See this "Rat breeding guide" at:

http://ratguide.com/breeding/postpartum/maternal_behaviors.php

Maternal Infanticide

The term infanticide is defined as the killing of an infant. It is hard to determine, unless you actually witness it, whether or not a female has committed infanticide or if the pup has died naturally. It is not typical for mother to kill their own offspring. Mother rats may kill the offspring of other females and virgin females will often kill babies (suggesting that hormones play an active role in determining female behavior to offspring). Yet there certainly are scenarios when mothers kill their own.Some hairless mothers who are unable to lactate may kill their entire litter. Hairless females with diminished lactation may cull their own litter one by one until they reach the number of babies that they can provide nourishment for.

Typically infanticide is committed during the first few days of life and the killing of an individual pup is more likely than the killing of an entire litter. Some factors that may lead to maternal infanticide include: hormonal imbalances, environmental stress, hunger, protein deficiencies, vitamin deficiencies, and obesity.

While I have not experienced this myself, I was taught about this phenomenon in mice during my FELASA training. Supposedly, it happens when the mother is very stressed. Extra care has to be taken when switching cages, because if the pups and mother are not handled properly, she can eat them. I understood that this was some sort of protection mechanism, where she essentially considered the pups expendable in favour of her own life, as she can always produce another litter in the future.

However, I received all this information verbally and cannot provide any references to back this up. I also do not know if it also applies to rats.

@ Espen A. Sjober, I tried to look for what happens about this topic online, and saw this film on Youtube from a lab (I guess). If you wish, you can look into it (towards to the end of the film). (due to the graphic nature of this film, viewer discretion is advised.)

https://www.youtube.com/watch?v=uV5MElxWrXg

It is may be because many of the infants with disabilities as human some times gets from the birth,. we can take care of our own babies as we have ability to that for a long time with the culture and the advanced social system. But for the animals its different, infants should live their own soon after they born. So animals frequently kills their infants if they have any kinds of disabilities as they will not survive in the nature with the disabilities. 

Do animals also have PPD like human beings?

"Postpartum depression (PPD) is a complex mix of physical, emotional, and behavioral changes that happen in a woman after giving birth." (from below link)

http://www.webmd.com/depression/guide/postpartum-depression#1