Transmission
The main mode of transmission for MAP is faecal- oral, although cows in late stages of infection may excrete MAP in colostrum and milk (Pithua et al., 2011; Stabel et al., 2014).
Furthermore, late-stage cases can transmit infection in utero, with 20-60% of calves born to clinical dams being infected (Donat et al., 2016). Although direct excretion of MAP from the mammary gland does occur, it is believed that poor udder hygiene and inadequate milking routines result in significant contamination of milk and colostrum with faecal material, and are thus the mechanism for potential transmission (McAloon et al., 2016a). Calf-to-calf transmission can occur; experimentally inoculated calves infected exposed pen mates, causing faecal shedding, highlighting the utility of individual housing of calves as a means of decreasing the risk of infection (Mortier et al., 2014; Corbett et al., 2017). It is believed that young animals are at greater infection risk due to their ‘open gut', specifically the presence of specialized lymphoid tissue (Peyer's patches) that accept maternal antibodies early in life. Although very young animals are still believed to be most susceptible to MAP infection, there is an apparent dose dependence, with older animals being susceptible to infection with higher doses (Windsor and Whittington, 2010; Mortier et al., 2013). There is also evidence that mature cows can become infected by exposure to clinically affected animals, with newly infected‘Corresponding author: imadaj@uoguelph.ca © CAB International 2020. Paratuberculosis: Organism, Disease, Control, 2nd Edition (eds M.A. Behr et al.)
cows diagnosed by tissue culture at slaughter (Schukken et al., 2015). The infective dose also seems to affect onset of clinical paratuberculosis; high-prevalence herds with high infective doses in the environment usually have a faster onset of clinical disease than low-prevalence herds (Lombard et al., 2005; Weber etal., 2010).
Recent evidence that MAP may be able to sporulate under certain conditions (Lamont et al., 2012), be carried in dust and infect animals through the respiratory tract (Rowe and Grant, 2006), suggests alternate transmission routes. Infected animals may not manifest clinical signs until 2-10 years after infection, often at times of physiological stress, e.g. calving (Tiwari et al., 2006). This long period between infection and clinical signs allows subclinical animals that were infected early in life to excrete the organism intermittently in their faeces; thereby acting as carriers, exposing their herd mates and avoiding detection until their immune system reacts and they either produce detectable antibody or display clinical signs. As infection progresses, the rate of faecal excretion increases, with advanced clinical cows representing a large source of infectious material (Mitchell et al., 2015).
With clinical cows representing a major source of contamination for an infected herd, test and cull can be useful for paratuberculosis control programmes. In addition, based on susceptibility of young stock to infection, management practices, biosecurity and biocontainment also have important roles. However, the more recent evidence that mature cows are also susceptible to infection may reduce effectiveness of traditional control practices.
1.3
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