Horse News & Views - July 2003

Ministry of Agriculture, Food and Rural Affairs

Ontario Association of Equine Practitioners

• In some areas, horses are plagued by summer seasonal recurrent dermatitis (SSRD) or "sweet itch." SSRD is provoked by biting midges. Allergic skin reactions have been attributed to allergens present in various Culicoides species(1). Researchers found that 72% of the midges were collected from the belly and 27% from the dorsal aspect of the body. Predilection to these areas may be associated with skin temperature, since the belly was shown to be the warmest part of the body(2). Practices to reduce exposure to midges include: stabling of horses from half an hour prior to half an hour after sunset, since midges are most active at this time and pasturing on exposed fields, since wind speeds of 2 to 3 knots reduced the number of midges collected(2). (1. Ungar-Waron H et al. Zentralbl Veterinarmed B. 1990; 37 (1,Feb): 64-72. 2. Braverman Y. Equine Vet J. 1988; 20 (6,Nov): 426-429.)

• Researchers in the United Kingdom have been investigating the role of cyanide and thiocyanate, the main metabolite of cyanide, and their presence in white clover. These chemicals are thought to have a potential role in equine grass sickness which is seen sporadically in the UK(1). Animals may ingest significant levels of cyanogens without detrimental effect. However, cystine, a sulphur amino acid, is required in the detoxification process. The production of cyanide is controlled by two independently inherited genes in white clover. Only plants possessing at least one dominant functional allele of both genes liberate hyrocyanic acid (HCN) when damaged(2). Some cultivars are capable of liberating HCN and others are not. (1. McGorman BC, Anderson RA. Vet. Record 2002; 151 (15): 442-445. 2. Canadian Poisonous Plant Identification System http://sis.agr.gc.ca/pls/pp/poison?p_x=px)

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