Download Bacterial Pathogens Associated With Bovine Respiratory Disease

Bacterial Pathogens Associated With Bovine Respiratory Disease Complex
Pneumonia is an inflammation of the tissues of the lungs that results from the response of the animal to
an infectious agent, either a virus or bacteria, or in most cases both. Common viruses that can initiate
pneumonia in cattle include: IBR (infectious bovinerhinotracheitis virus; a herpes virus), BRSV (bovine
respiratory syncytial virus), PI3 (parainfluenza 3 virus), and BVD (bovine virus diarrhea virus). Often the
virus infection will cause damage to the lung tissue and then bacteria will invade the compromised
tissues. Lung damage from bacterial pneumonia, in the form of lesions, is generally the precipitating
factor in BRDC related mortality. The four main bacterial pathogens involved in BRDC are opportunistic
pathogens, normally residing in the respiratory tracts of healthy cattle, and becoming pathogenic when
stress and secondary infection impairs immune function1. Three of the main bacterial pathogens
(Mannheimia haemolytica, Pasturella multocida, and Histophilis somni) frequently associated with BRDC
are all generally susceptible to the same types of antibiotics2. However, Mycoplasma bovis is a more
genetically distinct bacterial pathogen involved in BRD.
It lacks cell walls and is therefore not
susceptible to some common types of antibiotics, like penicillin3. The importance of knowing the
bacteria involved in BRDC is that it can influence treatment options.
Mannheimia haemolytica
Mannheimia haemolytica (formerly named Pasturella haemolytica ) is commonly isolated from the
lungs of cattle with pneumonia2. Mannheimia haemolytica is comprised of 12 capsular serotypes, with
serotype A1 involved in most cases of pneumonia. Serotype A1 alone can cause pneumonia, but
pneumonia symptoms are difficult to replicate without adding environmental stress or viral infection4.
The switch from commensal to pathogenic organism is considered density dependent, because virulence
factors are stimulated when M. haemolytica is able to grow rapidly in the lung5. One important
virulence factor, leukotoxin, is excreted during the log phase of bacterial growth6. Leuktoxin is thought
to play a major role in lung injury 7 due to its ability to damage white blood cells 8 and elicit a vigorous
inflammatory response9.
Pasturella multocida
Pasturella multocida subsp. multocida ranks only below Mannheimia heamolytica in involvement in fatal
cases of BRDC2,10, but the involvement of P. multocida in BRDC has risen in recent years2. The presence
of P. multocida in the upper respiratory tract is not always associated with disease11,12. It is not clear if
commensal P. multocida converts to a pathogen in a density dependent manner, or if differences in the
lung environment after viral infection favor the growth of more pathogenic isolates13. P. multocida does
generate a density dependent signal similar to M. haemolytica5, but no toxins are known to be secreted
from P. multocida involved in pneumonia14,15. Other virulence factors, such molecules on the bacterial
surface, may be important for P. multocida pathogenicity15. Pasturella multocida is the only common
BRDC pathogen that is zoonotic, or infectious to humans. P. multocida is generally transmitted to
humans by animal bite, scratch, or lick16.
Histophilis somni
Histophilis somni (formerly named Haemophilus somnus) is commonly isolated from the lungs of cattle
with BRDC2. In addition to being associated with of BRDC, H. somni is involved in bovine infertility,
abortion, septicemia, arthritis, myocarditis, and thrombotic meningoencephalitis 17. H. somni infections
are characterized by inflammatory destruction of blood vessels18. H. somni virulence stems from
molecules embedded on the bacterium’s cell surface. A primary virulence factor is lipoologosaccaride
(LOS) which, along with other molecules on the cell surface, can mimic eukaryotic cell coatings, and may
help hide the bacteria from the bovine immune system17,19. LOS can also damage endothelial cells and
activate platelets, inducing vascular inflammation and coagulation20,21. Vascular inflammation is further
exacerbated by the ability of H. somni to produce histamine22.
Mycoplasma bovis
For dairy calves, there are limited data on the prevalence of M. bovis. This bacterium has been the
subject of considerable investigation; however, its primary role in bovine bacterial pneumonia is
controversial23. M. bovis has been isolated from up to 45% of grossly and histologically normal bovine
lungs 24. The nasal prevalence of M. bovis in Californian dairy calves up to 8 months of age was 34% in
herds with M. bovis-associated disease and 6% in nondiseased herds25. A longitudinal study showed that
almost all calves in diseased herds became infected with M. bovis26. M. bovis is also associated with
bovine mastitis, arthritis, and ear infections. M. bovis is the predominant cause of ear infections in
cattle27, and infected animals may display ear droop, head tilt, and ear drainage due to infection of the
middle ear28. Arthritis and lameness generally occur after prolonged M. bovis infections29. M. bovis
should be suspected in cases of pneumonia that are unresponsive to antibiotic treatments, especially if
accompanied by ear infection or arthritis30. Because mycoplasmas lack a cell wall, the β-lactam
antimicrobials are not effective against these pathogens. Similarly, mycoplasmas do not synthesize folic
acid and are therefore intrinsically resistant to sulfonamides. Mycoplasmas as a class are generally
susceptible to drugs that interfere with protein (tetracyclines, macrolides, linosamides, and florfenicol)
or DNA (fluoroquinolones) synthesis. However, M. bovis is resistant to erythromycin31. Unlike the other
bacterial pathogens associated with BRDC, M. bovis is shed in the milk of infected cows. Feeding calves
unpasteurized milk is thought to play a role in the transmission of M. bovis32.
Treatment of Bacterial Pneumonia
Early detection and treatment of BRD is a priority to quickly reduce the impact on infected bovine. Initial
clinical signs include an elevated temperature, nasal and eye discharge, walking with a stiff gait, a crusty
muzzle, salivation and mild diarrhea. Rapid shallow breathing and coughing are also early signs. Affected
animals will often hang their heads and look lethargic. Their unwillingness to eat is closely tied to fever
and depression. Evaluating calves for treatment using a screening system, such as the calf respiratory
scoring
chart
which
was
developed
at
the
University
of
Wisconsin,
(http://www.vetmed.wisc.edu/dms/fapm/fapmtools/8calf/group_pen_respiratory_scoring_chart.pdf),
and is based on rectal temperature, character of nasal discharge, eye or ear appearance and presence of
coughing, has been recommended for dairy calves33.
Cases of bacterial pneumonia resulting from BRDC are typically treated with antibiotics, sometimes in
conjunction with non-steroidal anti-inflammatories (NSAIDs). Success of antibiotic treatment is
dependent on proper timing and dosage of drug administration and susceptibility of the bacterial
pathogen(s) to the administered antibiotic. It is important to follow labeling instructions and veterinary
guidelines as to the proper usage of antibiotics.
Vaccination Against Bacterial Pneumonia
USDA approved vaccines are available to help protect cattle against the four main bacterial pathogens
involved in BRDC34. The practice of vaccination against bacterial BRDC pathogens is common, but less so
than vaccination against viral BRDC pathogens35. However, the efficacy of vaccination in preventing or
reducing the severity of BRDC is not well established36. No decrease in treatments for respiratory
disease was observed in young dairy calves vaccinated with a modified-live Mannheimia haemolytica
and Pasteurella multocida vaccine37. Therefore vaccination should be considered only one element of
BRDC prevention, along with efforts to minimizing calf stress, colostrum feeding, and hygiene. Consult
your veterinarian to determine a vaccination protocol that matches the needs of your herd with
location-specific disease pressures.
Further reading and internet resources:
 Mycoplasma: Calf to Cow. http://www.vetmed.ucdavis.edu/vetext/INF-DA/Wust-mycoplasma.pdf
 Calf notes by Dr. Jim Quigley: English http://www.calfnotes.com
Spanish/Español http://www.calfnotes.com/CNnotasterneros.htm
 Calf facts by Dr. Sam Leadley: English and Spanish/Español http://atticacows.com/
 University of Wisconsin Dairy Calf Clinical Information and Forms including respiratory scoring chart
http://www.vetmed.wisc.edu/dms/fapm/fapmtools/calves.htm
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The “Integrated Program for Reducing Bovine Respiratory Disease Complex (BRDC) in Beef and Dairy Cattle”
Coordinated Agricultural Project is supported by Agriculture and Food Research Initiative Competitive Grant no.
2011-68004-30367 from the USDA National Institute of Food and Agriculture.”