Do macrolides resolve clinical signs in sheep with infectious keratoconjunctivitis more rapidly than oxytetracycline?

You are called to visit a flock of ewes that are six weeks away from lambing. A significant number of these ewes are affected with varying clinical signs of ovine infectious keratoconjunctivitis. Following diagnosis and a plan for treatment using injectable oxytetracycline, the farmer suggests that a ‘stronger’ drug that had been used on a neighbour's farm might speed up the recovery process. A discussion on a vet chat forum suggests treatment success when a macrolide antibiotic is used instead of the cheaper, more commonly used injectable oxytetracycline therapy. You wonder if there is any benefit in recommending the use of an injectable macrolide for a quicker resolution of clinical signs of ovine infectious keratoconjunctivitis. In [sheep with infectious keratoconjunctivitis], does the [administration of injectable oxytetracycline compared with an injectable macrolide] result in a [more rapid cessation of clinical signs]? The search strategy can be viewed at https://bestbetsforvets.org/bet/439, and is also available as a supplement to this article on Vet Record's website at https://bvajournals.onlinelibrary.wiley.com/toc/20427670/2021/188/8 Sheep with severe clinical signs of infectious keratoconjunctivitis Search last performed: 29 March 2021 Paper 1: ‘Pink eye’ or ‘zere oogjes’ or keratoconjunctivitis infectiosa ovis (KIO). Clinical efficacy of a number of antimicrobial therapies1 Patient group: Eight farms with a recent outbreak of ovine infectious keratoconjunctivitis (morbidity greater than 20 per cent) were included in the study, which compared multiple antimicrobial treatments over four different experiments. However, only two of these experiments (experiments 3 and 4) are relevant for the purpose of this evidence evaluation. In experiment 3, 30 sheep with clinical signs of infectious keratoconjunctivitis from a single farm (‘N at E’) received an intramuscular injection of either spiramycin (10 mg/kg) or oxytetracycline (5 mg/kg). In experiment 4, a total of 32 affected sheep from two farms (‘W at W’ and ‘S at W’) were injected with either spiramycin at 25 mg/kg or oxytetracycline at 10 mg/kg. All sheep included were of the Texel breed and were at least a year old. Study type: The study has some features of a randomised controlled trial but has some omissions. Outcomes: Each sheep's eyes were examined at enrolment, and a coded scoring system was used to assign a combined clinical score for both eyes. In experiment 3, the sheep's eyes were re-examined five days after treatment. In experiment 4, re-examination occurred 2.5 and five days after treatment on farm ‘W at W’ and four and nine days after treatment on farm ‘S at W’. Key results: In experiment 3, there was no significant difference in clinical ocular scores between sheep treated with spiramycin and those treated with oxytetracycline (P=0.93). There was also no significant difference in clinical ocular scores between treatment groups on either farm in experiment 4 (P=0.56 for ‘W at W’ and P=0.23 for ‘S at W’). Study weaknesses: The number of sheep included in each experiment is relatively low, and no information was provided regarding whether sample size calculations were carried out before the start of the experiments. There is also limited information about factors such as the age ranges of the sheep in each group, their bodyweights and the type of farming system used. The statistical methods used were not described, and it is unclear how the comparisons were made between the treatment groups. For example, the paper does not state how the codes for each eye were aggregated and how the scores were stratified before comparison. The interpretation of the coding for disease severity is also unclear, and no information is provided about who made the assessments of the clinical scores. The dosages of the antibiotics used in experiment 3 were much lower than those used in experiment 4, making it difficult to directly compare the results. Furthermore, these two experiments used different oxytetracycline preparations, with the preparation used in experiment 4 also containing another antibiotic (polymyxin B). The degree to which this secondary antibiotic may have influenced the results is unclear. There were multiple comparative studies within this paper, and it is unknown how similar the general conditions on each farm were, which could affect the results. Due to the omission of critical information, it would be difficult to replicate this study. This evidence evaluation looks at the only published study comparing the use of oxytetracycline and a macrolide in the treatment of ovine infectious keratoconjunctivitis. The paper references another experiment in which the levels of secretion of these drugs into the lacrimal fluid was assessed. This could be another outcome of interest for clinicians treating these patients. The ‘Evaluating The Evidence'section of Vet Record aims to answer specific clinical questions using a systematic approach to identify and succinctly summarise the relevant evidence from the scientific literature. The shortcomings of this evidence are also taken into account, thereby enabling vets to incorporate the best available evidence from the literature when making clinical decisions. Please contact us at vet.research@bvajournals.com if you have an article you would like us to consider for publication in this section. Mycoplasma conjunctivae is usually the primary pathogen in ovine infectious keratoconjunctivitis cases, with secondary involvement from other bacterial pathogens such as Chlamydophila. These organisms are typically susceptible to both oxytetracycline and the macrolide class of antibiotics. As such, antibiotic selection should be guided by responsible antimicrobial use guidelines in these cases. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.