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This article was published in 1979
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Oedematous Condition of Ovine Foetus
S.N. Carroll, B.V.Sc., M.R.C.V.S. Veterinary Inspector, Yass
Preamble
During the winter of 1978 an unusual disease manifestation affecting two mobs of lambing ewes was investigated on a local Yass property. The investigations were carried out by myself with back-up assistance given by officers from the Veterinary Research Station, Glenfield, who also carried out all laboratory tests on samples submitted from the field. V.0. S. King from Goulburn assisted on one occasion when I was absent from the district on leave.
Over all, five visits were made to the property over a three month period, including one visit by V.O. King. Samples were forwarded to Glenfield on four occasions.
DISEASE
The disease manifested itself by affected ewes becoming grossly distended in the abdomen in late pregnancy to the point, in some cases, where the abdominal muscles became ruptured. This distention was due to extreme swelling of the foetus with oedema in the subcutaneous tissues to the point where the foetus took on the appearance of a 'wombat'. As well, the foetal fluids were increased and in a number of cases were gelatinous. The weights of the lambs examined ranged from 1.2 kg. to 13.5 kg and in one instance it was estimated that the fluids and the placenta would have weighed 9 kg. In one case the ewe was bearing twins, one apparently normal and the other oedematous.
Odd ewes managed to lamb normally with smaller affected foetuses, but in general the ewes were destroyed before attempting to lamb due to their being unable to stand and feed on account of the extreme weight of the uterine contents.
PATHOLOGY
On gross pathology, the lambs examined at Glenfield were all estimated to be 4-4½ months gestation, having a normal skeleton. The main abnormalities noted, other than the oedema of the subcutaneous tissues which ranged from 2.5-5 cms. thick all over the lamb's body, were hypoplasia of the respiratory tree and a developmental abnormality of the peripheral lymphoid tissue. The major lymphoid organs, the thymus and spleen, appeared to be relatively normal and there was no evidence of cardiovascular abnormality.
BACTERIOLOGY
All tests carried out on foetal and maternal tissues failed to produce any significant findings including tests for Vibriosis.
VIROLOGY
Blood samples were submitted from affected ewes on two occasions and in both instances serology for Mucosal Disease and Bluetongue were negative although, in the case of one ewe, a neutralisation titre of 3 was obtained for Akabane. However, this titre is extremely low and it was not considered to be of any real significance.
HISTOPATHOLOGY:
Inflammatory changes were noted in the liver with megakaryocytes and large numbers of lymphoid cells, the picture suggestive of granulomatous immunological proliferation.
The lungs were hypoplastic with a thick fibrotic capsule and poor alveolar development, while the pleura showed focal fibrotic nodules with lymphoid and inflammatory cells in foci at the base of the pleural membrane.
Lymph nodes showed acute lymphadenitis with lymphoid depletion and there was a low cortical cell population in the thymus. The kidneys contained odd haemorrhages and cortical autolysis with mild pigment accumulation in the tubules.
This picture was suggestive of an aberrant immunological response.
HISTORY OF THE FLOCKS AFFECTED
The condition was first noted in late June in a mob of 98 ewes which had been purchased in January at a property clearing sale in the Yass district. The mob consisted of 4 tooth first-cross ewes many of which had already had one lamb and some of which were in lamb. These ewes were joined to Poll Dorset rams purchased from a local stud on 5th January. A total of 21 oedematous lambs occurred from the end of June until mid-August when Lambing finished (21.4% of oedematous lambs/ewes mated). The lamb marking percentage for the mob was about 100%.
No further cases were seen until mid-September, when another case was found in a separate mob of 122 maiden and 4 tooth first-cross ewes which had been bred on the property. These ewes had originally been joined in November to Poll Dorset and Dorset rams bred on the property from a different blood line to those above. Of the 260 ewes joined, only 138 lambed in April, so the dry ewes were pulled out and rejoined to the same rams. These ewes commenced lambing in Late September, although the first affected ewe was seen in mid-September. From this time until the end of October a total of 10 oedematous lambs occurred (8.2% oedematous lambs/ewes mated). Up to the end of December a total of about 100% of lambs were marked.
FLOCK MANAGEMENT
The first mob was drenched on arrival with Nilzan and then at the end of February with Ripercol and again in mid-June with Ripercol. Originally they were kept on natural pasture while joined and then for a short period on lucerne paddock. After this they were then moved to a natural clover and grass paddock where they remained until they finished lambing.
The second mob were drenched in mid-June with Ripercol and then again with Ranizole in mid-august. Throughout, the mob was kept on natural clover and grass paddock except for a very short spell on a phalaris paddock.
The only possible contact between the two mobs occurred when they were in neighbouring paddocks separated by a road during the time the second mob were lambing in April. After this they were kept on differert blocks of land separated by 1-2 kilometres.
Other mobs of Lambing ewes were in the paddocks neighbouring the first mob and no instances of oedematous lambs were seen in these, nor were there any oedematous lambs reported on any of the neighbouring properties, or on other properties in the Yass district.
CHEMICAL USAGE
2-4-D is used on variegated thistles on the property every year during July. The rate used is 1½ pints/10 gallons/acre which is about the recommended rate of 2.8 litres/hectare. However, there were no thistles in the lucerne paddock or the grass paddock in which the first mob was kept and these have not had 2-4-D used in them.
In the paddock where the second mob was kept there were large areas of thistles and these were sprayed in July while the mob was in the paddock.
There was no apparent aphid infestation in the lucerne paddock last year and so no insecticide spray was used.
The only chemicals which these sheep are known to have come in contact with, other than 2-4-D above, are the various drenches.
DISCUSSION
The condition described appears to affect the unborn foetus later in gestation, the main manifestations being pronounced oedema of the subcutaneous issues and increased foetal fluids, often gelatinous in nature. On post-mortem the main abnormalities are an hypoplasia of the respiratory tree and a defect in the normal development of the lymphatic system, suggestive of an aberrant immunological response.
A literature search has failed to find a record of a similar defect in lambs.
The defects to the foetus would seem to develop some time after development begins as the thymus and spleen are relatively normal. The condition becomes obvious around 4-4½ months gestation when the ewes appear to be physically unable to carry the oversize foetus and uterine contents. Only a small number of ewes managed to lamb affected foetuses normally, most being destroyed prior to the commencement of lambing.
Only two mobs were affected, even though there were a number of other lambing mobs on the property. One mob consisted of purchased ewes joined to purchased rams whilst the other consisted of station bred ewes joined to station bred rams. The age structure of both mobs was similar. Contact between both mobs was minimal.
Possible aetiology of this condition includes genetic, infectious and teratogenic factors. The presence of one normal lamb and one abnormal lamb in a set of twins has been recorded with infectious agents (Toxoplasma gondii and Akabane virus) and with teratogenic agents (Veratrum californicum) (J.W. Plant - pers. comm.).
Genetic Factors
These can virtually be eliminated immediately due to:
a. there is no relationship between either mobs of ewes,
b. the rams used with each mob are from completely different bloodlines,
c. only two lambing mobs on the property were affected,
d. there is no history of the condition having previously occurred on the property.
A congenital dropsy in Ayrshire calves (Donald et al.) has been described due to an autosomal, recessive gene. However, it does not resemble this condition in that the calves were full term and the oedema often generalised. As well, the lymphatic system was developed in these calves
INFECTIOUS CAUSES
Two factors point to an infectious agent being involved, the age of the ewes and the inflammatory changes in the liver of affected lambs. However, a number of points contradict this:
a. a viral infection, especially if arthropod borne, would have almost certainly occurred in other flocks in the district. Also it would have to have the ability to affect the lymphatic system at a late stage of development. Consequently, it would almost have to be similar to Mucosal Disease virus infection of the pregnant ewe in which infection anywhere between 12 to 70 days gestation could produce a similar picture. There was no indication on serology of Mucosal Disease virus being present.
b. there was no indication of a bacterial infection being present on either bacteriology or histopathology.
c. the duration of the period over which lambs were affected was extended over 3-4 months.
d. there was no direct contact between the two mobs.
TERATOGENIC FACTORS
Any agent causing teratogenic effects would have to be very specific in its action and would have had to be present over a reasonable period of time. Also it would have had to occur on the property for the first time this year and only in the two paddocks involved, separated by a considerable distance. This would virtually rule out teratogenic plants.
2-4-D has been implicated in causing birth defects, although this is still unproven, and this has been used on the property. However, no spraying occurred in either of the paddocks associated with the first mob in recent history. Also the chemical has been used on this property and many other properties in the district for many years without apparent ill-effect.
As far as I can ascertain there were no alterations to the general management carried out on the property last year which might have brought teratogenic factors into play.
The season over the time the disease was manifested was relatively favourable for sheep following a dry summer and autumn period. Regular falls of light rain, high humidity and lack of heavy frosts in the early winter period produced short green pasture adequate for sheep nutrition. No hand feeding was found necessary.
CONCLUSION
No diagnosis to the cause of the condition has been made on the available data. It would seem that an infectious agent could be the most likely cause, but all the reasons given above tend to discount this.
It is unfortunate that the four lots of specimens forwarded to Glenfield Veterinary Research Station were examined by four different people with the result that the examinations were not as complete as they might have been.
It is unlikely that the problem can be further followed up in the future, as the property on which the outbreak occurred was virtually burnt out in a major bushfire in February this year, resulting in the death or destruction of 1,009 sheep. The remaining sheep have since been boxed and sent to agistment on other properties in the district.
REFERENCES:
Dennis (1970) Cornell Vet. 60 : 9
Dennis (1975) Aust. Vet. J. 51 : 347
Donald et al. (1952) Vr. Vet. J. 108 : 227
Hughes et al. - Teratology 5 : 5-10
Saperstien et al. (1975) Journal of the American Veterinary Medical Association 167 : 314