Bovine TB in foxhounds
Back in 2017 there were several articles in the press on the subject of TB in foxhounds, specifically at the Kimblewick hunt. This resulted in an investigation by APHA, the results of which were published in a recent paper in the journal ‘Transboundary and Emerging diseases’’.
The publication of this paper resulted in a second wave of articles in the press and (rather predictably) a second wave of discussion on social media, much of it questioning whether fox hounds may play a significant role in the transmission of TB to cattle. The paper is open access, so available for anyone to read (click on the link above), but I still thought it might be useful to produce a quick summary of what the paper says and what the results mean for the spread of TB in the UK.
What did the study involve?
The investigation was triggered because a hound was put down (it was suffering from anorexia, excessive thirst and urination) and post mortem suggested that this animal was infected with Mycobacteria. APHA then became involved and confirmed that the animal was infected with Mycobacterium bovis (which causes TB in cattle), specifically genotype ‘10:a’.
Following this a “test and cull” policy was applied at the kennels (test the hounds and remove those with a positive reaction) with the aim of containing the spread and removing any other potentially infected animals. Hounds were tested using two tests:
- IGRA (interferon gamma release assay), essentially a modified version of the interferon gamma or blood test used on cattle (click here for factsheet on the gamma test).
- DPP (dual path platform) Vet TB test an antibody (serological) blood test developed for deer but regularly used by APHA to test badgers.
All dogs testing positive for either of these tests were euthanased and then subjected to gross post-mortem examination (PME – carcass inspected for lesions). Samples from some of these euthanased animals (but not all) were also taken for mycobacterial culture (where M. bovis is grown from samples in the lab).
Both the IGRA and DPP test for an immune response which indicates that the animal has been exposed to M. bovis. Although, it should be noted that neither of these tests have been specifically validated for use on dogs.
What where the results?
The results for this study are quite complicated as different numbers of hounds tested positive for various combinations of IGRA, DPP and culture. Overall the results suggest that large numbers of hounds at the kennels were infected with M. bovis and several were in advanced stages of the disease, with the bacteria cultured from several organs and also from urine. Several people in contact with the hounds were also tested, and one was diagnosed with TB (it is not possible to know if this was from the hounds or other sources). The main results are sumarised in the infographic below:
The testing at the kennels ran from the end of 2016 to July 2017. Following the results above it was deemed that the remaining 57 hounds were unlikely to be infected/infectious, so voluntary restrictions at the kennels were lifted.
How did the hounds become infected?
The hunt operates across several counties in the edge and low risk area in England, where TB incidence in cattle is relatively low (compared to the high risk area – for a map of the risk areas click here). It is difficult to identify an exact source of infection, but various options were considered and ranked as part of an epidemiological investigation conducted by the authors of the study.
The potential sources of infection were:
- Movement of infected hounds from other kennels within the high risk area. Hounds at these kennels may have become infected from eating fallen stock (raw meat from culled cattle) and then carried the infection with them. This was viewed as the most likely route.
- Feeding fallen stock infected with M. bovis. Rather than the infection moving in from other areas it is possible that the hounds at this kennel were directly fed meet from cattle infected with M. bovis. This was viewed as possible, but relatively unlikely, based on the TB history and status of where cattle fed to the hounds were sourced.
- Exposure to infected wildlife or cattle (either directly or by M. bovis contaminated environment). According to the authors, most of the area operated by the hunt has low levels of TB in cattle and no evidence of TB in wildlife (although little data is available on this). This was viewed as an unlikley source of infection, partly because it is very rare to find dogs infected with M .bovis, even in areas with high levels of TB in badgers and cattle (which suggest that transmission risks via these routes are low).
It seems likely that the feeding of infected fallen stock to hounds was a likely source. This has resulted in the following changes outlined in the paper….
“As a proportionate risk mitigation strategy, DEFRA has introduced tighter restrictions on the collection and feeding of fallen stock to hounds in registered kennels. Since 10 October 2017 the feeding of offal from livestock species to dogs from recognized kennels or packs of hounds has been banned in England (Anonymous, 2017). Hunt kennel operators must also carry out additional examinations for lesions of TB in fallen stock originating from “high risk” premises”
What do the results of this study mean?
TB has been previously found in small numbers of dogs, but the general assumption has been that dogs are dead end hosts which are unlikely to spread infection further. This study challenges this previous assumption – it provides evidence of transmission between dogs and demonstrates excretion of M. bovis by infected animals. BUT it is important to stress that this study is highly unusual and is the first example of such a large outbreak of TB with evidence of spread through a dog population. It may be that this was the result of the specific conditions at these kennels.
This study therefore raises the possibility that dogs may be able to infect other species such as cattle or wildlife. It also highlights that current practices of feeding fallen stock to dogs has a potential risk of disease transmission. However, it would be wrong to conclude from this one study that large numbers of hounds are infected in the British countryside and are contributing significantly to the hundreds of TB breakdowns that occur every year. If other kennels are infected to the same extent as in this study (with multiple animals with clinical signs of TB) then this should be detected, as happened here. Even if there is a risk from infected hounds, it seems likely that this risk is localised and significantly lower than the risks from other infected cattle, or from infected wildlife (principally badgers), both of which are far more numerous, widespread, and are the main sources of TB in the UK.