Objective—To develop a mathematical model to simulate infection dynamics of Mycobacterium bovis in cattle herds in the United States and predict efficacy of the current national control strategy for tuberculosis in cattle.
Design—Stochastic simulation model.
Sample—Theoretical cattle herds in the United States.
Procedures—A model of within-herd M bovis transmission dynamics following introduction of 1 latently infected cow was developed. Frequency- and density-dependent transmission modes and 3 tuberculin test–based culling strategies (no test-based culling, constant [annual] testing with test-based culling, and the current strategy of slaughterhouse detection–based testing and culling) were investigated. Results were evaluated for 3 herd sizes over a 10-year period and validated via simulation of known outbreaks of M bovis infection.
Results—On the basis of 1,000 simulations (1,000 herds each) at replacement rates typical for dairy cattle (0.33/y), median time to detection of M bovis infection in medium-sized herds (276 adult cattle) via slaughterhouse surveillance was 27 months after introduction, and 58% of these herds would spontaneously clear the infection prior to that time. Sixty-two percent of medium-sized herds without intervention and 99% of those managed with constant test–based culling were predicted to clear infection < 10 years after introduction. The model predicted observed outbreaks best for frequency-dependent transmission, and probability of clearance was most sensitive to replacement rate.
Conclusions and Clinical Relevance—Although modeling indicated the current national control strategy was sufficient for elimination of M bovis infection from dairy herds after detection, slaughterhouse surveillance was not sufficient to detect M bovis infection in all herds and resulted in subjectively delayed detection, compared with the constant testing method. Further research is required to economically optimize this strategy.
Objective—To determine sources and amounts of
variation in a kinetics ELISA (KELA) and results of culture
of fecal samples for Mycobacterium avium subsp
paratuberculosis (MAP) in repeated tests of individual
Animals—112 cows on 6 commercial dairy farms in
Procedure—A nonrandom longitudinal study was
conducted from January 2001 to March 2002. A KELA
was performed monthly, and MAP culture was performed
bimonthly. Cow- and herd-level data were collected.
The KELA and culture results were analyzed by
use of models that corrected for clustering within
herds and repeated measures on cows.
Results—Cows of second or higher lactation had
increased KELA values, compared with values for
first-lactation cows. Cows had lowest KELA values
during the first 15 days in milk; KELA values increased
until 60 days in milk and then stabilized. Moderate
and heavy shedders had significantly higher KELA values
than culture-negative cows, and KELA values of
shedders progressively increased over time. On average,
the KELA value was significantly increased 132
days after a cow was first detected to be a moderate
shedder and 236 days after a cow was first detected
to be a low shedder.
Conclusions and Clinical Relevance—Analysis suggests
that KELA results vary on a cow-level on the
basis of lactation number and stage of lactation. High
KELA values indicate heavy fecal shedding, but the
KELA is not useful in identifying low and moderate
shedders that can require up to 236 days to have a
significant increase in KELA value. (Am J Vet Res
Objective—To identify management factors associated with veterinary usage by organic and conventional dairy farms.
Design—Prospective case-control study.
Procedures—Organic farms in New York, Oregon, and Wisconsin were matched to conventional farms on the basis of location and herd size. During a single herd visit, a questionnaire was administered, information about animal disease incidence and number of veterinarian visits in the preceding 60 days was collected, and forms to record similar information during the 60 days after the visit were left for the herd manager to complete. For analysis, conventional herds were classified as either grazing or nongrazing. Multiple correspondence analysis was used to assess relationships among management factors and selected outcomes for frequency of veterinary usage.
Results—Intensive management practices were closely associated with frequent veterinary usage. Generally, organic management practices were associated with less frequent veterinary usage than were conventional management practices. Conventional grazing practices were associated with intermediate veterinary usage (more than organic practices but less than intensive practices), whereas conventional nongrazing practices were associated with frequent veterinary usage. Cost of routinely scheduled veterinarian visits/45 kg (100 lb) of milk produced/y was greater for small farms than that for large farms.
Conclusions and Clinical Relevance—Results suggested that management intensiveness was more closely associated with frequency of veterinary usage than was organic status; therefore, veterinarians should characterize farms by factors other than organic status when investigating which farms are most likely to use their services. Economic factors substantially affected routine veterinary usage on small farms.
Objective—To assess seasonal variation in prevalence
of Listeria monocytogeneson ruminant farms and identify
management practices associated with ruminant listeriosis
and fecal shedding of L monocytogenes.
Study Design—Case-control study.
Sample Population—2,056 samples of feces, feed,
soil, and water from 24 case farms with listeriosis
and 28 control farms without listeriosis.
Procedure—Samples were collected and evaluated
via bacterial culture for L monocytogenes. Univariate
associations between farm management practices
and listeriosis and fecal shedding of L monocytogenes
were assessed. Multivariate models were
developed to identify farm management practices
associated with listeriosis and fecal shedding of L monocytogenes.
Results—The prevalence of L monocytogeneson cattle,
goat, and sheep farms was seasonal, especially in
fecal samples, with peak prevalence in winter. Although
the prevalence of L monocytogenes in feedstuffs from
small-ruminant farms also peaked during winter, the
bacterium was detected at a constant rate in cattle farm
feedstuffs throughout the year. Farm management
practices, animal health and hygiene, and feedstuff quality
and storage were associated with ruminant listeriosis
and fecal shedding of L monocytogenes.
Conclusions and Clinical Relevance—Results suggest
that the prevalence of L monocytogenes on ruminant
farms is seasonal, management practices are
associated with ruminant listeriosis and fecal shedding
of L monocytogenes, and the epidemiologic features
of listeriosis differ in cattle versus small ruminants.
Awareness of risk factors may be used to develop control
measures to reduce animal disease and introduction
of L monocytogenes into the human food chain.
(J Am Vet Med Assoc 2005;227:1808–1814)