Gram-Negative Bacteria from Pharynx and Nasal Cavity of Domestic Goats in Grenada, and Resistance of Mannheimia haemolytica and Bibersteinia trehalosi to Tulathromycin and Trimethoprim-Sulfamethoxazole

Aim: The present study was conducted to evaluate the occurrence of bacterial respiratory pathogens, particularly members of the family Pasteurellaceae in healthy domestic goats in Grenada, and to determine the antimicrobial susceptibility of the predominant species.

Study Design: Nasal and pharyngeal swabs from 161 adult goats from the six parishes of Grenada were collected during a ten month period from May 2012 to March 2013 and examined for potential bacterial respiratory pathogens.

Methodology: Bacteria resembling Pasteurellaceae, and Corynebacterium spp. were presumptively identified by phenotypic characteristics. For definitive identification to species level, DNA from the isolates were subjected to 16s ribosomal RNA sequencing. The closest matches to sequences in GenBank, and their percentage identity were the criteria used to determine the bacterial species. The major members of Pasteurellaceae were tested for antimicrobial susceptibility to 11 antibiotics using the disk diffusion method.

Results: Of a total of 98 Gram-negative isolates, 41% were Mannheimia haemolytica, followed by Bibersteinia trehalosi (37%), Mannheimia glucosida (9%), and the remainder comprising of 11 different species, including five species of Moraxella. Of the three Gram-positive isolates, two were Rhodococcus equi, and one was Trueperella pyogenes. Antimicrobial susceptibility tests on a total of 73 isolates of M. haemolytica and B. trehalosi showed that 18% isolates were resistant to tulathromycin, a recently introduced drug for use in goats. Moreover, 77% of isolates were resistant to trimethoprim-sulfamethoxazole, another drug with application in goats. Tulathromycin resistance was accompanied by resistance to trimethoprim-sulfamethoxazole in 12 of the 13 isolates. Resistance to these two drugs is not in accordance with published data, and need detailed further investigation. Resistance to ceftiofur, a drug used for pneumonic pasteurellosis was minimal (one isolate only), and none of the isolates were resistant to amoxicillin-clavulanic acid or enrofloxacin.

Conclusion: In conclusion, our study, first of its kind in the Caribbean, showed that M. haemolytica and B. trehalosi, two major respiratory pathogens of ruminants colonize nasal cavity and pharynx of healthy goats in Grenada. Both organisms showed uncommon high resistance to tulathromycin and trimethoprim-sulfamethoxazole, the reasons for which are not understood, and need further investigation.