Medical Corner: Gentamicin and Tylan

With good medical advice so hard to find for rats and mice, I was extremely saddened, and even a little angry, to read Dr. Carmen Jane Booth's statement regarding gentamicin in the Medical section of the American Fancy Rat and Mouse Association's newsletter, Rat & Mouse Tales (Summer I 1997 issue). It is obvious that her opinion came from lack of experience with this drug in treating rats with respiratory disease, but in my opinion, that is no excuse. No experience should equal no opinion when you are counseling others about the care of their beloved pets.

Dr. Booth's opinion is as follows: "I would not use gentamicin in rodents for respiratory disease. The bacteria that commonly cause respiratory disease in cats are different from those in rodents (i.e., not mycoplasma pulmonis). Also, Gentamicin is in the antibiotic family called aminoglycosides and can be extremely nephrotoxic (cause severe kidney damage) and/or ototoxic (ear toxic, associated with causing deafness). This antibiotic is used to treat Gram (-) bacteria and crosses the placenta. In summary, this antibiotic preparation should not be used in rats to treat respiratory problems."

Mycoplasma pulmonis is the most common cause of respiratory disease in rats, but it is almost always accompanied by one or more other pathogens. Many of these multi-factorial infections lead to pneumonia quickly and must be treated very seriously. If Dr. Booth had had any actual experience using gentamicin to treat these respiratory conditions in rats, she would know just how truly effective it can be, but in her career as an industrial laboratory animal veterinarian/oncology scientist at a biotechnology company, she would not be looking for treatments for respiratory disease in rats. In fact, laboratory rats are free of mycoplasma and, therefore, rarely show the type of symptoms common in pet rats. Besides that, in research situations, rats who become ill with respiratory disease would most likely be put to sleep instead of being treated.

Although other drugs such as tetracycline and tylosin are listed in most rodent books as being more effective than gentamicin, we have found that gentamicin has been extremely effective in severe cases of pneumonia. Many, many rats have showed significant improvement after only one or two injections of gentamicin (of course, a full course of the drug must still be administered). Whether or not the gentamicin is actually attacking the mycoplasma pulmonis organism or not is up for debate, but one theory is that it destroys other pathogens that are accompanying the mycoplasma pulmonis which allows the rat's own immune system to attack and keep the mycoplasma under control.

Within the class of drugs known as aminoglycosides, there are drugs that fall into the group classified as 'Specific Aminoglycoside Antibiotics - Narrow- spectrum Aminoglycosides' and some that fall into the group classified as 'Broad-spectrum Aminoglycosides'. Gentamicin falls into the latter group and is described as possessing a broader spectra than some other aminoglycosides that often include several gram-positive as well as many gram-negative aerobic bacteria. Therefore, Dr. Booth's statement regarding this matter should not be used to determine whether or not gentamicin should be used.

It is true that all members of the aminoglycoside family of drugs can be ototoxic and nephrotoxic, but side effects and toxicity should not be a reason to state that a drug "should not be used". It should only be part of the decision making process based on the age and overall condition of the rat in question. Obviously, if the rat in question has reduced kidney function, you probably wouldn't want to use gentamicin. However, steps can be taken to help ensure that no kidney damage will occur as a result of using gentamicin. Making sure the rat is getting enough fluids to flush out the kidneys will help accomplish this goal. Fluids can simply be from the rat drinking a normal amount of water from his water bottle. If the rat isn't drinking on his own, it can be fluids you administer orally or fluids injected subcutaneously. Preventing kidney damage from gentamicin is not difficult.

Making sure you don't overdose the rat on gentamicin is also an easy way to ensure no toxic results from using gentamicin. The recommended dose for rats is 1 to 2 mg per pound twice daily, injected subcutaneously for 7 to 14 days. Although the book Veterinary Pharmaceuticals and Biologicals doesn't give us any treatment suggestions for rats, it does give us the results of a toxicity study using gentamicin in rats. It is as follows: "No toxic effects were observed in rats given gentamicin sulfate 20 mg/kg/ day for twenty-four days". Translated from kilograms to pounds that is 9.09 mg per pound! That is far more than double the highest dose we recommend and a full 10 days beyond recommended treatment length. No comparison and still NOT toxic according to this study.

During the years I have been using gentamicin to treat rat respiratory disease at the instruction of my vet, Dr. Daryl Mabley, I have been able to save numerous rat lives. Of course, it doesn't work in every case, but it has worked for me in a very high percentage of cases. At no time have I ever experienced a case of kidney damage (nephrotoxicity) and I have only seen one case of ototoxicity. In that case, the rat was not rendered deaf; instead, her equilibrium was damaged resulting in her being a little off balance. I would much rather have a rat have an ototoxic reaction to gentamicin and be a bit off balance or deaf and alive than to have a dead rat, and telling people not to use gentamicin is giving many people's pets a death sentence. I strongly object to that and immediately wrote to AFRMA upon reading the article asking them to correct the information. To date, I have still not seen a correction.

With respect for gentamicin, and a little knowledge about the drug, nobody should ever be afraid of using it in under the proper circumstances. Any reaction in rats would be isolated cases of non-tolerance (possible allergic reaction) or from misuse.

In the Fall 1997 issue of AFRMA Rat & Mouse Tales, Dr. Booth made the following statement regarding Tylan. "My recommendation would be to make the Tylan up daily and change the water daily. Many antibiotics precipitate in water and are destroyed by light."

Yes, some antibiotics do precipitate in water and are destroyed by light, but Tylosin (Brand name: Tylan) is not one of them. The instructions on the bottle from the manufacturer clearly state to prepare a fresh Tylan solution every three days, so there is no need to waste your expensive drug by mixing it fresh daily, unless you are adding vitamins to the water/Tylan mixture. In that case, the vitamins will need to be mixed fresh every day anyway, so it is recommended to mix a smaller amount of Tylan and water when mixing it with vitamins to avoid waste.