and depression may lead to underestimation of the extent of infection, thus resulting
in animals in the early onset of the disease failing to receive therapeutic interven-
tion. For early identification and treatment of affected calves, rectal temperature
exceeding 39.6C is often used as a threshold for initiation of antimicrobial therapy
(Potter 2007 ). Adjunct therapy with non-steroidal anti-inflammatorry drugs
(NSAIDs) can be useful in reducing pyrexia, providing analgesia and, most impor-
tantly, in limiting the acute pulmonary inflammatory response characteristic of the
BRD complex (Lockwood et al. 2003 ; Friton et al. 2005 ). The lung inflammation
can be so severe as to create a life threatening oedematous response.
Swine respiratory disease (SRD) is a complex condition involving a range of
viral, bacterial, and mycoplasmal infections in combination with various environ-
mental factors. Husbandary practices are also important in pathogenesis (Done and
White 2003 ). In the UK, it has been estimated that up to 80% of lungs may show
signs of enzootic pneumonia and that 20% may show signs of either acute or
chronic pleurisy (Done 1991 ). Two of the most prevalent non-viral respiratory
pathogens isolated during SRD outbreaks areActinobacillus pleuropneumoniae
andMycoplasma hyopneumoniae, the causative agents of porcine pleuropneumonia
and enzootic pneumonia, respectively.P. multocidais also frequently isolated from
pneumonic lungs, often as a secondary agent. Atrophic rhinitis is caused by
toxigenicPasteurella mutocida type D, often in co-existence withBordetella
bronchiseptica.The latter agent causes the initial damage after whichP. multocida
attaches to the epithelium and releases its toxin (Done and White 2003 ). Therapeu-
tic approaches involve broadly the same antibacterial classes as indicated above for
BRD. In addition, drugs of the pleuromutilin group are also commonly used in the
control of SRD. The in vitro susceptibility of SRD pathogens to antimicrobial
compounds is presented in Table 9. Antimicrobial drugs are usually administered
orally via water or feed for ease of delivery on a population basis and to reduce
the stress of animal handling (see Chap. 1 for discussion of population pharmaco-
kinetics). It should be noted, however, that a pig suffering from acute respiratory
disease may not eat or drink initially, and those that do will normally be the animals
with least signs of disease. Inter-animal variation in dosage received may therefore
be considerable, and parenteral antimicrobial therapy, in these circumstances, will
be a more effective drug delivery method. The main pharmacokinetic properties of
parenterally administered antimicrobial agents are summarised in Table 10.
5 Conclusion
Notwithstanding the importance of vaccination, biosecurity and good husbandry
practices in animal disease control, the strategic and integrated use of chemothera-
peutic agents continues to play a cardinal role in mitigating the impact of many
infectious diseases on welfare, production and public health.
132 H. Benchaoui