Mycobacterium spp in Horses (Equis) | Vetlexicon
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Mycobacterium spp

ISSN 2398-2977


Introduction

Classification

Taxonomy

  • Order:Actinomycetales.
  • Family:Mycobacteriaceae.
  • Genus:Mycobacterium- closely related toCorynebacterium,NocardiaandRhodococcus.
  • Mycobacteria can be classified into three groups:
    • Strict pathogens, egM. bovis.
    • Opportunistic, egM. avium.
    • Rare pathogens or saprophytes, egM. smegmatis.

Etymology

  • Gr:myces- a fungus;bakterion- a small rod.

Distribution

  • Worldwide.
  • Mycobacterial infections of animals are extremely common.

Significance

  • Mycobacteriumspp are the agents of tuberculosis   Mycobacteriosis  .
  • The tubercle bacilli areM. tuberculosis,M. aviumandM. bovis.
  • M. paratuberculosiscauses Johne's disease in ruminants.
  • M. aviumis associated with tuberculous lesions in horses.
  • OtherMycobacteriumspp ('atypical or opporunistic mycobacteria') may occasionally cause tuberculous lesions in various species.

Active Forms

Active Form 1

Morphology

  • Thin rods of varying lengths, straight or slightly curved, occurring singly and in occasional threads.
  • The cell envelope is crucial in adaptation of mycobacteria to intracellular growth.
  • Pathogenic mycobacteria have recently been discovered to have a capsular structure.

Taxonomy

  • Cytochemically gram-positive, butMycobacteriumspp do not take up this stain due to the high lipid content of the cell wall, chiefly mycolic acid.
  • Strongly acid-fast and alcohol-fast as demonstrated by Ziehl-Neelsen or flourochrome procedures. Some beaded forms due to irregular staining.
  • Non-motile, aerobic.
  • Species can be distinguished by biochemical reactions or using DNA/DNA hybridization, ribosomal RNA probes or polymerase chain reaction.
  • Can be differentiated from related species, egRhodococcus  Rhodococcus equi  , on the basis of mycolic acid synthesis.
  • Spoligotyping is used to investigate the molecular epidemiology of mycobacteria, especiallyM. bovis. It is based on polymorphism of the direct repeat region of the bacterial genome; the spacer sequences between the repeats are used as genetic markers.

Color

  • Light brown = 'buff'.

Tolerances

Temperature
  • Grow at 33-39°C.
Humidity
  • Resistant to drying.
Ultraviolet
  • Killed by sunlight or ultraviolet radiation.
Other
Very resistant to disinfectants; fenolic disinfectants are effective and should be used in the laboratory when working with mycobacteria.
  • Survive exposure to sodium hydroxide or hydrochloric acid.
  • Killed by pasteurization.

Development

Growth
  • Best in airtight containers.
  • Most species are obligate aerobes, but some are microaerophilic.
  • Mycobacteriumspp are grown on solid slants of Lowenstein-Jensen incorporating malachite green to inhibit contaminants or Stonebrinks media (complex organic media).
  • M. tuberculosis forms 'rough buff and tough' eugonic colonies on glycerol containing medium.
  • Most pathogenic species grow slowly; culture requires 15-28 days.
Reproduction
  • Binary fission.
  • Generation times range from 12 h upwards.
Longevity
  • Survives for long periods in soil.

Resting Forms

Clinical Effects

Epidemiology

Habitat

  • Tubercle bacteria are obtained from infected individuals: humans withM. tuberculosis, cattle and wildlife withM. bovis,and birds withM. avium.
  • Non-tuberculous mycobacteria are saprophytic, but some are commensals in various animal species.

Lifecycle

  • Multiplication occurs both intracellularly in macrophages and extracellularly.
  • The organism is able to survive in phagosomes and phagolysosomes.

Transmission

  • Infection in horses usually contracted from contaminated feed or water.
  • By aerosols or fomites - mainly from respiratory discharges from infected animals.

Pathological effects

  • The organism gains access to the body, usually via the respiratory tract or digestive tract (horses), and avoids initial killing by host phagocytes.
  • The subsequent lesions produced are in part due to the cell-mediated immune response which is generated after the infection has become established.
  • Acquired resistance depends on cell-mediated responses.
  • Antibodies are not protective.
  • The virulence of the organism is due to the lipids of the cell wall, which protect the bacilli from phagocytosis.
  • Initially, the organism proliferates and lymphatic spread may occur at this stage. Acute or subacute inflammation occurs with polymorphonuclear infiltration.
  • After delayed hypersensitivity develops, granulomatous inflammation supervenes and the macrophages become elongated and are concentrically arranged to form a tubercle.
  • Outside these epithelioid cells a fibrous layer builds up and caseous necrosis occurs at the center of the lesion.
  • Liquefaction of the caseous lesion occurs and a cavity develops in which further proliferation of the organism takes place.
  • Further spread may occur via the erosion of bronchi or viscera to new areas or via the bloodstream.

Equine cases

  • Horses are rarely affected, but more often byM. avium  Mycobacteriosis  thanM. bovis.
  • Usually infected via the gastrointestinal tract, primary lesions occur in the pharynx and intestine.
  • Lesions in horses often do not exhibit caseation or calcification.
  • Cutaneous forms, arthritis/synovitis, abortion, guttural pouch infection   Guttural pouch: mycosis  and ocular lesions have also been reported.

Other host effects

  • Some non-tuberculous bacteria are commensals.

Control

Control via animal

  • Most equine cases diagnosed at necropsy.
  • Treatment rarely attempted because of the zoonotic risk.
  • Treatment has been successfully attempted in non-human primate colonies.

Control via chemotherapies

  • Mycobacteria are resistant to most antimicrobials because of the high lipid content and complexity of their cell walls, together with their ability to reside within macrophages.
  • First-line drugs for tuberculosis therapy are isoniazid, pyrazinamide, ethambutol and rifampicin    Rifampicin  .
  • Second-line drugs include streptomycin    Streptomycin  , against which resistance is now common, rifapentine, kanamycin   Kanamycin  /amikacin   Amikacin  , capreomycin and fluoroquinolones   Therapeutics: nitrofurans / nitroimidazoles / quinolones  .
  • Combinations of drugs are usually used because resistance often develops under a single-drug regime.
  • Long-term therapy is required to effect a cure and eliminate the organism (9-24 months).
  • Short or incomplete courses of therapy are an important cause of the development of resistantM. tuberculosis.
  • Prophylactic treatment with isoniazid may be considered for pets exposed to tuberculosis.
  • Genomic and proteomic technologies can be used in drug development to analyze the metabolic patterns of mycobacteria and predict the activity of drugs.

Diagnosis

Useful samples

  • Live animal: aspirate, lymph node, biopsy tissue, urine, feces.
  • Dead animal: tissue from lesion or a selection of lymph nodes.

Isolation

  • Mycobacteriumspp are grown under aerobic conditions on solid slants of Lowenstein-Jensen incorporating malachite green to inhibit contaminants, or on Stonebrinks media (complex organic media).

Strict safety precautions are required when dealing with material suspected of containing these organisms. All handling of specimens must take place in a safety cabinet in a separate room. Effective disinfectants must be used and all contaminated material autoclaved.

Laboratory diagnosis

  • Ziehl-Neelsen staining slender red-staining rods, some beaded against a blue/green background.
  • Fluorescent dyes/staining, eg auramine, rhodamine are examined under a UV microscope.
  • Specimens must be treated before culture media are inoculated. This involves selective decontamination, liquefaction of mucus and concentration by centrifugation.
  • Biochemical tests: to distinguish species, eg tests for niacin production, pyrazinamidase, urease and nitrate reduction.
  • Molecular-based diagnostic methods are available.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Monreal L, Segura D, Segales J, Garrido J M & Prades M (2001) Diagnosis of Mycobacterium bovis infection in a mare. Vet Rec 149 (23), 712-714 PubMed
  • Booth T M & Wattret A (2000) Stifle abscess in a pony associated with Mycobacterium smegmatis.Vet Rec 147 (16), 452-454 PubMed
  • Leifsson P S, Olsen S N & Larsen S (1997) Ocular tuberculosis in a horse. Vet Rec 141 (25), 651-654 PubMed
  • Thorel M F, Huchzermeyer H, Weiss R & Fontaine J J (1997) Mycobacterium avium infections in animals. Literature review. Vet Res 28 (5), 439-447 PubMed
  • Hart C A, Becking N J & Duerden B I (1996) Tuberculosis into the next century. J Med Microbiol 44, 1-34 PubMed
  • Helie P & Higgins R (1996) Mycobacterium avium complex abortion in a mare. J Vet Diagn Invest (2), 257-258 PubMed.
  • Gunnes G, Nord K, Vatn S & Saxegaard F (1995) A case of generalized avian tuberculosis in a horse. Vet Rec 136 (22), 565-566 PubMed
  • Cline J M, Schlafer D W, Callihan D R et al (1991) Abortion and granulomatous colitis due to Mycobacterium avium complex infection in a horse. Vet Pathol 28 (1), 89-91 PubMed.
  • Flores J M, Sanchez J & Casta M (1991) Avian tuberculosis dermatitis in a young horse. Vet Rec 12 8(17), 407-408 PubMed.

Other sources of information

  • Manning E J B & Collins M T (2001) Mycobacterial infections in domestic and wild animals. Rev Sci Tech 20 (1), 331. ISBN: 929044519X.
  • Biberstein E L (1990) Mycobacterium species: The Agents of Animal Tuberculosis. In: Review of Veterinary Microbiology. Eds: Biberstein E L & Zee Y C. Blackwell Scientific, USA. pp 202-212. ISBN: 0865420858.