Can M. bovis survive in silage?
TB transmission via contaminated feed
Bovine TB is caused by Mycobacterium bovis (M. bovis), but it is unclear how the bacteria spreads between hosts, whether that’s between cattle, or between cattle and wildlife. One potential route is by ‘indirect’ transmission, where an infected animal contaminates grass, stored feed, water or other parts of the farm environment, and then another animal interacts with this contamined material and becomes infected.
Studying these indirect transmission routes is very difficult, but research has shown that M. bovis can survive in different environments such as water, soil, and faeces and a range of stored feed types (Fine et al. 2011). Research from the United states has also shown that M. bovis can spread from infected deer to cattle via contaminated troughs and housing (Palmer, Waters & Whipple 2004). This is why current guidance in the UK is to keep badgers out of stored feed and housing to reduce opportunities for disease spread.
But what about silage?
Badgers may be attracted to open silage clamps, particularly those containing whole crop wheat or maize silage (click here for a list fo feed attractive to badgers), so it is advisable to reduce badger access if possible. BUT – silage could also potentially become contaminated in the field, either from infected badgers, or if contaminated slurry is spread on the field (click here for a factsheet about TB in slurry and faeces). Silage goes through an ensiling, or fermentation process, where oxygen levels are low and the pH becomes acidic. Previous reviews of the subject have highlighted that M. bovis could potentially survive in these conditions, but these reviews have also highlighted the lack of science in this area.
Recently a study was published in the United states by (Grooms et al. 2019), which directy addresses the question of M. bovis survival in silage:
Grooms et al. (2019) Survival of Mycobacterium bovis during forage ensiling. American Journal of Veterinary Research, 80, 87-94.
Given the interest in this subject, and the fact that this paper is a difficult to access journal, I have produced a summary of the paper below.
What did the study invovle?
The study involved experimentally adding M. bovis to samples of cut forage which was then ensiled in the lab.
Three different feed types were tested:
- Alfalfa – dry matter content at ensiling 36%
- ‘Mixed mostly grass’ – mainly orchard grass (not specified, but I believe this is cocks foot), with some alfalfa and clover. – 65% dry matter content at ensiling (this was higher than intended, the target was 40%)
- Maize (‘chopped corn’) – 32% dry matter content at ensiling Samples were spiked with M. bovis prior to ensiling, with samples arranged in replicate groups of six; four samples containing M. bovis and two with no M. bovis ( to act as controls).After each period of time the samples were then tested for M. bovis using two methods
Each group of six samples were then ensiled for one of ten different periods spanning 0 – 112 days (meaning 60 samples of each forage type). Samples (250g of each) were placed in thick vaccuum packed polythene bags, wrapped in black liners and and stored at room temperature (18-22°C) to simulate the ensiling process.
After each period of time the samples were then tested for M. bovis using two methods:
- Culture – M. bovis is grown in the lab
- PCR – tests for the presence of M. bovis DNA (click herefor a factsheet on PCR testing)
What did the study find?
This study had three key results:
- The study found that M. bovis could be cultured from Maize and Alfalfa silage up to two days into the ensiling process, and up to 28 days in grass silage. All samples tested after this period tested negative. The authors suggest that the longer survival in grass could be due to the higher (less acidic) pH recorded in these samples (pH was around 5 while maize and alfalfa was closer to 4). This could also be due to the higher dry matter content in the grass samples in the study.
- M. bovis DNA could be found in some samples of all three silage types up to the end of the study (day 112).
- None of the control samples tested positive on either culture or PCR
What do these results mean?
To quote the authors “these results suggest that properly ensiled forages would be an unlikely source for M bovis transmission to cattle”, as M. bovis could not be cultured after 28 days in any of the samples. M. bovis DNA was detected beyond this date, right up to the end of the study. However, it is not clear whether the bacteria are viable (ie if they could grow or infect an animal). One possibility is that the bacteria are ‘dormant’ and that changes in conditions in the future could make them infectious, but it is unclear if this is possible (and what condictions could causes this).
Although this study has some very interesting results, it is important to acknowledge that this is an experimental study, and no experiment is perfect. M. bovis was added at very high concentrations to the forage samples (tens of millions of backeria in each 250g sample), which is likely much higher than contamination would occur naturally. The primary method of testing for M. bovis (culture) is also not 100% perfect, so could potentially fail to detect the bacteria, even though it was present. It is therefore possible that M. bovis survival in natural conditions is longer or shorter than stated here. Nevertheless, this study provides the best information to date on the likely survival of M. bovis in silage.
Overall conclusion : The study suggests that properly ensiled forage is unlikely to be a source of infection in cattle, although the detection of M. bovis DNA up to the end of the experiment means that the risk cannot be totally ruled out.
The factsheet on TB survival in feed, water and soil has now been updated to reflect these new results.