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99 problems...part 3: sensitivity, MIC, and the antibiogram

  How can the laboratory help me make better choices with antimicrobials? Sensitivity Analysis: What does it mean? Sensitivity analysis determines how well that organism can grow in the presence of a particular antimicrobial at varying concentrations in vitro (on the culture plate). But this is not reality: the bacteria has been removed from the site of infection (e.g. uterus, lung, skin etc) and conditions have changed.   Even if an organism is sensitive to the antimicrobial tested on the culture plate inside the horse it be unaffected. The goal of sensitivity analysis is to assess at what concentration of antimicrobial we would reasonably expect treatment success, and with knowledge of the conditions at the site of infection, whether is it reasonable to assume we can achieve that concentration.   This is a judgment call based on our experience and knowledge of biology.   The conditions at the site of infection can render the antimicrobial ineffective, or where a barrier has forme
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What to do when the pub has no beer. Or milk.

Sometimes mother’s milk is not available.   The mare cannot provide for the foal and there is no readily obtainable source.   Perhaps a nurse mare is not accessible or not affordable. As replacers are more concentrated than mare milk, correct dilution with water is essential.   Dry matter percentage at final dilution should attempt to match that of mare’s milk at approximately 10-15%. 1   This may differ from the targeted concentration of the milk replacer.   Make sure you calculate well.   Increased osmolality (concentration) of milk replacer as compared to mare milk may lead to constipation, hypernatremia (elevated sodium concentrations in the blood), and dehydration in the compromised foal.   Additionally, the higher osmolality of the milk replacer may induce an osmotic diarrhea (concentrated ingesta draws water into the gut).   Bovine-based milk replacers contain sugars not normally found in mare’s milk or in different quantities.   This may promote diarrhea as foals may not have
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Got milk?

Compared to adult horses, foals are poorly adapted to cope with periods of nutritional deficit.  Energy stores in the neonatal foal are minimal when compared to the neonates of other species.  Glycogen stores in the neonate, contained in liver and muscle, are only sufficient to maintain normal body temperature for less than one hour. 1   Hypoglycemia is therefore common in even the healthy neonate.  The ingestion of adequate amounts of colostrum (see Colostrum it’s what’s for dinner), not only for immunological needs but also for caloric, is therefore essential to carry the neonate over the initial neonatal period until intake is sufficient to meet needs.  In situations where neurological or physical compromise to the ability of the foal to suckle the mare are present, energy levels can rapidly diminish. Considerable calories are needed not only to support basal metabolic needs, but to maintain a growth rate that approaches 2.5% of bodyweight daily in the healthy foal during the neon
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99 problems...part 2

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So, there you are having to decide how to treat before you have all the information you need. A bit of local knowledge and a close look at the horse will get you in the ballpark (see 99 problems...part 1 ). How do you take that first step? Begin appropriate empiric treatment before going to narrow spectrum It may take between 24 and 72 hours (plus transportation time and non-working days because some people have weekends) to obtain positive identification of the pathogen. While you are waiting, broad-spectrum therapy can begin. In horses, the combination of a beta-lactam like penicillin or ampicillin with an aminoglycoside, whether it be amikacin in foals or gentamicin in adults, is widely practiced with good reason. This will likely cover most common bacterial infectious agents. Alternatively, other broad spectrum antibacterials can be used. Knowledge of regional sensitivity and resistance patterns, gained from previous culture results, is invaluable to minimize chances of delayin
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Bleeding mares...

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Rupture of and subsequent hemorrhage from the uterine artery is the most common cause of death in mares after foaling 1 .  In a review of central Kentucky mares, reproductive complications accounted for the majority (57 of 98 cases) of deaths in mares around the time of foaling 2 .  Of those that died, rupture of a uterine artery was determined to be the cause of death in 40 cases (70%).  The incidence of peripartum hemorrhage in the mare has not been determined by retrospective studies of large numbers of mares, instead reports of clinical cases are found in the literature.  Peripartum hemorrhage has been reported to occur at any age, however older mares are considered to be at greater risk 3 .  Age-related degeneration of arterial vessels associated with the reproductive tract is suspected to be the reason for the increased incidence in older mares, coupled with the increased mechanical stresses imposed by the gravid uterus.  Uterine contractions and obstetrical manipulations further
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99 problems but antimicrobials shouldn't be one of them...part 1

You are faced with a horse, a fever, exam findings suggestive of bacterial infection, and an anxious owner.   Where do you begin? Even when we have already initiated treatment, we have all been in the position where our carefully thought-out plan has not achieved our desired goals.   It is prudent to assess whether our understanding of the disease process at hand is truly complete, if we will choose or have chosen an appropriate therapeutic, and whether we fully appreciate the difficulties our chosen medications may have in meeting our treatment goals.   As we likely use antimicrobials daily in situations where a positive outcome for the animal and client is both needed and expected, a better understanding of the factors governing success of antimicrobials is the first step in ensuring that success. Consider the most likely organism(s) involved, if any When first asked to make a decision you won’t have the benefit of bacterial culture or polymerase chain reaction (PCR) results. Tha
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Time waits for no foal.

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Start your engines. For the foal, a successful transition to extrauterine life may be complicated by events both before and during parturition.   Some of these can be predicted, or at least you are on notice (see I don’t think it’s right… ). Once delivered, the neonate must rapidly develop altered cardiovascular, respiratory and gastrointestinal function; stand; suckle; achieve coordinated limb movements, and then prepare for a myriad of infectious challenges.   Although born immune deficient through a lack of pre-suckle antibodies, the foal can successfully mount a response to many infectious challenges once sufficient colostrum has been ingested and immunoglobulins absorbed.   Should this fail, common signs include depression, generalized weakness, weak or absent suckle reflex, fever or hypothermia, signs of infection, and abnormal neurological function. Not really that specific for any one condition as we could ascribe these events to almost every sick foal regardless of the cause
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