What caused baby mouse death?

Question
I really need an accurate as answer as you can possibly give.  I don't know if it's impossible to know without a necropsy/histo examine, but I am interested in knowing what you think the most likely answer is.

I found a new born mouse on the tile floor of my building.  ( I would love to know your best guess as to how it got there.  There is nowhere close by there could have been a nest, and would the mother have just dropped it on the floor and left it?)

When I first saw her, she was barely moving and freezing cold.  I heavily diluted some half and half (bc it was late at night) and fed her by wicking it into her mouth with a thick string. She mad a very long brown poop. By the end of the feeding, she was moving, and had a lot of color.
A few hours later I gave her another feeding and another belly and genital rub.  But this time she only peed.  The next day I watered down some KMR and fed her the same way.  To be honest, I don't think I fed her every two hours, but more like every 4 hours.  Do you think that could eventually could have killed her?  

I had her for a total of 4 days.  My best guess is that she was about 2 days old when I found her.  She was pigmented a dark gray normal, and was at the end sprouting little hairs (no real fur)

On the next day and everyday after I fed her every two hours.  The thing is, she never really pooped again.  I rubbed her genitals and belly, and she would always pee a lot, but in the few days (after that first initial big poo) I had her, she only pooped a few watery yellow drops.  Definitely nothing solid, like the first poop.  the poop was bright mustard yellow.  Is that a normal color? I would fed her a little, then rub her to stimulate her, then repeated until she yawned to go to sleep and I would feed her one more time, before putting her in her fluffy sock.

Every few feedings, I would give her a few drops of unflavored pedialyte, bc I she was peeing so much, I wanted to make sure she had enough electrolytes.

The last time I fed her, she was more energetic than ever.  I seemed like I fed her forever.  Everytime I tried to quit feeding, I would put her down and she would frantically sniff her sock, my guess, was bc she was looking for a nipple.  Then I would pick her up and feed her again, and she ate.  I  don't think I forced it down her throat, bc if I pushed too much at anytime, it would just drip all over her face and body.  I didn't see her aspirate any formula.  She never blew bubbles and I was very careful not to get it on her nose.  
When I picked her up for her last few feeding, she would make a clicking noise while sniffing around.  I think she was purring, but I have read some times they do that when in respiratory distress.  But she did not seem to have any problems breathing.  

Anyway, on her last marathon feeding, I put her down for ten seconds and when I came back, she was gasping, wide with her mouth.  She did this every few seconds 5 more times, and went limp.  I was worried maybe she was choking, and tried to hold her head down and gently pat her back to dislodge anything.

She only had a definite milk belly on the last day, but I assumed bc I was really looking the first day, and She didn't eat as much as the last two feeding throughout the other 3 days I had her.  Did the two small feeding of watered down half and half clog up her so she died of not being able to defecate?   

This whole thing has been very hard. Your best informed educated thoughts on why you think she died would be so helpful.  


I have done a ton of research and I still don't really have anything conclusive, but I have some guesses of Tyzzer's disease, Mouse Hepatitis virus, or just inablity to use the bathroom. Did she not get enough to eat?  Was it something else I'm not seeing?  If you think it will be helpful, I can send you some pictures pre and post mortem.

Thank you for your help.

Answer
Hi Happygurl,

Pictures won't be necessary, and on a mouse that young, so many things could cause death that even a gross necropsy probably wouldn't be sufficient.  If you refrigerated the body immediately, you might be able to get a histopathology necropsy, but those can be quite expensive and need done very, very quickly.

In answer to your question about how it got there on the floor - yes, it's very common for mice who have pups with congenital abnormalities or illnesses to abandon them outside of the nest.  It's also possible the nest was on something moving, such as a cart or in a hamper, and the baby fell out in transit.

Did you weigh her at all?  The best way to know if a baby is getting enough nutrition is to weigh them before and after each meal, or at least once a day to watch for weight gain.  A healthy baby mouse should gain in grams every single day, whereas a lethargic mouse may not wake frequently enough to eat, and therefore become more lethargic.  Pups that small should eat every 2 hours, 3 at the longest, and should poop a few times a day.  The fact that she did not poop but the one watery stool implies that she was both dehydrated and not receiving sufficient nutrition, but whether that was due to feeding or her body is unknown.  Raising orphaned pups is extremely difficult even if we know they are healthy!  It sounds like you did the best that you could.

The clicking is, in fact, indicative of a respiratory illness, and is probably the only sign you'd get from a pup that young.  The gasping is not, however, and is a pretty common sign of extreme distress when pups start to fade.  The URI could have resulted from an external illness, aspiration of formula (even if it didn't bubble), or an insufficient immune system.  I'm guessing it could also have resulted from when she chilled on the floor when you first found her.

So while I cannot guess at what specific disease, condition, or occurrence resulted in her death, I can address some potential causes:

- Failure to Thrive:  Failure to thrive is the generic term for any condition which results in stunted development, either externally or internally, of a mouse pup.  This usually results in death.  The causes are varied and not completely understood.

- Congenital Disorders:  This means any genetic problem that the mouse inherited, such as poor organ development, internal deformities, or even genetically prone tumor growth.  This could also affect the stomach and intestines.

- Lethal Genetics:  Some pairings of mouse genes are lethal.  These are usually recessive traits that hide in otherwise normal parents, and just pair up in a way in the pups that results in death.  Some pairings result in death in the womb, causing mice to have very small litters.  Others result in death within the first 3-4 weeks of life.  Mice with a pair of lethal genes (inherited from both parents) do not survive, ever.

- Tyzzer's and Hepatitis:  Tyzzer's is something she would have had to come into contact with, which is pretty unlikely with her being so very young.  I would have expected you to find a lot more dead mice.  Hepatitis can be spread either by respiratory means or enteric (digestive system).  Because of her age, I would not expect it to have intestinal involvement.  Which leaves it as a respiratory agent - whenever we have an infection in the lungs or throat, regardless of the name of the virus or bacteria, we call it an upper respiratory infection (URI).  It's not necessary to know which one, because she was too young to have received treatment.  Unfortunately, contracting a URI as a pinkie is lethal.



Hopefully this helped you figure out what happened.  Of course, we can only narrow it down so much.  Mouse pups are extremely fragile, and even a draft can be a death warrant.  I'm sorry for your loss, but thank you so much for having a heart big enough to take her in.  You were her hero.

-Tam