Abortion storms are not a common occurrence in the northern slopes of NSW. Investigations into low calf numbers generally involve looking into the causes of failure to conceive, other causes of embryonic and foetal loss, and then looking into losses between calving and weaning. Similarly, disease due to Salmonella is uncommon in this region, presumably due to the more extensive nature of beef cattle enterprises and the relatively dry conditions experienced here over the last 10 years. There have been sporadic cases of salmonella in young animals but there are no records to indicate any previous cases of abortion due to Salmonella. Of the Salmonella organisms known to occur in cattle and cause disease, Salmonella chester is relatively unknown and a literature search found no evidence of it as a cause of abortion in cattle.
A mob of 80, mostly 4yr old, cows that were pregnancy tested in-calf, were purchased and moved 200km to a 4000 acre property between Bingara and Narrabri. The property has had previous investigations into sub-fertility which revealed a marked selenium deficiency and possibly less significant copper deficiency at the time. These cows were moved to this property in April and subsequently given a 7-in-1 vaccine and a pestiguard vaccine. They were revaccinated with 7-in-1 approximately 4 weeks later.
The cows started calving in July and of the first 15 calves, 13 were found dead. Of these 13, 12 were considered late term abortions and 1 a mid-term abortion (approximately 6 months gestation). Veterinary advice was sought from the local private practitioner who performed an autopsy on an aborted foetus. Coincidently this foetus was the mid-term aborted foetus. The private practitioner referred the clients to the LHPA for further work up and subsequently they found another aborted late-term foetus.
Of the 13 cows that had aborted, 8 cows were briefly examined and bloods taken. A significant number of these cows had temperatures above normal and were suffering from retained afterbirth. There were no other abnormalities detected in the cows. No pregnant cows were examined at the time but they were observed to be well.
Causes of late term abortion include leptospirosis, neosporosis, salmonellosis, pestivirus, akabane, trichomoniasis, bovine vibriosis, IBR (exotic strain) and a range of environmental and feed related toxins. Given that almost all of these abortions were late term, in the first instance it was prudent to exclude all but neosporosis, leptospirosis and salmonellosis. The recent vaccination for leptospirosis made it easy to exclude this also although it remained a possible diagnosis.
Fresh liver, spleen, lung and foetal stomach contents were taken as well as a full range of fixed tissues, including the brain. Initially an IgG test was perfomed on pericardial fluid followed by a PACE test for pestivirus, culture of the stomach contents and histopathology of the brain to look for lesions consistent with neosporosis. Further testing for pestivirus, akabane, leptospirosis and salmonellosis proceeded from the initial results.
Histopathology on the calf that was autopsied by the private practitioner reported a hepatopathy consistent with either vibriosis or salmonellosis. Nothing was cultured from the fresh stomach contents of this calf. There were no other significant findings
The calf autopsied by the North West LHPA had a negative IgG test, a negative PACE test, no lesions consistent with neosporosis in the foetal brain, however, culture of the fresh stomach contents was positive for salmonella. Further typing of the Salmonella by the Institute of Medical and Veterinary Science Adelaide confirmed the type as S. chester. Further serological work on the cows was done and revealed recent pestivirus exposure (three samples tested with results of two samples at level three antibody and one sample at level two antibody), akabane seronegative and leptospirosis titres consistent with vaccination. In addition the stomach contents were negative for spirochaetes and two swabs from cows were examined for trichomoniasis with no positives.
Further serology for Salmonella was ordered for five out of eight blood samples from the aborted cows. Four samples had positive titres to S. typhimurium, with two of the samples expressing high titres, and all samples expressing mild positive titres to S. dublin.
From the results it seems that exposure to an infection with S. chester is the most likely cause of the abortions. A paddock inspection was not performed and so an environmental source of Salmonella has not been identified. Pestivirus could not be totally ruled out based on the results but the pattern of abortion does not seem to fit with pestivirus.
S. chester is a group D Salmonella, which is the same group as S. typhimurium. Therefore, the high titres to S. typhimurium, which will have crossover reaction to serological testing, are consistent with infection. The mild titres to S. dublin are also consistent with exposure to S. chester, as S. dublin, a group B Salmonella, shares some minor antigens with Group D and could cause some serological crossover reaction.
The question remains in this case about the source of the Salmonella and a possible source is increased shedding from a carrier animal around the time of calving or abortion may have been the source of contamination. However, Salmonella can also be spread by wildlife, including birds, by feral animals including pigs, and by carrier animals within the herd. S. chester has been isolated from cases of clinical enteritis in pigs. Birds or feral pigs could have contaminated the cattle environment such as their water source.
The water source in this case was from dams but the owners noted that, despite the overall dry conditions this paddock did contain some swampy areas. Notably the owners moved the cattle to another paddock following the property visit by the North West LHPA. That the outbreak stopped after this movement supports a location within the paddock as a key source for the outbreak.
Subsequent to the investigation only one further calf was found dead but it was not submitted for autopsy or testing. Calving proceeded normally after this period and as of September 2011 there were forty healthy calves on the ground.