Ewes affected in this outbreak were full-mouthed Merinos in good condition, running in a stubble paddock containing a fair amount of straw and a fresh growth of trefoil, crowfoot (Erodium spp.), a little couch grass, young capeweed and saffron thistle, with also a little self-sown wheat. They had been mated with Border Leicester rams and about one week prior to the date that lambing was due to commence the owner found the first dead lamb in the paddock. Thereafter the ewes were seen daily; no live lambs were born, and in the ensuing fortnight 50 dead ones were found.
The dead lambs ranged in degree of development from about a 16-weeks foetus to almost full term. The great variation was due to the fact that the mating period was prolonged with a consequent protracted lambing period. Only on rare occasions did the owner see a ewe in labour, and in most cases the ewe abandoned the lamb as soon as it was dropped. Over the same period of a fortnight only three ewes died after having lambed. Ewes which had lost their lambs showed no vaginal discharge and did not appear to be affected in any way; there being no pyrexia.
With the exception of the lungs, post-mortem examination of aborted lambs revealed no visible lesions. The lungs were non-aerated and were marked by extensive petechiation along the dorsal third on both sides. Pipettes were taken from kidney, spleen, lung and heart blood and submitted for bacteriological examination. The Director of Veterinary Research reported that cultures from the pipettes of kidney and heart blood were sterile, but those sown from the pipettes of spleen and lung revealed the presence of an organism morphologically identical with Listeria monocytogenes. Further cultural work and animal inoculation confirmed this identification, but the confirmation was delayed owing to the virulence of the organism becoming attenuated on sub-culture.
At the end of the fortnight the trouble ceased as suddenly as it commenced and normal lambing began with no further losses occurring.
Hindmarsh and Blumer (1932) state: "It would appear that outbreaks of Listeria infection occur only sporadically, affecting only a small percentage of animals in a flock or herd and then disappearing spontaneously." Such was the case in this particular outbreak as there were 1,100 ewes in the flock and only 50 were affected.
Paterson (1940) reported that intravenous injection of this organism into pregnant sheep was followed by the localisation of the organism in the uterus, foetal membranes and foetus, finally causing expulsion of the foetus. He was unable to provoke aborption by the oral administration of cultures to pregnant ewes. The intravenous inoculation of cultures of Listeria caused an immediate, usually transient, pyrexia; with abortion occurring in all cases seven to twelve days after inoculation. The author also discusses evidence pointing to the possibility of certain cases of listeriosis in human infants being due to intra-uterine infection.
The actual path of infection of sheep is not known, but (1938), in discussing Bacterial Encephalitis, which is caused by the same organism, states, "the evidence suggests that in rabbits, guinea pigs and the ground rodents of Africa (gerbilles), infection takes place through the alimentary tract, but the path of infection in the various other species that are affected is as yet unknown." He goes on to say that evidence in New Zeaiand suggested that injection did occur through the nasal passages and that in almost all cases examined the minute, first stage larvae of Oestrus ovis were present in the nasal mucous membrane and that these larvae may play a part.
The case reported occurred late in May, when one would expect the fly to be inactive, but the weather up till that time had been very mild and the fly could have been displaying some activity. This is supported by Monnig (1934) who states, "In South Africa, and probably in other warm climates, the flies infect sheep all the year round." However, the mode of infection still remains unsolved.
(1) A case of abortion in ewes, due to Listeria monocytogenes, is reported.
(2) Some discussion of the disease is presented.
(3) The path of infection is discussed.
Hindmarsh, W.L., and Blumer. C. (1932)—Aust. vet. J., 8 : 149.
Paterson, J.S. (1940)–Vet. J., 96 : 327.
Gill, D.A. (1938)–Yearbook. Inst. Insprs. Stock. N.S.W. : p. 49.
Mennig, H.O. (1934)—Veterinary Helminthology and Entomology Bailliere, Tindall & Cox. London. p. 294.