Transmission of Paratuberculosis in Cattle
11.2.1 Routes of transmission
The vast majority of new infections occur via the faecal-oral route, although other routes, including the vertical route, are possible (Seitz et al., 1989).
Calves born to seropositive dams were shown to be 6.6 times more likely to be seropositive compared with calves born to seronegative dams (Aly and Thurmond, 2005). A meta-analysis estimated that 9% of fetuses born from subclinically infected dams and 39% born from clinically affected dams were infected in utero (Whittington and Windsor, 2009). These findings indicate that the likelihood of fetal infection is dependent on the severity of the dam's infection. As the infection status of the dam represents a major risk factor for the newborn calf, it is not advised to keep a clinical cow within the herd in the hopes of obtaining the calf prior to culling, as the cow will continue to shed MAP organisms into the environment and the calf has a good chance of being infected.MAP has been isolated from uterine flush fluids of infected cattle, and MAP organisms have been shown to adhere to embryos in vitro (Rohde and Shulaw, 1990). Therefore, an embryo obtained from an infected cow could, in theory, result in an infected fetus; however, oocytes and embryos processed according to current embryo transfer recommendations are unlikely to result in infected calves (Kruip et al., 2003). Semen from clinically or subclinically infected bulls can be infected with MAP (Ayele et al., 2004; Buergelt et al., 2004). However, keeping in mind some of the key features of the pathogenesis of MAP, such as preferred route of infection (with oral being the main route of transmission) and age susceptibility (breedingage cattle are less likely to become infected), the probability of transmission of MAP via infected semen should be considered low. Additionally, bulls kept in commercial bull studs are typically rigorously tested for MAP.
Herd sires (dairy and beef) should be tested annually to ensure their negative status. In 2004, the European Food and Safety Authority Scientific Panel on Animal Health and Welfare (AHAW) publication addressed the risk of transmission of MAP via bovine semen (European Food and Safety Authority Scientific Panel on Animal Health and Welfare, 2004). More specifically, the working group asked about (i) the probability that a bull selected and used at semen collection centres in the European Union (EU) is infected with MAP, and (ii) the probability that the use of semen from that bull results in at least one infected animal. The AHAW panel concluded that given the current lack of sufficiently sensitive diagnostic test methods for subclinically infected animals, the likelihood of detecting MAP infection in a bull at 12-18 months of age (the typical age at which a bull would enter a semen collection centre) is very low, and that the selection of bulls should be based on considering only bull calves from MAP-negative cows within herds with low test-prevalence based on a repeat testing regimen. The panel also proposed that because of the lack of data on what represents an infective dose of MAP-infected semen, whether used for artificial insemination or in vitro fertilization, a probability of infection can't be established. They however concluded that infection of reproductive age cattle with MAP-infected semen is an event of low probability.Most infections with MAP occur in the early neonatal period and are often associated with the calf coming into contact with manure when ingesting colostrum from the dam. Multiple-use maternity pens can serve as focal points to spread the infection to the neonates. MAP may also be passed directly through the colostrum and milk of cattle in the later stages of infection (Sweeney et al., 1992a; Streeter et al., 1995). It has been demonstrated that colostrum from known infected cows is a much greater risk to spread MAP to calves compared with colostrum from known negative dams (Nielsen et al., 2008).
The practice of feeding pooled colostrum or waste milk may help spread the infection from infected adults to many calves in the herd and should be discouraged. It is also important to note that MAP can survive pasteurization temperatures (Chiodini and Hermon-Taylor, 1993); therefore, colostrum and milk from known infected cows, even pasteurized, should not be used. Following birth, physical separation of the dairy calves to calf hutches, or better yet, to another property such as a heifer-raising facility, reduces the risk of MAP transmission to the replacement stock.11.2.2 Susceptibility to Infection
Resistance to MAP infection increases with age (Windsor and Whittington, 2010). Possible explanations for the increased susceptibility of young calves may include: (i) the ‘openness' of
the newborn gut, which allows the calf to absorb immunoglobulins in colostrum could represent a permissive barrier for MAP organisms; (ii) the immaturity of the calf's immune system when compared with that of an adult cow (Sweeney,
2011) ; and (iii) the presence of the oesophageal groove, which allows the ingested MAP (typically via milk feeding) to bypass the rumen and be directed to the susceptible small intestine. Age- related resistance to infection can however be overcome by pressure of infection. Replacement stock and adult cattle that are exposed to MAP- contaminated forages, water and extremely contaminated environments can get infected with MAP. For example, the practice of offering feed refusals from adult cattle to younger heifers has been shown to be a significant risk factor to spread MAP (Rossiter and Burhans, 1996). Additionally, dairy herds using an intensive grazing system (also known as a leader-follower system) may have an increased risk of transmission to the replacement stock, due to the continuous low-level exposure to MAP organisms on pasture (Fecteau et al., 2010). As noted above, unless massive and repeated doses of MAP are consumed, this is likely to be a relatively low risk for adult cows but could represent an important risk to younger replacement stock. One could also argue that cattle that become infected later in age may not start shedding MAP and develop clinical signs of JD before they leave the herd for other unrelated reasons.
Susceptibility to infection with MAP can also be genetically influenced. It was suggested that certain breeds of cattle (Guernsey, Jersey and Bos indicus breeds) and certain family lines may have a genetic predisposition to becoming infected with MAP (Chiodini et al., 1984; Roussel et al., 2005; Elzo et al., 2006; Osterstock et al., 2008; Kirkpatrick and Lett, 2018; Kiser et al., 2018). Genetic and host susceptibility to MAP infection in cattle is discussed in more detail in Chapter 4 in this volume.
11.3