Studies on the development of a microencapsulated delivery system for norbormide, a species-specific acute rodenticide.

Norbormide, a selective rat toxicant, was microencapsulated to both mask the flavour and delay the release until after a lethal dose has been ingested by the rat. To this end, gelatine microspheres containing norbormide were made and over coated with either shellac resin or an equal mixture of shellac and Eudragit RS in a fluid-bed coating machine. The microcapsules absorb water, swell and burst to release their contents. In rats an 8 h window is available to delay the release of encapsulated material. In initial experiments, a shellac coating of 20% w/w was established as suitable for delaying the release. A capsule size range of 200-400 microm was selected, from capsule mastication experiment, for oral gavaging and feeding studies in rat. Oral gavage study has demonstrated for the first time that a substantial delay in release of a lethal dose of an acute poison has been achieved by microencapsulation. Feeding test has demonstrated that there is a fine balance between the size and density of the capsules in the bait to overcome mastication of the capsules by rats. A combination of shellac and Eudragit RS resins is a viable polymeric wall material to control the rate of penetration of water in to microcapsules.