LHPA district veterinarians approach disease investigations by progressively evaluating the likely significance of the disease event. At each stage of the diagnostic process the veterinarian must determine whether the investigation could be of:
1. National interest? (exotic disease)
2. State interest? (notifiable in NSW)
3. Regional interest? (alert needed for North Coast)
4. Farmer interest? (mainly important for individual farmer)
This case study provides an example of how this hierarchy is applied to a typical disease investigation.
In late October 2011 a dairy/beef producer at Rous (between Ballina and Lismore) reported the deaths of two Australian Illawarra Shorthorn (AIS) mature cows from a mob of 35 cows and calves. The death of these cows followed an acute illness characterised by a reluctance to move, drooling, mild scouring and inappetence. Stillbirth was observed in two other cows and two neonatal calves were also found dead.
The two cows had both calved a few days before they showed signs of illness. The cows had been moved to the 'less scrubby' front paddock a few weeks earlier to reduce the risk of tick paralysis in their newborn calves. There was no obvious access to chemicals. Pasture was a mix of kikuyu and naturalised grass. Potentially poisonous weeds such as fireweed, bracken fern, tobacco bush and red cotton bush were present on the property. Mechanical removal of some weeds was occurring on the property at the time of deaths. The sick cows were treated with a long-acting dose of oxytetracyline when signs of disease were observed but they still progressed to die the following day. A veterinarian was not contacted to undertake a post-mortem on these cows.
A further case of disease was observed after the initial two deaths. This sick animal (ID3) was examined and found to be pyrexic (39.9) and dehydrated, with a scant, smelly and mucoid scour. There was also a mild purulent discharge from the vulva. Examination of the mouth was normal. The cow was not lame and no lesions were apparent on the feet. The urine was dark coloured but not red. A bacterial gastroenteritis or pyelonephritis was suspected and the farmer was instructed to administer an antibiotic he had on hand (ceftiofur). The cow died the next morning.
The liver was enlarged by at least 25% with rounded edges and mild accentuation of the normal lobular pattern. The carcass was mildly icteric. The jejunum and ileum were gas filled and in some areas contained petechial haemorrhage. The epicaridum had significant echymotic haemorrhages. Similar haemorrhages were evident on the pleural surface of focal areas of the caudal lung lobes. While not grossly swollen, the spleen had a 'jam' like consistency on the cut surface. The mammary gland had no evidence of acute mastitis.
Blood, faeces and urine were collected and submitted while cow ID3 was alive. A normal haematocrit and platelets were reported. There was an inflammatory leukogram including moderate toxic change within neutrophils. The cow had reduced renal function because the urine was minimally concentrated despite the cow being azotaemic. Increased bilirubin and GGT indicated a hepatopathy. Hypochloraemia was considered to be from upper gastrointestinal stasis. Neither Yersinia nor Salmonella were cultured from the faeces.
A range of fresh and fixed samples were collected during the necropsy on ID3. Liver and intestinal samples were negative on selective culture for Salmonella and Yersiniosis. Histopathology confirmed splenic congestion and extensive epicaridal haemorrhage. Autolysis made examination of intestinal samples difficult but there appeared to be haemorrhages in the submucosa and mucosa. Moderate multifocal acute hepatic necrosis was reported.
Originally the producer in this case reported to the North Coast LHPA that cows had been observed drooling and reluctant to move. At the National level this syndrome could suggest Foot and Mouth Disease (FMD). FMD was excluded by the clinical examination and necropsy on ID3 especially the mouth examination.
The jaundice, splenic changes and acute illness could be suggestive of Tick Fever, a notifiable disease in NSW. The lack of anaemia and parasites on haematology excluded Tick Fever and Theileriosis. A peripheral blood smear had been collected from ID 3 but was not submitted following the lack of anaemia and upon advice of the pathologist. The haemorrhagic changes in the intestines prompted the pathologist to suggest that arsenic poisoning could be considered. History and environmental examination suggests it is not likely.
The gastrointestinal signs suggested that Yersiniosis was a possibility. This is a disease of regional significance and the North Coast LHPA sends out warnings when the disease is being seen frequently. However cases of Yersiniosis are rarely seen after mid October and the culture was negative.
The cow with ID 3 died from multi-organ dysfunction and possible disseminated coagulopathy. The most likely diagnosis behind the death of ID3 and the other cows is salmonellosis or some other gram negative sepsis. This suspect diagnosis is supported by the history of recently calved cows, stillbirths, gastrointestinal signs, pyrexia, toxic neutrophils, necrotic hepatitis and intestinal haemorrhage. The recent movement of the cattle had mixed different cattle together and placed them near the old dairy and yards where there is a much higher environmental contamination of faeces. Spread of salmonella from a stressed carrier or from point source environmental contact could be hypothesised. Salmonella has a high mortality rate and may not always be responsive to oxytetracyline. The negative salmonella culture does not rule out the infection, especially as only one animal was tested.
The farmer in this case has been advised to increase stock observation, especially around calving. The case was discussed with the farmer's private veterinarian who would be able to prescribe Trimethoprim-sulphonamide should any stock display signs of fever. If signs of disease were to continue without a response to antibiotics the farmer has been asked to recontact the LHPA. No further illness has been seen up to late November, four weeks after the death of ID3.