VN May 2022

Vetnews | Mei 2022 37 study even showed that upon exposure to antibodies specific for the variable surface proteins (VSP’s) expressed by the Mycoplasma bovis isolate involved, a change in the pattern of VSP’s is induced. Other evasion tactics include penetrating and surviving in various host cells and inhibiting blood mononuclear cell proliferation. Evading the immune system successfully allows this organism to establish chronic infections and disseminate to joints. Macroscopic pathologic lung lesions present a typical caseonecrotic bronchopneumonia, with white pinpoint to coalescing, raised and friable caseonecrotic foci. Distribution is usually cranio-ventral, with the caudo-dorsal areas of the lung often unaffected. Affected lung area can vary, and large areas of affected lung tissue, up to 80%, have been reported. Demonstration of the agent in laboratory samples via culture, PCR and immunohistochemistry in conjunction with the clinical presentation can be used to confirm a diagnosis. It is important to note that in the case of BRD, Mycoplasma bovis is often a secondary pathogen or part of a multi-pathogen infection. As such, it is not always possible to determine the primary agent of such an infection. The current perception is that primary Mycoplasma bovis infections are increasing in feedlots, but whether this is indeed the case or is just a result of successful metaphylactic control of the other bacteria involved in BRD remains debatable. Treatment is often unrewarding, and this is (amongst other factors) attributed to the fact that these animals present with clinical BRD at a stage where there is already advanced tissue necrosis in the lungs; this creates areas with inadequate blood supply that limit antimicrobial penetration. It being said, logic dictates that it is imperative to pull the affected animals early for treatment to be effective. It is recommended to compile an antimicrobial regime that continues treatment for 7-14 days. Some of the antimicrobials that work against Mycoplasma bovis are tulathromycin, danofloxacin, enrofloxacin and florfenicol. A critical point to remember is that mycoplasmas do not have a cell wall. Thus, antimicrobials that target bacterial cell wall synthesis, such as the beta-lactam antibiotics (e.g. ampicillin and ceftiofur), will not be effective. Currently, there are no registered Mycoplasma bovis vaccines in South Africa. Control is aimed at general BRD control, such as scientifically appropriate metaphylaxis, vaccination against immunosuppressive viruses like IBR and BVD, and vaccination against Mannheimia haemolytica and limiting the multiple stress factors that could precipitate multifactorial BRD. v Regulars I Zoetis Livestock Column