In the early winter of 2012 a Lismore private veterinarian referred a disease matter to the North Coast LHPA. The private veterinarian had been prescribing oxytetracycline antibiotics to sick cattle owned by a local beef producer. The antibiotics were being used in an attempt to treat diarrhoea mortalities in both young and adult stock. Cattle in the region are prone to yersiniosis and it is common practice to prescribe oxytetracycline to local farmers once this syndrome has been identified. The antibiotics were usually not effective on this farm.
The beef producer had cattle on a number of properties in the Lismore region. Cattle were moved between the properties but only infrequently. The diarrhoea and mortalities was limited to a mob of about 50 breeders kept on a property at Corndale. The breeders had been bred from a herd of dairy cows previously milked by the father before the dairy ceased operation in the late 1990's. Consequently, almost 50% of the breeders were over ten years of age and were of mixed dairy and beef genetics. The older cattle had been retained because their teeth were sound and they were producing a calf each year.
Since taking responsibility for the cattle,the beef producer was surprised by the high mortality rate of young calves and the occurrence of diarrhoea not responsive to anthelmintics or prescribed antibiotics. The mortality rate of calves less than four months of age was sometimes as high as 10%. An occasional adult cow had also died after a brief illness. This pattern of disease was also occurring before the beef producer took responsibility for the mob from his father. Undiagnosed mortalities and scours were therefore occurring for over ten years.
A 3 month old calf was examined and found to be dehydrated, febrile and weak. There was rectal straining with passing of stands of mucous. This calf had been treated with three daily dosages of oxytetracycline with no improvement. Bloods were collected for haematology and biochemistry,
Five adult cows, including the mother of the calf, were also examined. They were of moderate body condition with no detectable abnormalities.
Following the release of biochemistry and haematology results, the calf was euthanased and a necropsy performed. Necropsy findings included; swollen kidneys with a pale cortex and a mottled appearance to the capsular surface, a liver that appeared to be orange in colour than normal and had randomly distributed 1-4mm white-yellow circular foci. The intestinal contents were scant and mucoid.
An in-house calf scour test on faeces and mucus from the calf was negative for E.coli, Cryptosporidia, Rotavirus and Coronavirus. The calf was negative for pestivirus antigen by hair follicle testing.
Haematology and biochemistry on the calf revealed moderate leucocytosis, marked neutrophilia, marginal anaemia, severe azotaemia, hypokalaemia and hypernatraemia.
At the necropsy urine was collected and tested to have a urine specific gravity of 1.010. This result in combination with the severe azotaemia indicated renal failure.
Histopathology of the liver and kidney identified marked chronic active portal hepatitis with cholestasis and marked subacute tubulointerstitial nephritis. The pathologist commented that the pattern of inflammation seen in the kidney could be seen with leptospirosis or other bacterial infection such as Salmonella and that the liver changes were consistent with an infectious process.
A leptospirosis PCR on the urine was negative. No growth was obtained with bacterial cultures of the liver and kidney. A mixed growth was obtained with bacterial culture of the intestines. Salmonella, Yersinia, Campylobacter and Shigella pathogens were not isolated from this mixed growth.
Laboratory testing was undertaken on four randomly selected breeders and the mother of the sick calf. Three out of these five cows had Salmonella dublin antibodies in a Serum Agglutination Test ranging from 20 to 40. The mother of the sick calf had the highest reading (40).
Subsequent faecal cultures on five other healthy cows in the herd were negative for Salmonella.
Despite a lack of a positive culture the herd was tentatively diagnosed with endemic salmonellosis. This diagnosis was made from the history, serological results on the cows, histopathology and diseases excluded. The case was discussed with a pathologist at the NSW State Diagnostic Veterinary Laboratory who agreed that the disease was most likely being caused by Salmonella.
It was difficult to isolate Salmonella on this farm because antibiotic treatment of the calf would have affected its bacterial culture and unfortunately, management factors prevented the sampling of the cows most likely to be carrying the bacterium. If the opportunity arises bacterial culture will be undertaken on any future clinical cases.
Disease is likely persisting this herd as older carrier cattle shedding Salmonella in their manure, especially during times of stress such as calving (Parkinson TJ, Vermunt JJ and Malmo J, 2010). The wet muddy environment of the farm would favour its survival. Young calves would be most susceptible to infection. Sometimes under periods of stress adult cattle may be affected.
The farmer has been advised to gradually remove older cows from the herd especially if there is clinical or laboratory evidence that they are a Salmonella carrier. A commercially available Salmonella vaccine will be used in the herd. The findings were discussed with the private veterinarian who will prescribe Trimethoprim-sulfa for future suspect Salmonella cases.