1. 1. KVF Jubb, and PC Kennedy : Pathology of Domestic Animals . New York, Academic Press, 1970 , p494 .
2. 2. SJ Roberts : Veterinary Obstetrics and Genital Diseases (Theriogenology) . Ithaca, New York, published by author, 1971 , p536 .
3. 3. HA Smith, TC Jones, and RD Hunt : Veterinary Pathology . 4th ed . Philadelphia, Lea and Febiger, 1972 , pp1307 -1308 .
4. 4. Cohrs P. Nieberle, and P. Cohrs : Textbook of the Special Pathological Anatomy of Domestic Animals . Revised by. New York, Pergamon Press, 1967 , p722 .
5. 5. K. McEntee : Personal communication.