Sterility has many causes.
Most dogs, if not neutered or spayed, are apt to produce too often and too many pups; however, there are those dogs who are sterile and can't reproduce even though their owners want them to.
Males
Sterility in a male dog can be caused by azoospermia, which means that the amount of sperm produced is insufficient. This can be caused by previous infections. In addition, sometimes a male dog simply isn't attracted to the female dog.
Mycoplasma
An organism called Mycoplasma can result in kennel-wide sterility, according to ShowdogsSupersite.com.
Brucellosis in Females and Males
A disease of the reproductive tract called Brucellosis can cause sterility in both female and male dogs and cause a female to abort her pups, according to WildsideKennels.com.
Other Causes
Sterility can be the result of inbreeding, trauma, drugs or toxins, hypothyroidism, hyperadrenocorticism (excessive production of adrenal hormones), environmental damage, heat stress, systemic disease, prostate disease, presence of a dominant pet guardian or female dog, inflammation, or infection or cancer of the reproductive tract.
Discomfort
Incomplete ejaculation can occur because the stud is uncomfortable in the breeding situation, according to Rural Area Veterinary Services.
Tags: reproductive tract