Transmission parameters of highly pathogenic avian influenza (H7N1) among industrial poultry farms in northern Italy in 1999-2000

We estimated between-farm transmission parameters of the highly pathogenic avian-influenza (HPAI) epidemic that struck the poultry industry of northern Italy (including turkeys, layer hens, broilers, gamebirds, and waterfowl) from December 1999 through April 2000. We estimated the average number of susceptible farms that were infected with HPAI virus by each infectious farm during a day (β) with a generalised linear model (GLM). The HPAI's reproductive ratios (R_h; the average number of new infected farms (IFs) that were caused by an infectious farm) were calculated separately for the regions of Lombardy and Veneto, where 382 out of 413 (92.5%) of IFs were located. In both regions, R_h decreased to 1 during the second month of the epidemic (showing that its containment had been initiated). Subsequently, during the last two months of the epidemic, β and R_h were reduced to 0.04/day and 0.6, respectively, in Veneto and to 0.07/day and 0.8 in Lombardy. The reduction of the susceptible population through strict control measures, including pre-emptive slaughter of at-risk poultry flocks, was implemented to a greatest extent in Veneto and this might have been associated with a more rapid control of the epidemic in this region than in Lombardy.     

The classical swine fever epidemic 1997-1998 in The Netherlands: descriptive epidemiology

The objective of this paper is to describe the severe epidemic of classical swine fever (CSF) in The Netherlands in 1997–1998 under a policy of non-vaccination, intensive surveillance, pre-emptive slaughter and stamping out in an area which has one of the highest pig and herd densities in Europe.

The primary outbreak was detected on 4 February 1997 on a mixed sow and finishing pig herd. A total of 429 outbreaks was observed during the epidemic, and approximately 700 000 pigs from these herds were slaughtered. Among these outbreaks were two artificial insemination centres, which resulted in a CSF-suspect declaration of 1680 pig herds (mainly located in the southern part of The Netherlands). The time between introduction of CSF virus (CSFV) into the country and diagnosis of CSF in the primary outbreak was estimated to be approximately 6 weeks. It is presumed that CSFV was spread from The Netherlands to Italy and Spain via shipment of infected piglets in the beginning of February 1997, before the establishment of a total stand-still of transportation. In June 1997, CSFV is presumed to be introduced into Belgium from The Netherlands.

Pre-emptive slaughter of herds that had been in contact with infected herds or were located in close vicinity of infected herds, was carried out around the first two outbreaks. However, this policy was not further exercised till mid-April 1997, when pre-emptive slaughter became a standard operational procedure for the rest of the epidemic. In total, 1286 pig herds were pre-emptively slaughtered. (approximately 1.1 million pigs). A total of 44 outbreaks (10%) was detected via pre-emptive slaughter.

When there were clinical signs, the observed symptoms in infected herds were mainly atypical: fever, apathy, ataxia or a combination of these signs. In 322 out of 429 outbreaks (75%), detection was bases on clinical signs observed: 32% was detected by the farmer, 25% by the veterinary practitioner, 10% of the outbreaks by tracing teams and 8% by screening teams of the veterinary authorities. In 76% of the outbreaks detected by clinical signs, the farmer reported to have seen clinical symptoms for less than 1 week before diagnosis, in 22% for 1–4 weeks before diagnosis, and in 4 herds (1%) the farmer reported to have seen clinical symptoms for more than 4 weeks before diagnosis.

Transportation lorries played a major role in the transmission of CSFV before the primary outbreak was diagnosed. It is estimated that approximately 39 herds were already infected before the first measures of the eradication campaign came into force.

After the first measures to stop the spread of CSFV had been implemented, the distribution of the most likely routes of transmission markedly changed. In most outbreaks, a neighbourhood infection was indicated.

Basically, there were two reasons for this catastrophe. Firstly, there was the extent of the period between introduction of the virus in the region and detection of the first outbreak. As a result, CSFV had opportunities to spread from one herd to another during this period. Secondly, the measures initially taken did not prove sufficient in the swine- and herd-dense region involved.     

Implications derived from a mathematical model for eradication of pseudorabies virus

Simple mathematical models based on experimental and observational data were applied to evaluate the feasibility of eradicating pseudorabies virus (PRV) regionally by vaccination and to determine which factors can jeopardise eradication. As much as possible, the models were uncomplicated and our conclusions were based on mathematical analysis. For complicated situations, Monte-Carlo simulation was used to support the conclusions. For eradication, it is sufficient that the reproduction ratio R (the number of units infected by one infectious unit) is < 1. However, R can be determined at different scales: at one end the region with the herds as units and at the other end compartments with the pigs as units. Results from modelling within herds showed that contacts between groups within a herd is important whenever R between individuals (R_ind) is 1 in one or more groups. This is the case within finishing herds. In addition, if the R_ind is more than 1 within a herd, the size of the herd determines whether PRV can persist in the herd and determines the duration of persistence. Moreover, when reactivation of PRV in well-vaccinated sows is taken into account, R_ind in sow herds is still less than 1. In sow herds with group-housing systems, it is possible that in those groups R_ind is 1. Results from modelling between herds showed that whether or not R_herd is < 1 in a particular region is determined by two factors: (1) the transmission of infection between nucleus herds and rearing herds through transfer of animals and (2) contacts among finishing herds and among rearing herds. The transmission between herds can be reduced by reduction of the contact rate between herds, reduction of the herd size, and reduction of the transmission within herds.     

Transmission of classical swine fever virus within herds during the 1997-1998 epidemic in The Netherlands

In this paper, we describe the transmission of Classical Swine Fever virus (CSF virus) within herds during the 1997–1998 epidemic in the Netherlands. In seven herds where the infection started among individually housed breeding stock, all breeding pigs had been tested for antibodies to CSF virus shortly before depopulation. Based upon these data, the transmission of CSF virus between pigs was described as exponential growth in time with a parameter r, that was estimated at 0.108 (95% confidence interval (95% CI) 0.060–0.156). The accompanying per-generation transmission (expressed as the basic reproduction ratio, R₀) was estimated at 2.9. Based upon this characterisation, a calculation method was derived with which serological findings at depopulation can be used to calculate the period in which the virus was with a certain probability introduced into that breeding stock. This model was used to estimate the period when the virus had been introduced into 34 herds where the infection started in the breeding section. Of these herds, only a single contact with a herd previously infected had been traced. However, in contrast with the seven previously mentioned herds, only a sample of the breeding pigs had been tested before depopulation (as was the common procedure during the epidemic). The observed number of days between the single contact with an infected herd and the day of sampling of these 34 herds fitted well in the model. Thus, we concluded that the model and transmission parameter was in agreement with the transmission between breeding pigs in these herds.

Because of the limited sample size and because it was usually unknown in which specific pen the infection started, we were unable to estimate transmission parameters for weaned piglets and finishing pigs from the data collected during the epidemic. However, from the results of controlled experiments in which R₀ was estimated as 81 between weaned piglets and 14 between heavy finishing pigs (Laevens et al., 1998a. Vet. Quart. 20, 41–45; Laevens et al., 1999. Ph.D. Thesis), we constructed a simple model to describe the transmission of CSF virus in compartments (rooms) housing finishing pigs and weaned piglets. From the number of pens per compartment, the number of pigs per pen, the numbers of pigs tested for antibodies to CSF virus and the distribution of the seropositive pigs in the compartment, this model gives again a period in which the virus most probably entered the herd. Using the findings in 41 herds where the infection started in the section of the finishers or weaned piglets of the age of 8 weeks or older, and of which only a single contact with a herd previously infected was known, there was no reason to reject the model. Thus, we concluded that the transmission between weaned piglets and finishing pigs during the epidemic was not significantly different from the transmission observed in the experiments.     

Quantification of the spread of Mycoplasma hyopneumoniae in nursery pigs using transmission experiments

Mycoplasma hyopneumoniae (M. hyopneumoniae) is present in almost all swine herds worldwide, but transmission of the pathogen through herds is not yet fully clarified. The aim of this study, performed in 2003, was to investigate and to quantify the transmission of M. hyopneumoniae under experimental conditions by means of an adjusted reproduction ratio (R_n). This R_n-value, calculated according to the final size method, expresses the mean number of secondary infections due to one typical infectious piglet during the nursery period. The period lasted from 4 to 10 weeks of age, corresponding with the nursery period used in most European production systems. Additionally, a comparison was made between transmissions of highly virulent and low virulent isolates.

Forty-eight weaned piglets, free of M. hyopneumoniae, were housed in six separate pens. During 6 weeks, two animals experimentally infected with M. hyopneumoniae were housed together with six susceptible piglets. At the end of the study, the number of contact-infected animals was determined by the use of nPCR on bronchoalveolar lavage fluid. The R_n-values of the highly virulent and the low virulent isolates were estimated to be 1.47 (0.68–5.38) and 0.85 (0.33–3.39), respectively. No significant difference between the groups was found (P = 0.53). The overall R_n was estimated to be 1.16 (0.94–4.08). Under the present experimental conditions, the transmission of M. hyopneumoniae, assessed for the first time by a reproduction ratio, shows that one piglet infected before weaning will infect on average one penmate during the nursery phase.     

Simulation model of within-herd transmission of bovine tuberculosis in Argentine dairy herds

Transmission of bovine tuberculosis was quantified in three dairy herds located in south Santa Fe Province, Argentina. Using estimates of Mycobacterium bovis transmission (β) and a Reed–Frost simulation model, the prevalence of tuberculosis infection in the study herds over time was investigated. The Reed–Frost model was modified by incorporating randomness in both β and the incubation period () of M. bovis. The mean estimated herd β was 2.2 infective contacts per year and did not differ significantly between the study herds. Modeling as Poisson distributed (mean 24 months) best fit the observed prevalences. Infection was predicted by the model either to spread quickly (<10 years) within a herd and reach a high prevalence (>50%), or to persist at a low prevalence (<5–10%). The model was robust, predictions were realistic and the mean β estimated was consistent with previous studies of bovine tuberculosis.     

Herd demographics correlated with the spatial distribution of a foot-and-mouth disease epidemic in Buenos Aires province, Argentina

During a recent foot-and-mouth disease epidemic in Argentina, cattle herds affected in 2001 were located mainly (69%) in Buenos Aires province. The densities of outbreaks (no. of outbreaks per km²) and cattle-demographic variables in the province were estimated using a geographical information system and kernel function. Before the epidemic officially was recognized, the density of outbreaks was correlated (r_sp = 0.28–0.47) with the geographic distribution of small (≤100 cattle), dairy and fattening herds. During the mass-vaccination campaign to control the epidemic (April–July), the density of outbreaks was most strongly correlated (r_sp = 0.20–0.25) with the distribution of large (>500 cattle) and breeding herds. After the end of the mass-vaccination campaign, large herds and number of cows were most strongly correlated (r_sp = 0.16–0.26) with outbreak density. These relationships might indicate that: (1) the disease spread more rapidly or was more easily detected in intensive production systems at the beginning of the epidemic; (2) vaccination and other control methods applied were less effective in large, semi-intensive production systems; (3) incomplete vaccine protection was responsible for herd outbreaks that occurred after the end of the mass-vaccination campaign.     

Neighbourhood infections of classical swine fever during the 1997-1998 epidemic in The Netherlands

Data of the 1997–1998 epidemic of classical swine fever (CSF) in The Netherlands were analysed in survival analysis to identify risk factors that were associated with the rate of neighbourhood infections. The study population consisted of herds within 1000 m of exclusively one previously infected herd. Dates of virus introduction into herds were drawn randomly from estimated probability distributions per herd of possible weeks of virus introduction. (To confirm the insensitivity of the results for this random data-selection procedure, the procedure was repeated 9 times (resulting in 10 different datasets).) The dataset had 906 non-infected and 59 infected neighbour herds, which were distributed over 215 different neighbourhoods. Neighbour herds that never became infected were right-censored at the last date of the infectious period of the infected source herd. Neighbour herds that became empty within the infectious period or within the following 21 days due to preventive depopulation or due to the implemented buying-out programme were right-censored 21 days before the moment of becoming empty. This was done as a correction for the time a herd could be infected without being noticed as such.

The median time to identified infection of neighbour herds was 2 weeks, whereas the median time to right censoring of non-infected neighbour herds was 3 weeks. The risk factors, radial distance ≤500 m, cattle present on source herd and increasing herd size of the neighbour herd were associated multivariably with the hazard for neighbour herds to become infected. We did not find an association between time down wind and infection risk for neighbour herds. Radial dispersion of CSFV seemed more important in neighbourhood infections than dispersion along the road on which the infected source herd is situated. The results of this study support the strategy of preventive depopulation in the neighbourhood of an infected herd. Recommendations are presented to adapt the applied control strategy for neighbourhood infections.     

The 1997-1998 epidemic of classical swine fever in the Netherlands

In 1997, the pig husbandry in the Netherlands was struck by a severe epidemic of classical swine fever (CSF). During this epidemic 429 CSF-infected herds were depopulated and approximately 1300 herds were slaughtered pre-emptively. In addition millions of pigs of herds not CSF-infected were killed for welfare reasons (over crowding or overweight). In this paper, we describe the course of the epidemic and the measures that were taken to control it.The first outbreak was detected on 4 February 1997 in the pig dense south-eastern part of the Netherlands. We estimate that CSF virus (CSFV) had already been present in the country by that time for 5-7 weeks and that the virus had been introduced into approximately 39 herds before the eradication campaign started. This campaign consisted of stamping-out infected herds, movement restrictions and efforts to diagnose infected herds as soon as possible. However, despite these measures the rate at which new outbreaks were detected continued to rise. The epidemic faded out only upon the implementation of additional measures such as rapid pre-emptive slaughter of herds in contact with or located near infected herds, increased hygienic measures, biweekly screening of all herds by veterinary practitioners, and reduction of the transportation movements for welfare reasons. The last infected herd was depopulated on 6 March 1998.     

The 1997-1998 classical swine fever epidemic in The Netherlands--a survival analysis

The aim of this analysis was to characterise the temporal pattern of infection during the 1997/98 classical swine fever (CSF) epidemic in The Netherlands and hence identify and quantify risk factors for infection in different enterprise types and areas. Survival analysis and Cox proportional hazards regression were used to describe the epidemic. Substantial differences in temporal survival patterns (herd breakdown rate) were found between areas where different control policies operated. Factors with a significant influence on the infection hazard of individual herds included: sow numbers as a percentage of total sows and fatteners (HR = 3.38 for mixed herds (0.1–60% sows) vs. fattening herds (0% sows) and HR = 2.74 for breeding herds (60–100% sows) vs. fattening herds), the number of ‘transport contacts per month’ (>0.3 vs. <0.3; HR = 4.11), pig density (pigs/km²) in the area (HR_{1000 pigs} 1.48) and herd size (HR_{100 pigs} = 1.01).

Pre-emptive slaughter in an area appeared to be associated with lower subsequent disease levels. Higher frequency of transport contacts for welfare slaughter during the epidemic, however, well regulated and controlled, was associated with a substantially higher risk of becoming infected. The positive association of a higher pig density with CSF indicates the potential importance of local spread as a factor in disease transmission and emphasizes that dilution of the pig population can contribute to reduction in CSF occurrence. This analysis suggests however, that if pre-emptive slaughter can promptly be applied effectively in an area after initial diagnosis, pig density is then not a significant factor. Mixed and breeding herds had a higher probability of becoming infected than fattening herds, possibly due to different types and frequencies of inter-herd contacts. These contacts continue to some extent during the epidemic, despite the standstill of animal movements.     

The leukocyte count is a valuable parameter for detecting classical swine fever

In this paper we describe a study of the use of the white blood cell count (wbcc) as a parameter for detecting outbreaks of Classical Swine Fever (CSF). Meta-analysis of the results of challenge experiments revealed that oronasal infection of SPF-pigs with the virulent CSF virus (CSFV) strains Brescia or NL9201 resulted in a significant decrease in the average white blood cell count during the first week after inoculation of the virus. Challenge of conventional finishing pigs and sows with the moderately virulent strain Paderborn also resulted in a significant decrease in the average wbcc. However, this decrease was not observed after inoculation of SPF pigs with the mildly virulent CSFV strains Henken, Zoelen, or Bergen. The usefulness of clinical inspection in combination with wbcc to detect CSF outbreaks in the field was examined using the results of 214 EDTA blood specimens collected from 22 infected herds and 7250 EDTA blood specimens collected from 1450 non-infected herds. Half of the infected herds had been infected with the moderately virulent CSFV strain Venhorst (closely related to strain Paderborn) during the 1997-98 epidemic in the Netherlands. The other half had been infected with the moderately virulent CSFV strain Loraine. Using these data as a starting point, 1000 samples of one to ten specimens were generated by Monte Carlo simulation. These simulated samples and the samples of the non-infected herds were analysed by use of Receiver Operating Characteristic curves. On the basis of that analysis, the optimal number of animals whose wbcc needed to be determined to detect a CSF outbreak was five. With this number of animals, in conjunction with the threshold of 8000 white blood cells per mm3 (meaning that a herd is designated as CSF suspect if one or more of the five specimens has a white blood cell count of 8000 leukocytes/mm3 or less), the test procedure had a herd sensitivity (HSE) of 94.5% and a herd specificity (HSP) of 97.2%). The HSE is defined as the percentage of samples of infected herds with a positive result of the test procedure; HSP is defined as the percentage of uninfected herds with a negative result of the test procedure. We conclude that the wbcc can help the veterinary practitioner to detect outbreaks of CSF caused by (moderately) virulent CSFV strains. However, for the detection of outbreaks caused by mildly virulent CSFV strains, the contribution of the wbcc is doubtful. Development of additional tools that can improve the clinical diagnosis of the veterinary practitioner remains desirable.     

A method to calculate—for computer-simulated infections—the threshold value, R0, that predicts whether or not the infection will spread

Computer-simulation models of infections can easily incorporate heterogeneity among animals (important for the effect of control measures) by allocating animals to various classes. These classes are termed ‘states’ and the change from one state to another, during a unit of time, is termed a ‘transition’. Hence, most computer models are state-transition models. Using a fairly universal representation state-transition models, we derived an analytic expression (a formula) for the basic reproduction ratio of infection (R₀), i.e. the number of cases caused by one typical infectious animal. When R₀>1, the infection can spread; when R₀<1, the infection will disappear. Therefore, a strategy for controlling an infective agent is effective when, and only when, the ratio for that strategy is less than one.

Using the reproduction-ratio formula derived in this paper (instead of interpreting results from a large number of simulations) has several advantages for veterinary researchers. The structure of the formula allows investigators to understand how certain parameters (e.g. infection, demographic or control-strategy parameters), if changed, will influence the ratio. Moreover, the ratio can be calculated directly and quickly, hence whether an infection will spread or disappear can be determined quickly. In addition, because certain parameters present in the original model disappear in the formula for the ratio, they can be eliminated as influencing the effectiveness of the control strategy. Finally, the value of R₀ can be interpolated for parameter values (e.g. rates of infection, contact rates, replacement rates and herd sizes) other than those evaluated originally.     

Control of a foot-and-mouth disease epidemic in Argentina

A major epidemic of foot-and-mouth disease affected Argentina during 2001. The epidemic was controlled by mass-vaccination of the national herd and movement restrictions. The median herd disease reproduction ratio (R_H) decreased significantly from 2.4 (before the epidemic was officially recognized) to 1.2 during the mass-vaccination campaign and <1 following the mass-vaccination campaign. The largest distance between two outbreaks was similar during (1905 km) and after (1890 km) the mass-vaccination. However, after mass-vaccination was completed, the proportion of herd outbreaks clustered decreased from 70.4% to 66.8%, respectively. Although a combination of vaccination and livestock-movement restrictions was effective in controlling the epidemic, 112 herd outbreaks occurred up to 6 months after the end of the mass-vaccination campaign. Mass-vaccination and movement restrictions might be an effective strategy to control FMD; however, the time taken to end large, national epidemics might be >1 year.     

The effect of an outbreak of respiratory disease on herd-level milk production of Norwegian dairy farms

This study was done to evaluate the effect of an outbreak of acute respiratory disease associated with bovine respiratory syncytial virus (BRSV) on the daily milk yield per cow in Norwegian dairy-cattle farms. Retrospective data from 184 dairy herds located in two neighbouring veterinary districts during the study period (December 1994–May 1995, during which an epidemic of acute respiratory disease associated with BRSV occurred in this area) were analysed. Data on the bulk-milk deliveries and the date of the outbreak were collected at herd level, whereas information on calving dates and parity was collected at cow-level. The effect of the herd outbreaks on the daily milk yield was analysed with a repeated-measurement approach. The average daily milk loss was estimated to be 0.70 kg per cow for 7 days after a herd outbreak (compared with the period >1 week prior to an outbreak), adjusted for the herd-level lactation stage, parity and their interaction term. We consider the estimated milk loss associated with a herd outbreak of epidemic respiratory disease to be of minor importance.     

Analysis of the risk of introduction and spread of porcine reproductive and respiratory syndrome virus through importation of raw pigmeat into New Zealand

AIMS: To determine the frequency with which porcine reproductive and respiratory syndrome (PRRS) virus (PRRSv) would become established in a non-commercial pig herd in New Zealand due to illegal feeding of uncooked food waste containing virus-contaminated pigmeat. To determine the likelihood of a single incursion resulting in a multi-farm outbreak of the disease, and describe the spatio-temporal characteristics of such an outbreak. METHODS: A Monte Carlo simulation model was constructed to determine the expected annual frequency of PRRSv infection being initiated in a non-commercial pig herd as a result of inadvertent feeding of pigmeat imported from countries endemically infected with the disease. Once the likelihood of PRRSv becoming established in a single pig herd was determined, stochastic spatially explicit infectious disease modelling software was utilised to model the temporal and spatial characteristics of the resulting epidemic. RESULTS: Assuming the proportion of imported pigmeat remained at current levels, consumption patterns of pigmeat in households in New Zealand remained steady, and limited compliance with recently reintroduced regulations to prevent feeding of uncooked food waste, at least 4.3 pig herds per year were predicted to become infected with PRRSv. Simulation modelling of PRRSv epidemics related to initial infection of a non-commercial farm produced an estimate that 36% of these incursions would spread from the initial herd, and that these outbreaks would involve 93 herds on average in the first year. By increasing the estimated persistence of PRRSv infection in small herds, an average of 205 herds became infected in the first year. CONCLUSIONS: Given a mean of 4.3 infected premises per year and a 36% probability of infection spreading beyond the initial infected herd, there was a 95% likelihood of a multi-farm PRRS outbreak occurring within 3 years. CLINICAL RELEVANCE: Introduction of PRRSv through importation of virus-contaminated pigmeat presents a high risk for establishment of the disease in the pig industry in New Zealand.     

Estimating the number of undetected multi-resistant Salmonella Typhimurium DT104 infected pig herds in Denmark

In Denmark, the detection of multi-resistant Salmonella Typhimurium DT104 (MRDT104)-infected pig herds relies on the national Salmonella surveillance programme at the farm and slaughterhouse levels of production. With the surveillance sampling protocol and the diagnostic methods currently used, some herds might remain undetected. The number of undetected Danish pig herds infected with MRDT104 in the period 1 August 2001–31 July 2002 was estimated and compared with the number of culture-confirmed detected herds. A flow chart was constructed to illustrate where infected herds will go undetected in the surveillance system and Monte Carlo simulation was used to model the actual number of pig herds infected with MRDT1104. We estimated that 52 (90% CI [28, 178]) finisher herds were infected with MRDT104 compared to 23 (44%) detected. Among sow herds with production of weaners or growers, we estimated that 38 (90% CI [23, 74]) were infected with MRDT104 compared to 7 (18%) actually detected. Among breeder and multiplier herds, we estimated that five (90% CI [3, 8]) herds were infected with MRDT104 compared to three (60%) detected. In total, we estimated that 102 pig herds were infected with MRDT104 from 1 August 2001 till 31 July 2002 (90% CI [63, 228]). In comparison, 33 (32%) infected herds were detected in this period. The predicted proportion of undetected herds varied considerably with herd type. We infer that the proportion of detected MRDT104 infected herds depended on the intensity of the combined serological and bacteriological testing.     

Prevalence of bovine herpesvirus-1 in the Belgian cattle population

The national bovine herpesvirus 1 (BHV-1) seroprevalence (apparent prevalence) in the Belgian cattle population was determined by a serological survey that was conducted from December 1997 to March 1998. In a random sample of herds (N=556), all cattle (N=28 478) were tested for the presence of antibodies to glycoprotein B of BHV-1. No differentiation could be made between vaccinated and infected animals, because the exclusive use of marker vaccines was imposed by law only in 1997 by the Belgian Veterinary Authorities. Twenty-one percent of the farmers vaccinated continuously against BHV-1.

In the unvaccinated group, the overall herd, individual-animal and median within-herd seroprevalences were estimated to be 67% (95% confidence interval (CI)=62–72), 35.9% (95% CI=35.0–36.8) and 33% (quartiles=14–62), respectively.

Assuming a test sensitivity and specificity of 99 and 99.7%, respectively, the true herd, individual-animal and median within-herd prevalence for the unvaccinated group of herds were estimated to be 65, 36 and 34%, respectively. The true herd prevalence for dairy, mixed and beef herds were respectively, 84, 89 and 53%; the true individual-animal prevalence for those types of herds were, respectively, 35, 43 and 31%; whereas, the true median within-herd prevalences were 36, 29 and 38%.     

Parameters to monitor dairy herd fertility and their relation to financial loss from reproductive failure

Relationships between herd fertility measurements and financial loss from reproductive failure in dairy herds were studied. Financial losses attributable to prolonged calving interval and forced replacement from reproductive failure were considered. Herd fertility parameters were calculated from artificial insemination and calving data (i.e. calving to first service interval, non-return rate 56 days after first service, percentage of correct inseminations carried out in the interval 18–24 days, fertility status, calving interval, an oestrus index and number of insemination per average cow present in the herd.

The herd fertility parameters were moderately-highly related to loss due to suboptimal calving interval (r=0.20−0.79 in absolute values), but only slightly related to losses due to forced replacement (r<0.17 in absolute values).

Repeatabilities, calculated over a 3-year period, were high for the interval to first service, non-return rate and the oestrus index (0.52−0.67) and moderate for percentage correct reinseminations, fertility status, calving interval and loss due to suboptimal calving interval (0.38−0.48). Repeatability of loss due to forced replacement was low (0.20).

In a regression analysis no herd fertility parameter was fitted with respect to loss from forced replacement. Loss due to suboptimal calving interval at herd level was best estimated by the oestrus index (R²=0.63), the addition of the interval to first service to the regression equation explained a further 10% of the variation between herds. It is suggested that the oestrus index and the interval to first service should be presented as management aids to monitor herd fertility.     

Seroprevalence of and risk factors for infectious bovine rhinotracheitis in beef cattle herds of Yucatan,Mexico

The main objective of this cross-sectional study was to estimate the seroprevalence of infectious bovine rhinotracheitis (IBR) in a population of non-vaccinated beef cattle in the livestock region of Yucatan, Mexico and to determine potential risk factors related to the seroprevalence. Also, we estimated the intraherd correlation (r_e) and design effect (D) of IBR seropositivity. Cattle were selected by two-stage cluster sampling. Blood samples were collected from 564 animals from 35 herds. Sera were tested for antibodies against IBR using the serum-neutralisation test. Information regarding the herd and each animal sampled were recorded through a personal interview with the farmer or farm manager. The data were analysed using fixed-effects logistic multiple regression. Thirty-four of the 35 herds had at least one seropositive animal. The animal true seroprevalence was 54.4%. Animals in large herds or in production had higher odds of seropositivity than those in small herds or growing. The r_e and D were 0.17 and 3.62, respectively.