As used in sections 273.400 to 273.405, the following words mean:
(1) "Adopter", a person who is legally competent to enter into a contract and who is adopting or buying a dog or cat from a releasing agency;
(2) "Adult animal", any dog or cat that has reached the age of one hundred eighty days or six months or more;
(3) "Releasing agency", an animal pound, shelter, humane organization, or animal control agency, whether public or private, but not including an individual who occasionally renders humane assistance or shelter in his * home to a dog or cat;
(4) "Sterilization", the surgical removal of the reproductive organs of a dog or cat in order to render the animal unable to reproduce, or the use of an approved serum which will permanently render the animal unable to reproduce.