Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease encountered predominantly in the tropics. This saprophyte can adapt to a wide range of environmental conditions and infect humans and a multitude of animal hosts. Because of its potential use as a bioweapon, this organism is classified by the US government as a Risk Group 3 pathogen and a Tier 1 select agent.
Melioidosis is often associated with suppurative or caseous lesions, comprising a mixed purulent and granulomatous response that can affect any body organ. Clinically infected individuals can shed the agent via urine, feces, milk, sputum, and purulent material. The clinical presentation can take various forms, from fulminant septicemia, pneumonia, local infection, subacute illness, and chronic infection to subclinical disease. Positive culture is the diagnostic standard; no rapid and reliable commercially available tests currently exist, but DNA-based assays, such as PCR assay and sequencing, are also suitable for bacterial identification. Serologic tests are less advisable because of their low sensitivity and specificity. Treatment requires longterm antimicrobial courses that are expensive and frequently unsuccessful. Human treatment guidelines for melioidosis can be followed. Vaccines are not available.
Etiology and Epidemiology of Melioidosis in Animals
The etiologic agent for melioidosis is Burkholderia pseudomallei, an oval, motile, gram-negative, facultative anaerobic bacillus with bipolar staining. The organism is ubiquitous throughout Southeast Asia, northern Australia, and the South Pacific. Its distribution is predominantly tropical and subtropical, with “hyperendemicity” in the northernmost regions of Australia and northeastern Thailand. The true boundaries of its endemicity are ambiguous because of movement of the organism and its ability to travel to and exist in temperate regions (eg, southwestern Australia, France), where it may cause sporadic disease and outbreaks. B pseudomallei has been introduced to new environments with the export of animals, and shipments of contaminated soil, water and commercial products have expanded the organism's range. Reports of melioidosis have also come from India, some Pacific islands, the US, Mexico, Central and South America, the Caribbean, Africa, and the Middle East.
B pseudomallei is a widespread saprophyte and has been isolated from various soil types and surface water of varying depths. Melioidosis outbreaks have coincided with heavy rainfall and flooding associated with high humidity or temperature. Major excavations and disturbances in plumbing resulting in contamination of water supplies have also caused outbreaks.
Melioidosis occurs most commonly in sheep, goats, and pigs; other affected species include cattle, buffalo, horses, mules, deer, camels, alpacas, dogs, cats, dolphins, wallabies, koalas, human and nonhuman primates, birds, tropical fish, and reptiles. Laboratory animals affected by melioidosis include hamsters, guinea pigs, rabbits, mice, and rats. Host susceptibility and clinical signs vary between species. The introduction of naive production animals to endemic regions may predispose them to disease, as observed with sheep, goats, pigs, and camelids. Other species (eg, dogs and cats) may succumb to infection because of immunocompromising conditions.
Transmission of Melioidosis in Animals
Infection by B pseudomallei is thought to be opportunistic and primarily a result of transmission from the environment (eg, contaminated soil and surface waters) rather than from animal to animal. The most common routes of infection are percutaneous inoculation; contamination of wounds; ingestion of contaminated soil, water, or carcasses; and inhalation. Transplacental infection resulting in abortion has been reported in goats and B pseudomallei can be transmitted during breeding. Laboratory-acquired infection and iatrogenic infection via contaminated antiseptics, injections, or hospital or surgical equipment have been reported.
Pathogenesis of Melioidosis in Animals
The virulence of B pseudomallei appears to vary between isolates and also between affected individuals. Molecular-typed clonal outbreaks have produced a range of different clinical signs, which indicate that host factors and infecting dose may be just as important as isolate type in determining the severity of disease. The incubation period ranges from a few days to months or even years. B pseudomallei is a facultative intracellular pathogen that can remain dormant for many years before initiating infection
Clinical Findings of Melioidosis in Animals
Clinical signs of melioidosis in animals can vary widely within a species depending on the site of infection. Fever, anorexia, or swollen glands may be noted. Subclinical infection is common. Infection may be associated with single or multiple suppurative or caseous nodules or abscesses, which can be located in any organ. Disease is acquired by percutaneous inoculation or inhalation and often spreads to distant sites with or without evidence of active infection at the inoculation site. The organs most commonly affected include the lungs, spleen, liver, and associated lymph nodes.
Goats frequently develop mastitis or pneumonia; aortic aneurysms have also been reported. The respiratory system is involved preferentially in sheep; clinical signs can include fever, severe coughing, respiratory distress, and mucopurulent nasal and ocular discharge. CNS disease, with signs that include circling, incoordination, blindness, nystagmus, and spasms, has been observed in cattle, horses, sheep, and goats. Pigs often have subclinical lesions on the spleen that are incidental findings at slaughter. Lameness due to septic arthritis and osteomyelitis can occur. Fatalities often occur in association with acute fulminating infections or when vital organs are affected. Various forms of melioidosis have been reported in horses; clinical signs may include weakness, edema and lymphangitis of the limbs, mild colic, diarrhea, coughing, or nasal discharge. Skin infections may resemble fungal eczema initially, progressing to become papular. In dogs, disease may be acute, subacute, or chronic. In acute cases, septicemia with fever, severe diarrhea, and fulminant pneumonia are common. Subacute cases may present as a skin lesion with lymphangitis and lymphadenitis; untreated cases may progress to septicemia. Chronic disease can occur in any organ with clinical signs that include anorexia, myalgia, edema of the limbs, and skin abscesses. Cats may present with localized ocular disease.
Multiple abscesses that contain thick, caseous, greenish-yellow to off-white material are noted at necropsy. The organs most commonly involved are the lungs, spleen, lymph nodes, liver, and subcutaneous tissues. Exudative bronchopneumonia, consolidation, and abscesses may be found in the lungs of animals with respiratory disease. Nodules and ulcers may be found on the nasal mucosa and septum and on the turbinates, and they may coalesce into irregular plaques. Meningoencephalitis, unilateral ocular infection ("red eye"), severe enteritis, suppurative polyarthritis, and other syndromes also have been reported.
Diagnosis of Melioidosis in Animals
Culture is the diagnostic standard
PCR assay and sequencing for identification and typing
The clinical signs of melioidosis are not diagnostic, because of the variable nature of the disease. For a definitive diagnosis, the organism must be isolated and identified from lesion or discharge samples. It is possible to culture the organism on routine diagnostic media; however, Ashdown’s medium is preferred because of a consistently distinctive colony morphology. Gram-stained smears of exudate or pus can sometimes identify bipolar “safety pin”–shaped, gram-negative rods. Serologic tests such as complement fixation and indirect hemagglutination must be evaluated with caution in endemic regions and should not be used as the sole diagnostic tool. Diagnostics based on PCR assay and target gene sequencing (eg, 16S rRNA) are recommended for identification and typing. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry can also be used for bacterial identification, provided that sufficient reference spectra are in the database.
Treatment and Prevention of Melioidosis in Animals
Administration of antimicrobials, dependent on culture and susceptibility testing
Prevention of exposure to contaminated soil and water
Antimicrobial treatment should be based on culture and susceptibility results. Treatment may be expensive, prolonged, and possibly unsuccessful, with the risk of recrudescence after treatment is discontinued. The possibility of underlying immunosuppressive conditions should be considered. With severe disease, treatment regimens can follow guidelines for human melioidosis: intravenous administration of beta-lactams or a carbapenem for 2-4 weeks, followed by trimethoprim-sulfamethoxazole, for 3-6 months. Treatment using chloramphenicol, cotrimoxazole, and doxycycline or amoxicillin/clavulanate have also been reported. B pseudomallei is resistant to penicillin, ampicillin, first- and second-generation cephalosporins, gentamicin, tobramycin, streptomycin, macrolides, and polymyxins. Surgical drainage of abscesses may also be required.
Preventive measures are more practical and economical in intensive farming environments; these include raising animals off the soil, avoiding exposure to muddy or water-inundated regions and providing clean drinking water via chlorination and filtration. Minimization of environmental contamination by diseased animals is also an important control measure. There is no effective vaccine.
Zoonotic Risk of Melioidosis in Animals
Melioidosis is an environmental pathogen and is not contagious, but has zoonotic potential. Animal-to-human transmission and animal-to-animal transmission of B pseudomallei have occurred after exposure to an environment contaminated by the bacteria because infected animals can shed the organism in wound exudates and body fluids, including sputum, nasal secretions, milk, feces, and urine. B pseudomallei has been isolated from goat's milk, prompting the requirement for pasteurization of commercial goat's milk in the tropics. Infected animal carcasses are condemned at the abattoir. Importation of infected animals into nonendemic regions could lead to establishment of the organism if the environmental conditions are suitable.
Melioidosis is an infectious disease affecting a broad range of animal species and humans with very low zoonotic potential.
The clinical diagnosis is difficult because of the multitude of clinical signs and syndromes. Isolation and identification of the organism are required.
DNA-based assays are suitable for isolate identification. Serologic tests display low sensitivity and specificity and must be interpreted with caution.
Treatment is prolonged and not always successful; however, selection of appropriate antimicrobials can decrease the mortality rate. No effective vaccines exist. In endemic areas, prevention of exposure to contaminated environments is the best management strategy.
For More Information
Wiersinga WJ, Virk HS, Torres AG, et al. Melioidosis. Nat Rev Dis Primers. 2018;4:art.17107. doi:10.1038/nrdp.2017.107
Sprague LD, Neubauer H. Melioidosis in animals: a review on epizootiology, diagnosis and clinical presentation. J Vet Med B Infect Dis Vet Public Health. 2004;51(7):305-320. doi:10.1111/j.1439-0450.2004.00797.x
Centers for Disease Control and Prevention: Melioidosis
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