Mass mortalities in cattle due to botulism occur sporadically usually associated with feed contamination. Management of these outbreaks is a major logistical exercise and highly traumatic for all involved. While botulism is not notifiable in Australia animal carcasses have to be disposed of ensuring environmental impacts are minimised and a range of government agencies are usually involved in management of the outbreak response. The emotional impact for all involved is significant and agencies with expertise in counselling and support services are needed to assist with recovery.
We report on a large outbreak of botulism in a dairy herd during the summer of 2018 and how that outbreak was managed.
In early January 2018 veterinary assistance was sought for around 6 recumbent milking cows in a 450 pasture based dairy herd on the NSW south coast. One of the recumbent adult cows had died by the time the veterinarian visited the property and was autopsied with non-specific gross findings. Clinical pathology showed elevated enzymes but normal mineral levels.
All the affected cows were in the milking herd and had been fed silage (both maize silage and round bale silage) using a mixer.
A preliminary diagnosis of botulism was made and a range of laboratory samples were submitted for histopathology and toxin testing.
Three days later the number of dead cows had increased to 10 and there were a further 20 recumbent cows and by 7 days the deaths had reached 80 and there were 150 animal showing clinical signs. The last animal died around day 30 and all up 275 adult cows died.
Autopsies were carried out on a further 7 cows and samples submitted for laboratory testing. Botulinum toxin Type D was detected in several samples from one of the sampled cows.
The clinical cases were generally similar. The animals did not have fever and they were found recumbent. In the initial stages of the outbreak animals were mostly in lateral recumbency having spent only a short period in sternal recumbency and death from respiratory failure followed after a short period usually within 1-2 days. Later on in the outbreak animals were in sternal recumbency for longer periods and towards the end of the outbreaks some of the cases could be lifted and walked short distances but always relapsed becoming recumbent again.
A frequent observation of affected cows in sternal recumbency was that they were often seen in the “frog leg positon” where the hind limbs were stretched out behind the cow presumably to make breathing a little easier.
It was also noted that none of the cases examined showed evidence of tongue paralysis a common finding in some clinical botulism cases reported by others.
The outbreak occurred in early January when daytime temperatures reached 35 degrees Celsius on some days. It was impossible to provide shade for all the cases so access to drinking water was a priority. Animals alive but in lateral recumbency were euthanased. Those in sternal recumbency were treated with symptomatic therapy including fluids, metabolic treatments and activated charcoal given by stomach tube. Despite symptomatic treatment no affected animals survived in this outbreak.
At the start of the outbreak when the number of dead cattle was relatively low the herd managers made the decision that animal carcasses would be dealt with on property. The options for on property disposal include burning, composting, burial and surface disposal. As the property was located in a built up area leaving the carcasses above ground to decompose would generate odours and exudates so that option was excluded. Burning was also discarded because of the location, the large amount of fuel required, the time of the year and the pollution aspects associated with burning 275 carcasses weighing over 500 kg each. Composting was an option but would require extensive organic material (approx. 4000-5000 m3) and is a technique that has not been tested in large scale livestock outbreaks although it has been used in managing poultry carcasses successfully.
The owners initially opted for onsite burial and dug pits to accommodate the carcasses which were moved by machinery and covered over with soil. Unfortunately the water table made on farm disposal too much of an environmental risk and after consultation with State government agencies a decision was made to transport the carcasses by road transport to a nearby approved landfill site.
Digging up and relocating the carcasses already buried was completed over a 7 day period using road transport using a private contractor. The remediated burial site was inspected by the local council.
Vaccination of the herd was undertaken before botulism was confirmed at around 10 days into the outbreak. A short acting vaccine was used and a second dose of vaccine administered 2 weeks later. The onset of clinical signs in the last clinical case occurred before administration of the second dose of vaccine. Some level of protection would be expected from the priming vaccine dose but full immunity would not be expected until at least 5 days after the second dose of vaccine. In this case report where the exposure date is known and no further exposure occurred vaccination had little impact on development of cases.
During the outbreak extensive laboratory testing was undertaken to exclude alternative diagnosis and to confirm botulism. Ten cows were autopsied over the course of the outbreak and a wide range of fresh and fixed samples submitted. The gross and histopathological findings were non-specific as was the clinical pathology. Two samples of gut contents from 1 cow were positive on PCR for Botulism toxin and genetic analysis of the PCR product was consistent with Toxin Type D. Thirteen samples were submitted for testing and only 2 of the samples were positive.
When the outbreak first started the owners were content to manage the situation in conjunction with their private veterinarian. The private vet discussed the case with the District veterinarian who provided technical assistance.
When the number of deaths had reached 50 NSW DPI were officially informed and a formal Emergency Animal Disease response was initiated and resources allocated to Planning, Operations and Logistics functions involving NSW using staff from a range of government agencies. Once deaths continued to climb particular attention was paid to ensuring adequate support services were available to the families. The importance of these support services cannot be underestimated. An After Action Review was undertaken to systematically evaluate what went well and what could be improved in the management of the outbreak and recovery.
Dr Kate Sawford provided the authors with information in preparing this report.