Control Of Infectious Diseases Biology Essay


Mastitis is an inflammatory reaction of the mammary gland (Gieseeke et al, 1994). The most important mastitis pathogen worldwide is S. aureus (Lammers et al, 2001; Slobodanka & Pavlovic, 2009). The virulence of S. aureus cannot be explained in terms of a single virulence determinant (Albus et al, 1991). Complex interactions that occur among a variety of adhesion mechanisms, extracellular toxins, surface proteins, and enzymes are responsible for staphylococcal virulence (Albus et al, 1991). Typically, S. aureus requires some breach in the primary host defence, commonly the skin of a mucosal barrier in the teat of a cow, in order to cause disease (Forbes, 1968). From the point of entrance the microorganism must then fight to remain in the cistern, dodging being flushed out during the milking process (Hogeveen, 2005). This is only possible through adherence to the epithelial cells and extracellular matrix (ECM) proteins allowing colonisation of the mammary gland (Anderson, 1983; Forbes, 1968; Lammers et al, 2001; Kerro Dego et al, 2002). Fibronectin binding protein A (FnBPA) mediates bacterial attachment to immobilised fibronectin and contributes to S. aureus in udder epithelial cells (Anderson, 1983) (Hogeveen, 2005) (Olmsted & Norcross, 1992).

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Shkreta et al 2004 developed a vaccine based on bicistronic plasmid encoding for FnBPA and a plasmid encoding for granulocyte - macrophage colony stimulating factor (GM-CSF) gene. The results of the "challenge trial showed some protection against infection and improvements in cow physiological parameters and a lyphoprolipherative and humoral immune response" (Hogeveen, 2005). Brown et al, 1998 high light that with development of a vaccine which elicits a humoral and cellular immune response similar to Shketa et al it is also important to manipulate the vaccine to direct responses towards a proper immune response. Brown et al, 1998) research discusses many molecules which could possibly be manipulated to develop an effective vaccine, however the best candidate was believed to be cytokines. Slobodanka & Pavlovic, 2009 followed though with this concept developing a vaccine which modulated the immune response in 30% of their tests cases towards a TH-1 dominated response instead of a TH-2 one. TH-1 response is more efficient in controlling invading pathogens such as S. aureus by inducing a strong IL2 and IFNÏ’ response stimulating IgG2 production and improving phagocytosis and intracellular killing (Shkreta et al, 2004; Lammers et al, 2001). Olmsted & Norcross, 1992 have demonstrated that the ability of S. aureus to bind to epithelial cells of the ductules and alveloli in the bovine mammary gland is an important virulence factor, and that antibodies against whole cells inhibit the adherence therefore the development of infection. Wedlock et al, 2000 have also shown that application of rBo-IL2 induces higher levels of phagocytosis also reducing the development of infection and in doing so successfully controlling the infectious disease.

It is well documented that the best control measure for S. aureus infections is prevention (Gieseeke et al, 1994; Hogeveen, 2005). Masitis infections will only arise if the S. aureus pathogen is given the opportunity to enter the teat duct (Frost et al, 1977). Pieterse & Todorov, 2010 report that since the implementation of hygiene control strategies in milking parlours and dairy farms in the late 60's "the number of incidences of contagious pathogens responsible for clinical mastitis" (Pieterse & Todorov, 2010) such as S. aureus have decreased significantly, "accounting now for only 10% of the total reported cases" (Pieterse & Todorov, 2010). Gieseeke et al, 1994 discuss the "five point plan for mastitis control" (Gieseeke et al, 1994), which Pieterse & Todorov, 2010 consider to be the "gold standard for many years, and has been successful in reducing the incidences of mastitis" (Pieterse & Todorov, 2010). The five point plan high lights that the most effective way to prevent mastitis is to "eliminate conditions that expose teat ends to bacteria and reduce the possibility of spread" (Gieseeke et al, 1994) from cow to cow in the milking parlour. By reducing exposure of the teat to S. aureus the hygiene procedures which are outlined briefly in the list below, reduce the possibility of S. aureus travelling up the milk duct and establishing epithelial cell attachment. Blocking the pathogen attachment to the host cell is considered paramount when controlling mastitis (Hunter & Jeffrey, 1975; Hogeveen, 2005).

Hygiene procedures outlined by (Gieseeke et al, 1994).

Milkers should always wear disposable gloves or disinfect them frequently

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Dirt should be brushed off with the use of dry single-use towel

Apply a pre and post dip to each teat before and after milking

Clean milking clusters

Flushing of the liking lines/units regularly

The above points were adapted from (Pieterse & Todorov, 2010).

Mycobacteria bovis and tuberculosis control in cattle.

Mycobacteria bovis is a slow growing nonphotochromogenic (Dennis, 1991) organism is the etiologic agent of bovine tuberculosis and causes disease in a wide range of domestic animals, however predominately in cattle (Laneelle & Daffe, 1991). Development of mycobacterial disease in animals depends on the "ability of mycobacteria to survive and multiply within macrophages of the host" (Dennis, 1991). Pathogenicity of mycobacteria is a multifactorial phenomenon, requiring the participation and cumulative effects of several components (Laneelle & Daffe, 1991). Intact mycobacteria are nontoxic, and the clinical symptoms and lesions that develop depend upon the types of immune responses that predominate in response to antigen (Brown et al, 1998). Mycobacteria have a wide repertoire of antigens, many of which are modifiers of the host responses or responsible for the pathologic manifestations of disease (Rastogi, 1991).

Immunity to mycobacterial infections is dependent on cell mediated responses; humoral immune factors are of little importance in protection of the host (Momotani et al, 1988). Development of cell mediated responses to facultative intracellular organisms involves the cooperative action of T lymphocytes as specific inducers and macrophages as non specific effector cells (Rastogi, 1991; Romagnani, 1992). T lymphocytes recruit and assemble mononuclear phagocytes and release cytokines that activate macrophages for enhanced bactericidal activity (Romagnani, 1992; Dennis, 1991). It was Waddington and Elwood, 1972 who first experimented with inducing cell mediated responses via vaccination to stimulate immunity to M. bovis. They demonstrated "that the subcutaneous application of Bacillus Calmette Guerin (BCG) vaccine in newborn calves could induce an immune response that protected against M. bovis infection" (Waddington & Elwood, 1972). More recently field evaluations by (Lopez-Valencia et al, 2010) revealed results that the "tuberculosis frequency in the non-vaccinated group was 22.7%, while in the vaccinated group it was 9.2%. The BCG vaccine efficacy indicates that it protected 59.4% of the vaccinated calves that were exposed to TB up unitl they were one year old" (Lopez-Valencia et al, 2010). Waddington & Elwood, 1972 and Lopez-Valencia et al, 2010, vaccination trails were successful as they both stimulated TH-1 cells within the hosts. This subsequently resulted in secretion of interferon - gamma which suppressed TH-2 cell activity and in doing so decreased antibody production in reaction to the M. bovis antigen (Brown et al, 1998; Dennis, 1991). Comparatively trail results from Lopez-Valencia et al, 2010 were more convincing in providing adequate host response to M. bovis than Waddington & Elwood, 1972. However the percentages are still low when compared to human vaccine efficacy to Tuberculosis, which has been reported by Bass, 2005 to reach up to 90%. Dennis, 1991 believes that an understanding of the pathogenesis of mycobacterial infections is continguent upon comprehension of the architecture and function of mycobacterial antigens. This theory and further research could potentially result in a new vaccine being developed which is capable of protecting 100% of vaccinated animals.

Currently the control management for M. bovis is controversial as animals are tested for the disease using a Tuberculin skin test (TST) described by Schiller et al, 2010 and positive reactor animals are slaughtered. The TST measures dermal swelling caused by a cell mediated immune response to a protein derivative which acts as M. bovis antigen in the host (Rastogi, 1991). A study by Monaghan et al, 1994 revealed that TST techniques showed 68-96.8% sensitivity which followed through to 100% flock eradication of bovine tuberculosis following a slaughtering action plan. Similarly Pollock et al 1998 observed 80-98.2% sensitivity followed by 100% flock eradication via slaughter. Recently antibody detection assays have been shown to provide a more reliable, convenient and cost effective way to effectively observe and therefore control bovine tuberculosis. However, (Pollock, et al., 1992) states that the development of these assays has not been easy due to the fact that early M. bovis infections yield weak antibody responses (Schiller, et al., 2010). Preliminary studies by Waters et al, 2006 have "indicated the potential for antibody-based tests, however large scale field trails are required to clearly define serological test accuracy, especially in direct comparison to cell mediated based test" (Schiller, et al., 2010; Rastogi, 1991).

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Understanding and critically analysing the pathogenesis of infectious diseases is crucial in developing appropriate and successful control measures. Control measures will only be successful if specific virulence factors of pathogens are targeted, therefore reducing and where possible cease the ability to cause disease. Successful control strategies for S. aureus mastitis infections target the adherence of the microorganism to host epithelial cells, either by preventing entrance to susceptible tissues lining the teat or by antibody blocking via vaccination. The ability of the host to form immunity to M. bovis is exploited in control strategies for bovine tuberculosis, by vaccinations designed to stimulate a cell mediate immunity response opposed to a humoral response. However due to the complexity of M. bovis pathogenesis, slaughter of positive tested animals is currently the most effective control measure.