Natural Habitat

The mollicutes occur worldwide as free-living saprophytes or as parasites of animals. Both pathogenic and non-pathogenic species are found as commensals on the mucous membranes of the upper respiratory, intestinal and genital tracts, on articular surfaces and in the bovine mammary gland. Outside the host, the pathogenic species can survive in microenvironments, protected from sunlight, for several days.

Pathogenesis

The most important species of mycoplasmas and the diseases that they cause in poultry, other domestic animals and laboratory animals are listed in Table 35.2. The mycoplasmas tend to be fairly host-specific although it has been suggested that interspecies transmission can occur and may be of pathogenic importance in immunosuppressed hosts (Pitcher & Nicholas 2005). In addition to the mycoplasmas shown in Table 35.2, there are many species isolated from birds and animals, whose disease status is at present uncertain. These are listed in Table 35.3.

The parasitic mycoplasmas tend to adhere firmly to the mucous membranes of the host and some species have been shown to affix to cells by specific attachment structures. A number of surface proteins play a role in adhesion, including the P26 antigen and variable surface proteins of M. bovis (Caswell & Archambault 2007), variable lipoprotein haemagglutinin of the avian mycoplasmas (Noormohammadi 2007) and P116 of M. hyopneumoniae (Seymour et al. 2010). Although the production of a cytotoxin (CARDS toxin) has been identified in the human pathogen, Mycoplasma pneumoniae (Kannan & Baseman 2006, Techasaensiri et al. 2010), production of specific toxins has not been demonstrated for pathogenic mycoplasmas of animals. It is considered that many of the toxic effects on host cells result from the action of toxic metabolic products which diffuse into the tissues of the host following adherence of the organisms. These metabolic products include H₂O₂ and other reactive oxygen species. Enzymes such as proteases and haemolysins may also be involved in the production of tissue damage.

Variable surface proteins (Vsps) have been identified in many mycoplasmal species and these proteins play a role in adhesion and colonization and, most importantly, in the evasion of the humoral response of the host. The production of a number of different Vsps has been demonstrated in many of the major mycoplasmal pathogens of animals, including Mycoplasma mycoides subspecies mycoides (Persson et al. 2002), M. bovis (Sachse et al. 2000) and the avian pathogens M. gallisepticum and M. synoviae (Noormohammadi 2007). The similarity between some mycoplasmal antigens and host cell antigens may also contribute to persistence of mycoplasmas in the host because of the inability to induce an effective immune response due to a failure in antigen recognition. However, the sharing of antigenic determinants between the organism and host cells may also lead to the development of autoimmune disease. It is thought that immunological mechanisms may be involved in the destruction of red cells in cats infected with Mycoplasma haemofelis although direct damage by the organism may also play a role. Important virulence factors of Mycoplasma species are listed in Table 35.4.

Mycoplasma species can affect may tissues of the animal body. Some pathogenic species have a predilection for mesenchymal cells lining joints and serous cavities. The respiratory tract and lungs are frequent sites of infection. Mycoplasmas are capable of destroying the cilia of cells in the respiratory tract, thus predisposing to secondary bacterial invasion. Latency can occur and various stresses predispose to mycoplasmal diseases. The infections are frequently chronic or low grade. Infections are either endogenous or exogenous. Transmission is usually by aerosol, venereal or vertical and the important avian mycoplasmas are egg-transmitted. Transmission of haemotropic mycoplasmas occurs through fighting and biting arthropods.