Antistreptolysin O titer in horses

Antistreptolysin O (ASO) titers were determined in the sera of 532 horses from stud farms and 436 working horses from small farms. A statistically significant correlation was seen between the ASO titer and the age of the horses. There was a significant difference between mean ASO titer in horses 0-2 years and horses 2-10 years In horses older than 10 years the titer was significantly higher than mean ASO titer for the group. Twenty four of 30 horses in which ASO titer was higher or equal to 80 I.U. had histories which suggested that streptococcal infection had occurred. Clinical and bacteriological examination showed that in all examined stud farms cases of Streptococcus C infection was noted during the 2 years before serum was collected. Streptococcus C was isolated usually from horses which had low ASO titer. Our results are in agreement with the suggestion that S. zooepidemicus infections increase the ASO titer in serum of horses.