Sterilization of Animals

Shalom!

Thank you for your question!

As a general rule, sterilizing animals is a Torah prohibition derived form the fact that only animals that are able to reproduce may be offered as sacrifices. Even fish are included in the ban. It is interesting to note, however, that there is some discussion whether sterilizing female animals is a biblical transgression or a rabbinical one. This is because females are not truly required to reproduce.   

In some cases, one who ignored this halacha and sterilized his animals is required to sell the animal as a penalty.

There are different views on whether the ban on sterilizing animals also applies to non-Jews with most authorities ruling that they are not included in the ban. Le'halacha, a Jew should not instruct a non-Jewish veterinarian to sterilize a male animal but one may have a non-Jew sterilize a female one.

Both active sterilization and passive sterilization through giving medication are forbidden. 

As one can see, this is a very serious halachic issue, and the best thing to do is to avoid the issue altogether by not acquiring a pet that requires sterilization or by acquiring one that was already sterilized by a non-Jew.

Sanhedrin 56b; Shabbat 110b; Chagiga 14b; EH 5:11-14; Shulcha Aruch Even Ha'ezer Siman 5 se'if 14.