Background Numbers and proportion of cases of bovine tuberculosis (BTB) in cattle are increasing, largely in the west country and Wales with few if any cases in the south and north east. Work by DEFRA demonstrates that TB in cattle is mutifactorial, as in humans, making some herds at greater risk of TB than others. Micronutrients could be one of these factors. Deficiencies, but also excesses, may cause secondary immunodeficiency and infection related morbidity in man. Different soil types contain different quantities of these trace elements and this will be reflected in pasture, hay and silage.
Objectives To determine whether certain rock and soil types predominate in areas with high incidence of BTB, and whether there is a case for investigating micronutrients in cattle.
Methods Information on cases of BTB and latest mapping of cases was provided by the Veterinary Research Laboratories (UK). Using information from the British Geological Survey and geological mapping, rock and soil types and their mineral contents were compared with density of cases of BTB.
Results There is a significant difference in the proportion of confirmed new incidents of BTB in the west of England (6.8/100 herds) and all other areas (p<0.001), and Wales (3.2/100) and the north (0.8/100) and east (0.3/100) of England (p<0.001). The rock types present in the west of England and Wales are rich in iron and aluminium while those in areas largely free of BTB are rich in calcium salts and oxides of silicon. Soils reflect the mineral content of the underlying rocks.
Discussion Iron deposits are common in areas with high numbers of cases of BTB, affecting quantities in pasture, hay, silage, and earthworms, a staple component of the diet of badgers, who are frequently blamed for this disease. Excess iron is associated with increased susceptibility to TB and more aggressive disease in man, and mycobacteria responsible for BTB need iron for their survival within the host.
Conclusions There is an association between BTB and high levels of iron in the soil. Further work is needed to determine levels of iron in affected cattle and its effect on immune responses.