Objective—To determine the sample size necessary
to evaluate the efficacy of a vaccine in a population.
Procedure—An equation was coded into a computer
spreadsheet to compare the traditional sample size
calculation with that needed when evaluating the efficacy
of a vaccine applied in a population.
Results—The traditional approach used to conservatively
estimate sample size necessary to detect a
given difference in group proportions potentially
greatly underestimates the number of animals needed
for vaccine efficacy (VE) trials. In VE trials, it is necessary
to estimate the effect of population-level vaccination
prior to estimating sample size. In VE trials,
as incidence proportion in the population or herd
decreases or VE decreases, necessary sample size
Conclusion and Clinical Relevance—In designing a
clinical or field trial, such as one to evaluate the efficacy
of a vaccine against an infectious disease in a
population, one needs to approach sample size calculations
in a nontraditional manner. The proportion of
the population vaccinated, disease transmission
dynamics, and VE will affect the incidence in the nonvaccinated
and vaccinated groups and, hence, sample
size. Thus, estimation of the effect of the vaccination
on the population must be made prior to calculating
sample size. Otherwise, sample size and the power
to identify VE will be insufficient. (Am J Vet Res
Objective—To evaluate the application of a vaccine in
a population of animals.
Sample Population—Field-trial data from the literature.
Procedure—A spreadsheet simulation model was
constructed to estimate the impact of a vaccination
program, assuming various population sizes, transmission
rates, and vaccine efficacies.
Results—Total effectiveness (proportion of affected
animals [ie, cases] avoided) increased with the vaccinated
proportion of the population. However, with a
highly efficacious vaccine, this relationship discontinued
after a sufficient vaccination proportion was
reached, reflecting herd immunity. Evaluation of a
case study indicated that what may be considered a
poor vaccine on the basis of its low efficacy may protect
a substantial portion of the population if the vaccine
is administered to a sufficient number of susceptible
animals. Further investigation of a case study
of horses indicated that evaluating a vaccine based
solely on its efficacy could greatly underestimate its
Conclusions and Clinical Relevance—When evaluating
a vaccine applied to a population, in addition to
the vaccine efficacy, the vaccination rate, cost of the
vaccine, potential disease transmission rate, and
number and cost of cases avoided must also be considered.
Efficacy may underestimate vaccine value in
terms of the reduction of indirect cases typically
avoided when vaccination is applied in a population.
(Am J Vet Res 2001;62:202–205)
Objective—To determine biosecurity practices and travel history of individuals exhibiting livestock at the 2005 California State Fair.
Study Population—137 individuals exhibiting livestock at the fair.
Procedures—Exhibitors were asked to complete a survey to gather information about the animals they exhibited, the biosecurity practices they used, and the distances they traveled to exhibit their animals.
Results—132 of the 137 (96%) respondents came from California, with respondents representing 40 of California's 58 counties. Median number of livestock exhibitions attended by respondents during the past 12 months was 3 (range, 1 to 7). Respondents indicated that 787 of the 812 (97%) animals they exhibited would be returned home after the fair. Nine (7%) respondents indicated that they did not take any particular biosecurity precautions before arriving at the fair, and 14 (10%) indicated that they did not take any particular biosecurity precautions while at the fair. Only 36 (26%) respondents indicated that they quarantined their animals when returning to their farm of residence after the fair.
Conclusions and Clinical Relevance—Results indicated that there was extensive movement of livestock among exhibitions in California, posing a potential threat for widespread dispersion of disease throughout the state and beyond, particularly given the low percentages of exhibitors who used various biosecurity measures.
Objective—To estimate direct and indirect contact
rates on livestock facilities and distance traveled
between herd contacts.
Sample Population—320 beef, dairy, goat, sheep,
and swine herds, 7 artificial insemination technicians,
6 hoof trimmers, 15 veterinarians, 4 sales yard owners,
and 7 managers of livestock-related companies
within a 3-county region of California.
Procedure—A questionnaire was mailed to livestock
producers, and personal and telephone interviews
were conducted with individuals.
Results—Mean monthly direct contact rates were
2.6, 1.6, and 2.0 for dairies with < 1,000, 1,000 to
1,999, and ≥ 2,000 cattle, respectively. Mean indirect
contact rates on dairies ranged from 234 to 743 contacts/
mo and increased by 1 contact/mo as herd size
increased by 4.3. Mean direct monthly contact rate
for beef herds was 0.4. Distance traveled by personnel
and vehicles during a 3-day period ranged from
58.4 to 210.4 km. Of livestock arriving at sales yards,
7% (500/7,072) came from ≥ 60 km away, and of
those sold, 32% (1,180/3,721) were destined for a
location ≥ 60 km away. Fifty-five percent (16/29) of
owners of large beef herds observed deer or elk within
150 m of livestock at least once per month.
Conclusions and Clinical Relevance—Direct and
indirect contacts occur on livestock facilities located
over a wide geographic area and at a higher frequency
on larger facilities. Knowledge of contact rates may
be useful for planning biosecurity programs at the
herd, state, and national levels and for modeling
transmission potential for foot-and-mouth disease
virus. (Am J Vet Res 2001;62:1121–1129)
Objective—To develop a model to evaluate the effect
of vaccination against Tritrichomonas foetuson reproductive
efficiency in beef herds.
Sample Population—A beef herd of 300 cows and
12 bulls (8 bulls ≤ 3 years old and 4 bulls > 3 years
Procedure—The model was developed by use of
data for various risk factors and vaccine efficacy. The
reference herd was considered to be one in which T
foetus had been diagnosed and bulls were tested for
T foetus before the breeding season. Five thousand
iterations were run for each of 13 simulations, with
each simulation representing a separate combination
of risk factors.
Results—In all simulations, vaccination resulted in
significantly higher calving incidence than nonvaccination.
Shared grazing was found to be the most significant
risk factor for a decrease in calving incidence
attributable to T foetus infection, followed in importance
by lack of testing before the breeding season
and a higher proportion of old bulls. Combinations of
risk factors contributed to a loss of income of up to
22%, some of which could be blunted by vaccination.
Conclusions and Clinical Relevance—Highest calving
incidence is achieved when all bulls are tested for
T foetus before the breeding season and all bulls with
positive culture results are culled. Avoiding all risk factors
is better than vaccinating, but when this is not
feasible for a given herd, the results of this simulation
indicate that proper vaccination can decrease economic
losses attributable to abortions caused by T
foetus. (Am J Vet Res 2004;65:770–775)
Objective—To develop a spatial epidemic model to
simulate intraherd and interherd transmission of footand-
mouth disease (FMD) virus.
Sample Population—2,238 herds, representing
beef, dairy, swine, goats, and sheep, and 5 sale yards
located in Fresno, Kings, and Tulare counties of
Procedure—Using Monte-Carlo simulations, a spatial
stochastic epidemic simulation model was developed
to identify new herds that would acquire FMD following
random selection of an index herd and to assess
progression of an epidemic after implementation of
mandatory control strategies.
Results—The model included species-specific transition
periods for FMD infection, locations of herds,
rates of direct and indirect contacts among herds, and
probability distributions derived from expert opinions
on probabilities of transmission by direct and indirect
contact, as well as reduction in contact following
implementation of restrictions on movements in designated
infected areas and surveillance zones.
Models of supplemental control programs included
slaughter of all animals within a specified distance of
infected herds, slaughter of only high-risk animals
identified by use of a model simulation, and vaccination
of all animals within a 5- to 50-km radius of infected
Conclusions and Clinical Relevance—The FMD
model represents a tool for use in planning biosecurity
and emergency-response programs and in comparing
potential benefits of various strategies for control
and eradication of FMD appropriate for specific populations.
(Am J Vet Res 2003;64:195–204)
Objective—To assess estimated effectiveness of
control and eradication procedures for foot-andmouth
disease (FMD) in a region of California.
Sample Population—2,238 herds and 5 sale yards in
Fresno, Kings, and Tulare counties of California.
Procedure—A spatial stochastic model was used to
simulate hypothetical epidemics of FMD for specified
control scenarios that included a baseline eradication
strategy mandated by USDA and supplemental control
strategies of slaughter or vaccination of all animals within
a specified distance of infected herds, slaughter of
only high-risk animals identified by use of a model simulation,
and expansion of infected and surveillance zones.
Results—Median number of herds affected varied
from 1 to 385 (17% of all herds), depending on type
of index herd and delay in diagnosis of FMD.
Percentage of herds infected decreased from that of
the baseline eradication strategy by expanding the
designated infected area from 10 to 20 km (48%),
vaccinating within a 50-km radius of an infected herd
(41%), slaughtering the 10 highest-risk herds for each
infected herd (39%), and slaughtering all animals
within 5 km of an infected herd (24%).
Conclusions and Clinical Relevance—Results for the
model provided a means of assessing the relative merits
of potential strategies for control and eradication of
FMD should it enter the US livestock population. For the
study region, preemptive slaughter of highest-risk herds
and vaccination of all animals within a specified distance
of an infected herd consistently decreased size and
duration of an epidemic, compared with the baseline
eradication strategy. (Am J Vet Res 2003;64:205–210)
Objective—To assess relative costs and benefits of
vaccination and preemptive herd slaughter to control
transmission of foot-and-mouth disease (FMD) virus
Sample Population—2,238 herds and 5 sale yards
located in Fresno, Kings, and Tulare counties of
Procedure—Direct costs associated with indemnity,
slaughter, cleaning and disinfecting livestock premises,
and vaccination were compared for various eradication
strategies. Additional cost, total program cost,
net benefit, and benefit-cost value (B/C) for each supplemental
strategy were estimated, based in part on
results of published model simulations for FMD.
Sensitivity analyses were conducted.
Results—Mean herd indemnity payments were estimated
to be $2.6 million and $110,359 for dairy and
nondairy herds, respectively. Cost to clean and disinfect
livestock premises ranged from $18,062 to
$60,205. Mean vaccination cost was $2,960/herd.
Total eradication cost ranged from $61 million to $551
million. All supplemental strategies involving use of
vaccination were economically efficient (B/C range,
5.0 to 10.1) and feasible, whereas supplemental
strategies involving use of slaughter programs were
not economically efficient (B-C, 0.05 to 0.8) or feasible.
Conclusions and Clinical Relevance—Vaccination
with a highly efficacious vaccine may be a cost-effective
strategy for control of FMD if vaccinated animals
are not subsequently slaughtered and there is no
future adverse economic impact, such as trade
restrictions. Although less preferable than the baseline
eradication program, selective slaughter of highest-risk herds was preferable to other preemptive
slaughter strategies. However, indirect costs can be
expected to contribute substantially more than direct
costs to the total cost of eradication programs. (Am J Vet Res 2003;64:805–812)
Objective—To estimate contact rates and movement variables for shipments of beef cattle to and from producer premises in California.
Sample Population—583 beef producers in California.
Procedures—Questionnaires were developed and distributed to beef cattle producers in California. The study period was from April 20, 2005, through September 7, 2006. Data from completed questionnaires were entered manually into an electronic format. Descriptive statistical analyses were performed.
Results—Median number of interstate animal movements (movement of cattle into or out of California) was 0.17/mo; on the basis of this median, beef cattle were moved interstate > 2 times/y. Respondents kept beef cattle at up to 5 locations throughout the year. More than 40% of the movements from the respondents' premises were to a sale yard or auction facility.
Conclusions and Clinical Relevance—Frequency of animal movements in this statewide study differed from values in another study of monthly shipments of animals to and from beef operations in 3 counties of California. The survey reported here revealed more frequent movements of animals to and from beef operations of all sizes. In addition, there were more high-risk indirect contacts on beef operations than has been reported previously. However, the number of low-risk indirect contacts was similar for small beef operations but less for large beef operations than has been reported elsewhere. Epidemic simulation models for California based on data in earlier studies likely underpredicted disease transmission involving beef herds.
Objective—To evaluate a Markov-chain model for the
development of forelimb injuries in Thoroughbreds
and to use the model to determine effects of reducing
sprint distance on incidence of metacarpal condylar
fracture (CDY) and severe suspensory apparatus
Sample Population—Weekly exercise and injury
data for 122 Thoroughbreds during racing or training.
Procedure—Weekly data were used to construct a
Markov-chain model with 5 states (uninjured [UNINJ],
palpable suspensory apparatus injury [PSAI], SSAI,
CDY, and lost to follow-up [LOST]). Transition probabilities
between UNINJ and PSAI were estimated as a
function of weekly sprint distance by use of linear
regression analysis. The model was used to predict
distributions of annual CDY and SSAI incidences in
southern California racehorses and was validated by
using CDY incidence reported by racetrack practitioners.
The model was modified by reducing the number
of sprint distances that were > 6 furlongs (> 1.20 km)
by 20%, and CDY and SSAI incidences were compared
with those generated by the baseline model.
Results—The model accurately fit development of
injuries in the sample population but overestimated
development of injuries in the southern California
racehorse population. Development of and recovery
from PSAI were correlated with distance run at high
speeds. Reducing by 20% the number of sprints run
at distances > 6 furlongs significantly reduced modeled
annual CDY and SSAI incidence by 9%.
Conclusions and Clinical Relevance—Reducing the
number of sprints at distances > 6 furlongs, particularly
among horses with PSAI, reduces risk of CDY
and SSAI. (Am J Vet Res 2003;64:328–337)