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A longitudinal study of respiratory infections in British National Hunt racehorses
  1. J. M. Cardwell, MA, VetMB, PhD, PGDip(VetEd), FHEA, MRCVS1,
  2. K. C. Smith, BVM&S, PhD, FHEA, FRCPath, MRCVS2,
  3. J. L. N. Wood, BSc, BVetMed, MSc, PhD, MA, MRCVS3 and
  4. J. R. Newton, BVSc, MSc, PhD, FRCVS4
  1. 1Department of Production and Population Health,
  2. 2Department of Pathology and Pathogen Biology, Royal Veterinary College, Hawkshead Lane, North Mymms,
    Hatfield AL9 7TA, UK
  3. 3University of Cambridge
    Department of Veterinary Medicine Madingley Road, Cambridge,
    CB3 0ES, UK
  4. 4Epidemiology Unit, Animal Health Trust, Lanwades Park, Kentford CB8 7UU, UK
  1. E-mail for correspondence: jcardwell{at}

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Lower airway inflammation is a much investigated multifactorial problem in racehorses in training (Burrell 1985, Wood and others 1993, Burrell and others 1994, Burrell and others 1996, Chapman and others 2000, Holcombe and others 2001, Christley and others 2001a, b, Holcombe and others 2006, Wood and others 2005a, b). Studies in young (yearling to four-year-old) racehorses indicate that bacterial infections are more common and aetiologically more important than viral infections (Burrell and others 1996, Christley and ­others 2001a, Wood and others 2005a). Bacterial species consistently associated with disease are Streptococcus zooepidemicus (Streptococcus equi subspecies zooepidemicus), Actinobacillus/Pasteurella species, Streptococcus pneumoniae and Mycoplasma equirhinis (Burrell and others 1996, Wood and others 1993, Chapman and others 2000, Christley and others 2001a, b, Wood and others 2005a). Disease likelihood increases with increasing counts (cfu/ml) of these species. Other commonly isolated species such as non-haemolytic streptococci (NHS) and coagulase-negative staphylococci are not associated with disease (Wood and others 1993, Burrell and others 1996, Christley and others 2001a). Both disease and infection rates decrease with age, consistent with the development of immunity (Wood and others 2005a, b). Reduced disease rates with increasing time in training, adjusted for age, are reported (Christley and others 2001b, Newton and others 2003). Lower disease prevalence in four-year-olds in training for several years compared with those new to training (Chapman and others 2000) suggests that immunity develops with increasing time exposed to the training environment, rather than simply with age. The structure of the British National Hunt (NH) racehorse population allows further examination of …

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