Do mild winters lead to high TB?

Introduction

This page examines how bovine TB levels correlate with average monthly temperature in Southern Ireland to see if there is a link between mild winters and high levels of TB.

Southern Ireland TB levels

Unlike in England, Wales and Northern Ireland, TB levels in Southern Ireland have remained high ever since herds became atested in the early 60's.


During the 50 years since then, TB levels have remained level except in the mid 70's, end of the 80's and end of the 90's. In the mid 70's there was a veterinary strike which interupted testing and was likely to have caused the variation in disclosed TB levels³. At the end of the 80's and 90's however TB levels jumped up and then gradually recovered. As can be seen in the graph below, particularly mild winters occurred in 1989/90, 1998/99 and 2007/08 according to average monthly temperature¹.

The image below is a thumbnail. Click it to see a larger, more detailed graph.



For each of these mild winters, TB levels rose.

Now let us look at it the other way around. Do cold winters cause low TB? Low TB occurred in 1979, 2004 and 2006. A cold winter occurred in 1979 but not in 2004 and 2006. In addition to this very cold winters occurred in 1984/85 and 1985/86 but this did not result in low TB in 1985. How about 2009 to 2011 when 3 consecutive cold winters occurred? It would appear that TB levels dropped. However from about 2000 onwards a large number of badgers started to be culled reaching a maximum cull rate from 2008 onwards. Low TB could be due to badger culling. Let us look at England and Wales to take a look at the impact of these cold winters in the absence of culling.

England and Wales TB levels

In England and Wales, the cold winters from 2009 to 2011 did indeed appear to have caused TB levels to level off and perhaps drop in the case of Wales although the drop in Wales may be due to the delayed benefit of adopting 1-year testing throughout the country.

It would appear from TB levels in England, Wales and Ireland that cold winters may eventually cause TB to reduce if a sufficient number of them occur together but the association does not appear to be strong.

Impact of increased testing in Wales

The following graph shows the impact of increased testing on the number of TB restricted herds in Wales. Unlike in previous graphs the point is shown at the end of the year rather than midway through.

How could mild winters cause TB levels to rise?

Transmission of bovine TB is very complex so the reason why TB levels may be dependent on temperature is not clear. However milder winters and longer springs/summers favour badger survival and increase the numbers of cubs born the next spring².

The following is an extract taken from Reference 4.

The key is in mild winters. When the average January temperature is 2.5 C, the average weight of the badgers is 8kg. When the average temperature is 6.5 C, the average weight is 10kg. The reason for this difference is that, in mild winters, when the soil is not frozen solid, badgers can still forage for worms at night. When the weather is colder, they can't get to the worms. They must live off the body fat they stored the previous autumn. This is even more important for the females, which might be pregnant during the winter months. If they have plenty of food, they are much more likely to produce healthy cubs and be able to feed them well. If winters are short and mild, so much the better, as more cubs are born. (Of course, if the following summer is dry, many more will die!) It also seems that, in good years, more males are born, but in poor years, more cubs are females. This might also happen in other species, but no one is quite sure why. So: Mild winters lead to increasing population. In dry summers, more badgers die. At the moment, this seems to be just in the badgers' favour, but, if summers carry on becoming hotter and drier...

The above account is a simplified view of a badger's dependence on winter temperature. The different account given in Ref 6 implies that a badger has a remarkable ability to adapt when winters are cold and can go for long periods without emerging from the sett.

How mild winters can lead to increased TB levels is subject to speculation. However when the density of a badger population increases, the opportunity for badger-to-badger and badger-to-cattle transmission of bovine TB also increases.

How well can M bovis survive cold temperatures

M. bovis is very resilient. Survival is affected greatly by temperature and moisture. When exposed to summer temperatures, sunlight and drying conditions, the bacteria dies, but away from sunlight and in moist conditions in soil and manure, M. bovis can survive for many months, especially in the cold.⁷

Potential for badgers to become as populous in Cumbria and Scotland as in South West England

The following graph compares temperatures in Scotland when downloaded from Ref 5 with those in South West England and South Wales.



Evidently minimum temperatures in South West England and South Wales are about 1.5 to 2 degrees centigrade warmer than in Scotland. If badger populations are largely influenced by minimum temperatures reached in winters, this perhaps could be used to estimate the number of years needed for climate change to increase temperature before badgers become as populous in Scotland as they are today in South West England. It should be noted however that habitant and abundance of food is considered to be a prime driver for influencing badger populations.

The following graph shows that there is only a one degree difference between minimum temperatures in SW England & S Wales and NW England & N Wales where the cattle-dense and largely-unaffected counties of Lancashire and Cumbria lie.

Conclusion

A succession of mild winters is usually followed by high levels of TB in cattle. Although rather a tentative and an incomplete explanation, one possible factor in the link between mild winters and increased TB levels (if there is one) is the large increase in badger density which mild winters bring and increased opportunity for badger-to-cattle transmission.

References

1. Met Office Hadley Centre Central England Temperature Data (cetml1659on). Hadobs. Hadcet.
2. CLIMATE CHANGE: the impact on biodiversity. 2006. Earthwatch Institute.
3. Veterinary handbook for herd management in the bovine TB eradication programme. M Good et al. Department of Agriculture Fisheries and Food. Ireland. June 2010.
4. BADGERS AND CLIMATE CHANGE - A STUDY OF JUST ONE ANIMAL. Durham County Badger Group. 2012.
5. Met Office climate summaries in defined regions. Datasets. UK and regional series.
6. Wildlife Online - Natural History of the European Badger. Updated: 6th August 2010.
7. Frequently Asked Questions about Bovine Tuberculosis. MSU Extension. Michigan State University. Posted 31 May 2013 by Phil Durst.