Analysis: Badgers and bovine TB. (Or, Everything eventually comes back to parasites)

Badgers! Image via BBC News website

I’ve been interested in the ongoing debate around the culling of badgers in the UK as a control method for bovine tuberculosis (Mycobacterium bovis). European badgers (Meles meles) have been known to be infected with bovine TB since the 1970s, and other British wildlife have also been identified as carriers, e.g., deer, foxes and rats (Gallagher & Clifton-Hadley 2000). Close proximity of badgers and cattle can result in bovine TB being spread back and forth, usually via eating contaminated grass, or inhaling bacteria released via aerosol (e.g., sneezing). Bovine TB is an important disease of livestock, with around 4% of the national cattle herd infected. It has cost the UK around 500 million pounds to control bovine TB over the past 10 years (stats from www.defra.gov.uk).

Now, that ongoing debate about culling. The UK government wanted to conduct a pilot study to see how culling of badgers would reduce rates of bovine TB, but the plan is currently being reviewed by the High Court. Wales has already decided against culling, instead opting for vaccination of badgers to reduce the spread of the disease. Vaccinating would take longer and involve higher costs for materials and manpower than culling, which would be faster and cheaper. So why not just cull?

Because badgers are territorial, and have varying sized home ranges. In areas where badgers are not culled, they move around less, whereas badgers who have fewer neighbours are more likely to move around. So, if an infected badger is not culled, but its neighbours are, it may end up with a larger home range and still be busily infecting any cattle that also exist in its range. Modifications to badger home ranges caused by culling strategies that remove small proportions of badger populations means that it is less likely that long-term benefits on cattle health will be observed (Woodroffe et al. 2006).

A key aspect of control is understanding the movement of the pathogens of the disease through a population. Cows can infect badgers TB, and the badgers can return the favour to other, uninfected cattle. Testing cattle herds for the presence of bovine TB is one measure that can be used to monitor the spread and prevalence of the infection, in order to manage it. Identification of carriers of TB is conducted by the single intradermal comparative cervical tuberculin (SICCT) test (Claridge et al. 2012), where a small amount of tuberculin (= M. bovis bacteria) is injected under the skin of the neck of a cow (or person). The immune systems of infected cattle (or people) will mount a response in the region of the injection, causing swelling.

Now comes the part about the parasites. One pathogen that has been implicated in compromising the sensitivity of the SICCT test is the cattle liver fluke, Fasciola hepatica. This fluke is common in the UK, and has had quite a spectacular increase in distribution over the past 10-20 years (due, in part to climatic changes) (Kenyon et al. 2009). Infection with F. hepatica causes an anti-inflammatory response in the host, which not only increases host susceptibility to infection with other pathogens, but would also interfere with tests that involve the immune system function as an indicator. New research published in Nature Communications indicates that cattle infected with F. hepatica are less likely to produce the typical immune system response to the SICCT test, thus producing a false-negative result (Claridge et al. 2012).

What on earth does this all mean?

This means that, when considering appropriate control of a disease such as bovine TB, it is important to understand as much of the problem as possible. It is well-known that badgers can transmit TB to cattle, yet a straight-out cull may not be the answer as this may actually compound the problem via the changes to the movements of the un-culled badgers. Further, understanding the infection rate and prevalence of cattle infected with TB is also integral to controlling the spread of the disease. Cattle that are falsely negative for TB because their co-infection with F. hepatica masks the immune response of the SICCT test may be moved around and potentially infect new herds, or badgers. Culling wildlife is always controversial. This is an example of how important it is to understand all the interacting factors of a particular problem before making a decision.

References

Claridge et al. (2012) Fasciola hepatica is associated with the failure to detect bovine tuberculosis in dairy cattle. Nature Communications 3, doi:10.1038/ncomms/1840

Gallagher and Clifton-Hadley (2000) Tuberculosis in badgers: a review of the disease and its significance for other animals. Research in Veterinary Science 69, 203-217.

Kenyon et al. (2009) Sheep helminth parasitic disease in south eastern Scotland arising as a possible consequence of climate change. Veterinary Parasitology 163, 293-297.

Woodroffe et al. (2006) Effects of culling on badger Meles meles spatial organisation: implications for the control of bovine tuberculosis. Journal of Applied Ecology 43, 1-10.

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