How many intramuscular (IM) injections has your horse received in his lifetime? Dozens, probably, or maybe even hundreds, between regular vaccinations, treatments for illness or injury, medication for pain and possibly supplementation of vitamins or minerals. And how many times has he suffered anything beyond a bit of soreness and swelling at the injection site? Probably never. When administered properly into the muscle, IM injections do little harm and lots of good. Every now and then, however, injection sites become breeding grounds for life-threatening clostridial infections.
Isn't it likely, then, that injection technique contributes to these lethal responses by introducing the causative anaerobic bacteria into the muscle? When a team of researchers at Michigan State University conducted a survey of equine veterinarians looking for that link, the results showed, rather surprisingly, that scrupulous cleanliness during IM administration had no significant effect in preventing clostridial infections. Nor did less careful injection techniques. The survey asked if they
- used a separate needle and syringe for each injection
- clipped the injection site
- cleansed the site with an alcohol- or disinfectant-soaked swab
- similarly cleansed the tops of multidose vials before withdrawal of each dose
- had had a patient develop clostridial myositis (muscle inflammation) following such injections.
Since clostridial organisms are everywhere in the horse's environment, the injection paraphernalia as well as the horse's hair and skin could be contaminated. You would think that disinfecting the injection site, the needle and syringe and the vial top would wipe out these external contaminants, but clostridial spores are undamaged by routine surface cleaning. The surveyed practitioners generally employed at least some of these precautions. Of the 439 respondents, 414 used a new, sterile syringe and needle for each injection, and half of the group disinfected both the injection sites and the tops of multidose vials. Horses' hairiness imposes a barrier to thorough cleansing of the skin, however, and only two of the respondents routinely took the added precaution of clipping IM-injection sites before disinfecting the skin.The two site clippers reported no cases of clostridial infections among their patients, but the sample was too small to be considered statistically significant. Of the rest of the practitioners, 6.4 percent said they had had at least one clostridial incident, and their injection technique had no bearing on the incidence. Just two of the 25 veterinarians who reused syringes and needles reported this postinjection complication, making the incidence among that group, at eight percent, not significantly greater than that of their more meticulous peers.The survey results are not to be taken as a license for unclean injection methods, despite the study's failure to show a direct link between environmental contamination and the incidence of clostridial infections following IM injections. Instead, the researchers suggest that their findings may point to a more complex and circuitous route of infection in which IM administration of nonantibiotic medications in conjunction with gastrointestinal disruptions may trigger the condition. A similar infection in cattle, called blackleg, is thought to arise from clostridial spores that are absorbed from the gut to lie dormant in other tissues until a muscle trauma creates the ideal environment for the bacteria's proliferation.
Perhaps horse muscles harbor the spores as well, and IM injections of medications other than antibiotics cause enough tissue death, without providing the bacteria-killing effects, to set the clostridia brewing. Colic patients and horses that receive injectable dewormers may be at particular risk in this sort of infection scenario. Not only are they receiving nonantibiotic IM medications, but gastrointestinal inflammation is also encouraging the absorption of resident bacteria into the general circulation.The moral of the study, then, is that IM injections pose a small but real risk for deadly clostridial infections in horses, and though clean, careful injection techniques are certainly desirable, scrupulous administration is not a surefire preventive. Instead, the researchers suggest avoiding IM injections of nonantibiotic medications in cases where the drugs can be given orally or intravenously. Other routes of administration sidestep the clostridial complication altogether.
This article first appeared in the June, 1990
issue of EQUUS magazine.