Answer
No, the herbal horse wormer is not safe to use, bin it. Black Walnut in any form is toxic for horses. You were quite right to be concerned about the herbal wormer containing Black Walnut. Black Walnut (Juglans Nigra) not to be confused with the European Walnut, is toxic for horses, resulting in laminitis. The Black Walnut grows across the United States from the North Eastern states to Texas. The wood is used in furniture manufacture and the shavings, as a by product for animal bedding. Horses bedded on shavings with only 5% Black Walnut were still affected by laminitis within 8 hours.
Laminitis occurs within 8 hours of giving either an aqueous extract or introducing the shavings as bedding. Other symptoms include; filling and pitting edema of the legs, increased heart rate, increased respiratory rate, a raised temperature and pounding digital pulses
Juglone, an alkaloid chemically similar to iodine, found in the hulls was once thought to be the cause. Trials in America have established that Juglone alone is not the sole cause. In fact, all parts of the Black Walnut tree are toxic for horses and will cause laminitis. Trials using aqueous extracts of the heartwood which contains no juglone were given, resulting in laminitis, raised hoof temperature and pounding digital pulses.
The results of these trials were reported in the American Veterinary Journal 1990:51:83-88 by Galey F D,Beasley V R, Schaeffer D, et al. 'Effect of an aqueous extract of Black Walnut'.
Another study by Uhlinger C. 'Black Walnut toxicosis in ten horses' was reported in 1989 in the Journal of the American Veterinary Medical Association 1989:195: 343-344. | 
