CASE NOTES
An outbreak of anthrax at Bourke
Brian Hodge, District Veterinarian, South East Livestock Health and Pest Authority, Cooma
Posted Flock & Herd March 2011
Introduction
Anthrax occurs in a belt of country which extends from a line between Albury and Tocumwal on the Murray River, northwards through Narrandera, Condobolin, Nyngan and Bourke which seems to be the northern limit of its incidence. I had attended a number of smaller outbreaks of anthrax while I was in my own practice in Bourke.
The anthrax bacillus is a spore former and the spores can remain in a viable condition in the ground for many, many years. The incidence of the disease on sparse pasture is probably due to the ingestion of contaminated feed and soil and to injury of the oral mucosa facilitating invasion by the organism.
The incident
There was no history of an infection on the property in the past 80 years and numerous drafts of unvaccinated sheep had grazed this paddock without contracting the disease.
In this incident 1800 merino sheep were running in a red soil forested paddock during a fairly hot dry period. When the grazier phoned me on a Saturday afternoon I went straight out to the property and it seemed like most of the sheep in the paddock were dying. This is a far higher occurrence than in any anthrax outbreak which I had experienced. I took about 12 blood smears from dead and dying sheep.
Believing that the earliest possible control was necessary in such a big outbreak I immediately drove the smears to the Regional Veterinary Laboratory at Orange where Graham Bailey found anthrax bacilli on all the slides. This provided us with an official diagnosis so that control measures were implemented immediately. Within 2 days we received the vaccine and about 1,200 surviving sheep were vaccinated. In spite of the early vaccination eventually 1,200 of the original 1800 died, about 600 before vaccination and 600 after vaccination before immunity could develop.
There were enormous numbers of wild pigs throughout the area and I would estimate that 10 per cent of the carcasses were completely consumed by the pigs before we could burn them.
We did not have the machinery to bury any of the carcasses in the hard soil in such a large heavily timbered paddock so we burned them using six car tyres per carcass. We had a team of 4 personnel from the local fire group to burn carcasses. Before their work started, I insisted that everyone who had contact with carcasses consult with the local doctor and obtain a prescription for penicillin to protect them from the disease. The people who burned carcasses in the outbreak could easily inhale anthrax spores and a fellow working in the area could for example eat his lunch with carcass residues on his hands, even if he was told not to. Their clothes had to be washed in a strong antiseptic solution.
To my knowledge there have not been any more anthrax deaths in this paddock in the years since but I suspect that only vaccinated sheep are run there these days.
Discussion
The wild pigs on this Bourke property were ingesting an estimated 20 sheep carcasses each night.
I was worried that a few of the wild pigs with a large content of ingested anthrax might escape the paddock and die on other properties in the area, potentially spreading the disease. Although one property 2 properties away did contact me about 10 sheep in a mob of 215 which had died I did not diagnose anthrax. As well as the pigs there was a concern about the large number of eagles in the area. Anthrax bacilli have been found in the crops of eagles in some overseas outbreaks. Overall there was no apparent spread beyond the original paddock.
Being responsible for a large disease outbreak was a very memorable life experience for me. I believe that so many sheep died because there was an enormous load of anthrax bacilli which suddenly became infectious after 80 years. It was the biggest outbreak which I have heard of anywhere.
I particularly acknowledge the support of Wayne O'Malley who was the chairman of the Bourke RLPB in the year 2000 who offered to help in the outbreak. He organised the people who assisted us locally and also the large number of old tyres which were required. Sadly this was a great loss of sheep for the owner and he sold his property within a few years. Early in the outbreak he had hoped that his losses would not be too great. Many of his sheep which were vaccinated obviously died before the vaccine immunity could be established. Had each vaccinated sheep received antibiotic at the time I believe that hundreds more sheep might have survived. Vaccination would then have to be delayed until the antibiotic had cleared the system. Unfortunately such a strategy was not recommended at that time.
Reference
- Radostits, Gay CC, Blood DC and Hinchcliff KW (2000) Veterinary Medicine. pp 747-751 Diseases caused by Bacillus spp. Anthrax