Arthritis continues to play a significant role within the Australian sheep industry from both an economic and welfare perspective. Arthritis is not a new to topic to the NSW District Veterinarians Conference with several presentations in the past including those by Watt (2007, 2010 and 2011) and Walker and Polkinghorne (2016). In 2015, arthritis was ranked 10th in a priority list of 23 diseases that affect the red meat sheep industry, highlighting its significance as a disease (Jubb et al., 2015). Despite this, it has received relatively little attention in the development of prevention and treatment options compared to its counterparts on that priority list.
Arthritis is a disease primarily caused by bacteria in young sheep. According to Farquharson (2007), there are 18 bacterial species or strains known to cause arthritis. In Australia, three distinct forms of the disease are generally considered including: erysipelas, chlamydial (polyarthritis) and purulent arthritis, the latter being caused by a range of pyogenic bacteria, but relatively infrequently compared to the first two forms (Watt, 2010). For a more detailed description of the disease and the different forms see Watt (2007) and Farquharson (2007). Although a number of treatment options are available, these too have production considerations including increased cost and withholding times. Due to the nature of the disease, prevention far outweighs treatment and this is reflected in the annual costs associated with these options (Jubb et al., 2015).
At present, a vaccine is only available for erysipelas arthritis but adoption has not been high to date (Jubb et al., 2015). However, properties/farms that experience erysipelas arthritis and use the vaccine for prevention report ongoing arthritis cases of a different clinical presentation, more consistent (and confirmed to be) with the chlamydial form (McRae, personnel communication).
The primary source of the economic impact associated with arthritis is direct production loss, whereas, prevention and treatment contribute only around 15% to total losses. Several reports have indicated the economic impact of arthritis on the Australian sheep industry since the early 2000s including those shown in Table 1. Whilst there is some variation in these figures, the consensus suggests the economic impact is continuing to grow. The generally accepted figure of the economic impact now seems to be $39.5m annually as reported by Jubb et al., (2015). It is likely that this has escalated since this report with the ongoing drought conditions which seem to favour the manifestation of the disease.
|Economic Impact in|
|Report Author||Year||Prepared for||2012||2013||2014||2015|
|Sackett et al.||2006||MLA||$25,953,763 annually|
|Jubb et al.||2015||MLA||$39,440,000 annually|
|Bryan et al.||2016||AHA||$2,020,219||$18,990,162||$28,942,910||$25,586,354|
Research into chlamydial diseases is complicated due to the bacteriologically recalcitrant nature of these bacteria. However, ongoing research into them, including Chlamydia pecorum (the species responsible for arthritis in sheep), continues to expand our understanding at the cellular level and how this may influence the host/pathogen relationship (Polkinghorne, 2011). However, field-based research has been limited. Recently, a collaborative effort between the University of the Sunshine Coast and NSW DPI was undertaken to reproduce a 1965 study (Storz, 1965) where chlamydial arthritis was experimentally induced in sheep. The study was highly successful as can be seen in Figures 1 and 2. This study confirmed that we can now use laboratory grown isolates of C. pecorum in future studies to evaluate vaccine candidates for chlamydial arthritis.
Arthritis continues to be a major source of economic loss and an ongoing welfare concern for Australian sheep producers. Although vaccination for the erysipelas form now exists, arthritis persists on many properties. Has the presence of erysipelas infection masked the presence of other bacterial pathogens or has vaccination against erysipelas opened a niche for other pathogens such as C. pecorum to exploit? These are critical questions that need to be addressed in an attempt to minimise the impact on production and economic loss resulting from this disease. Either way, a vaccine for C. pecorum and other pathogens is paramount to strategies to control arthritis in lambs.
The authors would like to further acknowledge staff of the Elizabeth Macarthur Agricultural Institute, NSW Department of Primary Industries and the staff and students of the University of the Sunshine Coast.