Ovine Observer

Causes of abortion and lamb mortality for maiden ewes in WA

Introduction

Lamb survival typically refers to the survival of foetuses between scanning in mid-pregnancy and lamb marking. Lower lamb survival has been reported for maiden ewes compared with mature and mixed age flocks, but the causes of this lower survival for lambs born to maiden ewes are not well studied. Australian studies using mostly mature or mixed age flocks have shown starvation-mismothering-exposure complex, stillbirths, and dystocia (difficult births) are the most common causes of lamb mortality during the perinatal period during or soon after lambing.

Infectious diseases may contribute to lamb mortality through abortion, stillbirths and birth of weak lambs that are more like to die soon after birth. Maiden ewes can be more susceptible to infectious diseases resulting in abortion or birth of compromised lambs because these ewes are younger and less likely to have developed immunity to infection prior to pregnancy. Relatively few studies include lamb necropsies (post-mortem examination) in conjunction with laboratory testing for infectious diseases. Similarly, relatively few investigations for abortion and perinatal lamb death are submitted to veterinary diagnostic laboratories for exclusion of infectious diseases. It is possible that the contribution of infectious disease to poor lamb survival may be under-recognised.

Therefore, the aim of this study was to determine the causes of abortion and perinatal deaths for lambs born to primiparous ewes in Western Australia and identify if infectious diseases are implicated.

Key findings

• Starvation-mismothering-exposure, dystocia and stillbirth were the most common causes of perinatal lamb death.
• There was no evidence of exotic diseases in aborted and stillborn lambs.
• Chlamydia pecorum was detected in 43% of aborted and stillborn lambs, including 64% of the aborted or stillborn lambs from ewe lamb flocks. Infections may contribute to foetal and lamb mortality for maiden ewes in Western Australia.

Materials and methods

Study location and sheep flock characteristics

This study was conducted at 10 farms in southern Western Australia using 11 maiden flocks each with approximately two-hundred Merino or non-Merino maiden ewes randomly selected at mating.

Maiden ewes were mated as either ewe lambs (7 flocks, 7-10 months old at mating) or maiden hogget ewes (4 flocks, 18-20 months old at mating) and monitored between mating and lamb marking. Two farms used an artificial insemination program on all or some of the ewes, followed by a period of natural back-up mating. All other flocks were mated naturally with an average mating period of 38 days (range 32-46 days).

Reproductive data

Foetal count (number of foetuses) were determined using pregnancy ultrasounds at 62–87 days (scan 1).

Farmers checked the lambing mobs once or twice daily throughout the lambing period. On most farms, lambs were ear tagged at birth, and their birth type and dam were recorded. The number of lambs born was calculated using records of the number of lambs tagged plus the number of dead lambs collected. On farms where tagging at birth was not performed, number of lambs born was calculated based on number of lambs present at marking (approximately 6 weeks from start of lambing) plus the number of dead lambs collected. Number of lambs born may have been underestimated at these sites because it is unlikely that all lambs that died were recovered for necropsy. There was no evidence of widespread abortion or ewe illness noted in any of the flocks.

Lamb necropsies and laboratory testing

Lambs that died in the first 3 days of life were collected for necropsy to determine cause of death. Dead lambs were either refrigerated (4⁰C) or frozen (-20⁰C) for up to 5 days.

One aborted foetus and one foetal membrane were also recovered prior to the start of lambing in flock 3a.

Lamb necropsies were performed using a previously described method to determine cause of death (Table 1)

Table 1 - Cause of death classifications (modified from Everett-Hincks and Duncan, 2008)

Classification	Post-mortem observations
Dystocia	Evidence of oedema to the head or neck
Stillborn	Full-term appearance; not walked or breathed
Abortion/prematurity	Pre-term appearance (size, wool covering); not walked or breathed
Starvation-mismothering-exposure complex	Evidence that had walked and breathed Empty stomach contents Mobilisation of peri-renal (kidney) and peri-cardial (heart) fat
Other	Cause of death determined based on the gross appearance of affected organ systems

Tissue samples from aborted or stillborn lambs from flocks with at least two lambs classified as abortion or stillbirth were submitted to the Department of Primary Industry and Regional Development Diagnostic Laboratory Service (South Perth, Western Australia). The testing methods included histology (examination of tissues with microscope), bacterial culture (growth of bacteria) and molecular diagnostics (for example, PCR and sequencing) and are outlined in more detail in the scientific paper (see further information).

Statistical analyses

Lamb mortality (%) between birth and marking for single-born lambs and multiple-born lambs (twins and triplets) were compared using two-tailed two-sample z-test. Only farms where lambs were tagged at birth were included in calculation of mortality for single- and multiple-born lambs. The proportion of cases with C. pecorum detected for each ewe age category (ewe lambs and yearling ewes) were compared using two-tailed two-sample z-test.

Results

Lamb mortality in maiden ewe flocks

Lamb mortality for study flocks are outlined in Table 2. Overall, lamb mortality from birth to marking ranged 13-27% for Merino hogget flocks and 9-41% for non-Merino ewe lamb flocks.

Table 2 - Lamb mortality for 11 flocks of maiden ewes in Western Australia

		Mortality between birth and marking (%)
Ewe age category	Lambs born (n)	All birth types	Singles	Multiple
Hogget	249	19.7	9.8	29.4
Hogget	210	27.1	17.2	42.7
Hogget	277	23.5	16.0	29.1
Hogget	151 A	12.6 A	Unknown	Unknown
Ewe lamb	89	18.0	7.0	25.0
Ewe lamb	169 A	29.0 A	Unknown	Unknown
Ewe lamb	150 A	40.7 A	Unknown	Unknown
Ewe lamb	197	16.8	15.4	17.2
Ewe lamb	196 B	10.7 B	15.8 B	14.5 B
Ewe lamb	145	20.0	15.7	24.2
Ewe lamb	130	24.6	13.0	29.2
TOTAL	1963	22.0	14.0	26.4

A Lambs not tagged at birth (number lambs born may be underestimated)

B Not all lambs tagged and assigned to birth type and/or dam

Necropsies and cause of death

A total of 298 lamb necropsies were performed. These represented 69% of lambs that died between birth and marking. Remaining cases without necropsy either were not recovered by the farmers or died after 72 hours of age.

The cause of death assigned at necropsy are shown in Table 3. The starvation-mismothering-exposure complex, dystocia, and stillbirths accounted for 96% (218/227) of cases where cause of death was identified.

Table 3 - Cause of death identified at lamb necropsy

	Count (n)	% necropsies	% necropsies with diagnosis
Necropsies performed
Total	298	-	-
Cause of death
Abortion or premature birth	4	1	1.8
Stillbirth	44	15	19.4
Dystocia	73	24	32.1
Starvation-mismothering	101	34	44.5
Infection/inflammation	5	2	2.2
Undetermined	71	24	-

Infectious diseases detected in aborted and stillborn lambs

Infections detected for 35 cases classified as abortion or stillbirth from six farms are shown in Table 4.

Chlamydia pecorum DNA was detected by qPCR in 39% (13/33) of stillborn or premature cases and 100% (2/2) of abortion cases. C. pecorum DNA was detected at five of the six farms, and was more commonly detected in cases from ewe lambs (64%, 14/22) compared with hoggets ewes (8%, 1/13; P = 0.001).

Genetic characterisation identified the C. pecorum from aborted and stillborn lambs strain sequence types (STs) as ST23. This genotype was identical to C. pecorum strains associated with sheep abortions in NSW, arthritis in sheep, conjunctivitis (pink eye) in sheep, and sporadic bovine encephalomyelitis (in cattle from WA, NSW and USA). This sequence type ST23 is distinct from sheep and cattle rectal strains, and strains from pigs and goats.

The only other bacteria detected that was likely to be associated with abortion or birth of compromised lambs was Trueperella pyogenes. This was detected in two lambs, including one lamb where C. pecorum was concurrently detected.

Other infections that are endemic in WA and may cause abortion (such as Toxoplasma gondii, Listeria spp., Campylobacter fetus, Campylobacter jejuni) were not detected. Exotic diseases including Chlamydia abortus, Brucella melitensis, Salmonella enterica serovar Abortusovis were not detected.

Table 4 - Detection of infectious agents from aborted or stillborn lambs in Western Australia

	Ewe lamb flocks	Hogget flocks	TOTAL
Cases submitted (n)
Total	22	13	35
Aborted foetus & membranes	1	0	1
Aborted membranes only	1	0	1
Stillborn lamb	18	13	33
Chlamydia spp.
C. pecorum (qPCR)	14	1	15
Other
Listeria (culture)	0	0	0
Salmonella (culture)	0	0	0
Trueperella pyogenes (culture)	2	-	2
Campylobacter (culture)	0	0	0
Campylobacter (PCR) A	2 A	0	2
Leptospira (PCR)	0	0	0
Toxoplasma (qPCR)	0	0	0
Coxiella (qPCR)	0	0	0
Brucella ovis (PCR + sequencing)	0	0	0
Pan-pestivirus (qPCR)	0	0	0

A C. sputorum and C. mucosalis by sequencing (suspected contaminant)

Discussion

Chlamydia pecorum was detected in aborted and stillborn lambs from maiden ewes on multiple farms in WA. The same sequence type identified in aborted and stillborn lambs can be associated with arthritis and pink eye in sheep. Other C. pecorum sequence types have been detected in the gut of sheep without evidence of disease. C. pecorum should be considered as a differential diagnosis for abortion and perinatal mortality in Australian sheep, particularly ewe lambs.

C. pecorum was detected in aborted and stillborn lambs from five out of six farms, and was more commonly detected in offspring from younger ewes (ewe lambs). However, the degree to which C. pecorum ST23 contributed overall lamb mortalities could not be determined. Infectious disease screening was not conducted for lambs that died from causes other than abortion or stillbirth, including those classified as starvation-mismothering. Inflammation of the placenta caused by C. pecorum infection may result in a spectrum of outcomes. This could include abortion of the foetus, stillbirth, birth of live but compromised lambs (weak and with low birth weights) with poor survival, or normal offspring. The outcome is likely to depend on the severity of disease in the placenta and the lamb’s organs. More research is needed to determine whether infection contributes to reduced lamb viability (presenting as starvation-mismothering), as well as abortion or stillbirth, and the factors that impact the outcome for the lamb.

Overall lamb mortality for flocks in this study was comparable with other Australian studies. Stillbirths accounted for 19% of necropsies where cause of death was determined. This was not markedly different to stillbirths as proportion of total losses reported in other Australian studies despite detection of C. pecorum in most flocks. Notably, stillbirths and abortion associated with C. pecorum ST23 were detected in flocks without obvious signs of abortion storm (i.e. farmers detecting abortions when inspecting ewes) or illness in ewes that would have triggered alarm. This suggests C. pecorum may be associated with losses that go undetected on Australian farms, and explains why C. pecorum abortion has not been more widely reported in the past.

The starvation-mismothering-exposure complex and dystocia accounted for most lamb mortalities that occurred in the perinatal period and mortality was higher for multiple-born lambs compared to single-born lambs. Strategies to reduce dystocia and starvation-mismothering-exposure, including provision of adequate shelter for lambing ewes and managing ewe nutrition during pregnancy to optimise lamb birthweights may help optimise survival for progeny of maiden ewes.

Conclusion

Starvation-mismothering-exposure complex, dystocia and stillbirths accounted for most lamb mortalities for lambs born to maiden ewes. Chlamydia pecorum was detected in aborted and stillborn lambs born to maiden ewes in WA. The C. pecorum strains detected from abortions and stillborn lambs belong to the ST23 sequence type that has also been associated with abortions in sheep and cattle, and other diseases including arthritis, pink eye and sporadic bovine encephalitis. Further investigation to quantify impact of C. pecorum as a cause of abortion, stillbirth or poor lamb viability in sheep, and determine factors that impact outcomes for pregnant ewes.

Further information

Project final report: https://www.mla.com.au/research-and-development/reports/2022/reducing-foetal-and-lamb-losses-in-young-ewes2/

Scientific paper (free access): https://doi.org/10.1186/s13567-021-00950-w

DPIRD Sheep Abortion Surveillance Scheme: https://www.agric.wa.gov.au/livestock-biosecurity/ewe-abortion-and-newborn-lamb-deaths-surveillance-program

Acknowledgments

This project was funded by Meat and Livestock Australia. We thank the participating farmers who provided access to their animals and facilities. Laboratory diagnostic testing was performed under the Western Australian Ewe Abortion and Newborn Lamb Death Surveillance Program and supported with funding by Australian Government Department of Agriculture in partnership with Australian Wool Innovation as part of the 2020 Science and Innovation Awards for Young People in Agriculture, Fisheries and Forestry awarded to Tom Clune, 2019 Australian Research Council Discovery Early Career Research Award awarded to Martina Jelocnik and Murdoch University Seed Grants. Tom Clune was supported with post graduate scholarships from Meat and Livestock Australia and Sheep Industry Business Innovation (Department of Primary Industries and Regional Development, Western Australia).