With this bait, TB won’t play possum

From New Orleans, La., at Experimental Biology 2002

With this bait, TB won’t play possum

Tuberculosis isn’t a problem just for people. It strikes animals, and curtailing its spread in the wild can prove daunting. The only effective tuberculosis (TB) vaccine must be injected, so prevention requires capturing uninfected animals.

That obstacle may disappear, however, thanks to a new, oral form of the vaccine that can be put in animal baits. Created to treat wild possums, this new preparation may also be useful for vaccinating people.

In New Zealand, an estimated 3 percent of the nation’s 70 million brushtail possums are infected with Mycobacterium bovis, the germ that causes TB in these animals and cattle (SN: 4/6/02, p. 218: Aerial War against Disease). Like Mycobacterium tuberculosis, the TB germ that infects people, the animal-infecting M. bovis can be prevented by the injectable Bacille Calmette-Guerin vaccine, or BCG. In fact, BCG is made of live M. bovis germs that have had their harmful properties deactivated. BCG must be administered under the skin instead of orally because the protective microbes in the vaccine are killed when they enter the stomach.

Immunologist Frank E. Aldwell of the University of Otago in Dunedin, New Zealand, has been exploring ways to protect cattle that might run into TB-infected possums. Ranchers don’t vaccinate livestock they intend to sell because BCG causes an animal to test positive on diagnostic TB tests, making it unmarketable. The challenge, therefore, was to vaccinate the nocturnal, tree-dwelling possums.

Aldwell’s group decided to mix live BCG bacteria into edible pellets of triglycerides and fatty acids. The lipid mix protects the bacteria as they move through the stomach but breaks down in the intestines and releases the vaccine’s bugs there, the team reports. The scientists also found that the beneficial microbes stay viable in the pellets if possums don’t find and eat them right away. “When you take [the edible vaccine] out of the fridge,” Aldwell notes, “it still lasts 30 to 40 days–which may be long enough.”

Tests with mice–and later captive possums–showed that although the animals’ immune systems responded more slowly to lipid-encapsulated BCG than to injections of the standard vaccine, “levels of protection against TB were similar,” Aldwell reports. His group has now doctored the lipid recipe with anise oil and sugar to make its taste and smell irresistible–at least to brushtail possums. A patent is pending on this orally active BCG, which is slated to undergo field-testing at possum-bait stations later this year.

Last week, Aldwell also met with his colleagues at Colorado State University in Fort Collins, where the U.S.-New Zealand team will soon begin efficacy tests of the oral vaccine against animals that will be exposed to germs causing human TB.