On the etiology of epizootic or infectious equine abortion

Abstract

1. Two outbreaks of infectious equine abortions are reported; both with a very high abortion incidence. 2. In the majority of the abortions studied the foetal organs were found to be extensively invaded by S. abortus-equi but in others this organism could not be recovered from the foetus or after-birth. 3. Both donkey and horse mares were found to be susceptible, donkeys being on the whole more resistant than horses. 4. Abortion was successfully produced in both horse and donkey mares by means of (a) the oral administration of minced abortus-equi infected foetal organs (4 cases), (b) intravenous inoculation of collodion membrane or Berkefeld candle filtrates of these organs (9 cases, including 2 foals that died within 48 hours of birth) and (c) the oral administration of minced foetal organs that were culturally free from abortus-equi (one case). 5. The twelve aborted foetuses and the two foals that died within 48 hours of birth all presented lesions typical of infectious equine abortion. 6 . Abortus-equi was recovered from the organs of ten of the aborted foetuses and one of the dead foals, but not from two aborted foetuses and one dead foal. It was also isolated from the after-birth of two mares that had given birth to live foals (Nos. 704 and 732). 7. All the mares gave negative agglutination reactions at the time of abortion or parturition, but in those cases where abortus-equi was recovered from the foetus-not the foal-the reaction became positive in about 2 weeks. When the organs of the foetus were free from abortus-equi or when a live foal was born, whether its organs contained abortus-equi or not, the agglutination reaction of the mare generally remained negative. 8. The aborting mares did not suffer any ill effects as the results of the abortions and the after-births were expelled normally. 9. It is concluded that the primary cause of infectious equine abortion is an infecting agent that will pass either through collodion membrane, with the size of the pores 810 milli-micra, or through Berkefeld candles. 10. It was not possible to produce abortion in three pregnant donkey mares dosed with large amounts of live abortus-equi culture. 11. The significance of S. abortus-equi in equine abortion is comparable with that of S. cholerae in swine-fever. Like cholerae-suis, abortus-equi is regarded as a saprophyte which frequently occurs in the body of the horse without causing any obvious disturbance, invading the tissues of the body only when conditions become favourable. When the resistance of the foetus, foetal membranes and uterus has been lowered by the primary cause of infectious abortion, viz., the filterable infecting agent, this organism enters and causes a secondary infection. Alternatively abortus-equi plays the role of a synergist as H. influenzae suis. 12. The advisability of further investigating the etiological significance of salmonella encountered in epizootic diseases like calf paratyphoid and fowl typhoid is suggested.