Horse shipping infections...are drugs the answer?

Sometimes, I get a little concerned when research papers get picked up by the press. It's not necessarily because the research is weak, it's just that results sometimes get overstated or misinterpreted when they work their way outside of scientific forums.

A paper published in the latest edition of the American Journal of Veterinary Research is one of those. The paper (Tsuchiya et al. 2012) describes a study that looked at the impact of interferon-alpha (used to stimulate the immune system) and enrofloxacin (an antibiotic) on body temperature and lung fluid white blood cell counts in 32 horses that were shipped for approximately 26 hours in commercial vans. Horses either received just interferon or interferon and enrofloxacin before being shipped.

  • After shipping, 3 antibiotic-treated and 9 untreated horses developed fevers. That's actually not statistically significant, by my calculations, and it's quite strange that the authors didn't do that analysis (and that the reviewers didn't pick that up).
  • Two antibiotic-treated and 7 untreated horses were treated with antibiotics after arrival because of concerns about infections. Again, that's not statistically significant and it's surprising (and concerning) no one pointed that out.
  • Overall, the average temperature of horses in the treatment group was significantly lower after arrival, but the clinical relevance of that is questionable since it was only a 0.4 C difference. Further, it's hard to say what a temperature immediately after arrival really means, since that's pretty early for a bacterial infection to have developed.
  • There were significant differences in tracheobroncial fluid (fluid collected from the airways) between the groups, with lower white blood cell counts in the treated group. That's an interesting finding and is consistent with less inflammation. What that means in terms of disease prevention is harder to say, but it's something worth investigating further.
  • There does not appear to have been any difference between the two groups in the ultimate health status of the horses.

This study provides some interesting information to help us think about how, when and why infections and inflammation develop after shipping. Results suggest that antibiotics might be useful in certain situations, but many questions remain. Any antibiotic use runs some risk of complications such as antibiotic-associated diarrhea. It also increases the risk of antibiotic resistance (and ultimately more problems trying to treat disease). We have to remember these issues when considering these results. Further, while it is typically much better to prevent disease than treat it, in situations like this, it's hard to say whether mass prophylactic treatment is actually preferable to early treatment, since horses can be observed closely after arrival and treated when early signs of disease develop. Ultimately, it's still not even clear from these data whether pre-treatment with antibiotics actually does have a positive clinical effect.

It's important to remember what this study tells us, and what it doesn't. Despite what some lay articles that have picked up the story say, it doesn't mean that antibiotics are broadly useful for keeping shipped horses healthy. The authors address this by stating "The use of enrofloxacin raises concerns regarding the emergence of antimicrobial-resistant bacteria, and it is important that antimicrobials such as enrofloxacin are not used inappropriately. The guidelines for enrofloxacin use in the Japan Racing Association’s medical office require that it is only administered as prophylaxis against transportation-associated fever when the duration of transportation is expected to be ≥ 20 hours and the horse has had clinical signs of transportation-associated fever before or is considered to be at risk for developing transportation-associated fever (eg, if the horse has undergone laryngoplasty or has a history of pneumonia)."

Regarding the big picture, however, this should make us think again about how we manage horses. Antibiotics should never be used as a crutch in place of good management. In a situation like this, where 19% of horses treated with interferon and enrofloxacin and 56% of horses treated with interferon alone get sick, something's wrong. Antibiotics may be an easy way to try to reduce the likelihood of disease in some situations, but that doesn't mean it's a good idea. Considering the number of horses that get sick (and die) every year from shipping-associated illness, maybe we need to rethink how they are transported. Is lack of antibiotics the problem, or is it how (and how long) horses are shipped? Maybe long, interrupted trips aren't a good idea, antibiotics or not.

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Comments (2) Read through and enter the discussion with the form at the end
Debbie Racklyeft - July 26, 2012 7:22 AM

Dear Scott - I love your blog. I have a special interest in LRT infections in horses assoc with travelling. I did research on head posture (followedup by my colleague Sharanne Raidal) way back in the nineties(!!). I agree SO much with all of your comments. Management, not antimicrobials is the most important preventative strategy. Travel sickness is a polymicrobial disease caused by "aspiration" of normal pharyngeal flora and any antibiotic use merely changes the flora and cannot eliminate all organisms. An additonal thing to consider is that enrofloxacen has poor efficacy against anaerobic bacteria, which as you know are often the ultimate culprits in severe pleuropheumonia. There are some impressive statistics on morbidity and mortality from pleuropneumonia in army remount horses in the early 20th century before and after simple management strategies were put in place - long before antibiotics were available. Here is an extract from the introduction to my thesis

Army veterinarian, Dr. Watkins-Pitchford recognised that there was a preliminary period of time, before pneumonia became established, where fever was the only evidence of disease to an unskilled observer. He recommended that body temperature should be measured at least daily and that no horse with an elevated temperature be allowed to travel. He stressed the importance of sufficient skilled observers to accompany travelling horses. These personnel would become acquainted with physical peculiarities and habits of horses in their care, and therefore could detect readily any slight departure from normal health. He also recommended that newly purchased horses be kept under clinical observation for at least two weeks and not travelled unless they were normal and that no army remount should be subject to any rail journey of more than 50 miles within 21 days of a previous rail journey. Other recommendations were added to these: 1) that horses transported by sea be placed on board as late as possible before leaving, 2) that the number of ports visited before reaching their destination be kept to an absolute minimum, 3) that the horses were unshipped as soon as possible after arriving, 4) that there be a 30 hour maximum period between stops to offload to feed, rest and water during journeys by rail and 5) that horses not be exercised for the first three weeks following arrival.

The application of these measures dramatically reduced the number of cases of, and losses due to, pneumonia in transported horses. In an English army remount hospital during Jan 1915, 50 horses per week died from pneumonia. Following institution of the above recommendations, losses were less than ten per week by January 1916. Similarly in North America between December 1917 and November 1918, cases of pneumonia were reduced from 500 to less than 80 per week and mortality from 213 to 31 per week.

Impressive eh!

Best regards

Debbie Racklyeft
Scone Australia

Kate Allison - July 30, 2012 9:11 AM

I think that a enro will drive higher mutations based on structure of the drug and how close the "reaction point" of the bacteria is to the "mutation button." It certainly correlates with increased anti-microbial resistance as use of that class of drugs became more common place.

"Shipping fever" is so multifactorial, including the state of gut health, general health, etc of the animal (I am including cows too), that anti-biotics as prophylaxis makes little sense with in the context of public health and indeed herd health. I like the idea of checking for abnormals before pneumonia is a problem.

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