Biotech Approach To Feral Cat Problem Devised

February 18, 1998

BLACKSBURG, VA, February 18, 1998--A student in the Virginia-Maryland Regional College of Veterinary Medicine has used a summer fellowship grant from the Geraldine R. Dodge Foundation (Morristown, NJ) to develop a genetically engineered bacterium to serve as an oral contraceptive that may one day help solve a major animal overpopulation problem.

Experts estimate that anywhere from 30- to 60-million stray cats roam the United States, according to second-year student Michelle Meister-Weisbarth. These feral cats are wreaking havoc on the nation's songbird population and raising public health concerns as they spread infectious diseases and alter delicate ecological balances.

Controlling the birth rates of feral cats has proven a vexing task, since conventional spay and neuter techniques require surgery in a controlled environment. Animal control experts also note that eliminating or removing the feral cats does not work well since others seem to migrate into the recently vacated niches.

Working with faculty mentor Dr. Stephen Boyle in the Virginia-Maryland Regional College of Veterinary Medicine's Center for Molecular Medicine and Infectious Disease (CMMID), Weisbarth conducted research which suggests the viability of a provocative new immuno-contraceptive approach for controlling reproduction in these feral cats.

The method involves the use of genetic engineering technology to modify a strain of the bacterium, Salmonella, which could then be delivered to feral cats in the wild via a vaccine-laden bait. Work in other laboratories around the world has demonstrated the viability of using genetically altered strains of Salmonella as vehicles for delivering vaccines, including oral contraceptives, explains Boyle, a molecular biologist at the CMMID.

Here's how the process works: Scientists used genetic engineering techniques to remove specific genes on the Salmonella genome making it unable to cause disease. Then, Meister-Weisbarth introduced a gene encoding a protein derived from the zona pellucida (ZP) surrounding the vertebrate egg into the Salmonella. The bacterial vaccine is capable of inducing the production of antibodies which recognize the ZP and block the ability of a sperm to fertilize the egg.

While many people recognize Salmonella as a bacterium which causes disease in people and animals, Boyle says, the attenuated strain cannot cause disease. The Salmonella bacterium is especially useful as a vehicle for delivering an immuno-contraceptive agent since once ingested, it survives in the stomach and crosses the intestinal tract to cells in the immune system, according to Boyle. Once in the cells of the immune system, the Salmonella are killed and the ZP antigen is released and stimulates production of antibodies against ZP. Because these antibodies bind to the ZP surrounding the egg, they inhibit the binding of sperm and thus block fertilization.

Scientists have looked at various techniques for delivering oral rabies vaccines to wild animals in the form of bait which Boyle and Meister-Weisbarth believe could easily be modified to deliver the genetically engineered Salmonella as an oral immuno-contraceptive vaccine.

With the preliminary work accomplished, Meister-Weisbarth and Boyle are seeking funding which will finance the next phase of the work: testing the attenuated Salmonella based oral contraceptive in laboratory animals.

The Geraldine R. Dodge Foundation's Frontiers in Veterinary Medicine program is designed to promote veterinary research into advancing the humane treatment of animals as well as support nontraditional scholarship, including human-animal relationships, veterinary ethics, international issues, and conservation biology.
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Virginia Tech

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