Cellulitis is a condition commonly seen in meat type poultry at processing characterized by extensive lesions of the skin and subcutaneous tissue associated with bacteria E. coli. This condition is the principal cause of carcass condemnations in the Canadian meat–type poultry industry. It is estimated that losses to the Canadian meat type poultry producers are in the vicinity of C$15 to 20 million annually. The economic impact of cellulitis is obvious but because this condition is associated with bacteria, issues concerning meat hygiene and potential health risk to the consumer have been raised. Due to the significance of the problems associated with cellulitis, finding the means for controlling this condition has been deemed a high priority.

A research project aimed at developing practical means for better control of the incidence of cellulitis in commercial broilers has been completed. This study has provided novel and practical information on the patho-physiology of this condition. The predisposition of broilers to pathogen invasion of the skin tissue, and eventual development of cellulitis lesions, is associated with insufficiency of the host's first line of defence mechanisms. However, the most important and far reaching observation is that the identified weaknesses (mechanical barrier, chemical, and cellular) in defenses have underlying metabolic causes.

It is noteworthy that all important risk factors identified in our previous study using experimentally induced cellulitis lesions, have been validated by the findings from this project in spontaneous cases of cellulitis in commercial broilers. However, a major step forward was the discovery of a diagnostic means for identification of broilers at high risk of cellulitis prior to slaughter. This added a new perspective to the investigation of etiological factors predisposing broilers to cellulitis.

The Saskatchewan study revealed that many broilers with leg bone degeneration are also at high risk to be affected by cellulitis lesions. Closer examination of the femoral bone cavity revealed that changes in bone marrow may be at the core of impaired defense mechanisms associated with poor functional quality of phagocytic cells, where degenerative changes in precursors of these cells were observed. The observation of degenerative changes in precursors of immune cells in bone marrow provides a logical explanation for the observed poor functional capacity of phagocytic cells in broilers. This apparent weakness of the broiler chickens' first line defence mechanisms is most likely associated with fundamental changes at the level of development and maturation of the immune system cells.

Biochemical analysis of the skin revealed that the protein content and profile of amino acid in the skin tissue differ between broilers resistant to cellulitis and broilers susceptible to cellulitis. Based on amino acid profile, it appears that impaired metabolism of skin matrix proteins is one of the key factors responsible for poor structural quality of the skin in fast growing broilers. Lowered content of amino acid and cross-linking molecules in the skin is indicative of systemic inadequacy, and therefore may be implicated in several metabolic, structural, and functional changes of the skin. The apparent deficit of some amino acids observed in broilers' skin and blood plasma is indicative of biochemical insufficiency of these metabolically important compounds.

Interestingly, further biochemical and structural analysis of bone degeneration patho-physiology revealed that the insufficiency in protein metabolism is most likely a primary etiological factor in the natural history of femoral bone degeneration. This indicates that generalised metabolic problems resulting in insufficient protein synthesis represents a systemic weakness underlying the aetiology of both cellulitis and femoral bone degeneration. The investigation revealed that many broilers with leg bone degeneration are also at high risk of being affected by cellulitis lesions. Degenerative changes of bone may help to explain the pathology of bone marrow, and taken together this provides logical evidence that the apparent weakness of the broiler chickens' cellular defense mechanisms is most likely associated with pathological changes during development and maturation of the immune system cells in the bone marrow.

Dr Olkowski and his co-workers compared structural and metabolic characteristics of skin samples from broilers that developed cellulitis lesions (susceptible to cellulitis) and samples from normal broilers (resistant to cellulitis). They identified poor structural quality of the broiler skin tissue as a major risk factor predisposing broilers' skin to bacterial invasion. Reduced cellular innate immunity (microbiocidal activity of phagocytes) was identified as a major risk factor facilitating pathogen colonisation, and eventual development of the lesions.

Although development of cellulitis lesions is directly associated with bacterial infection, the underlying pathogenesis of this condition is clearly related to metabolic problems. Of particular interest are the findings that in broiler chickens, challenge with E. coli induces significant systemic changes in homeostasis of several amino acids. Moreover, evidence from our lab indicates that some nutrients (amino acids, vitamins) generally considered as non-essential on the assumption that they are synthesised in adequate amounts by chickens, in fact, may be deficient in the susceptible broilers during rapid growth phase, simply because they are not synthesised by the host in sufficient amounts. In view of the present study, nutritional biochemistry of amino acids such as serine, glycine, proline, alanine, isoleuine, leucine, and lysine must be considered. In the authors' opinion, the aspect of 'non-essentiality' of glycine or proline – the key building blocks of structural proteins in skin and bone – or vitamin C in fast growing broilers must be re-examined based on metabolic demand of all organs.

The Saskatchewan researchers achieved encouraging results were obtained from the experiments where they investigated effects of dietary treatments. Supplementation of glycine in the diet and ascorbate (vitamin C) in water resulted in improvement of structural qualities of the skin and increased resistance to bacterial infection. Also, it is of interest to note that acidification of water decreased the risk of colonisation of broiler skin by E. coli.

Overall, Dr Olkowski and co-authors state they have made significant progress in understanding the pathogenesis of cellulitis in broilers. Definition and elimination of all weak points of host defenses is crucial for the development of a strategy for the management of cellulitis in commercial broiler flocks. Their findings are of practical importance because they show that dietary intervention may be exploited to control cellulitis in the commercial situation.

Summary of Facts Relevant to Control of Cellulitis in Commercial Broiler Flocks

• Cellulitis continues to be one of the principal grounds for condemnation in the broiler industry, and this appears to be directly associated with generalized inability of broilers to deal effectively with opportunistic bacterial infection.
  
  
• The protective mechanisms of the skin against pathogens in fast growing broilers are generally weak, and these systemic weaknesses of antimicrobial defenses render many broilers in commercial situations susceptible to otherwise innocuous bacterial pathogens that are normally present in the environment.
  
  
• The systemic weaknesses of antimicrobial defenses in broilers appear to be associated with:
  1. inferior structural characteristics of the skin in fast growing broilers which provide a poor protective barrier,
  2. physico-chemical and biochemical conditions of the skin conducive to attachment and proliferation of pathogens that are the primary cause of cellulitis, and
  3. poor responses of phagocytic cells in the skin and, thus, inability to contain the initial invasion of pathogens.
  
  
  
• Although development of cellulitis lesions is directly associated with bacterial infection, the underlying pathogenesis of this condition is clearly related to metabolic problems.
  
  
• Elimination of all weak points of host defences – structural, chemical, and cellular – is crucial for the development of a strategy for the management of cellulitis in commercial broiler flocks.
  
  
• This project provided evidence that supplementation of some substrates improved the structural quality of the skin and resistance to bacterial infection. These findings are very encouraging. However, it is important to note that our research also identified several other metabolic substrates that warrant thorough attention.

Further research strategy should be focused on correction of metabolic insufficiency associated with rapid growth, concluded Dr Olkowski and colleagues. This approach will provide a practical means of decreasing the incidence of cellulitis in fast growing commercial broilers.

December 2010