Scabs and Itchy Rats plus

Scabs and Itchy Rats
plus: how to double check a dosage

Mites are a common cause of itching and scabs on rats. Other causes include poor diet and skin conditions. There are various methods of treating mites and the following story describes just one of those methods.

A few months after I first got Rimsky and Korsakoff I noticed that they each had a few small scabs on the skin in their shoulder regions. I didn't know what was causing the scabs.... the boys didn't seem overly itchy and they only had one or two small scabs at a time so I decided that maybe a few of the wrestling matches might have gotten out of hand.

When Rimsky developed the early signs of a respiratory infection we went to the vet for some antibiotics and I asked about the scabs at the same time. The vet said that they were probably bite marks. This made me unsure of what to do. The two brothers had always seemed to get along really well and loved cuddling with each other. The wrestling matches were always playful and never aggressive... I decided to 'wait and watch'.

Gradually, both boys started developing more and more small scabs on their shoulders, faces, and underneath their chins. They also started scratching a lot. During our next vet trip, the vet wanted to inject some Ivermectin in case it was mites. I declined the injection (because I've heard of rats having bad reactions to ivermectin injections) so instead I bought a package of Revolution, a brand of flea/mite medication used to treat puppies and kittens which is applied on the skin. Revolution is the brand name for Selamectin, which is a derivative of Ivermectin. The box contains three separate packets of fluid and cost about $30 Canadian.

Revolution is a topical parasiticide and comes in different coloured packages depending on the size of the animal being treated. For rats, the mauve packet that has 15mg per tube is the only suitable one because it is for puppies and kittens (be careful because there is a purple packet too).

The instructions for Revolution state that it should be applied once a month, but in my experience just one dose can fix a mite problem in rats. I have heard that rats normally have some mites, but that these mites only sometimes become a problem. Sometimes stress can cause a 'flare up' of mites or, sometimes contact with material (such as bedding) that is contaminated with a high number of mites can cause rats to acquire a higher than normal number of mites on their skin. For whatever reason, Rimsky and Korsakoff had a gradual 'flare up' of mites and one dose of Revolution cured it. Rimsky and Korsakoff never developed any more scabs the rest of their lives (they lived to be 2 years-3 months and 2 years-7 months respectively). I did not have a skin scraping done to test for mites because I've heard that they tend to give false negatives.

My 3 new boys were treated with Revolution shortly before I got them because of scabs and itching and again the medication has completely cleared up the problem with a single dose. The scabs disappeared very quickly but boys continued to be itchy for a week or two after their treatment.

Note: some pet owners feel that Revolution and some of the other 'flea-fighting' medications on the market are not safe for pets. Discuss any concerns you have with your veterinarian. Be VERY VERY careful if you give your rats this type of medication because an overdose can do a lot of harm.

I always like to double check the vet's dosage so here's how I did it for Rimsky and Korsakoff:

Vet's dosage: 0.05cc (same as 0.05mL)
The weight of my two rats: 500g (half a kg) each
Concentration of the medication: 60mg/mL (from the package)
Known Recommended dosage (from sources such as the RMCA drug chart) = 6 mg/kg

Since my rats are half a kg each, they should each get 3 mg (half of 6mg/kg) according to the known dosage.

0.5kg x 6mg/kg = 3 mg

I know my rats should be getting 3 mg so I want to make sure there is 3 mg in the 0.05mL that I have been told to give them.

Mass Divided by Volume = Concentration so....

Volume x Concentration = Mass

0.05 mL x 60 mg/mL = 3 mg (the mL units cancel out), the vet's dosage agrees with the known recommended dosage.

Or, I sometimes like to use ratios for this type of problem:

I want to know how many mg are in 0.05mL of medicine. I know that there are 60mg in 1mL.

60mg : 1mL = ? mg : 0.05mL "there are 60mg in 1 mL, so ? mg in 0.05mL"

Another way to write it is:

60mg = x mg or, alternatively: 1 mg = 0.05 mL
1mL 0.05 mL 60 mg x mg

Always make sure the units on the bottom of the fractions (mL in the first case) are the same, and make sure the units on the top are the same. Multiply the two diagonal numbers together and then divide by the third number (ie. multiply 60x0.05 and then divide by 1). This solves for x.

x = 3 mg, so the dosage my vet gave me is correct.

So, my rats should each get 0.05cc applied to their skin. I use a very small empty syringe supplied by the vet to measure this tiny amount, part the fur on my rats' shoulders and apply the tiny drop to the skin.

There is 0.25mL in each tube and I only used 0.1 (0.05 per rat). I tried to store the extra selamectin in case it was needed in the future but it evaporated. I still have 2 unopened tubes left in case I ever need them.

The instructions for Revolution state that it should be applied once a month, but I think that one dose can fix a mite problem. I have heard that rats normally have some mites, but that these mites only sometimes become a problem. Sometimes stress can cause a 'flare up' of mites or, sometimes contact with material (such as bedding) that is contaminated with a high number of mites can cause rats to acquire a higher than normal number of mites on their skin. For whatever reason, Rimsky and Korsakoff had a gradual 'flare up' of mites and one dose of Revolution seems to have cured it.

I did not have a skin scraping done to test for mites because I've heard that they tend to give false negatives.

Note: some pet owners feel that Revolution and some of the other 'flea-fighting' medications on the market are not safe for pets. Discuss any concerns you have with your veterinarian.

Sometimes lbs are used instead of kg. use the following conversion: 1 kg = 2.2lb)

Convert from kg to lbs: 0.5kg x 2.2lb = 1.1 lb
1 kg

Convert from lbs to kg: 1.1lb x 1 kg = 0.5 kg
2.2 lb

To convert 6 mg/kg to ? mg/lb:

6 mg x 1 kg = 6 x 1 / 2.2 = 2.727 mg/lb
1 kg 2.2 lb

To covert 2.727 mg/lb to ? mg/kg

2.727 mg x 2.2lb = 2.727 x 2.2 = 6 mg/kg
1 lb 1 kg

A less common scenario is when you are given the medication, but not a dosage (volume). Here's how to figure out the dosage using instructions in the medication:

Instructions inside the packet state: Recommended dosage is 6mg selamectin per kg of body weight.

My rats are 500g (half a kg) each so that means that I should administer 3 mg per rat.

My problem is, I don't know how much fluid contains 3 mg.

The package says the concentration of selamectin is 60mg/mL which means 1 mL contains 60mg but I want to know the volume (? mL) that contains 3mg.

I use a ratio mathmatical technique to solve type of problem.

1 mL : 60 mg = ? mL : 3 mg "1 mL is to 60 mg as ? is to 3 mg" or "1 mL has 60 mg so ? mL has 3 mg"

Another way to write it is:

1 mL = x mL or, alternatively: 60 mg = 3 mg
60 mg 3 mg 1 mL x mL

Always make sure the units on the bottom of the fractions (mg in the first case) are the same, and make sure the units on the top are the same. Multiply the two diagonal numbers together and then divide by the third number (ie. multiply 1x3 and then divide by 60). This solves for x.

x = 0.05 mL which is the same as 0.05 cc. This means that 0.05cc contains 3 mg which is the dose I need.

For more information on external parasites, read this excellent article:
http://www.rmca.org/Articles/bugs.htm

Disclaimer:
This information is by no means intended to replace the advice of a veterinarian. I strongly suggest that anyone dealing with scabs and itchy rats discuss all treatment options with their vet. If you have a question regarding the dosage you have been prescribed, always phone and discuss this with your vet before making any changes yourself. Under no circumstances can anything found in the forum, or anywhere else on the Pet Rats Canada site, be construed as professional veterinary advice or instruction. All vet care discussion at Pet Rats Canada is intended for the purpose of assisting you in discussing options with your local vet. Any other use is done at your risk and is solely your responsibility.