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Abstract

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 increases.

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 2001;62:1582–1584)

Full access
in American Journal of Veterinary Research

Abstract

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 value.

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)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To determine biosecurity practices and travel history of individuals exhibiting livestock at the 2005 California State Fair.

Design—Survey.

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.

Restricted access
in Journal of the American Veterinary Medical Association

Abstract

Objective—To estimate contact rates and movement variables for shipments of beef cattle to and from producer premises in California.

Design—Cross-sectional survey.

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.

Restricted access
in Journal of the American Veterinary Medical Association

Abstract

Objective—To characterize husbandry practices that could affect the risks of foreign animal disease in miniature swine.

Design—Survey study.

Study Population—106 owners of miniature swine.

Procedures—An online survey of owners of miniature swine was conducted to obtain information about miniature pig and owner demographics; pig husbandry; movements of pigs; and pig contacts with humans, other miniature swine, and livestock.

Results—12 states, 106 premises, and 317 miniature swine were represented in the survey. More than a third (35%) of miniature swine owners also owned other livestock species. Regular contact with livestock species at other premises was reported by 13% of owners. More than a third of owners visited shows or fairs (39%) and club or association events (37%) where miniature swine were present. More than 40% of owners fed food waste to miniature swine. Approximately half (48%) of the veterinarians providing health care for miniature swine were in mixed-animal practice.

Conclusions and Clinical Relevance—Results of this study indicated that miniature swine kept as pets can be exposed, directly and indirectly, to feed and other livestock, potentially introducing, establishing, or spreading a foreign animal disease such as foot-and-mouth disease. In addition, the veterinary services and carcass disposal methods used by miniature swine owners may reduce the likelihood of sick or dead pigs undergoing ante- or postmortem examination by a veterinarian.

Restricted access
in Journal of the American Veterinary Medical Association

Abstract

Objective—To estimate potential spread of foot-and-mouth disease (FMD) if introduced from wild pigs in California and to evaluate efficacies of various control strategies.

Sample Population—Data for California livestock and from hunter surveys on wild pigs in California.

Procedures—A spatial, stochastic simulation model was used to simulate FMD epidemics that might occur if a dairy or beef herd were infected from contact with a wild pig. Index herd location and type were examined, in addition to different statewide movement ban (SWMB) durations, to determine their effect on extent of the epidemic.

Results—Duration, number of infected premises, size of simulated outbreak, number of culled animals, and spatial distribution of infected herds resulting from the simulated outbreaks varied considerably among geographic regions, depending on index case type and location. Outbreaks beginning in the southern region of California were consistently longest, whereas those beginning in the northern region were shortest. The largest outbreaks resulted from index cases located in the southern and valley regions, whereas outbreaks were smallest when originating in the Sonoma or northern regions. For all regions, when the index herd was a dairy herd, size and duration of the outbreak were consistently reduced with implementation of an SWMB ≥ 3 days.

Conclusions and Clinical Relevance—Introduction of FMDV from wild pigs into a dairy or beef herd could result in a large and rapidly spreading outbreak, potentially affecting large numbers of herds. Size and duration of the outbreak might be reduced with an SWMB; however, the impact is highly dependent on the index herd type and location.

Full access
in American Journal of Veterinary Research

Abstract

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 California.

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 herds.

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)

Full access
in American Journal of Veterinary Research

Abstract

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)

Full access
in American Journal of Veterinary Research

Abstract

Objective—To assess relative costs and benefits of vaccination and preemptive herd slaughter to control transmission of foot-and-mouth disease (FMD) virus (FMDV).

Sample Population—2,238 herds and 5 sale yards located in Fresno, Kings, and Tulare counties of California.

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)

Full access
in American Journal of Veterinary Research

Abstract

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)

Full access
in American Journal of Veterinary Research