into adjacent cells, forming multinucleated giant cells (MNGCs) that facilitate cell-to-cell spread [99, 141, 134]. The type VI secretion system (T6SS) is critical for MNGC formation and virulence in animal models [126, 99]. Additional virulence factors include the polysaccharide capsule, lipopolysaccharide (LPS) with variable O-antigen structures that modulate innate immune recognition, quorum-sensing molecules (e.g., 3-hydroxy-C10-HSL) that trigger host cell organelle stress, and a two-component regulatory system (IrlS2-IrlR2) governing biofilm formation and stress adaptation. The Trigger Factor protein (TF) contributes to host cell internalization, cytotoxicity, and flagellar motility [4]. B. pseudomallei also subverts host lipid metabolism via NR1D2-mediated suppression of PNPLA2, blocking lipophagy and autophagy-dependent killing. These pathogenic mechanisms are broadly conserved across mammalian hosts, including equids, although detailed equine-specific studies are limited.

Clinical Manifestations in Equids

Equine melioidosis presents a spectrum of clinical syndromes analogous to those seen in other susceptible mammals [2, 62]. The incubation period ranges from days to months, and latent infections can reactivate under stress or immunosuppression. Acute presentations include high fever, depression, anorexia, tachypnea, and septic shock, often with bacteremia [5, 6]. Respiratory involvement is common, manifesting as pneumonia, pleuropneumonia, or lung abscesses, and may progress to severe acute respiratory distress syndrome (ARDS) [7, 8]. Chronic disease is characterized by weight loss, intermittent fever, and localized abscesses in internal organs (especially spleen, liver, lungs) or superficial tissues [9, 10]. Osteomyelitis and septic arthritis have been reported, leading to lameness and joint swelling [6, 11]. Neurological signs due to brainstem or spinal cord involvement (neuromelioidosis) include ataxia, paresis, and cranial nerve deficits [5, 12, 13]. Parapharyngeal or retropharyngeal abscesses cause dysphagia and respiratory stridor. Cutaneous and subcutaneous abscesses may resemble strangles, but B. pseudomallei lesions are often multiple, non-painful, and yield purulent material with a characteristic musty odor [2, 111]. Subclinical infections are common, with horses seropositive but clinically normal, complicating disease surveillance [76, 43].

Diagnostic Approaches

Accurate diagnosis of equine melioidosis is critical for timely therapy and infection control, but remains challenging in tropical regions due to limited laboratory resources and the pathogen's fastidious nature [14, 15]. The following sections detail the principal diagnostic modalities, organized by method.

Culture and Isolation

Culture of B. pseudomallei from clinical specimens is the gold standard for definitive diagnosis. Appropriate samples include blood, pus, tracheal washes, synovial fluid, tissue biopsies, and cerebrospinal fluid [16]. The bacterium grows on standard media such as blood agar and MacConkey agar incubated aerobically at 37°C, but is often overlooked due to slow growth and pleomorphic colonial morphology [17]. Colonies are typically smooth, cream-colored, and may develop a metallic sheen after 48-72 hours; some isolates produce a characteristic earthy odor [17]. Selective media incorporating gentamicin, colistin, or other inhibitors (e.g., Ashdown's agar) improve recovery from contaminated specimens. Confirmation of B. pseudomallei requires demonstration of biochemical profiles (positive oxidase, positive arginine dihydrolase, negative for lactose fermentation) and resistance to polymyxin B. Latex agglutination using monoclonal antibodies provides rapid presumptive identification. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) can rapidly and accurately identify B. pseudomallei from cultured isolates, with reported sensitivities of 100% in multi-country studies [115, 135, 143]. However, misidentification as B. thailandensis or B. cepacia complex is possible if the spectral database lacks sufficient diversity. Blood culture systems (automated, continuous-monitoring) detect bacteremia, but the organism may be flagged as "non-fermenting Gram-negative rod," prompting further workup [18].

Serological and Antigen Detection

Serological assays detect antibodies against B. pseudomallei in equine serum or plasma, but have limited utility in endemic areas due to high background seropositivity and cross-reactivity with other Burkholderia species [15, 76]. Enzyme-linked immunosorbent assays (ELISAs) using whole-cell lysates or purified antigens (e.g., O-polysaccharide, Hcp1, FlgL) show variable sensitivity (50-80%) and specificity when applied to human sera, with similar performance anticipated in horses [81, 144, 78]. Novel recombinant multi-epitope antigens have been developed in silico and tested in human cohorts, but equine validation is lacking. Lateral flow immunoassays (LFI) detecting B. pseudomallei antigen in pus, sputum, or urine offer rapid point-of-care screening; a commercial LFI (Active Melioidosis Detect, InBios) demonstrated 99% sensitivity on turbid blood cultures and 94% positive predictive value on urine samples in a human study. In children with skin abscesses, antigen detection from pus showed moderate sensitivity but excellent specificity. A portable multiplex vertical flow immunoassay platform for biothreat detection, including B. pseudomallei, has been developed for biofluids and environmental matrices, and could be adapted for field use in veterinary settings. Urinary antigen detection remains a promising but insufficiently evaluated tool for equine melioidosis [15].

Molecular Assays

Nucleic acid amplification tests (NAATs) offer high sensitivity and rapid turnaround. Real-time PCR assays targeting the orf2, tssM, or wcbI genes of B. pseudomallei are well validated for human and environmental samples, with detection limits as low as 20 fg (~25 genome equivalents) [129, 90]. A novel real-time PCR method differentiating B. mallei from B. pseudomallei is particularly relevant for differential diagnosis of glanders in horses [19]. Isothermal amplification, such as recombinase polymerase amplification combined with lateral flow dipsticks (LF-RPA), enables detection within 30 minutes without thermal cyclers, achieving a limit of detection of 25.6 copies of genomic DNA and good tolerance to inhibitors in blood. CRISPR-based platforms, including a Cas14a-based electrochemical biosensor with PtPd@PCN-224 nanoenzymes, demonstrate attomolar sensitivity and specificity for B. pseudomallei DNA. Environmental and clinical CRISPR-based diagnostics have identified sanitation gaps in endemic areas and could be adapted for equine samples [20]. Multiplex PCR targeting phage terminase genes uses phage-based detection to signal the presence of B. pseudomallei in clinical specimens. Shotgun metagenome sequencing can accurately assemble B. pseudomallei metagenome-assembled genomes (MAGs) from complex samples, providing a culture-independent option for public health response, though cost and bioinformatics expertise limit routine use.

Computational and Genomic Tools

Whole-genome sequencing (WGS) of B. pseudomallei isolates enables high-resolution molecular epidemiology, antimicrobial resistance prediction, and source attribution [122, 130]. Core genome multilocus sequence typing (cgMLST) schemes provide standardized classification for outbreak investigations and global surveillance. The Antimicrobial Resistance Detection and Prediction (ARDaP) tool identifies the full spectrum of acquired resistance determinants (SNPs, indels, copy-number variations, inversions, gene loss) from WGS data and outperforms legacy software for B. pseudomallei. Genome-wide association studies (GWAS) have identified genes associated with clinical versus environmental isolates, potentially informing equine virulence markers [130, 103]. Within-host evolution during chronic infection, as documented in cystic fibrosis patients, reveals pathoadaptation patterns (antibiotic resistance, virulence attenuation, metabolic shifts) that may also occur in equine chronic melioidosis [136, 148]. Transcriptomic approaches (e.g., TRANSITome) map gene expression flux during intracellular transit, pinpointing stage-specific virulence factors.

Point-of-Care and Emerging Technologies

Given the limitations of laboratory infrastructure in many tropical regions, point-of-care tests (POCTs) are crucial. The lateral flow immunoassay (described above) is simple and rapid for pus and urine. LF-RPA combines isothermal amplification with dipstick readout, requiring minimal equipment. The CRISPR-Cas14a electrochemical biosensor provides an ultrasensitive, equipment-dependent but highly specific option. Phage-based assays, such as multiplex PCR targeting terminase genes from induced prophages, offer an innovative approach to detect viable bacteria through lytic induction. Breath analysis (volatile organic compounds) using gas chromatography-mass spectrometry has been explored for human melioidosis and could have veterinary applications. These emerging tools must undergo field validation in equine populations in tropical settings before widespread adoption.

flowchart TD
 A["Equine suspect case: fever, abscess, respiratory/neurologic signs, lameness"] --> B["Sample collection: blood, pus, BAL, synovial fluid, CSF, tissue"]
 B --> C{Initial laboratory access?}
 C -->|Yes| D["Gram stain: small Gram-negative rods"]
 D --> E[Culture on blood agar + selective media 48-72h]
 E --> F["Colonial morphology: smooth, cream, earthy odor"]
 F --> G["Presumptive ID: oxidase+, arginine dihydrolase+, polymyxin R"]
 G --> H[MALDI-TOF MS confirmation or Latex agglutination]
 H --> I[Definitive diagnosis]
 C -->|Limited| J[Point-of-care testing]
 J --> K["Lateral flow antigen test: pus/urine"]
 K --> L[Positive?]
 L -->|Yes| I
 L -->|No| M[Consider LF-RPA or CRISPR assay on sample]
 M --> N[Positive?]
 N -->|Yes| I
 N -->|No| O[Perform blood culture if available, or refer to reference lab]
 O --> P["'Serology (ELISA') for retrospective/epidemiological use"]
 P --> I
 I --> Q["Antimicrobial susceptibility testing: broth microdilution, ECOFF interpretation"]
 Q --> R[Report and treat with ceftazidime/meropenem followed by TMP-SMX]

Differential Diagnosis

Equine melioidosis must be distinguished from other bacterial infections common in tropical regions. Glanders (Burkholderia mallei) presents with similar respiratory and cutaneous signs, but B. mallei is generally non-motile and lacks the characteristic wrinkled colonial morphology on glycerol agar; real-time PCR assays differentiating the two are essential [19]. Streptococcus equi subsp. equi causes strangles, with typical lymph node abscessation but usually in younger horses and with a more acute course. Mycobacterium bovis and M. avium complex infections produce chronic wasting, but acid-fast staining and mycobacterial culture differentiate them. Rhodococcus equi is a cause of pneumonia and abscesses in foals but can be distinguished by its distinct colony appearance and susceptibility patterns. Other differentials include Staphylococcus aureus abscesses, Actinobacillus spp. infections, and tick-borne diseases such as anaplasmosis (e.g., [Anaplasma phagocytophilum](/knowledge/bacteria/Equine Granulocytic Anaplasmosis/anaplasma-phagocytophilum-equine-granulocytic-anaplasmosis-tick)) that cause fever and lethargy [see Anaplasma phagocytophilum in Livestock and Companion Animals].

Treatment Challenges and Antimicrobial Resistance

Treatment of equine melioidosis follows principles derived from human medicine and small animal studies: an intensive intravenous phase (ceftazidime or meropenem for at least 10-14 days) followed by oral eradication therapy (trimethoprim-sulfamethoxazole, TMP-SMX, for 12-20 weeks) [82, 95]. However, B. pseudomallei exhibits intrinsic resistance to many antibiotics, including aminoglycosides and polymyxins, and acquired resistance to β-lactams and TMP-SMX occurs [109, 137]. Resistance to ceftazidime and amoxicillin-clavulanate is mediated by penA mutations, promoter changes, or gene amplification [109, 146]. Co-trimoxazole resistance involves mutations in bpeT, bpeS, and folM, leading to efflux pump overexpression or altered folate metabolism. A multi-center study establishing EUCAST epidemiological cut-off values for B. pseudomallei provides a standardized framework for MIC interpretation. The ARDaP tool facilitates rapid detection of resistance determinants from WGS. In horses, prolonged therapy is challenging due to cost, drug availability, and owner compliance; relapse is common if treatment is truncated [82, 97]. Novel therapeutic strategies under investigation include DNase I to disrupt biofilm and improve β-lactam efficacy, chitosan-drug conjugates, phage therapy using lytic podoviridae, and secondary metabolites from Bacillus amyloliquefaciens that kill B. pseudomallei including drug-resistant strains. High-throughput drug screening has identified repurposed agents such as auranofin and miltefosine with anti-persister activity [142, 33].

Conclusion

Equine melioidosis is an underdiagnosed but clinically significant infection in tropical regions, with manifestations overlapping those of glanders, strangles, and tuberculosis. Diagnosis requires a high index of suspicion, appropriate sample collection, and access to culture, MALDI-TOF MS, or NAATs. In resource-limited settings, point-of-care antigen tests and isothermal amplification assays offer promising alternatives, though further equine-specific validation is needed. The expanding geographic range of B. pseudomallei, driven by climate change and anthropogenic factors, underscores the urgency of strengthening veterinary diagnostic capacity in tropical livestock systems. Integration of genomic surveillance and antimicrobial susceptibility testing into routine veterinary diagnostics will improve therapeutic outcomes and contribute to One Health surveillance for this emerging zoonotic pathogen.

References

[1] Currie BJ, Kaestli M, Meumann EM. Global dispersal of Burkholderia pseudomallei and the evolving endemicity of melioidosis in the United States of America. PLoS Negl Trop Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42030350/

[2] Lee K, Shilton C, Benedict S et al. Melioidosis in companion animals: Analysis of 45 Australian cases (24 dogs; 21 cats) from 1997 to 2025 and a brief review of the animal and human literature. Aust Vet J. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42161377/

[3] Chaianunporn K, Wang-Ngarm S, Chareonsudjai P et al. Interaction effects of physicochemical factors on the growth of Burkholderia pseudomallei in soil microcosms. PLoS Negl Trop Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42149956/

[4] Bendo JB, Debowski AW, Bzdyl NM et al. Trigger Factor in Burkholderia pseudomallei is essential for key virulence determinants, including host cell internalization, cytotoxicity, motility, and stress resistance. J Bacteriol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42017692/

[5] Janet Preethi Philomina P, Sunitha VC, Adithan S. Neuromelioidosis in a nutshell-Pattern and Endemicity. Clin Radiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42284690/

[6] Sugumaran M, Selvam M, Ramachandran R et al. The Great Mimicker in Bone: A Rare Case of Melioidosis With Multifocal Skeletal Manifestations and Diagnostic Challenges. Cureus. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42255820/

[7] Wang F, Xie C, Zhao M et al. VV-ECMO-supported management of severe ARDS secondary to melioidosis sepsis: A case report and concise review. IDCases. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42222019/

[8] Mobio PMN, Ha VHT, Kouame IK et al. Severe acute respiratory distress syndrome due to Burkholderia pseudomallei pneumonia complicated by septic shock. IDCases. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42022536/

[9] Tatineni SP, Varadan S, Sekar V et al. Melioidosis Presenting As a Chronic Splenic Abscess: A Diagnostic Challenge. Cureus. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42220795/

[10] Pattiyakumbura T, Perera TS, Manilgama S et al. From chronic abscess to acute respiratory distress syndrome (ARDS): A case of recurrent melioidosis in Sri Lanka. Trop Doct. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42157547/

[11] Chhakchhuak Z, Hmar L. Tibial Osteomyelitis due to Burkholderia pseudomallei in a Non-diabetic Patient: A Case Report. J Orthop Case Rep. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42131008/

[12] Mehta SK, Mishra N, Prajapati A et al. Neurological and Systemic Manifestations of Melioidosis: A Three-Case Series From Eastern India. Cureus. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42220815/

[14] Merlino J, Koos G, Aitkens O et al. Melioidosis: Burkholderia pseudomallei technical challenges in identification and susceptibility testing in a non-endemic area. Diagn Microbiol Infect Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42105441/

[15] Kusuma SAF, Anggraeni SR, Sari QW et al. Current Melioidosis Diagnostic Landscape and Missed Opportunities in Biomarker Development. Diagnostics (Basel). 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42072872/

[16] Chowdhury FR, Zewdie M, Abraham P et al. Mucosal-associated invariant T (MAIT) cells are reduced and dysfunctional in acute melioidosis. Sci Rep. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42297953/

[17] Thacharakavil Variyath N, Mohanty S, Dutta RP. Unusual colony morphology of Burkholderia pseudomallei leading to initial diagnostic dilemma. Indian J Med Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42069083/

[18] Simsic B, Ewin T, Hennessy J et al. Remote regions, rapid recognition: performance and timeliness of BIOFIRE FilmArray BCID2 for diagnosing bloodstream infection in remote northern Australia. J Clin Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42283523/

[19] Koshikawa A, Ichikawa Y, Nakase M et al. Development of a Novel Real-Time PCR Assay Differentiating Burkholderia mallei From Burkholderia pseudomallei. Microbiol Immunol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42057371/

[20] Pakdeerat S, Chomkatekaew C, Boonklang P et al. CRISPR-based environmental detection of Burkholderia pseudomallei identifies sanitation gaps and melioidosis risk in northeast Thailand. Nat Commun. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42140954/

[21] Metrailer MC, Le Quyen TT, Van Pham K et al. Hospital-Based Cross-Sectional Study of Burkholderia pseudomallei Seroreactivity Among Febrile Patients in Northernmost Vietnam: Near-Neighbor Bloodstream Isolates, Environmental Correlates, and Spatial Clustering. Geohealth. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42255322/

[22] Singh H, Currie BJ, Woerle C et al. Epidemiology, Clinical Features, and Outcomes of Chronic Melioidosis in the Top End of Australia's Northern Territory, 1989-2023. Open Forum Infect Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42253281/

[23] Allu RK, Nalabothula R, Kannan K et al. Melioidosis: diverse clinical manifestations and patient outcomes in a case series. BMC Infect Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42143249/

[24] Melo G, Re F. Targeting inflammasome, IL-1β, and coagulation in melioidosis. Front Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42131212/

[25] Cruz SC, Raimundo C, Andela L et al. First culture-confirmed melioidosis case in Mozambique: a wakeup call for better diagnostics and clinical awareness. Oxf Med Case Reports. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42117134/

[26] Klotoe B, Vorimore F, Gasqué M et al. Genomic and morphological analysis of human clinical Burkholderia pseudomallei isolates from New Caledonia (2021-2022). Infect Genet Evol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42097259/

[27] Thanasai J, Chanthot C, Chittamma A et al. Serum Procalcitonin in Culture-Confirmed Melioidosis: A Systematic Review and Meta-Analysis with Narrative Evaluation of Clinical and Prognostic Implications. Diseases. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42041611/

[28] Peacey J, Tan C, Afolabi O et al. Prostate abscess in tropical northern Australia: drainage strategies and outcomes. Int Urol Nephrol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42012773/

[29] You G, Wang S, Hua Y et al. A four-year misdiagnosis of spinal Burkholderia pseudomallei infection: A case report and literature review. IDCases. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42004152/

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.