The project aimed to obtain contemporary information on levels of TB infectivity (Mycobacterium bovis) present in the faeces of cattle which test positive (react) to the tuberculin skin test. Up until now, published scientific information on M. bovis in cattle faeces has been very old, dating back to a time when the disease situation was different, or reliant on scientific methods which do not directly measure live M. bovis bacteria.
The researchers collected faecal samples from 72 cattle naturally infected with M. bovis and from 12 cattle experimentally infected with M. bovis. The samples were subjected to microbial (microbiological) culture and Polymerase Chain Reaction (PCR) testing to look for evidence of M. bovis. The study found that levels of M. bovis bacteria in the faecal samples of TB-infected cattle were extremely low. There were no positive cultures from any of the naturally infected animals. A single M. bovis colony was cultured from one of the experimentally infected animals. A single PCR positive result was obtained from the faecal sample of one naturally infected test reactor.
Although the results of this research suggest that the risk of spreading TB through using slurry or manure as an agricultural fertiliser is lower than suggested in some historical literature, the Animal & Plant Health Agency (APHA) still recommends that farmers take a precautionary approach and follow the guidance on handling, storing and spreading slurry and manure in the TB Biosecurity Five Point Plan. The results of this research could inform a reconsideration of current risk assessments and guidelines on the disposal of manure and slurry from TB-infected herds. The full scientific paper is published in the Journal of Applied Microbiology.
This was a collaboration involving research teams and animals in the United Kingdom and the Republic of Ireland.
The research involved collecting faecal samples from 84 cattle in total including:
- 42 cattle, which reacted to the tuberculin skin test and presented with visible lesions typical of bTB at post-mortem examination. These animals were sourced from 29 different farms and sampled at a slaughterhouse in the west of England.
- 30 cattle, which reacted to the tuberculin skin test and the interferon gamma blood test. These animals were sourced from farms and housed at a Department of Agriculture, Food and the Marine research facility in Ireland.
- 12 cattle experimentally infected with M. bovis as part of research being carried out at the Animal and Plant Health Agency (APHA) in Weybridge. No animals were killed specifically for this study.
The researchers deliberately sampled naturally-infected tuberculin skin test reactors including cattle with visible lesions of TB, as well as a smaller sample of experimentally infected animals. This ensured that faecal samples were taken from a representative population of TB reactors in England and Ireland.
Polymerase Chain Reaction (PCR) is a molecular-based laboratory technique that involves detecting small amounts of the genetic material (DNA) that is contained within all living organisms. This is done in the laboratory by making millions of copies of a specific sequence of the target DNA, a process called ‘amplification’. The PCR used in this study could detect as few as 100,000 M. bovis bacteria in a gram of sample.
Microbial (microbiological) culture means attempting to grow any live M. bovis bacteria present within the samples, in a laboratory under carefully controlled conditions. Culture can detect as few as 1 to 10 M. bovis bacteria in a sample.
- M. bovis was not grown from any of the faecal samples collected from the naturally infected TB reactor cattle from the UK and the Republic of Ireland
- A single colony of M. bovis grew from a faecal sample from one of the experimentally infected animals. This is indicative of growth from a single bacterium detected in the sample
- M. bovis was detected by PCR testing in one faecal sample from a TB reactor from the Republic of Ireland out of four such samples from that animal tested by PCR. However subsequent PCR testing of a duplicate set of samples from that animal produced negative results.
The major limitation of this study is that it is very difficult to prove a negative result. Detecting bacteria at the evidently extremely low amounts found in a single sample is working at the very limits of detection. It is, therefore, feasible that there were bacteria present in numbers below these limits.
The results of this research show that the frequency of shedding of M. bovis bacteria (above the limits of detection) in the faeces of naturally and experimentally infected cattle from the UK and the Republic of Ireland, including those with visible lesions of TB and those without, is extremely low. The results suggest that the risk of subsequently spreading TB through cattle slurry or manure use on agricultural land may be lower than previously suggested.
The results of this research could be used to inform risk assessments and guidelines on the disposal of cattle slurry and manure on agricultural land.
Although the results of this research suggest that the risk of spreading TB via cattle slurry and manure use on agricultural land may be lower than previously suggested, APHA still recommends that farmers take a precautionary approach and follow the guidance on handling, storing and spreading slurry and manure in the TB Biosecurity Five Point Plan.
The TB risk posed by cattle slurry and manure depends on multiple factors which can vary between farms:
- Severity of the TB breakdown from which the slurry/manure originate
- Rate of shedding of M. bovis bacterium in cattle faeces (found in this research to be extremely low)
- Number of cattle in the herd shedding M. bovis bacteria in their faeces (shedding can be intermittent)
- Size of the herd contributing faeces to the muck heap/slurry lagoon
- Dilution factor
- Bacterial die-off / inactivation during the composting/storage process
- Method of storage, handling and spreading
- Storage time before spreading
- Environmental conditions and survival of viable M. bovis bacteria on pasture.