Infectious Disease and Microbiology – Mycobacterium bovis

Overview

Mycobacterium bovis is an aerobic, acid-fast bacillus belonging to the Mycobacterium tuberculosis complex (MTBC). It primarily causes tuberculosis in cattle but can infect humans and produce disease that is clinically similar to tuberculosis caused by M. tuberculosis.

Human infection has classically been associated with consumption of unpasteurized dairy products, although transmission can also occur through inhalation of infectious aerosols.

Classification

Genus: Mycobacterium

Species: M. bovis

Complex: Mycobacterium tuberculosis complex

Type: Acid-fast bacillus

Microbiologic Characteristics

M. bovis is:

• Aerobic

• Slender bacillus

• Acid-fast

• Nonmotile

• Non-spore-forming

• Slow-growing

Like other mycobacteria, its cell wall contains large amounts of:

Mycolic acids

which contribute to acid-fast staining and resistance to many environmental stresses.

Acid-Fastness

The lipid-rich cell wall allows M. bovis to retain certain dyes despite acid-alcohol decolorization.

Therefore:

Acid-fast bacillus

• ●

Tuberculosis-like disease

→ Consider organisms within the M. tuberculosis complex

Epidemiology

M. bovis is primarily a:

Zoonotic pathogen

Cattle represent an important reservoir, although infection can occur in several other mammalian species.

Transmission

The classic route of human infection is:

Ingestion of contaminated, unpasteurized milk or dairy products

Another important route is:

Inhalation of infectious respiratory aerosols

particularly in settings involving close exposure to infected animals.

Importance of Pasteurization

Historically, bovine tuberculosis was an important source of human disease through contaminated milk.

Widespread:

Milk pasteurization

• ●

Control of bovine tuberculosis in cattle

have greatly reduced foodborne M. bovis infection in many countries.

Pathogenesis

After entering the body, M. bovis can survive within:

Macrophages

and produce granulomatous disease similar to M. tuberculosis.

The anatomical pattern can partly reflect the route of acquisition.

Route of Infection and Disease Pattern

Ingestion of contaminated dairy products

→ Oropharyngeal or gastrointestinal exposure

→ Regional lymphatic involvement

→ Cervical lymphadenitis or gastrointestinal tuberculosis

Whereas:

Inhalation of infectious aerosols

→ Pulmonary exposure

→ Pulmonary tuberculosis

Lymphadenitis

One important manifestation is:

Tuberculous lymphadenitis

Historically, cervical lymphadenitis associated with bovine tuberculosis was sometimes referred to as:

Scrofula

especially when cervical lymph nodes were involved.

Pulmonary Infection

Aerosol transmission can result in:

Pulmonary tuberculosis

Clinical manifestations may include:

• Persistent cough

• Fever

• Night sweats

• Weight loss

• Fatigue

• Hemoptysis in advanced disease

Pulmonary M. bovis infection may be clinically difficult to distinguish from pulmonary M. tuberculosis infection without microbiologic identification.

Gastrointestinal Infection

Ingestion of contaminated unpasteurized dairy products can produce:

Gastrointestinal tuberculosis

Potential manifestations include:

• Abdominal pain

• Fever

• Weight loss

• Intestinal inflammation

• Regional lymphadenopathy

High-Yield Exposure Pattern

Tuberculosis-like illness

• ●

History of unpasteurized milk or dairy consumption

• ●

Cattle exposure

→ Consider Mycobacterium bovis

Diagnosis

The source lists:

Culture

as the primary diagnostic method.

Culture allows confirmation of mycobacterial infection and can help differentiate organisms within the M. tuberculosis complex.

Additional Diagnostic Methods

Evaluation of suspected disease may include:

• Acid-fast staining

• Mycobacterial culture

• Nucleic acid amplification testing

• Species identification

• Drug susceptibility testing

• Histopathology of affected tissue

Because treatment differs in an important way from standard M. tuberculosis therapy, species-level identification can be clinically significant.

Histopathology

Affected tissues may demonstrate:

Granulomatous inflammation

with:

Caseous necrosis

similar to tuberculosis caused by M. tuberculosis.

Acid-fast bacilli may sometimes be demonstrated within tissue specimens.

Major Drug Resistance Feature

The most important pharmacologic characteristic of M. bovis is:

Intrinsic resistance to pyrazinamide

This is a classic distinction from typical drug-susceptible M. tuberculosis.

High-Yield Resistance Pattern

Tuberculosis

• ●

M. bovis identified

→ Remember:

PYRAZINAMIDE RESISTANT

This is one of the most important exam associations for M. bovis.

Treatment

The source describes treatment using:

Isoniazid (INH)

• ●

Rifampin

• ●

Ethambutol or streptomycin

Because M. bovis is intrinsically resistant to pyrazinamide, pyrazinamide should not be relied upon as an active drug.

Duration

The source states that:

Most infections require 9 months or more of therapy

The longer duration compared with conventional pyrazinamide-containing tuberculosis regimens reflects the absence of pyrazinamide from an effective treatment regimen.

Actual therapy should be determined by susceptibility testing, disease location, severity, and current tuberculosis treatment guidance.

Treatment Principle

M. bovis tuberculosis

↓

Confirm organism and susceptibility

↓

Isoniazid + rifampin + additional active agent initially

↓

Do not count pyrazinamide as active therapy

↓

Continue an appropriately prolonged tuberculosis regimen

M. bovis vs. M. tuberculosis

Mycobacterium bovis

→ Member of M. tuberculosis complex

→ Important animal reservoir, especially cattle

→ Zoonotic

→ Unpasteurized dairy is a classic exposure

→ Can also spread by aerosols

→ Intrinsically resistant to pyrazinamide

Mycobacterium tuberculosis

→ Primarily human reservoir

→ Mainly transmitted through respiratory aerosols

→ Pulmonary disease is classic

→ Usually pyrazinamide susceptible unless acquired resistance occurs

M. bovis and BCG

An important microbiologic connection is the:

BCG vaccine

BCG was developed from an attenuated strain of:

Mycobacterium bovis

Thus:

M. bovis

→ attenuation

→ M. bovis BCG

→ Bacillus Calmette–Guérin vaccine

Prevention

Prevention of human M. bovis infection includes:

• Pasteurization of milk

• Avoidance of unpasteurized dairy products

• Control of bovine tuberculosis

• Veterinary surveillance

• Appropriate precautions when working with potentially infected animals

• Respiratory infection-control measures when infectious pulmonary disease is present

High-Yield Clinical Pattern

Tuberculosis-like disease

• ●

Unpasteurized dairy or cattle exposure

• ●

Lymphadenitis, gastrointestinal disease, or pulmonary infection

• ●

Pyrazinamide resistance

→ Think Mycobacterium bovis

Exam Essentials

Organism: Mycobacterium bovis

Genus: Mycobacterium

Complex: Mycobacterium tuberculosis complex

Type: Aerobic acid-fast bacillus

Reservoir: Primarily cattle and other animals

Disease: Zoonotic tuberculosis

Classic transmission: Unpasteurized milk/dairy products

Other transmission: Respiratory aerosols

Major infections: Lymphadenitis, pulmonary tuberculosis, gastrointestinal tuberculosis

Diagnosis: Mycobacterial culture with species identification; molecular testing may assist

Key resistance: Intrinsic pyrazinamide resistance

Source treatment: INH + rifampin + ethambutol or streptomycin

Treatment duration: Typically prolonged; source describes ≥9 months

Prevention: Milk pasteurization and control of bovine tuberculosis

Vaccine association: BCG is derived from attenuated M. bovis

Key clinical pearl: Mycobacterium bovis is a zoonotic member of the M. tuberculosis complex classically acquired from unpasteurized dairy products or infected cattle. The single most important therapeutic clue is its intrinsic resistance to pyrazinamide, while the classic preventive measure is milk pasteurization.