Mycobacterium avium subsp. paratuberculosis: herd prevalence and calf-to-calf transmission
Creators: name: Corbett, Caroline Susan
Publisher: name: Graduate Studies
Year: 2018
Subjects: subject: calf
subject: transmission
subject: paratuberculosis
subject: Johne's disease
subject: group-housing
subject: prevalence
subject: fecal shedding
subject: environmental sample
subject: Veterinary Science
subject: Epidemiology

description: Johne’s disease results in a progressive chronic enteritis caused by Mycobacterium avium subsp. paratuberculosis (MAP), leading to economic losses among dairy producers worldwide. In the absence of an effective vaccine to prevent infection and treatment for infected animals, control is primarily based on decreasing the number of new transmissions within a herd. Control programs have been implemented in countries and regions around the world; however, comparisons among prevalence estimates of difference regions and control programs are difficult and unreliable due to different tests used to identify infected animals and herds. Therefore, the first objectives of this thesis were to elucidate the influences of environmental sample characteristics on the outcome status of a herd, and to estimate the prevalence of MAP based on 2 environmental samples (and 3 environmental samples when including young stock) in Canada. Six environmental samples were collected twice, 3.5 years apart, from 148 dairy farms to determine whether difference in prevalence between sampling periods were associated with herd size and sample characteristics. All environmental samples regardless of type, had decreased odds of testing positive in the second sampling, and the largest herds had increased odds of testing positive than smaller herds at both sample periods. Across 4 regions (10 provinces) in Canada, 2 environmental samples, one from the lactating cow area and one the manure storage, were collected from 362 dairy farms, with an additional sample collected from breeding age heifers. Prevalence was lowest among tie-stall herds, in herds ≤ 100 cows, and in Québec; and although breeding age heifer samples did not affect prevalence estimates, they provided additional evidence that young stock are shedding MAP on farm. Therefore, the second objectives of this thesis were to determine the extent to which calf-to-calf transmission occurs among group-housed penmates, and to quantify the amount of fecal shedding that occurs among these infectious calves. An experimental transmission study was conducted, where 32 newborn calves were grouped into 7 experimental groups of 4, consisting of 2 inoculated (IN), and 2 contact exposed (CE) calves, and 1 control pen with 4 non-exposed calves. Calves were group-housed for 3 months, during which fecal, blood and environmental samples were collected frequently. The based reproduction ratio (R0) was estimated as a parameter of transmission of MAP infection using a final size (FS) model with a susceptible-infected-recovered (SIR) model based on ELISA and tissue culture. In addition, transmission rate parameter () was estimated using a GLM with a susceptible-infected-susceptible (SIS) model based on fecal culture during group housing. Throughout group housing, all IN and CE calves had MAP-positive fecal samples, and although there was a difference between frequency of shedding, there was no difference between the quantities of MAP shed in feces. All IN calves had positive MAP-tissue samples, and 7 (50%) of CE calves had positive tissue samples. Based on fecal shedding, the basic reproduction ratio R0 for CE calves (R0CE) was 3.24 (95% CI: 1.14, 7.41). R0I (based on interferon- results from blood samples) was 0.90 (95% CI: 0.24, 2.59), and R0T (based on tissue) was 1.36 (95% CI: 0.45, 3.94). Additionally, the effects of freezing on the ability to identify MAP in tissue samples were found to be minor; however, there may be a greater effect for CE calves that should be considered when freezing tissue samples. In conclusion, environmental samples characteristics did not influence the infection status of a herd, and collecting 2 environmental samples could be used to estimate prevalence and compare differences among regions. Shedding calves transmit infection to fellow penmates; therefore, future control programs should consider monitoring and testing of young stock to further decrease new transmissions on farm.