I looked up Wormshield not sure about it never heard of it. On the website I went to it showed it has ivermectin/pyrantel. Do not know if that is good or bad for your dog.
It sounds like this is a risk for dogs/breeds that have the mutant MDR1 gene, but not a severe risk. I'm not aware of Basenjis having this problem. Someone correct me if I am wrong.
Approximately 3 of every 4 Collies in the United States have the mutant MDR1 gene. The frequency is about the same in France and Australia, so it is likely that most Collies worldwide have the mutation. The MDR1 mutation has also been found in Shetland Sheepdogs (Shelties). Australian Shepherds, Old English Sheepdogs, English Shepherds, German Shepherds, Long-haired Whippets, Silken Windhounds, and a variety of mixed breed dogs.
Ivermectin (antiparasitic agent)-While the dose of ivermectin used to prevent heartworm infection is SAFE in dogs with the mutation (6 micrograms per kilogram), higher doses, such as those used for treating mange (300-600 micrograms per kilogram) will cause neurological toxicity in dogs that are homozygous for the MDR1 mutation (mutant/mutant) and can cause toxicity in dogs that are heterozygous for the mutation (mutant/normal).