Ovine brucellosis is an important disease with economic implications for the Australian sheep industry. The aetiological agent, Brucella ovis, is shed in the semen of infected rams and is transmitted horizontally in the flock. Rams contract the bacterium directly from other rams, or indirectly via a ewe mated by both rams.1 The bacteria undergoes haematogenous spread to the male reproductive tract with sequelae including interstitial abscesses and spermatic granulomas.2 The presence of epididymal lesions and inflammatory cells in the ejaculate and possible blockage of the epididymides causes infertility in both clinically and sub-clinically seropositive rams.3, 4 Ewes can become transiently infected and can occasionally abort due to B. ovis, but are less important than rams in the epidemiology of the disease.1
Diagnosis of B. ovis is typically by combining flock history (extended lambing period and low marking success), scrotal palpation, serology and in some circumstances, semen culture.4
The authors believe this paper contains the first report of the results of an entire ram flock 'cull and replace' strategy during joining with a short sexual rest period.
The 7300ha western New South Wales enterprise consists of a cattle feedlot (500 head), 1400 merino ewes joined to Border Leicester rams, and cropping. The property was severely drought affected from 2017 to 2020 with the sheep flock receiving high level supplementary, to total, ration feeding for almost all the 24 months prior to drought-breaking rains in early March 2020. From March 2020 seasonal conditions were excellent. The first-cross lamb enterprise has two mobs joining in spring and autumn and two shearings. Ewes scanned as not pregnant from each joining are moved to the other mob for the subsequent joining period.
In spring 2019 the mob of 800 merino ewes for spring joining were in Body Condition Score (BCS) 1.5-2 (mostly the latter) and joined to 12 Border Leicester rams in BCS 2. Prior to joining, the rams were kept and fed in a pen near where the ewes were fed. This continual sight and smell contact with the rams may have contributed to the subsequent infertility by reducing the impact of the ram effect to initiate cycling in anoestrus ewes. The rams were left with ewes until removed at scanning on 17 March 2020. They were then immediately joined to the autumn-joining ewe mob.
The March 2020 pregnancy scanning of the spring 2019-joined ewes found no pregnancies in the first 200 head and scanning was discontinued. The North West Local Land Services District Veterinarian was contacted to investigate possible causes of the low pregnancy rate. The owner was advised of the possible differential diagnoses, which included: 1) Anoestrus (due to spring joining, lack of ram effect, low BCS and ongoing stress from dust storms), 2) Ovine brucellosis, and 3) Ovine campylobacteriosis (unvaccinated flock).
In March 2020, laboratory investigations were conducted to investigate the cause of the infertility. Blood samples were taken from the 12 Border Leicester rams for B. ovis complement fixation testing (CFT) at the State Veterinary Diagnostic Laboratory at Elizabeth MacArthur Agricultural Institute (EMAI).5 Testicular palpation was not performed due to COVID-19 concerns. The rams were B. ovis positive (7/12 positive, 2/12 inconclusive). Blood samples were also taken from 10 Merino ewes to test for Campylobacter spp. (Campylobacter fetus subspecies fetus and C. jejuni) via the agglutination test (ACE Laboratory Services, Bendigo East, VIC). The ewes had antibody titres consistent with historical exposure to C. fetus subsp. fetus (titres 1:10-1:80; 4/10 ewes) or were negative (titres <1:10; 6/10 ewes). Most ewes (9/10) had antibodies titres consistent with normal, background titres (1:10-1:80) for C. jejuni.
The owner was advised that due to the high prevalence of B. ovis in the ram mob, culling the entire ram flock was the preferred eradication method over repeated test and cull. The owner agreed.
Options were then considered for how this cull would occur as joining to the autumn-joined mob with the infected ram mob had already commenced and the owner also intended to immediately re-join the failed spring-joining mob. Central to planning this was determining what period between culling the current infected ram mob and introducing a new ram mob would minimise the risk of ewes carrying the bacteria and infecting new rams. Deciding this interval was important because the owners had to weigh the disadvantage of delayed lambing (heat stress, mismothering) with the risk of reinfection of new rams with B. ovis.
No information was found in the literature on the recommended period before re-introducing new rams during joining. Discussion by the investigating author with District Veterinarian colleagues elicited an unreferenced recommendation of a minimum period of 21 days, being a sufficient period for all ewes to have undergone oestrus twice. The owner decided on a 14-day period between culling infected rams and introducing new rams. This period would be sufficient for almost all ewes to have undergone oestrus.
On 8 April the 12 infected Border Leicester rams were removed and culled, and new rams introduced from 22 April. The 30 new rams were Border Leicesters from three studs; two of the three studs were accredited under the Ovine Brucellosis Accreditation Scheme.6 Most were virgin rams.
The testing protocol for ovine brucellosis in a ram flock are two consecutive negative B. ovis CFT tests for all individuals at a minimum of 60 days apart.6 The 60-day period is appropriate because it is the maximum period for a ram to become seropositive following infection.6
The first sampling occurred on 17 June (57 days after the new rams were introduced) when the rams were removed from the ewes. Twenty-nine rams were sampled; one ram had died. No testicular lesions were found on palpation, and all rams tested negative on B. ovis CFT. An incidental finding of ulcerative balanitis was found in 8/29 (28%) of rams.
The second sampling occurred on 17 August 2020 (61 days after first sampling). The 29 rams from the previous sampling, plus an additional two rams introduced since then but kept separate and not joined were seronegative for B. ovis. Two of the eight rams with ulcerative balanitis had testicular swelling that was not typical of ovine brucellosis.
The success of the B. ovis eradication in this current case study is likely due to the decision to cull the entire ram flock, as opposed to a salvage operation with repeated test and cull cycles. Culling all rams ensured that there was no possibility that CFT negative, infected rams would continue to shed the bacterium. Usually the culling of the entire ram flock as the eradication strategy has few complexities, but the unique situation in this case was that it was required mid-joining, with significant disadvantages with any delay to lambing. The key factor was consideration of the period between removal of the infected rams and replacement with uninfected rams. Planning this requires determination of the likely period ewes will maintain the B. ovis infection in the absence of rams and spread infection to the new, uninfected rams during servicing.
One hypothesis for why B. ovis may fail to be eradicated in flocks, despite testing and culling of infected rams, is that infected ewes are responsible for maintaining the infection in the flock. Investigation of the failure of a four-year test and cull program for B. ovis in two Spanish flocks found 19% (71/373 ewes) were positive for B. ovis via the gel diffusion test. Forty-four of these ewes were slaughtered and organs assessed for B. ovis infection; 16/44 of these ewes were culture-positive for B. ovis. The uterus was the most commonly infected organ (12/16 ewes).7 In another study, ewes were experimentally infected with B.ovis via the intra-vaginal route. The organism was demonstred to persist in ewe circulation for up to 98 days post infection. Based on this, a theoretical four months of sexual rest for ewes that have had contact with infected rams has been suggested as sufficient to eradicate B ovis.8 Based on these findings, a theoretical four months of sexual rest for ewes that have had contact with infected rams has been suggested as sufficient to eradicate B ovis.8 However, we failed to find reports in the literature of successful or failed sexual rest periods and the interaction with B. ovis infection.
The potential role of ewes in maintaining infection in the flock warrants consideration when deciding how long to wait between culling infected rams and introducing uninfected rams. However, this period is driven by a trade-off between minimising the risk of perpetuating the B. ovis infection and having a productive and profitable flock. In this case study, this period was necessarily short so as to not prolong the lambing period. Lambing too late in the year in north-western NSW could cause exposure, lamb losses and mismothering. The owner was prepared to risk infecting rams as the current economic returns for each additional first cross lamb were relatively high.
Differential diagnoses for the initial infertility in the flock included anoestrus and ovine campylobacteriosis. Anoestrus due to spring joining, lack of ram effect, low BCS and ongoing stress from dust storms were likely contributing factors for infertility and, together with B. ovis, as the most likely cause of the lack of detectable pregnancy at the first ultrasound scan. Ovine campylobacteriosis (formerly ovine vibriosis) is caused by C. fetus subsp. fetus or C. jejuni and causes late term abortions in sheep.12 Antibodies for both campylobacter species were detected in the ewes but at titres unlikely to be consistent with levels required for abortion.
Thanks to the property owner and farm staff. Also, a thank-you to Associate Professor Jenny-Ann Toribio (Sydney School of Veterinary Science) for facilitating this collaboration as part of the online Public, Industry and Community placement for final year Sydney University veterinary students during the COVID-19 pandemic.