The population of unowned, free-roaming cats and dogs in the world is unknown, but we know an overpopulation of these animals exists. They present a health threat to people and pets. Rabid dogs are the main source of rabies deaths in people worldwide.1 Free-roaming dogs and cats may also have a negative impact on wild species and the environment.
Furthermore, the welfare of free-roaming animals is debatable and likely varies with location, climate, and people's attitudes. Some evidence indicates that free-roaming cats have shorter life spans than owned cats. One study showed that 87 of 169 (51%) free-roaming kittens died before 6 months of age, and the most common cause of death was trauma from dog attacks or motor vehicle accidents.2
METHODS OF POPULATION CONTROL
The mainstay of population control for dogs and cats is surgical sterilization via ovariohysterectomy and orchiectomy. However, many reasons why surgical sterilization may not be effective as the sole method for population control exist. It requires anesthesia, medical equipment, a suitable surgical facility, adequate recovery time, and the advanced training of a veterinarian. It carries the risks inherent in any surgical procedure. Many people are unwilling to subject their pets to what they perceive to be a painful and invasive procedure. The cost of surgery is prohibitive for many owners, particularly in developing countries.
In addition, many dogs and cats are not owned; thus, shelters and other animal organizations are responsible for financing the surgeries. Neutering these animals involves some method of capturing or trapping the animals, which can be labor-intensive. Trap-neuter-return programs for feral cats and dogs are one such method.3
Nonsurgical forms of contraception are a promising additional method of population control (Table 1). According to the Alliance for Contraception in Cats and Dogs, an ideal nonsurgical product would be safe, effective, affordable, permanent, delivered in a single injection or treatment, available for males and females and for dogs and cats, and have documented effects on behavior and health.4 In this article, we will examine some of the nonsurgical methods currently being researched or developed that have the best chances of fulfilling these criteria.
Table 1: Comparison of Selected Nonsurgical Contraceptives in Dogs and Cats
A brief review of basic reproductive physiology is necessary to understand the mechanisms of action of these nonsurgical methods. Gonadotropin-releasing hormone (Gn-RH) is a decapeptide hormone produced in the hypothalamus. It is responsible for regulating the release of the glycoproteins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior lobe of the pituitary gland. In females, LH causes ovulation, corpora lutea formation, and progesterone secretion in the ovary. FSH is responsible for follicular development and estradiol synthesis. In males, LH stimulates testosterone production in the Leydig cells of the testes. FSH is necessary for Sertoli cell function.
CONTRACEPTIVE DRUGS AND CHEMICALS
These agents include implants that contain a Gn-RH-agonist, injections that contain a Gn-RH antagonist, and the chemicals vincyclohexene diepoxide or zinc gluconate.