The objective of this study was to estimate true animal-level and herd-level prevalence of
Paratuberculosis (PTB) is a progressive, debilitating, and production limiting disease of ruminants caused by
Even though goats are considered a minor species in the US, the goat industry is recognized as one of the fastest growing US livestock sectors [
Valid estimates of prevalence of MAP infection in goats at both the animal and herd level are needed by industry stakeholders to determine whether the disease warrants interventions to mitigate its negative impact on herd profitability. In MAP affected goat herds, possible intervention goals could include eradication efforts in the event of very low prevalence, institution of a long-term risk-based control program that emphasizes management changes in high prevalence herds, and surveillance in the event of likely absence of infection. The objective of this study was to estimate true animal, within-herd, and between-herd prevalences of MAP antibodies in Missouri Boer goat herds.
Herd prevalence of MAP infection in Boer goat herds in the state of Missouri was determined using a cross-sectional study approved by the Institutional Animal Use and Care Committee at the University of Missouri (Protocol no. 7395).
The number of herds required to determine the apparent herd prevalence of MAP infected Boer goat herds was calculated as follows [
Boer goat herds in the state of Missouri constituted the sampling unit. Herds that contained only Boer goats were eligible to participate in the study. Consequently, herds with multiple breeds of goats were excluded from the study.
The target population included Boer goats 24 months of age and older. Due to the long incubation period of PTB in ruminants and the well-recognized low sensitivity of ELISA (including the ELISA used for this study) tests for detecting MAP antibodies in nonfecal shedding and younger animals [
Prior to the study onset, herd addresses, and owner contacts were obtained from the membership list of the Missouri Meat Goat Producers Association. Based on the above inclusion criteria, a total 142 Boer goat herds in Missouri were determined to be eligible and were contacted with the request to participate in the study. Twenty-five (~18%) herd owners agreed to testing their herds for MAP representing 41% (25 of 61) of the estimated number of herds required to determine between-herd prevalence (see sample size calculations above). This relatively low response rate was expected given the sensitivity attached to data regarding MAP herd status by many producers in Missouri plus the voluntary nature of participation.
Consequently, all eligible animals (i.e., Boer goats ≥ 24 months old) present in agreeing herds were tested for MAP. Herd visits were completed between May and September, 2012. In total, 629 goats from 25 herds were tested for MAP antibodies. The mean ± SD (minimum, maximum) number of animals tested per herd was
Blood samples were collected for serology from all goats that met the inclusion criteria on a single scheduled visit to each participating herd via jugular venipuncture using plain 10 mL Vacutainer (Becton, Dickinson and Co., Franklin Lakes, NJ, USA) tubes. Samples were transported to the University of Missouri’s Veterinary Diagnostic Laboratory for further processing.
Blood samples were initially centrifuged for 5 minutes at 3,000 g. Sera were tested for MAP antibodies using a solid phase indirect enzyme immunoassay (Parachek, Johne’s Absorbed EIA, Prionics USA, Inc.) according to the manufacturer’s instructions. Presence of MAP antibodies in a sample was indicated by a sample absorbance value of 0.27 as the positive cut point [
A goat that tested positive for MAP antibodies using the Parachek, Johne’s Absorbed ELISA (Prionics USA, Inc.) was considered infected. Herds were declared positive for MAP if one or more goats from the herd tested positive for MAP antibodies on the Parachek, Johne’s absorbed ELISA (Prionics USA, Inc.).
The apparent animal, within-herd, and between-herd prevalences were calculated by dividing the number of test positive outcomes by the corresponding denominator (i.e., total number of goats tested from all herds, total number of goats tested within each herd, and total number of herds tested, resp.) for each measure as described [
True animal, within-herd, and between-herd prevalences were calculated using the Rogan-Gladen estimator [
In all calculations, the apparent sensitivity and specificity of the Parachek, Johne’s absorbed ELISA (Prionics USA, Inc.) for detecting MAP antibodies in goat sera were assumed to be 65% (range 65–88%) and 99%, respectively, based on a previous comparative study [
In total, 12 of the 629 goats originating from the 25 herds tested were positive for MAP antibodies. Nine of the 25 herds tested had at least one MAP positive goat and were declared infected with MAP.
The animal, within-herd, and between-herd apparent prevalences were 1.9% (95% CI = 1.1 to 3.3%), 2% (95% CI = 0 to 4%), and 36% (95% CI = 20.2 to 55.5%), respectively (Table
Apparent and true prevalence estimates for animal, within-herd, and between-herd prevalences derived from 629 Boer goats in 25 Missouri herds.
Prevalence type | Number tested | Number positive for MAP | Apparent prevalence | True prevalence | ||
---|---|---|---|---|---|---|
Estimate, % | 95% CI | Estimate, % | 95% CI | |||
Animal | 629 | 12 | 1.9 | 1.1 to 3.3 | 1.4 | 0.1 to 3.6 |
Within-herd | — | — | 2 | 0 to 4 | 3 | 0 to 6 |
Between-herd | 25 | 9 | 36 | 20.2 to 55.5 | 54.7 | 28.2 to 86.2 |
Interestingly, the true between-herd (54.7%; 95% CI = 28.2 to 86.2%) prevalence for MAP in this study was similar to related estimates reported for Cyprian (55.2%; 95% CI = 45.3 to 64.7%) and French (50%; 95% CI = 39 to 62%) dairy goat herds, respectively [
A possible reason for the above differences in true animal and within-herd prevalences estimates could be due to a breed predisposition to MAP infection with apparent risks being greater for dairy breeds of goats than meat breeds (i.e., Boer goats), although other herd factors cannot be discounted. In cattle, herds composed of predominantly Jersey breed were more likely to be infected with MAP than those herds in which other breeds predominated [
Other than the possibility of apparent breed susceptibility and the confounding effects by unrelated herd factors, a plausible reason in herd prevalence could be differences in the mobility patterns of dairy versus meat goats. For example, in dairy goat management, some goats may be moved to other farms over their lifetime as owners buy in or sell out animals. This apparent between-herds mobility is more likely a dairy goat phenomenon and may explain the higher animal and herd prevalences of MAP infection in dairy relative to the meat breeds of goats.
In this study, the Rogan-Gladen [
Finally, the results presented here must be interpreted cautiously given some obvious study limitations. First, given that the apparent sensitivity (i.e., 65%) of the assay used in the current study possibly represented an overestimate of the true sensitivity of the Parachek, Johne’s absorbed ELISA (Prionics USA, Inc.) for detecting MAP antibodies in caprine sera, the true prevalence estimates reported here may be an underestimate of the true MAP prevalence in Missouri Boer goat herds. Second, of the 61 herds initially required to estimate herd prevalence, only 25 (41%) agreed to be tested for MAP in this study. This limited number of participating herds coupled with the larger percentage of smaller herds (Figure
To our knowledge, this is the first study performed in Missouri to quantify prevalence of MAP infection in a caprine population. Findings in this study confirmed that MAP infection is endemic in Missouri Boer goat herds. With an estimated meat goat population of approximately 37,151 head [
Future studies are warranted to further validate current screening tests for MAP antibodies in caprine sera and to characterize specific risk factors associated with MAP prevalence in Missouri (and indeed US) goat herds in order to understand the specific impact of caprine PTB on profitability and performance levels of both meat and dairy goat enterprises under the current US goat production systems.
The authors declared no conflict of interests.
The paper is supported in part by a Committee on Research Grant from the College of Veterinary Medicine, University of Missouri, Columbia, USA.