I'm certainly not an expert on slow worms.....had to look it up! I would say that anything that eats is POTENTIALLY possible to carry an internal parasite. Since it's a reptile, rather than a worm, I think the likely hood of it being a large problem is probably small. While I don't have slow worms, my chickens do eat small snakes, lizards and frogs, and it's never been an issue. I would just treat it the same as everything else. Monitor your birds, if you develop a parasite problem then treat them. Any parasite the chickens could pick up could come from any number of things. Many parasites they can just pick up from the ground, tapeworms would be one that they could potentially pick up from eating something infected with them. Some kinds parasites are species specific, and some are not. Do you have a source like our local agriculture extension offices here in the US over there? They might be able to give you better information on the parasites common to your specific area and the sources they commonly come from. Everything I can find on reptiles and amphibians talks about the parasites they can be potentially infected with, but none discuss them as a vector for infecting another animal after being eaten. Like anything else I suppose, the higher you go on the food chain the more risk you are exposed because you are eating to some extent everything that that thing ate before it was caught and eaten. I honestly don't think it's something that's a bigger worry than any other possible source.