Historically infection by Chlamydia spp has been a relatively infrequent diagnosis in the southern Riverina. When it has been diagnosed it has been seen as a sporadic infection of young cattle. Morbidity has usually been less than 5%. Affected animals have presented as febrile and lame with variable swelling and pain in most joints, occasionally blind and sometimes with other nervous signs such has tremor. In the absence of treatment the case mortality rate was usually well over 50%. Treatment with oxytetracycline usually produced cure rates of over 75%.
Since 2011 Chlamydia spp have been associated with a syndrome which, although the clinical picture remained similar has a much wider distribution and higher morbidity. In the affected herds new cases have continued to occur for some years after the initial incident. It also appears to be more prevalent in dairy cattle.
A typical irrigated dairy farm in the southern Riverina running some 300 cattle on 130 hectares.
The first incident was in 2010 in a pen of 8 six-month-old Holstein heifers. Affected animals presented as lethargic and lame and eventually unable to stand. Initial treatment for hypocalcaemia produced no response and 7 of the 8 died.
Diagnosis was based on seroconversion.
Approximately 30 other heifers in closely adjoining paddocks appeared to be unaffected.
Since the initial incident subsequent cases have been mainly in adult cows. It usually affects 2-4 animals at a time and seems to follow a rainfall event. Affected animals present as lethargic and sore in the joints with concurrent mastitis. Some have had nasal and ocular discharges and some have appeared dull and sleepy. If treated early in the course of the illness (as soon as they lag behind coming in for milking) with standard doses of oxytetracycline they recover quickly and fully. If the treatment is delayed for several days they usually abort some 3 weeks later and if they are not treated before they go down they usually die.
Seroconversion was usually rapid but ephemeral with acute sera having titres up to 128 but falling several dilutions within weeks.
Only one animal was observed with neurological signs, notably blindness. This animal was treated and survived but the blindness persisted and it was destroyed.
Young animals do not appear to be affected in significant numbers in recent years: none of the latest calf drop (spring 2012) and only 2 from the autumn 2012 calf drop.
Another irrigated dairy farm approximately 20km distant from the farm above.
During the winter of 2010 the weaned calves were affected. There had been an increase in pinkeye in this group some time beforehand but it did not seem to be related to this incident. The weather was fairly wet at the time.
Approximately 30 of 150 4-7 month-old heifer calves were affected. They were lame and febrile. Treatment with oxytetracycline had variable results. Once an animal became unable to stand it did not survive. 5 of the affected calves died.
Diagnosis was based on seropositive acute sera.
Some time later two adult cows went down with apparent polyarthritis. They had low Chlamydia titres at the time of the episode but were euthanized.
There has been no recurrence of the calf syndrome.
Another dairy farm. Calves from 2-4 months old were febrile with nervous signs and some went down. They responded to oxytetracycline therapy and convalescent sera ranged from 8 to 128.
No similar syndrome was observed before or since.
Several other dairy farms have had individual adult animals that have been affected with lameness, recumbency, swollen joints, milk drop and strong seroconversion. One appeared partially paralyzed.
Historically the main syndrome associated with Chlamydia in cattle in the Riverina has been Sporadic Bovine Encephalomyelitis (SBE) with low and sporadic incidence and affected animals being blind, lame, febrile and with swollen joints. In the absence of treatment mortality was high but most affected animals responded to therapy with oxytetracycline.
Sherwin (1980) in a review of Chlamydial infections reported that polyarthritis was sporadic in newborn calves with weakness, fever, swollen joints and death within two to ten days. She did not report polyarthritis in weaned calves or adult cattle.
Reggiardo et al. (1989) reported that in the southwestern USA dairy calves were affected by a syndrome involving polyarthritis with high morbidity and mortality during the first few months of their lives which they diagnosed as being caused by Chlamydia.
Dannat et al. (1998) reported that in the United Kingdom they were seeing a syndrome associated with Chlamydia in adult dairy cows producing reduction in milk production, nasal discharge, anorexia and subsequent abortion.
These reports match some but not all of the signs seen in the Riverina in recent years and further investigation of the epidemiology, pathogenesis, clinical signs and pathology would be useful
Much of this data has been provided by the observant dairy farmers of the Finley area.