The Feral Cat Contraceptive Vaccine

A real chance to solve the homeless cat problem -- humanely.

Michelle Meister-Weisbarth, a student at the Virginia-Maryland Regional College of Veterinary Medicine, has developed a genetically engineered bacterium that can be used as an oral contraceptive to control the unwanted cat population.

Meister-Weisbarth has been working with faculty mentor and molecular biologist Dr. Stephen Boyle, using genetic engineering technology to modify an approved vaccine, a strain of the bacterium Salmonella, which could then be delivered to feral cats in the wild via a vaccine-laden bait.

The team is soliciting funding for the next phase of research, which will involve testing the attenuated Salmonella on lab animals.

For more information about this contraceptive vaccine and other approaches to non-surgical pet sterilization, please visit the Alliance for Contraception in Cats and Dogs' web site at

Answers to Typical Questions About the Vaccine
Provided by Dr. Stephen Boyle

1. How does this immunocontraceptive vaccine work ?
It causes the body's immune system to produce antibodies which block interaction of the egg and sperm and therefore block conception.

2. How is the immunocontraceptive vaccine produced ?
It involves placing the gene encoding a sperm receptor protein from the egg surface into an approved bacterial vaccine strain. This causes the bacterium to synthesize a sperm receptor protein which stimulates the vaccinated animal to produce specific antibodies which bind to the sperm receptors on its egg and block attachment of the sperm to the egg.

3. How is this immunocontraceptive vaccine administered ?
The approved bacterial strain will be placed in food which will be used to attract feral cats. Once the cat eats the food, the bacterial vaccine strain delivers the sperm receptor protein to the cat's immune system.

4. If the vaccine is given through food, which quantities are necessary to be effective and could there be any harmful side effects?
The doses have not yet been worked out - more research is necessary. If the dose is correct, there should be no harmful side effects.

5. Feeding in the wild means that male and female will eat the bait; isn't this harmful for the other gender?
No, the antibodies induced are specific for a sperm receptor protein produced by the female egg; if a male is vaccinated, he will produce the antibodies which will not react adversely with any of his tissues.

6. If this contraceptive vaccine is effective for stray cats, might it be used for the normal house cat?
Yes, it will be effective for house cats and presumably, if refined, veterinarians could use it when appropriate as an alternative in place of spay and neuter surgery.

7. Would the cat only have to take this vaccine only once?
We don't know the answer as there has not been sufficient research done with respect to dosage required to have an contraceptive effect.

8. Would all the cats have to eat this vaccine to be sterile?
The larger the number of cats in an area which consume the vaccine, the better the chances of them becoming unable to produce offspring.

9. Since the approved bacterial strain is Salmonella, could cats transmit this strain to each other?
The Salmonella vaccine strain is a non-disease producing strain and will not be easily spread from cat to cat as it does not survive in the cat which ate the baited food.

10. Is the immunocontraceptive effect reversible in the event that someone's pedigree cat eats the food containing the vaccine ?
It depends on the dose they eat; once the immune response is triggered by a sufficiently high dose, the state of contraception will essentially be irreversible.

11. What impact will this bacterial vaccine have on conception in other animal species other than cats?
The vaccine is being designed in such a way that it should have limited impact on other animal species who eat the bait.

12. Specifically, how can you ensure that only cats will eat the bait?
There are a number of bait attractants that have been developed to cause cats as opposed to other animals to find the bait by odor. However, additional research needs to be done to assure that this bait/vaccine/attractant is consumed only by cats.

13. How can you be sure the bacteria strain won't mutate to affect people?
The vaccine strain contains 3 regions which have been removed from its genetic makeup - which eliminates the possibility of a mutation triggering a disease producing strain.

14. Does the Salmonella make the cat sick by causing food poisoning?
No, the Salmonella strain has been altered such that it does not cause disease in any animal species (including humans).

15. Is the technology regarding immunocontraceptive vaccine already patented, or are there new patents which would cover this specific application?
Yes, the Salmonella vaccine delivery system is patented as is the intellectual property regarding the use of egg or sperm components as vaccines.

16. Have any companies expressed an interest in licensing the patents?
Yes, Megan Health in St. Louis, MO holds the liscensing to the Salmonella vaccine strain.

17. When will field tests on the vaccine be performed?
The controlled studies on cats will be done this summer and the field tests probably started and completed sometime in the next 1- 2 years.

18. How frequently would baits have to be re-applied to an area to keep the cat population under control?
The answer is not known but probably more than once in order to be sure all the feral cats consume the bait and become vaccinated.

19. Is there government/industry support for a large-scale deployment of this vaccine?
There is a significant amount of federally and privately funded immunocontraceptive work being carried out in the USA and elsewhere. In the US, the National Institutes of Health and the World Health Organization fund this type of research.

For more information about this contraceptive vaccine and other approaches to non-surgical pet sterilization, please visit the Alliance for Contraception in Cats and Dogs web site at