Herd-Level Prevalence

1.6.1 Dairy cattle herd-level prevalence

Due to the intensive nature of dairy farming and the long incubation period of paratuberculosis, MAP infection is prevalent and increasing in the dairy industry.

Fig. 1.1. Global dairy cattle herd-level prevalence of MAP infection. (Adapted from Whittington et al., 2019.)

Fig. 1.2. Global goat herd-level prevalence of MAP infection. (Adapted from Whittington et al., 2019.)

Italy, the Netherlands, New Zealand, Panama, Republic of Ireland, Spain, UK, USA, Uruguay) (Whittington etal., 2019). Lombard et al. (2013) reported an apparent herd-level prevalence of 70% among American dairy farms in 2007, with Bayesian estimates being closer to 91%. A true herd-level prevalence was determined to be 46% for Canadian dairy farms using en­vironmental cultures (Corbett et al., 2018). Elsewhere, true herd-level prevalence of MAP- infected herds was estimated to be 85% of dairy herds in Denmark, 20-71% of herds in the Netherlands and approximately 28-43% of UK herds (Geraghty et al., 2014). Some regions in Germany have an apparent herd-level seroprev­alence of up to 85% (Khol and Baumgartner,

2011). Channel Island breeds are often con­sidered at an increased risk for MAP infection; however, it is unclear whether this is due to in­creased susceptibility or due to increased expo­sure due to other farm-level factors (Sorge et al.,

Fig. 1.3. Global sheep herd-level prevalence of MAP infection. (Adapted from Whittington et al., 2019.)

2012).

1.6.2 Beef cattle herd-level prevalence

Research investigating the prevalence of MAP infection in beef cows is limited. Prevalence is believed to be lower than that of dairy herds, possibly due to more extensive management, re­ducing transmission risk. Furthermore, lifespan of feedlot cattle (15-28 months) (Drouillard, 2018) is often much shorter than that of dairy cattle (5-7 years) (Hare et al., 2006) providing less opportunity for the infection to progress to clinical manifestations. In contrast to this, cow­calf herds may keep brood cows much longer (8-12 years) (Rae et al., 1993; Selk, 2018), po­tentially allowing for the manifestation, persis­tence and spread of paratuberculosis. The use of cull dairy cattle as either nurse cows or embryo transfer recipients may be a potential risk factor for introducing MAP into beef herds (Roussel et al., 2005). Herd-level prevalence is often es­timated near 7% (7.9% Canadian and US beef herds) (Geraghty et al., 2014); however, these are likely underestimates (Dargatz et al., 2001). Current estimates are even higher for purebred cattle, with ~44% of American and British beef herds being positive (Roussel et al., 2005; Lawson and Caldow, 2014). This breed effect may be due to genetic susceptibility or the trade in purebred genetic stock.

1.6.3 Within-herd prevalence

A lower proportion of beef cattle is thought to be infected with MAP than dairy cattle; in a Western Canadian study, there was an appar­ent cow-level prevalence of 0.8% (Pruvot et al., 2014a). By comparison, apparent cow-level seroprevalence in Canadian dairy cows ranges from 1.3-7% (Tiwari et al., 2006). This lower prevalence in beef cattle could be due to the ex­tensive management style of beef herds (Dargatz et al., 2001). Infected herds with older animals can be expected to have more visible clinical signs than herds who remove animals earlier than the typical MAP incubation period. Herds with more intensive management may expect more trans­mission between animals and contamination of the environment.

1.6.4 Sheep, goats and other ruminants

Mortality due to paratuberculosis in Australian sheep flocks is considerable, ranging from 6-8% (Bush et al., 2006). Regional herd-level prevalence in Australia ranges from 0-60%, with >5% of their national herd believed to be infected (2117 known MAP infected flocks in 2011). Australia is success­fully using vaccination in paratuberculosis control (Windsor, 2015). There has been a relative lack of data on risk factors for paratuberculosis in goats (Gautam et al., 2018). However, there has been a successful eradication of MAP infection from a goat herd using extensive testing and removal of positive animals (Gavin et al., 2018). Similarly, through an intensive national campaign, Norway was able to eradicate MAP infections in their goat herds (Whittington et al., 2019). Infection with MAP in small ruminants often does not result in the same progressive diarrhoea as in cattle. These animals usually have progressive weight loss and, in sheep, impairments in their wool production (Salem et al., 2012). In New Zealand, 76% of sheep flocks and 46% of deer flocks are considered infected. In Ontario, Canada, true herd-level prev­alence was 67% for dairy sheep and 83% for dairy goats, whereas within-herd prevalence was 35% for MAP-infected goat flocks and 48% for infect­ed sheep (Bauman et al., 2016).

1.6.5 Wildlife

Mycobacterium avium subspecies paratuberculosis has been identified in >100 vertebrate species, of these, five (white tailed deer, roe deer, red deer, fallow deer, wild hare) are considered po­tential reservoirs for domestic cattle (Carta et al.,

2013). MAP was also isolated from rabbit faeces and urine; contaminated pastures could act as a potential reservoir for infection of cattle grazing (Daniels et al., 2003; Shaughnessy et al., 2013). There has also been a proposed risk for cattle and elk comingling (Pruvot et al., 2014b). Further, mature deer seem to be equally at risk for infec­tion as younger counterparts.

1.7