25. mycoplasmas and respiratory pathogens I (bacteriology)

what is special about the structure of mollicutes (mycoplasma spp.)?

lack cell walls → have a single trilaminar membrane rich in proteins and lipids

are mycoplasmas typically found as free-living organisms?

no → obligate parasite

• co-evolve with their host

• limited host range

what classes of antibiotics are ineffective against mycoplasmas?

• all beta lactam antibiotics

• sulfonamides

mycoplasma mycoides subsp. mycoides is the causative agent of what disease? what species does it affect?

• contagious bovine pleuropneumonia (CBPP)

• affects cattle and buffalo

mycoplasma bovis sources (infection in calves)

contaminated milk, possibly aerosols, or fomites

• almost all calves born on M. bovis infected farms end up colonized

mycoplasma bovis reservoir

mucosal surfaces (respiratory, genital, conjunctiva, mammary gland, alimentary canal, and joints(?))

how is mycoplasma bovis transmitted?

ingestion, possible inhalation, or direct contact

mycoplasma bovis virulence mechanisms

• suspect antigenic variation (variable surface lipoproteins)

• damage host cells (phospholipases, H₂O₂, and superoxide radicals)

• biofilm formation

• can adhere to neutrophils and inhibit respiratory burst activity

how does mycoplasma spread systemically? what are its predilection sites?

• hematological spread

• predilection sites:

  • nares

  • tonsils

  • eyes

  • tympanic bulla

  • meninges

  • lungs

  • udder

  • joints

  • genital area

what are possible outcomes of mycoplasma bovis infection in calves?

• clear infection

• chronic colonization of URT & intermittent shedding

• increased replication in URT → spread from URT → clinical disease

what are complications associated with myocplasma bovis infection in calves?

• otitis

• pneumonia

• arthritis

clinical signs of mycoplasma bovis infection in calves

progressive disease

• fever & anorexia

• otitis → scratching ear, head shaking, ear droop

• head tilt ± other neurologic signs indicate otitis interna (CN VIII) ± nystagmus

• advanced otitis media-interna → meningitis and/or other cranial nerve deficits

  • → spontaneous regurgitation, loss of pharyngeal tone, dysphagia, and aspiration pneumonia

• unilateral or bilateral neurologic deficits

• purulent aural discharge + ruptured tympanic membrane

• arthritis

mycoplasma bovis diagnosis (calves)

• based primarily on clinical signs

• pharyngeal swabs for culture or PCR

• synovial fluid from calves with arthritis can be aspirated for culture or PCR

treatment of mycoplasma bovis infection in calves

• antimicrobials are usually unrewarding but may be beneficial in early stage disease

• puncturing the tympanic membrane to allow for drainage may alleviate pain from trapped exudate

what age of animal does mycoplasma bovis mastitis affect? how is it transmitted?

• affects any age

• type of contagious mastitis (spread easily among animals by direct contact and fomites)

clinical signs of mycoplasma bovis mastitis

• infection usually subclinical

• clinical mastitis usually mild → may have some discharge; affected udders not typically painful

• only indicator may be sudden drop in milk production

• history of recurrent mastitis is common

clinical/lab findings of mycoplasma bovis mastitis

• somatic cell counts usually normal

• milk may appear discolored

• false negative cultures/PCRs common due to intermittent shedding (need to do repeat sampling)

• serology testing individual cow is useless due to high number of false negatives (m. bovis is intracellular)

mycoplasma bovis mastitis treatment

• intramammary antimicrobial therapy not effective

• if cow spontaneously resolves infection, she can become a carrier

mycoplasma bovis pneumonia source

carrier animals

mycoplasma bovis pneumonia transmission

ingestion or inhalation

mycoplasma bovis pneumonia predisposing factors

• all ages affected

• stressors (shipment, parturition, weather changes) that affect innate immunity (impaired mucociliary escalator)

what is a gross finding of mycoplasma bovis pneumonia? what is a common sequelae?

1. lung abscesses

2. arthritis

mycoplasma bovis pneumonia clinical signs

• fever, tachypnea, dyspnea, and decreased appetite ± nasal discharge and coughing

• poor weight gain in chronically affected animals

• can be accompanied by ear infection and/or arthritis

mycoplasma bovis pneumonia diagnosis

• presence of M. bovis in the respiratory tract of healthy animals confound diagnosis

• diagnosis will be presumptive and depends on clinical signs and presence of organism in lesion (necropsy) or transtracheal wash

mycoplasma bovis pneumonia treatment

antimicrobials administered early (e.g. tetracyclines) may help + supportive care → unrewarding with advanced disease

what is an important practice to reduce mycoplasma bovis infections?

mycoplasmas are sensitive to disinfection → good farm disinfection & biosecurity practices

rhodococcus gram stain

gram positive rods

rhodococcus equi is the causative agent of what disease? (what age/hosts does it affect?)

• pyogranulomatous pneumonia in foals < 6 months of age

• uncommon infections in other animals

• commonly causes pyogranulomatous pneumonia in immune compromised humans

rhodococcus equi virulence factors

• plasmid expressing virulence associated protein A (VapA) essential for intracellular survival in foal macrophages

  • helps resist phagolysosome fusion and keeps pH inside phagosome neutral

• lipid rich cell envelope protects from desiccation in environment and supports survival within macrophage

rhodococcus equi source

manure contaminated soils, hays, and sandy stalls (risk factor)

rhodococcus equi transmission

inhalation of organisms

predisposing factors to rhodococcus equi infection

heavy environmental contamination and inadequate colostrum

rhodococcus equi pathogenesis

bacteria engulfed by alveolar macrophages, where it persists → foal macrophages are inefficient at killing bacteria & VapA interferes with lysosome-mediated killing → accumulation of bacteria-laden macrophages → pulmonary pyogranulomas

• evades host immune response (i.e. no fever)

• can cause extensive, irreversible lung damage before foal shows signs of illness

• pyogranulomas can be found in other body sites (eyes, joints, intestines)

rhodococcus equi clinical signs

• subclinical until advanced disease

• based on location of organisms

  • respiratory: coughing, crackles, tachypnea, flared nostrils

  • GI: diarrhea with ulcerative colitis and mesenteric lymphadenitis if intestines become infected

  • osteomyelitis with bone infection (bone swelling, pain/lameness)

rhodococcus equi diagnostics

• clinical signs

• transtracheal or bronchiolar lavage for culture and cytology

• PCR detection of VapA gene is definitive (plasmid only present in virulent strains)

• nasal swabs are useless

rhodococcus equi prognosis/treatment

• prognosis is good if infection caught early; advanced infections have high mortality rates (40-50%)

• sick foals should be isolated and treated with antibiotics (macrolides + rifampin) for several weeks

what is a risk to consider when treating rhodococcus equi with antibiotics?

both mare and foal are at risk for clostridioides difficile associated diarrhea (CDAD)