Chlamydia pecorum (C. pecorum) is an obligate intracellular gram negative bacterium that is capable of causing disease in a range of species including sheep, goats, pigs, cattle and wildlife (Walker, 2018). In Australia it is estimated to be present in 30% of the nation's sheep flock (Yang, 2014) and has been associated with polyarthritis, conjunctivitis and occasionally abortions (Robson, 2003; Robson, 2004; Jelocnik et al., 2014; Tighe and Slattery, 2012; Slattery, 2008).
Bacterial arthritis is estimated to cost the industry $39M annually (Lane et al., 2015) with on-farm mortality, lower production in surviving lambs, treatment and prevention costs and post farmgate carcase condemnation (Walker et al., 2016; Watt, 2011; Lane et al., 2015).
This paper describes chlamydial polyarthritis in feedlot lambs on two separate properties in the Central Tablelands of NSW.
The producer operates a lamb feedlot as well as a commercial merino flock. The feedlot holds approximately 4000-5000 first- and second-cross lambs that are sourced from New South Wales and Victoria. The lambs are purchased at approximately 38kg and grown to market weight of approximately 51kg. Lambs are transitioned over 12-14 days onto a feedlot ration consisting of 83% barley, 15% lupins and 2% additives. The feedlot has been designed according to industry guidelines with a stocking density of 12m2/lamb. On induction to the feedlot, lambs are administered a 5-in-1 clostridial vaccine and a vitamin B12 injection and are drenched with a combination drench.
In late September 2018, a consignment of 800 approximately six-month-old first- and second-cross lambs was purchased from various sources but mainly Victoria. One month after introduction to the feedlot, 10-15 of these lambs were noticed to be reluctant to walk, preferring sternal recumbency. The lambs walked with a stiff gait and some were noted to be in poor body condition. Despite their lameness, the affected lambs remained bright, alert and responsive. Some had an ocular discharge.
Four of the lame lambs were clinically examined. Rectal temperatures ranged from 40.0-40.6˚C (mean 40.25˚C). All of the lambs were lame with stiff gaits. One lamb had moderate distension of the left hind fetlock and hock. There was no obvious swelling or effusion of the joints affecting the other three lambs. Three of the four lambs were fat score 2 and the fourth lamb was fat score 3. One of the lambs had been treated with long-acting oxytetracycline (30mg/kg) but had not yet responded to treatment.
The second feedlot feeds about 1,000 first-cross lambs that are mainly sourced from New South Wales. The ration and induction procedures were similar to the feedlot of Case 1. In late November 2018, a consignment of 475 Suffolk x merino lambs was purchased from a farm also located in the Central Tablelands. Approximately six weeks after introduction to the feedlot, 25 of these lambs were observed to be lame. They were reluctant to walk, appeared to be tucked up and had lost body condition. Prior to the lameness the lambs had performed well in the feedlot, gaining 15kg/head on average. Notably, the other 640 lambs in the feedlot and in close proximity to the recently introduced lambs remained unaffected.
Four affected lambs were examined. Rectal temperatures ranged from 39.8-40.5˚C and averaged 40.36˚C. All of the lambs were lame and walked with a stiff gait and arched backs. Three of the lambs had distension of one or more joints. The carpal joints were the most commonly affected joints, however one lamb had swelling of the fetlock and another had stifle distension. Dry, crusty bilateral conjunctivitis was noted in two of the lambs. Serology results were positive for C. pecorum in all lambs tested. (See Table 1 below).
|Lamb ID||C.pecorum titre
Lame lambs were treated with a single dose of long-acting oxytetracycline (30mg/kg). The producer was advised to monitor the feedlot for any lame lambs and to treat any cases with antibiotics as soon as they were detected. The producer commented that almost all treated lambs responded and resumed normal growth.
The epidemiology of C. pecorum is poorly understood (Walker et al., 2015).
Lambs are most likely born without the bacteria and become infected with C. pecorum by 2-3 months of age, presumably via faecal-oral transmission at first pasture grazing (Walker, 2015). It often resides asymptomatically in the gastrointestinal tract and can be readily found in the faeces or the urogenital tract of healthy animals (Bommana et al., 2018; Yang et al., 2014).
Chlamydial polyarthritis is typically seen in fast-growing lambs that are between four and eight months of age (Jelocnik et al., 2014). It is suggested that the bacterium targets rapidly growing tissues and joints (Walker et al., 2015). Feedlot lambs may be more at risk as they are young and growing rapidly on high-energy, high-protein rations. They are also likely to be from multiple sources with a range of prior environments, management practices and previous pathogen exposure. Lambs introduced to a feedlot face several stressors including transportation, increased human contact, changed nutrition, routine husbandry procedures, interaction with other lambs at high stocking rates and potentially a dry and dusty environment.
On both properties, polyarthritis was observed in single consignments and the disease remained confined to that cohort despite close contact with other groups. This observation suggests that the affected groups may have been predisposed to developing clinical disease. It also suggests that pathogenic strains of C. pecorum are not transmitted easily, which may be attributed to prior immunity through exposure to the non-pathogenic strain.
Diagnosis using quantitative PCR (qPCR) gives the most accurate picture of acute infection as serology using the complement fixation test (CFT) is unable to differentiate between past and current infection. However, the CFT is currently the most common diagnostic test used even though CFT titre levels have been found to poorly correlate with clinical disease severity (Walker et al., 2016). Consideration should be given to using qPCR with acute presentations. The recent development of loop mediated isothermal technology (LAMP) may be the future in terms of providing time-sensitive diagnostics in a point-of-care setting (Jelocnik et al., 2017)
Control of chlamydial infections in lambs is challenging. Most clinical cases respond to antibiotics but the use of antibiotics presents the problem of residue and antimicrobial resistance management. Antibiotic treatment may also induce latency rather than eliminate the infection (Walker et al., 2015). Chlamydial vaccines have been used successfully in other species and are being developed for sheep (Desclozeaux et al., 2017).