Horse News & Views - September 2004

Ministry of Agriculture, Food and Rural Affairs

Ontario Association of Equine Practitioners

• Giant hogweed (Heracleam mantegazzianum) is a large biennial. Sap from this plant on skin may cause blisters, scabbing and scarring in humans. It is unclear how this sap might affect horses. The first year, the plant is vegetative. The second year, it flowers and sheds seeds. It is very large, usually 12 to 16 feet tall in the second year with a flower head up to 4 feet across and loaded with seed. The leaves may reach 40 inches across and are divided into three leaflets which, in turn, are further divided into sharply pointed teeth. The main stem may reach 4 inches across, is hollow between the nodes and spotted with reddish purple sharp irregular bumps. The flower head could be compared to Wild Carrot in that the umbel divides into 30 to 50 branches, which, in turn, consist of another umbel of 30 to 40 flowers. The whole head is flat on the bottom with slightly rounded top and white. It could possibly be confused with a similar but slightly smaller plant called Angelica and also Cow Parsnip, again slightly smaller. Giant Hogweed should not be allowed to set seed or a grove of these plants can result. (John Benham, Weed Inspector, South Wellington County)
  
  
• Ivermectin is approved for use in horses not intended for food. It is available under a number of trade names, e.g., Eqvalan, as an oral paste and an oral liquid (not for injection). Ivermectin is commonly used in the fall because of its activity against bots in both the oral and gastric stages. It is also licensed for control of: large strongyles (adult), small strongyles, pinworms (adults and 4th-stage larvae), round worms (Parascaris equorum - adults), hairworms (adults), large-mouth stomach worms (adults), neck threadworms (microfilariae), lungworms (adults and 4th-stage larvae), intestinal threadworms (adults) and summer sore (cutaneous 3rd-stage larvae) secondary to Habronema or Draschia spp.. Ivermectin is not affective against tapeworms. A recent field study suggested that roundworms on one farm were also resistant to ivermectin.

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