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Pregnancy Prevention
 

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by Arnold Plotnick MS, DVM, ACVIM, ABVP

Cats have long been recognized for their fertility.  Bastet, the Egyptian goddess of fertility, is depicted as a cat in artwork, and the ancient Egyptians worshipped cats as a symbol of fertility. When it comes to making babies, cats are one of the most prolific domestic pets out there.  Like rabbits, they are capable of multiple pregnancies within a single reproductive season.  Although the average at which queens reach puberty and have their first heat cycle is between 5 and 9 months of age, some cats experience this as early as 3 ½ months, and at a body size as small as 4 ½ pounds. 

There are many factors that affect the onset of puberty.  General health, physical condition, nutritional status, social environment, time of year, and breed can all influence puberty.  In general, domestic shorthaired cats come into heat at an earlier age than domestic longhaired cats, and mixed breeds come into puberty earlier than purebreds.  Persian cats are especially late in reaching puberty, often not experiencing their first heat until 12 months of age. 

Here in the Northern Hemisphere, as the days get longer in late January and early February, queens begin to cycle, coming into heat approximately every two weeks. This usually continues until late September.  In October, November, and December, cats tend to stop cycling, until the new season resumes again in late January.  For housecats, the actual parameters of the reproductive season can vary due to the effects of artificial lighting on the reproductive cycle, although most housecats stop coming into heat during the winter months.  Siamese cats are less affected by photoperiod than other breeds and often cycle all year round. 

Cats are most fertile between the ages of 1 ½ and 8 years.  If allowed to mate naturally, a typical queen having 2 – 3 litters a year, with 3 – 4 kittens per litter, can have anywhere from 50 to 150 kittens in their lifetime.

Some aspects of the cats’ reproductive capabilities are truly remarkable.  If allowed, a female cat in heat may allow several males to mate with her, and it is possible for kittens in a litter to have different fathers. (The technical term for this is superfecundity.)  Lactation (production of milk) does not suppress the heat cycle, and cats that are actively nursing kittens can come into heat as soon as two weeks after giving birth.  Even being pregnant doesn’t necessarily suppress the heat cycle.  In fact, 10% of female cats come into heat between the 3rd and 6th week of pregnancy.  Although these cycles are rarely fertile, it is possible for a cat to be carrying fetuses of different ages, resulting from separate matings in different heat cycles! (The scientific term for this is superfetation.)

The unique reproductive features of the cat – polyestrous (multiple heat cycles during the reproductive season), early onset of puberty, extreme fertility, heat cycles not suppressed by lactation, and short gestation period (65 – 67 days, average) – all contribute to the sad fact that there are many many more cats than there are homes, and in the United States, an estimated 3 to 4 million cats are euthanized every year. 

Currently, the only method available to control reproduction in dogs and cats is surgical, i.e. spaying and neutering.  Surgical sterilization can be expensive, is irreversible, and engenders some risk due to anesthesia.  Several European countries have banned, or are in the process of banning, castration of domestic pets.  A contraceptive vaccine that could block the action of reproductive hormones or hormone receptors would be a major advance in the ability to keep pet populations under control.

When cats come into heat, the level of the hormone estradiol rises dramatically. After mating occurs, the pituitary gland releases luteinizing hormone (LH), and cats ovulate soon afterward.  If pregnancy occurs, the ovaries produce progesterone, which maintains the pregnancy until birth occurs.

One way to potentially interfere with the reproductive process would be to inhibit the binding of LH to its receptor, causing the ovaries to dysfunction and make the cat infertile.  The March 2003 American Journal of Veterinary Research contains a report describing the successful immunization of cats with a contraceptive vaccine. In the study described in report, 9 adult female domestic cats were immunized with LH receptors that were isolated from cows. The cats produced antibodies against the bovine LH receptors. Bovine LH receptors, fortunately, are very similar in structure to feline LH receptors, and the antibodies “cross-recognized” the cats’ own LH receptors.  When the antibodies bind to the cats’ LH receptors, the receptors are blocked, and cannot respond to any LH that is released from the pituitary.  If LH can’t bind to its receptors, cats cannot ovulate, and if they can’t ovulate, they can’t get pregnant.

As with most vaccines, antibody levels decline over time, and a booster is required to bring the levels back up.  In this study, as the antibody levels began to decline (approximately 500 days after vaccination), ovarian function was restored and cats began cycling again.  In other words, the effects of the vaccine were reversible. 

There are many questions to be answered and much work to be done before a vaccine like this can become commercially available.  How many vaccines should be administered initially?  How often is revaccination necessary?  What is the optimum antibody level to maintain infertility?  When ovarian function returns, is fertility affected in any way?  Research is currently being done to answer these questions, and I don’t imagine it will be too long before an anti-fertility vaccine becomes commercially available as a non-surgical means of sterilization.

Updated 2/9/06