The following comments are based on author recollections, discussions with producers and analysis of DMS data since 1992. These records contain sixteen reports of ovine chlamydiosis and thirty of sporadic bovine encephalomyelitis (SBE). On endemic properties reporting may be haphazard as producers are familiar with the condition.
The most interesting facet of the Narrabri data is the contrast in clinical and epidemiological appearance between most cases since 1992, and an epidemic in 2003.
Cases are almost always introduced weaners or yearlings from other districts.
Cases are usually confined to two areas of the previous Narrabri RLPB. These are the foothills of the Nandewar Ranges and the Turrawan to Baan Baa ridge. These two areas are also the only areas where chlamydial arthritis of lambs has been reported. These discontinuous areas share a soil type that is distinctly different to soil types in other parts of the former Narrabri RLPB. Both areas border very large tracts of native forest. Both areas are moderately hilly.
A very few properties may get a case or two every year, but others see it at intervals of several years.
Incidence is usually sporadic. A typical picture is one or two individuals, affected a week apart, out of the mob of 30-50 head (that is 1-5% of the usually sized mob of traded steers). On three occasions a much higher portion of the mob was affected (eg dozen out of 30). In these multiple animal cases, some clinically affected individuals were negative on CFT, while other individuals were strongly positive. This suggests that the sensitivity of the CFT is not high.
Clinically, most cases are very mild with slight to moderate depression and stiffness. Slight drooling is also present. Apart from the epidemic of 2003 (see below), overt central nervous system clinical signs have been seen only once.
A useful clinical sign is that the stiffness is greatest when disturbed from recumbency and then reduces with exercise. This can be to the extent that clinical animals walked to yards as part of a mob may no longer show sufficient stiffness to be differentiated from unaffected animals.
Affected animals show severe loss of condition within a few days of clinical signs. Recovery is usually uneventful with clinical signs disappearing within a week. In contrast, the severe weight loss may take months to be recovered. Any deaths are usually due to affected animals being stranded away from water in summer (with the exception of the winter 2003 epidemic see below).
Long acting oxytetracyline antibiotic, if given when clinical signs appear, may not make a significant difference to the recovery time. However it appears to prevent the severe weight loss.
Excluding the epidemic of 2003 (see below) cases occur throughout the year with a small rise in June.
There is a suggestion that some general climatic change may trigger cases. On three occasions, SBE cases occurred on a second distant property within 10 days. On three occasions, cases in cattle and sheep on distant properties occurred within a short period.
In the winter of 2003 seven cases of SBE presented with a very different appearance to the cases recorded outside of that winter.
During February 2003 the Narrabri district saw the end of a 15-12 month long drought. The resultant restocking saw the introduction of large numbers of weaner yearling cattle from still drought affected southern NSW.
Between 23 June and 22 July the author diagnosed SBE by serology or had reliable reports from seven properties in a small area, known locally as Yarrie Lake. This area had not had cases in the previous decade and had different lighter sandy Pilliga soils and a flat topography.
The cases were all similar:
Of the sixteen cases of ovine chlamydiosis, all but one have presented as gait stiffness in cross bred or Dorper lambs with concurrent weight loss. Some lambs subsequently developed chronic arthritis. Several animals are usually affected from a mob. Like cattle, affected lambs lose their stiffness on walking. Treatment with long acting oxytetracycline improves recovery time and prevents both the severe weight loss and chronic arthritis.
In a single investigation, chlamydia was associated with stillborns in a small dorper flock. PCR excluded Enzootic Abortion of Ewes.
Chlamydiosis of sheep generally occurs in the same areas as SBE. Similarly, cases tend to occur throughout the year.