Melioidosis in Dogs and Cats: A Short Review

Philip R Judge BVSc MVS PG Cert Vet Stud MACVSc (Vet Emergency and Critical Care; Medicine of Dogs)

Introduction

Melioidosis is a bacterial disease caused by Burkholderia pseudomallei, a Gram-negative, facultative intracellular pathogen that lives in soil and water in endemic regions, particularly Southeast Asia, northern Australia, and parts of South America^1,2​​. While extensively studied in human medicine, melioidosis remains underdiagnosed and underreported in veterinary medicine, despite being associated with significant morbidity and mortality in dogs and cats³​.

Melioidosis can manifest in a wide range of clinical syndromes, from mild localized infections to severe septicaemia⁴. Infected animals may also serve as sentinels for human exposure due to shared environmental risks⁵​. This short review consolidates current knowledge on the epidemiology, pathogenesis, clinical manifestations, diagnosis, treatment, and public health implications of melioidosis in dogs and cats, based on published scientific literature.

Epidemiology

Geographical Distribution and Risk Factors^2,5

Melioidosis is endemic to:

• Southeast Asia (Thailand, Malaysia, Singapore, Indonesia, Myanmar, and Vietnam)
• Northern Australia (Queensland and the Northern Territory)
• South Asia and the Pacific (India, Sri Lanka, Papua New Guinea)
• South America and the Caribbean

Recent seroprevalence studies in dogs in Thailand found a 5.77% prevalence, suggesting that exposure is relatively common​. In Australia, cases have been reported in multiple animal species, including livestock, wildlife, and companion animals​.

Transmission and Environmental Factors^1,3,6

Dogs and cats contract melioidosis primarily through:

• Direct contact with contaminated soil and water (most common)
• Ingestion of contaminated food or water
• Inhalation of aerosolized B. pseudomallei (during heavy rainfall and storms)
• Wound contamination (e.g., via scratches, bites, or surgical sites)

Environmental factors influencing infection risk:

• High rainfall and flooding: Increases bacterial dissemination in soil and surface water​.
• Outdoor lifestyle and rural settings: Dogs used for hunting or farm work are at greater risk​.
• Importation of animals from endemic areas: Documented in a public health case in the USA, where an adopted dog from Thailand was found to be infected​.

Pathogenesis and Clinical Manifestations^1-3,5

Pathogenesis

On entering the host, haematogenous spread occurs leading to infection +/- abscess formation in end-organs, including the spleen, liver, lungs, prostate, and CNS.

Clinical Syndromes in Dogs and Cats

Melioidosis presents in three clinical forms:

1. Acute Septicaemic Melioidosis
   • Rapid onset of fever, anorexia, lethargy
   • Respiratory distress, tachypnoea, and cyanosis
   • Disseminated abscesses (liver, spleen, lungs, prostate, brain, spinal cord)
   • Systemic inflammation, sepsis and septic shock 
   • Common in immunocompromised animals or those with co-existent infections e.g., Ehrlicia, Anaplasma etc.
2. Chronic Melioidosis
   • Localized abscesses in soft tissues or internal organs
   • Neurological signs (ataxia, seizures) if CNS is affected​.
   • Persistent or relapsing fever
   • Osteomyelitis or joint infections (rare)​.
3. Subclinical or Asymptomatic Infection
   • Detected via serology or culture in apparently healthy dogs
   • Potential latent infections that reactivate under stress​.

Cats are less commonly affected than dogs, but documented cases show similar syndromes, including cutaneous abscesses, pneumonia, and systemic infections​.

Diagnosis^1,3,5-7

Accurate and early diagnosis of melioidosis in dogs and cats is crucial, as prompt initiation of treatment significantly improves survival outcomes. However, diagnosing melioidosis can be challenging due to its diverse clinical presentations, which often mimic other bacterial, fungal, and neoplastic diseases. Diagnosis is typically achieved through bacterial culture, serology, molecular testing, and imaging.

A definitive diagnosis of melioidosis requires isolation and identification of Burkholderia pseudomallei from clinical samples. 

The following diagnostic tests are recommended:

1. Bacterial Culture 

Bacterial culture is described as the gold standard for confirming melioidosis, but its success depends on:

• Proper sample collection from sterile sites (blood, urine, abscess fluid, tissue biopsies).
• Use of selective culture media (Ashdown’s agar, which contains gentamicin to inhibit competing flora).
• Prolonged incubation (up to 5-7 days) due to the slow-growing nature of B. pseudomallei​.

Samples for Culture:

• Blood culture: Useful for detecting septicaemic melioidosis. Sensitivity is variable (30-70%)​
• Urine culture: Positive culture is achieved in 40-60% of cases of renal abscesses or subclinical urinary shedding​.
• Pus from abscesses: High sensitivity due to high bacterial load, making it a preferred sample for culture​.
• Aspirates from lymph nodes, liver, spleen, or lungs​.
• Tracheal wash or bronchoalveolar lavage (BAL) in dogs with pneumonia or respiratory involvement​.

Limitations of Culture:

• False negatives are common, especially in animals with low bacterial loads or those patients who have received prior antibiotic therapy.
• Contaminating flora in non-sterile sites – e.g. faeces or respiratory secretions.
• Requires biosafety level 3 (BSL-3) laboratory facilities due to the zoonotic risk of handling live bacteria​.

2. Molecular Detection (PCR)

Polymerase Chain Reaction (PCR) is a rapid and highly sensitive method for detecting B. pseudomallei DNA. PCR is particularly useful when:

• Culture results are delayed or inconclusive.
• Early diagnosis is needed in critically ill animals.

Types of PCR for Melioidosis:

• Conventional PCR: Targets B. pseudomallei-specific genes such as TTS1 (type three secretion system) or LpxO gene​.
• Real-Time PCR (qPCR): More sensitive and allows for quantification of bacterial DNA​.
• Multiplex PCR: Can differentiate B. pseudomallei from Burkholderia thailandensis (a non-pathogenic environmental related bacteria)​.

Advantages of PCR: 

• Faster turnaround time (results within hours)
• Higher sensitivity than culture, especially in early or localized infections
• Can be performed on a variety of samples, including blood, urine, pus, and tissue.

Limitations of PCR:

• Cannot differentiate between active infection and environmental exposure.
• Requires specialized laboratory facilities for molecular testing.
• False negatives may occur due to PCR inhibitors in clinical samples​.

3. Serology (Antibody Testing)

Serological tests detect antibodies against B. pseudomallei and can be useful in endemic areas or for epidemiological surveillance. However, serology alone is not confirmatory and should always be interpreted alongside clinical findings.

Common Serological Tests:

• Indirect Hemagglutination Assay (IHA): The most widely used serological test for melioidosis.
• ELISA (Enzyme-Linked Immunosorbent Assay): Detects specific IgG and IgM antibodies.
• Latex Agglutination Test: Rapid but less commonly used

IHA Interpretation:

Limitations of Serology:

• False positives occur due to cross-reaction with other bacteria (e.g., Pseudomonas spp.).
• False negatives can occur in immunosuppressed animals or those with early infections before seroconversion​.
• Cannot distinguish between active and previous infections.

4. Imaging Studies

Imaging may be indicated in patient showing abdominal, respiratory or musculoskeletal symptoms of illness.

Radiography

• Thoracic radiographs may reveal nodular lung lesions, pleural effusion, or alveolar infiltrates 
• Skeletal radiographs are useful for identifying osteomyelitis or joint involvement in chronic melioidosis​.

Ultrasound and CT 

• Abdominal ultrasound may detect hepatic, splenic, or prostatic abscesses, which are common in chronic melioidosis​.
• CT scans provide similar information to ultrasound, but may provide superior imaging for surgical planning​.

5. Hematology and Biochemistry Findings

While non-specific, bloodwork can help support a clinical suspicion of melioidosis. Common abnormalities that may be detected in routine screen include:

• Leukocytosis with a left shift.
• Mild to severe anemia in chronic or septicaemic cases​.
• Thrombocytopenia in animals with disseminated infection.
• Elevated liver enzymes (ALT, ALP) if hepatic involvement is present.
• Azotemia (increased BUN/creatinine) in cases with renal involvement​.

Differential Diagnoses

Melioidosis mimics several other bacterial, fungal, and neoplastic diseases, making differentiation critical.

Summary of Diagnostic Approach

1. Suspect melioidosis in animals from endemic regions with persistent fever, abscesses, or respiratory signs.
2. Obtain samples for bacterial culture (blood, urine, pus, BAL, organ aspirates).
3. Use PCR for early detection, especially in critically ill patients.
4. Perform imaging (X-rays, ultrasound) to assess organ involvement.
5. Confirm with serology, but interpret results cautiously.

Treatment^3,4,8,9

Treatment is divided into two critical phases – the acute phase, and the eradication phase:

1. Intensive Phase (Acute Infection)

Goal: To prevent septicaemia and reduce bacterial load.

• Ceftazidime (50 mg/kg IV q6-8h) – First-line therapy​. Continue for at least 14 days.
• Meropenem (20 mg/kg IV/SC q8h for at least 14 days) – For neurological cases, osteomyelitis, as well as severe, resistant, or non-responsive cases​.
• Trimethoprim-sulfamethoxazole should be added to neurological, arthritis, osteomyelitis, genitourinary infection and skin and soft tissue infection
• Alternative: Amoxicillin-Clavulanate – Second-line option if carbapenems are unavailable​.

Adjunctive Therapy

• Fluid therapy, correction of electrolyte abnormalities etc. 
• Abscess drainage to reduce bacterial load.
• Analgesic therapy
• Nutritional support
• Anti-inflammatory therapy?

2. Eradication Phase (Chronic or Asymptomatic Infection)

Goal: To prevent relapse by eliminating residual bacteria.

• Trimethoprim-Sulfamethoxazole (25 mg/kg PO q12h) for at least 20 weeks​.
• Doxycycline (10 mg/kg PO q24h) added for cases with intracellular infection or when co-infections like Ehrlicia or Anaplasma are present​.

When to Use Doxycycline?

• If co-trimoxazole intolerance is suspected.
• If intracellular penetration is needed (e.g., CNS or lung infections).
• If coinfections (e.g., Ehrlicia, Anaplasma) are present​.

Prognosis and Relapse Risk^1,3-5

The prognosis for melioidosis in dogs and cats depends on several factors, including the severity of infection at diagnosis, organ involvement, immune status, coexistent infections, comorbidities, and response to treatment.

Factors Influencing Prognosis

1. Disease Severity at Presentation
   1. Acute septicaemic melioidosis: The prognosis is poor, with reported mortality rates of 50-90% in untreated or late-stage cases​. Even with aggressive treatment, mortality can remain as high as 40-50% in animals presenting with septic shock or multi-organ failure.
   2. Chronic or localized melioidosis: Prognosis is fair to good, depending on the extent of organ involvement. Localised skin abscesses or subcutaneous infections have a better prognosis (survival rates of 70-90%), provided prompt surgical drainage and appropriate antibiotic therapy are initiated​.
   3. Subclinical infections: Animals diagnosed serologically without clinical signs may never develop clinical illness but should be monitored for any progression​.
2. Organ Involvement and Complications
   1. Pulmonary Melioidosis: If pneumonia is present, survival is variable, with a higher risk of prolonged illness and secondary bacterial infections​.
   2. CNS Involvement: Neurological signs (ataxia, seizures, or encephalitis) carry a poor prognosis, as B. pseudomallei is difficult to eliminate from the central nervous system​.
   3. Hepatic and Splenic Abscesses: Dogs and cats with multiple large abscesses may require prolonged antibiotic therapy (up to six months) and have a higher risk of relapse (30-40%)​.
   4. Bone and Joint Infections: Cases of osteomyelitis or septic arthritis are rare but require longer treatment courses (often beyond 6 months) and may result in chronic pain and/or altered mobility​.
3. Immune Status of the Animal
   1. Dogs with underlying immunosuppressive conditions (e.g., Ehrlichiosis, Anaplasmosis, or FIV/FeLV in cats) tend to have worse outcomes and a higher risk of relapse​.
   2. Geriatric animals or those with pre-existing comorbidities (e.g., diabetes, renal disease) have poorer prognoses due to a weaker immune response​.
4. Response to Treatment
   1. Early intervention significantly improves survival rates. If IV antibiotic therapy is started within 24-48 hours of symptom onset, survival rates improve to 60-80%, even in severe cases​.
   2. Duration of antibiotic therapy is critical: Inadequate treatment duration leads to persistent infection, recrudescence, or relapse.

Relapse Risk and Long-Term Management^3-5

One of the biggest challenges in treating melioidosis is relapse, with recurrence rates ranging between 10-30%, especially if the eradication phase is incomplete​.

Factors Contributing to Relapse

• Premature discontinuation of antibiotics (before 20 weeks).
• Failure to use an effective eradication-phase antibiotic (Trimethoprim-Sulfamethoxazole) during the second phase of treatment​.
• Deep-seated infections (e.g., splenic abscesses, bone infections, or neurological disease) that act as bacterial reservoirs​.
• Coinfections or immunosuppression delaying bacterial clearance​.

Strategies to Reduce Relapse Risk

• Strict adherence to the full antibiotic course (minimum 20 weeks).
• Regular follow-up cultures and imaging (ultrasound for abscesses, radiographs for lung involvement).
• Serological monitoring in endemic areas, especially for dogs with a history of previous infection​.

Prognosis by Disease Stage

Long-Term Outlook and Quality of Life¹

• Dogs and cats that recover fully from melioidosis can lead normal lives.
• Some animals may experience residual effects, including chronic organ dysfunction (hepatic or renal impairment from severe sepsis).
• Re-infection risk exists in endemic areas, particularly for outdoor dogs that continue to be exposed to contaminated environments​.

Melioidosis carries a high mortality rate in acute cases, but early intervention and strict adherence to treatment protocols can significantly improve survival outcomes. Relapse remains a major concern, and long-term follow-up is crucial for ensuring successful eradication of the infection​.

Public Health Considerations^6,10

• Zoonotic potential exists, but transmission risk is low.
• Proper handling precautions (gloves, PPE) recommended for veterinarians and pet owners​.
• Surveillance of imported dogs from endemic regions is crucial​.
• Relapse may occur in 6% of humans; can take several months – up to 2 years to occur.

Conclusion⁴

Melioidosis is a serious but underdiagnosed disease in dogs and cats, with high mortality in acute cases and significant relapse risks if not fully treated. Early diagnosis, aggressive treatment, and adherence to the full eradication phase are critical for successful management. Veterinary awareness is essential, especially in endemic regions, to prevent misdiagnosis and improve outcomes​​.

References: 

1. Sim, S.H., Ong, C.E.L., Gan, Y.H., Wang, D., Koh, V.W.H., Tan, Y.K., Wong, M.S.Y., et al. (2018). Melioidosis in Singapore: Clinical, veterinary, and environmental perspectives. Tropical Medicine and Infectious Disease, 3(1), 31. 
2. Choy, J.L., Mayo, M., Janmaat, A., & Currie, B.J. (2000). Animal melioidosis in Australia. Acta Tropica, 74(2-3), 153-158. 
3. Khrongsee, P., Lueangthuwapranit, C., Ingkasri, T., Sretrirutchai, S., Kaewrakmuk, J., Saechan, V., & Tuanyok, A. (2019). Successful treatments and management of a case of canine melioidosis. Veterinary Sciences, 6(4), 76. 
4. Dance, D.A.B. (2014). Treatment and prophylaxis of melioidosis. International Journal of Antimicrobial Agents, 43(4), 310-318. 
5. Fungwithaya, P., Boonhoh, W., Sontigun, N., Hayakijkosol, O., Klangbud, W.K., & Wongtawan, T. (2024). Seroprevalence of melioidosis and its association with blood profiles and pathogens in sheltered dogs in southern Thailand. Veterinary World, 17(3), 705-711. 
6. Ryan, C.W., Bishop, K., Blaney, D.D., Britton, S.J., Cantone, F., Egan, C., Elrod, M.G., et al. (2018). Public health response to an imported case of canine melioidosis. Zoonoses and Public Health, 65(4), 420-424. 
7. Khaejawat, K., Panichote, A., & Meesing, A. (2023). Development of a simple score for diagnosis of melioidosis. Open Forum Infectious Diseases, 10(Suppl 2), ofad500.291.
8. Bidgood, T., & Papich, M.G. (2002). Plasma pharmacokinetics and tissue fluid concentrations of meropenem after intravenous and subcutaneous administration in dogs. American Journal of Veterinary Research, 63(12), 1622-1628. 
9. Currie BJ, Janson S, Meumann E, Martin GE, Ewin T, Marshal CS. The 2024 revised Darwin melioidosis treatment guideline. The Northern Territory Disease Control Bulletin. 2023;30(4):3-12.
10. Alexander, A.D., Binn, L.N., Elisberg, B., Husted, P., Huxsoll, D.L., Marshall, J.D. Jr., Needy, C.F., & White, A.D. (1972). Zoonotic infections in military scout and tracker dogs in Vietnam. Infection and Immunity, 5(5), 745-749.