Investigating the magnitude, timing and causes of abortion and lamb mortality in maiden ewes

Thomas Clune

The reproductive performance of maiden (primiparous) ewes is an important component of overall sheep flock performance in Australia. However, the reproductive performance of maiden ewes that are mated for the first time either as ewe lambs at 7 to 10 months of age, or hoggets, at 18 to 20 months of age is poor and inconsistent. Lamb mortalities in the perinatal period are a major source of reproductive inefficiency and a major welfare concern, with approximately 20-30% of lambs born in Australia dying at or after birth. However, most industry data for lamb survival is based on mature or mixed age ewes and do not distinguish between in utero losses and those that occur at or after birth. Consequently, the timing, magnitude and nature of losses that occur between pregnancy scanning and lamb marking in maiden ewe flocks in Australia are not well described. Maiden ewes may be more susceptible to infectious diseases that cause abortion, stillbirths and the birth of weak lambs that are less likely to survive, as younger ewes are less likely to have developed immunocompetency to infection prior to pregnancy. In New Zealand, some studies have reported that infectious diseases are important contributors to the poor and inconsistent reproductive performance observed for maiden ewes, including an increased incidence of abortions in maiden ewe flocks. However, it is not clear if infectious diseases are an important contributor to foetal and lamb mortality for maiden ewes in Australia. The overall objectives of the research described in this thesis was to i) identify the most common causes of ovine abortion and stillbirths diagnosed in Australia, ii) quantify abortion and lamb mortalities that occur in maiden ewe flocks between pregnancy scanning and lamb marking and iii) determine the impact of endemic abortigenic pathogens including Toxoplasma gondii, Neospora caninum, Coxiella burnetii, Campylobacter fetus and Campylobacter jejuni on the reproductive performance of maiden ewes. A review of abortion and stillbirth investigations submitted to Australian veterinary laboratories revealed that an aetiological diagnosis was made for 57% of investigations. Of the investigations where an aetiological diagnosis was made, 81% involved infectious abortion, with Campylobacter spp. (32%), Listeria spp. (25%) and Toxoplasma gondii (9%) the three most common abortigenic pathogens implicated. A cohort study was conducted between 2018 and 2020 involving 30 maiden ewe flocks located across South Australia (n=9), Western Australia (n=11) and Victoria (n=10). Foetal and lamb mortality from pregnancy scanning to marking were determined for 19 ewe lamb flocks (including composite breed, Border Leicester, Dorper, White Suffolk and Merino flocks) and 11 Merino hogget flocks (approximately 200 ewes per flock) based on two ultrasounds, lambing round records and ewe lactation status. Seropositivity to T gondii, N. caninum and C. burnetii was determined for a subset of maiden ewes that were predominantly determined to be pregnant and subsequently failed to rear a lamb (n = 1279). For flocks that had not been vaccinated against Campylobacter spp. (n=22), antibody titres for C. fetus and C. jejuni of 10 ewes that raised lambs and 10-20 ewes that aborted or had lambs that died in the perinatal period were determined. Average foetal and lamb mortality from scanning to marking was 35.8% (range 14.3-71.1%) for ewe lamb flocks and 29.4% (range 19.7-52.7%) for Merino hogget flocks. Mid-pregnancy abortion was detected by sequential ultrasounds in 5.7% ewes (range 0-50%) in ewe lamb flocks and 0.9% ewes (range 0-4.4%) in hogget flocks. Lamb mortality from birth to marking represented the greatest contributor to overall lamb mortality after scanning. The proportion of sampled maiden ewes that were seropositive for T gondii, N. caninum and C. burnetti were 1.1% (95% confidence interval (CI) 0.6, 1.8), 0.16% (95% CI 0.03, 0.5) and 0.08% (95% CI 0.01, 0.36), respectively. Seropositivity to C. fetus and C. jejuni using titre cut-off ≥1:80 was demonstrated in 12% (57/462, 95% CI 9.6, 15.6) and 44% (204/462, 95% CI 39.7, 48.7) of maiden ewes, respectively. Campylobacter fetus-associated abortions were confirmed with microbial culture from aborted lambs in one flock in Victoria. The odds for failing to rear a lamb in maiden ewes with C. fetus titre ≥1:10 was 2.01 times that of seronegative ewes (95% CI 1.09, 3.77; P=0.027). However, there was no association between C. fetus titre ≥1:80 and failure to raise (OR 1.69 (95% CI 0.77, 3.76) P=0.191). Whilst C. jejuni was detected in all flocks, there was no evidence that seropositivity to C. jejuni was associated with increased odds of failing to rear at either titre threshold. Chlamydia pecorum was also detected via qPCR in tissue samples from aborted or stillborn lambs from five flocks in Western Australia. This research highlights the variability in reproductive performance for maiden ewes and indicates scope to improve overall reproductive performance by reducing foetal and lamb mortality between pregnancy scanning and lamb marking. The low seropositivity to T. gondii, N. caninum and C. burnetii suggests that exposure to these pathogens was not widespread in ewes on farms located over a wide geographical region and were unlikely to be important contributors to abortion and perinatal mortalities observed in these flocks of maiden ewes. Campylobacter fetus– associated reproductive losses were confirmed on one farm in this study, consistent with the acknowledged role of Campylobacter spp. as a sporadic cause of ovine abortions in Australia. However, we observed that associations between seropositivity to Campylobacter spp. and reproductive outcomes were inconsistent, emphasising the importance of considering risk factors specific to the flock and pathogen detection at necropsy to confirm reproductive disease. The detection of C. pecorum, a bacterium more commonly associated with polyarthritis and conjunctivitis in sheep, was a notable observation, especially given the lack of epidemiological and geographic linkages between sites where the pathogen was detected. Further investigation to understand C. pecorum epidemiology and impacts on sheep reproduction are required. Disease investigations to establish an underlying cause are warranted for maiden ewe flocks with disappointing or inconsistent lamb survival to inform targeted strategies for addressing lamb survival and reducing the incidence of abortion. Implementing strategies to improve perinatal lamb survival are a priority for maiden ewe flocks, but identifying flocks where abortion is contributing to poor lamb survival and addressing risks for reproductive disease can improve reproductive performance.

Investigating the magnitude, timing and causes of abortion and lamb mortality in maiden ewes

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