Scabs and Itchy Rats | Part 2

Scabs and Itchy Rats | Part 2

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.

    \[\frac {60 \: mg} {1 \: mL} = \frac {? \: mg} {0{,}05 \: mL}\]

“there are 60mg in 1 mL, so ? mg in 0.05mL”

Another way to write it is:

    \[\frac {60 \: mg} {1 \: mL} = \frac {x \: mg} {0{,}05 \: mL}\]

or, alternatively:

    \[\frac {1 \: mg} {60 \: mg} = \frac {0{,}05 \: mL} {x \: mg}\]

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 60×0.05 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)

    \[\ 0{,}5 \: kg * \frac{2{,}2 \: lb} {1 \: kg} = \ 1{,}1 \: lb \]

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:

    \[\frac {6 \: mg} {1 \: kg} = \frac {1 \: kg} {2{,}2 \: lb} \ = \ 6 * \ 1 \: /  \ 2{,}2 = \ 2{,}727 \: mg / lb \]

To covert 2.727 mg/lb to ? mg/kg

    \[\frac {2{,}727 \: mg} {1 \: lb} * \frac {2{,}2 \: lb} {1 \: kg} = \ 2{,}727 * \ 2{,}2 = \ 6mg/kg \]

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:

    \[\frac {1 \: mL} {60 \: mg} = \frac {x \: mL} {3 \: mg}\]

or, alternatively:

    \[\frac {60 \: mg} {1 \: mL} = \frac {3 \: mg} {x \: mL}\]

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 1×3 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.


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.

Written by Francisco Vietto

Francisco Vietto

Francisco Vietto is one of the chief specialists of The Getafe Veterinary Clinic. He had graduated from the university by the age 24. He is the author of many scientific works on the topic of microbiology in veterinary science. Now he is a thirty eight years old family man. In the free time he is also intrested in surfing and football