Problem Releasing Wild Mice

Question
QUESTION: Hi, Tamarah:
I hope you are able to answer this question, since your expertise is not in wild mice, but you seem to have
good insights on mice in general, and you are currently available. So, here goes:
I caught 11 deer mice in my home over the winter. I took care of them for months. I kept 5 in one small
cage, and 6 in another with at least one wheel and shelter with the idea of releasing them when the warm
weather came. ( NOTE: I also have other deer mice as pets that I raised from pups.)
I released the first cage of winter guests a while back. They seemed the most desperate to leave. They were
bouncing off the walls of the cage to get out. I took them somewhere outside, opened the top of the cage,
and walked away. At 5 am the next morning, all the mice were gone. No problem.
I hesitated to release the other cage immediately, because I grew a little more attached to those mice - one
mouse in particular - The mouse that I caught last, who was easily identifiable because of one bad eye. I
was struggling with whether to set all of them free, or keep that one mouse, and a companion.
Finally, I decided they all belonged free. That would be better for them, and in the long run, for me, as well.
It was a very difficult decision, but I made it.
I made a little wooden home for them as a temporary shelter, filled with food, and some of their scent from
their used litter as an enticement to get them to go in it. I then put their aquarium outside next to the
wooden shelter and slid the mesh lid off most of the way off so that they could jump out onto the lid and
leap to freedom. I even used another lid as a ramp for them to go down to the ground.
They seemed pretty active, and running about with all the night sights, and sounds.
My favorite even leaped at the top of the cage a few times. That said to me, that she must want to be free.
Even though, I would miss her, it made me feel a little better about setting her, and the others free.
I was worried about predators with the lid being open, and them being exposed, so I went and checked on
them a few times during the night. Each time when I did, I noticed at least three of the six were still in the
cage. One was running on the wheel, the other was moving around the cage, while another was going in
and out of their double decker house.
Finally, a little after 5 am, as the sun was starting to come out, about 5 hours after I opened their lid, I went
to see if anyone was still left in the cage.
To my surprise,  4 out of the 6 were still in the cage - including my favorite of the bunch. I'm thinking, now
what do I do? I'm happy to see my fav still there, but not sure what to do next. I didn't count on this. Are
they confused? My favorite always knew how to get to the top of the cage. Some decided to leave. Are the
others content to stay? Should I try to keep them as pets now if I decide I can? I don't know that I should at
all, but it was hard enough letting them go the first time. The cage is closed, and they are back in my house
at the moment. What is your opinion of why the majority still remain in the cage.

ANSWER: Hi Derek,

Wild mice almost always remain wild, but it seems to me that your mice have noticed they've got what they need and don't want to leave.  After all - food and safety can be hard to find in the wild!  Only you can decide if you want to keep them or re-release them, but if you do keep them there are a few things you should check on first.

Gender is the first thing that comes to mind - are the remaining mice of one gender?  I am assuming that were more than one male you probably would have realized it by now (fighting, etc.), but it would be an added stress to you were they to have litters.  Hantavirus is the second thing that comes to mind.  If you decide to keep them it may be wise to find a lab to test their poo for the virus.  It can be very dangerous to humans, so if it isn't possible to test them for it I would reconsider release for your personal safety.

After that it's just up to you.  Think about why you wanted to release them the first time.  You sound like a very responsible mouse fellow - I'm sure they'll be happy either way!

-Tam

---------- FOLLOW-UP ----------

QUESTION: Thanks for a response! Yes, I do worry about hantavirus, as well as, the
possibility of having both sexes in the same cage. I do have a lab that
can test the mice for me. I also have a couple vets nearby who said they
can handle exotics like deer mice.
My real problem is: Can I take on more deer mice, besides the 10 I've
raised from pups? Do I want the responsibility? I have gotten  attached to
the mice, particularly, one of them, but know there will be more time and
expense involved in taking care of them. Starting with the hantavirus test
which costs $100.00. Also, I could get a test that checks them for other
common diseases for another $100.00.
Do you know how deer mice get hantavirus? If the mice have any parasites,
like mites, would they likely be spread to my mice, if the aquariums are
in the same room? I assume they would.
One other question is, what sexes are they? If I had to split them up,
would they adapt to the split? I've had to do it with my other mice when
sexing them. Some were depressed for a while but, seemed to get over it.
A very difficult decision. Any experience, or opinion you can offer would
be helpful.

Answer
Hello again,

The expenses and responsibility are definitely a very real thing to consider, and I'm really glad you've thought about it.  As far as hantavirus, it's spread through fecal matter, urine, and saliva.  Sometimes it can be aerosolized by shaking up bedding, or being spread around a cage as a mouse walks around.  Bites and scratches can spread it between mice, if a mouse has been cleaning itself and is infected, or has been walking across an infected area.  However, to my knowledge, parasites do not spread the disease between mice or to you.  Washing your hands between handling different cages, and changing bedding separately and in a different area from the community, can help prevent spreading any issues between cages.  Nothing should physically go between cages without being disinfected first.  Within a cage - if one mouse has it, it's likely to give it to any others that have exposure to it's dirty bedding.  At that point it's a risk to you, but not to them (you can become infected by breathing it in, and as far as I know that's the only way).  This site has just about all the facts:  http://www.cdc.gov/ncidod/diseases/hanta/hps/noframes/FAQ.htm

Splitting is only required if there's a problem - fighting or pregnancies.  I'd hate to split up a well established colony, but it's better than having unplanned litters of babies to take on and care for as well.  If they are all one gender and are getting along, it's fine to leave them as they are already.  I've never kept deer mice, so I don't have too much wisdom to offer regarding their social habits.

Hope I helped!
-Tam