Saturday, December 15, 2007
Excenel - not all its cracked up to be?
This is Monique. She was born with some very serious conditions that forced her to spend her first couple months in a specialized horse clinic. Since, she has recovered to the point were she can now support herself and stabilize her metabolism without her mothers milk. The major problems leading up to this point in time have left her immune system suppressed and therefore susceptible to many common bacteria that can be found around a wet climate barn yard. It was no surprise that the "cold" that circulated briefly around the several other young horses, would reach Monique. But when the symptoms of her "cold" progressed far worse than those of the other horses, she was put on Excenel to help her get over the infection that was developing in her lungs.
Excenel is an antibiotic common among large animals that falls under Cephalosporins. Cephalosporins of this generation are classified as bactericidal, acting to inhibit the mucopeptide synthesis of the bacterial cell wall and creating an osmotically sensitive cell. They are different generations of chephalosporins, each with their own specificities and developed resistancies. For example, 1st generation Cephalosporins are more effective towards gram-positive bacteria such as S.intermedius and S.aureas (to name only two), and are consistently resisted by Streptococci, Enterococci, Methicillin-resistant Staphylococci, and Pseudomonas. 3rd generation cephalosporins are much broader spectrum with few recognized or tested resistant bacteria, while some antibiotics of the 3rd generation have proved particularly effective against Pseudomonas. Excenel is a new generation cephalosporin antibiotic used particularly in horses for its abilities against S.equi and Streptococcus zooepidemicus involved in common respiratory infections. Sounds promising right?
Wrong! Monique was given Excenel intramuscularly for a month straight, twice as long as Excenel advertises, but still, her symptoms persisted. A broad spectrum bactericidal drug that triggers common pathogens to horses should have worked well with Monique's suppressed immune system! What went wrong?
When a sterile swab was taken of Monique's mucus, Pseudomonas as well as S.aureus were isolated. Other forms of gram-positive mannitol fermenting Staphylococci were also present (could it have been possibly S.equi) as well as some very interesting other bacteria that could not be analyzed. The evidence of both S.aureus and Pseudomonas indicates the inevitable resistance factor. Has the bacteria causing the problem become resistant? It could be!
With the mucus sample we were able to take a look at the raw evidence through making a simple slide. The slide showed lots of puss, a very effective way to prevent antibiotics from reaching target areas of infection. Puss protects the bacteria causing the infection by acting as a barrier between antibiotics and the site of infection. Could this explain why such a highly regarded drug has failed in Monique's case? Possibly.
Monique was in a specialized hospital for months. Within the sterile walls of any medical institution, bacteria are discouraged in as many ways as possible creating a selective pressure. Many hospitals today are facing the man made circumstance of MRSA, Methicillin resistant Staphylococci. Recall that for first generation cephalosporins MRSA are resistant. Could it be that Monique actually suffers from something more than a common respiratory bacterial infection? Could she have contracted MRSA somewhere in the hospital? The other horses she has come in contact with have recovered meaning that she has most likely not contracted MRSA.
Monique's case is very complicated and no serious conclusions can be drawn without further extensive testing, however, some theories from micro biology 202 can be applied. The antibiotic was unable to penetrate the infection causing bacteria due to resistance factors or the antibiotic was unable to localize the infection site due to prevention variables. The possibility of MRSA exists but is unlikely. Excenel did not work and continues to not work. The veterinarians are now looking into some other options such as a tracheal flush. The bacteria that is causing the problem has evidently proved its virulence and therefore the next step is for the veterinarians to culture the bacteria from the lower respiratory track and examine the real "enemy". Email for updates!
Dr.Margret Clarins (JDF vet)
Veterinarians hand book of drugs (trust me - this is not a hand book)
Linda Scotten (Instructor at Camosun College) (THANK YOU!!)