Pig health and production surveillance in Denmark: sampling design, data recording, and measures of disease frequency

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PREVENTIVE VETERINARY MEDICINE Preventive Veterinary Medicine 20 ( 1994 ) 47-61

Pig health and production surveillance in Denmark: sampling design, data recording, and measures of disease frequency J. Christensen *'a'b, B. Ellegaarda, B. Kirkegaard Petersen ", P. Willeberg b, J. Mousing" "The Federation of Danish Pig Producers and Slaughterhouses, Veterinary Division, Axelborg, Axeltorv 3, 1609 Copenhagen F,, Denmark bRoyal Veterinary and Agricultural University, Department of Animal Science and Animal Health, Division of Ethology and Health, Biilowsvej 13, 1870 Frederiksberg C, Denmark (Accepted 12 November 1993)

Abstract

The Danish Swine Efficiency Control System and the Danish Pig Health Scheme are presented as the historical background for a 3 year pilot project called the Health and Production Surveillance system (HEPS). The overall objective of HEPS (launched in 1989) was to provide pig producers, their advisors, and other people associated with the pig industry with information about production performance, disease occurrence, and the impact of disease at the herd and national levels. This paper presents the sampling design, data recording and measures of disease frequency at the national level. The HEPS database includes information about production, clinical disease occurrence and routine veterinary slaughter inspection findings. All data concerning production and clinical disease were producer-recorded, while carcass lesions were recorded by meat inspectors at the abattoirs. A three-digit hierarchical disease coding system was designed for classification of clinical disease, mortality and culling reason. Disease frequency was measured on a daily basis. Smoothed estimates of the instantaneous incidence density showed that the most frequent diseases were: among sows, reproductive problems at farrowing (incidence 1.5 per 1000 sow-days); among non-weaned piglets, gastrointestinal diseases (incidence 0.8 per 100 piglet-days) and mortality (incidence 0.3 per 100 piglet-days); among fatteners, respiratory diseases (incidence 4 per 1000 fattener-days).

*Corresponding author. 0167-5877/94/$07.00 O 1994 Elsevier Science B.V. All rights reserved SSD10167-5877 (93)00343-4

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J. Christensen et al. / Preventive Veterinary Medicine 20 (1994) 47-61

1. Introduction

The need for valid information about animal disease occurrence, and cost of disease on the herd, regional, or national levels has been recognized by several authors (Van der Valk et al., 1984; Stein, 1986; Dohoo, 1988; Morley, 1988; Petersen et al., 1989; King, 1990). During the 1980s, several computer based programs were developed worldwide to monitor health and/or production in swine production.

1.1. Production monitoring systems in Denmark In Denmark, the predominant program for herd-level monitoring of pig production is the Efficiency Control System (Herl~v and Vedel, 1992 ). This system is one of the services offered by the local advisory service and consequently it is voluntary and paid for by the producer. It started as a manual control program in 1973. The first computerized Efficiency Control System designed for a mainframe computer was introduced in 1977 but since 1983, the system has been PCbased. Since 1980, the Efficiency Control System has covered 35% of all slaughter-pig producing herds which produce 55% of all pigs slaughtered in Denmark. In 1990, a new concept was devised in which all information concerning the farm was covered by a common PC-based information system. This Integrated Farm Management System consists of a joint module that integrates the modules for plant production, cattle production, pig production, buildings/machinery, and gross margin budget (Herlov and Vedel, 1992).

1.2. Slaughter check schemes in Denmark Since 1964, a central databank comprising slaughterhouse data from all Danish abattoirs has been maintained (Willeberg et al., 1984). In 1978, the Danish veterinary authorities and the pig producers agreed to develop and operate the Danish Pig Health Scheme (DPHS), based on data from this databank (Andersen et al., 1980). A pilot project was initiated in 1980 and the program gradually became a nationwide health surveillance scheme that, from 1982, covered 95-97% of all pigs slaughtered in Denmark. A producer-related element of the DPHS was designed to identify 'problem' herds. These herds were offered free advisory visits to improve their health status and production performance (Willeberg et al., 1984). In the mid-1980s, the DPHS was evaluated and improved (Mousing, 1986). However, further investigations revealed that the producers' willingness to pay for the DPHS was relatively low and that the advisory visits often were offered later than actions already taken to reduce the health problems in the herd (Mousing, 1988, 1990a,b). Also, the value of reporting the prevalence of carcass lesions from the DPHS to the producer became increasingly superfluous because they had access to the same information through the Efficiency Control System. Consequently in 1989, it was decided to close down the producer element of the DPHS.

J. Christensen et al. /Preventive Veterinary Medicine 20 (1994) 4 7-61

49

Therefore, free advisory visits are no longer offered (Mousing et al., 1990; Willeberg, 1992). However, the DPHS still provides information on the overall monthly prevalence of different types of carcass lesions by slaughterhouse and by health status, the categories of which are: ( 1 ) Danish specific pathogen free (SPF) (this category applies when a herd is declared free ofMycoplasma hyopneumon-

iae, Actinobacillus pleuropneumoniae, Serpulina hyodysenteriae, Pasteurella multocida toxin-producing, Haematopinus suis and Sarcoptes scabiei var. suis ); (2) SPF but infected with Mycoplasma hyopneumoniae (MS); (3) no declared disease status.

1.3. Initiation and objective of the Pig Health and Production Surveillance system In order to replace the producer element of the DPHS and to strengthen disease surveillance at the national level, a 3 year pilot project called the Health and Production Surveillance system (HEPS) was launched in 1989. The overall objective of HEPS was to develop an integrated system for monitoring health and production in Danish swine herds at the herd and national levels. The purpose of HEPS at the herd level (HEPS-herd) was to develop and test a PC-based program for local veterinarians for in-herd monitoring and analysis of health and production data (Ellegaard et al., 1992). (Since 1991, the program has been part of the Integrated Farm Management System. This program is available in Danish and can be bought at Landbrugets R~idgivningscenter, Landskontoret for Svin, Udk~ersvej 15, Skejby, DK-8200 Aarhus, Denmark. ) The objectives of HEPS at the national level (HEPS-n) were: ( 1 ) to develop a system for the collection and storage of data from three different sources: herds, abattoirs and other sources utilizing existing data from HEPSherd and routine veterinary slaughter inspection data; (2) to test and evaluate methods for national disease surveillance in a selected group of herds; ( 3 ) to provide data for analytical epidemiological studies. The purpose of this paper is to describe the design and to present preliminary experiences of the HEPS pilot project at the national level. The description will emphasize sampling design, method and frequency of data collection, types of data including the disease coding system, and measures of disease frequency. 2. Materials and methods

2.1. Sampling design Owing to economic and practical constraints, the sample size was set to 46 herds in the pilot phase of HEPS. The sampling of herds was performed in two stages. First, 12 pairs of local advisors (a local veterinarian and an agricultural advisor/swine specialist) were selected from a list of volunteers to represent different geographical areas in Denmark. Second, 46 herds (all kept completely indoors) were recruited from regu-

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J. Christensen et al. /Preventive Veterinary Medicine 20 (I 994) 4 7-61

lar clients of the 12 pairs. All participants were volunteers, as cooperation was considered vital in the pilot phase of the HEPS project. By 1 October 1990, all herds had used HEPS for at least 3 months and most herds started to use the Integrated Farm Management System in the autumn of 1991. Therefore, the preliminary results presented here represent the study period from 1 October 1990 to 1 July 1991 (273 days).

2.2. Types of data The data in the HEPS database originate from two main sources: abattoirs and farms. The slaughterhouse information on meat quality (meat percentage and weight) and routine veterinary slaughter inspection findings (pleuritis, acute pneumonia, atrophic rhinitis, abscesses, other lesions, and condemnations) is included in the Efficiency Control System and is regularly reported to the producer and his advisors. On the farm, production data and data on clinical disease occurrence are recorded by the producer. The production data may be classified into inventories and events (Table 1 ). All events are characterised by the relevant parameters, for example the farrowing statistics include sow identity, date, parity, number of piglets born alive, and number stillborn. Disease recordings may be grouped into the following events: mortality, culling, or treatment. Sows, gilts, and boars are identified by their individual number, and non-weaned piglets are identified by the number of the nursing sow. Weaned pigs are not individually identified but are grouped according to their weight. Only one clinical sign is recorded for an individual disease event. If a treatment was given for more than one condition at any given time, the producer would choose the most important of these. Table 1 An overview of the production and health data recorded in the Danish Health and Production Surveillance system for swine production Animal category

Inventory

Events

Sow/gilt/boar

Feed Stock

Entry/exit Culling/mortality Service Farrowing statistics Weaning Treatment Preweaning mortality Preweaning treatment Entry/exit Slaughter Mortality Treatment

Non-weaned piglets Weaning to slaughter

Feed Stock

J. Christensen et al. /Preventive Veterinary Medicine 20 (1994) 47-61

51

Table 2 Disease recordings in common for all herds participating in the Danish Health and Production Surveillance system for swine production Animal category

Event

Parameter

Sow/gilt/boar

Treatment Culling/death

Suckling piglets

Treatment/death

Weaned pigs

Treatment/death

Animal no., date, disease code Animal no., date, culling or death, culling/ death reason a Sow no., date, disease code, number of piglets treated/dead Date, disease code, number treated/dead, weight

aThe disease coding system is also used to identify culling or death reason.

In HEPS, a substantial number of optional health recording facilities is available to the individual herd (Ellegaard et al., 1992), but common to all participating herds are the disease recordings described in Table 2.

2.3. Method and frequency of data collection The data flow in HEPS is illustrated in Fig. 1. On the farm, the producer records disease events and production data. All events are recorded on the day they occur, and inventories of stock and feed are usually made every 3 months. Recording lists are sent to the local advisory service where the data are transferred into the Efficiency Control System program. At the local advisory service, information on entry, exit, farrowing, and weaning was compared with the inventory of stock in order to ensure data quality. The data were sent on diskettes to the HEPS database and to the local veterinarian for his/her advisory service at the herd level (Ellegaard et al., 1992 ). In the HEPS database, all on-farm recorded data as well as slaughterhouse information are stored in the Efficiency Control System software on a personal computer. ASCII files are defined and exported to the Statistical Analysis Systems Institute Inc. system (SAS Institute Inc., 1985) where basic data sets of animals-at-risk and cases per day are formed. Animals-at-risk per day is calculated from the information of entry, exit, farrowing, weaning and preweaning mortality events. Data from other sources are entered directly as SAS data sets.

2.4. Disease coding system Since HEPS is based on producer-recorded events, a disease coding system that could be applied easily by producers was important. At the same time, the intention was to develop a coding system which would also be useful in a problemsolving situation or during clinical trials. Hence, a three-digit hierarchical disease coding system was designed, in which the first digit describes a clinical sign, the next a crude disease classification and the third an etiological diagnosis, often

J. Christensen et al. / Preventive ~'eterina*T Medicine 20 (1994) 4 7-61

52

DATA SOURCES

~

....................... DATABASE

Farm

r ...................

! ........

Efficiency Control System ~ SAS data sets

IData for analysis or other research proj~cts~ !

V Infbrmation

Fig. 1. The data flow in the Danish Health and Production Surveillance system for swine production from the three sources of data to the database.

based on laboratory findings (Fig. 2). Diseases at farrowing and at service are defined as reproductive disorders at farrowing and at service, respectively. Clinical disease surveillance in HEPS is based on recordings of treatments, cullings, or deaths, grouped at the clinical sign level (first digit) of the disease coding system. Consequently, producers are required only to record clinical signs (first digit) but for their own purpose they are free to use the second or third digit also. For the estimation of incidence density, a new case was defined as a therapeutic treatment or the death of a gilt, sow, boar, non-weaned or weaned pig. Repeated treatments for the same clinical sign given within 6 days to an animal individually identified were not recorded and, where possible, a check was performed in the database. Preventive (strategic) treatments given regardless of the disease status were not recorded as new cases.

2.5. Measurement of diseasefrequency The HEPS database allows for disease surveillance in the study population on a daily basis. This may be described epidemiologically as a prospective space/

J. Christensen et al. /Preventive Veterinary Medicine 20 (1994) 4 7-61

53

CLINICAL SIGN: 100 200 300 400 500 600 700 800 900

Death Respiratory disease Gastro-intestinal disorders Lameness Disease at farrowing Disease at service Injury Systemic disease Other diseases

DISEASE CLASSIFICATION: 200 210 220

Respiratory disease Pneumonia Enzootic pneumonia

290

Other

ETIOLOGIC CLASSIFICATION: 210 211 212

Pneumonia Pasteurella pneumonia Streptococcus pneumonia

Fig. 2. The principle of the disease coding system in the Danish Health and Production Surveillance system for swine production. The first digit describes a clinical sign, the second a crude disease classification, and the third an etiologic diagnosis.

time cluster study of a dynamic population (Kleinbaum et al., 1982) in which the disease frequency is measured by the instantaneous incidence density at day b(2(b)) (Breslow and Day, 1980)

~,(tj) -/jdnj

(1)

where dj is the number of new cases at day tj, lj is 1 day (the length ofjth time interval), and nj is the number of animals under observation at day tj. The denominator is equal to the population time, and it reduces to nj for practical calculations since lj = 1 day. The incidence density in the study period was calculated as J

3(tj) =

~£(t,)

i=1

(2)

where j = 273 days. The standard error is calculated as [ . ,1/2 (3)

54

J. Christensen et al. / Preventive Veterinao' Medtcine 20 (1994) 47-61

where j = 273 days. A non-parametric smoothed estimate of the instantaneous incidence density was calculated as

1~

[t-t,\d,

where j = 2 7 3 days. Here, K is a function defined by K ( x ) = O . 7 5 ( 1 - x 2) for - 1 ~
1 F ~ K 2[t-t''~d~-]

1/2

(5)

3. Results

Of the 46 herds that initially volunteered, two never started to record disease data and one was sold. During the first year an additional two herds dropped out because they did not recognize sufficient benefit from the program. Thus, 41 herds (89%) remained in the study from the spring of 1990 until the end of the pilot project in the summer of 1992. Twenty-three herds were farrow-to-finish herds, 15 were farrow-to-feeder herds, and three were specialized fattening herds. At the start of the project, 23 herds had no declared disease status, five were SPF, seven were MS production herds, and the remaining six were multiplying herds with health status SPF or MS. The average ( + SE) herd size was 153 _+68 sows a n d / o r 780 + 423 fatteners. Disease at farrowing was by far the most frequent disease among sows (average incidence of 1.5 per 1000 sow-days), while all other diseases among sows were relatively rare (Table 3). Consequently, for sows all disease categories among sows except for disease at farrowing were grouped together. The 95% confidence intervals of the smoothed instantaneous incidence density were indicated in Fig. 3 and in Fig. 4 for gastrointestinal disorders. The estimated preweaning mortality and incidence density of clinical diseases in non-weaned piglets are shown in Fig. 4. During the period from 1 October 1990 to 1 July 1991, approximately 20 000 fatteners were at risk every day. In the same period, 22 503 treatments were given to pigs with a body weight of more than 20 kg. That gives the crude incidence of 4.1 per 1000 fattener-days (22 5 0 3 / ( 2 0 0 0 0 × 2 7 3 ) ) . Respiratory disease recordings represented 54.6% of all treatments given to fatteners, and respiratory disease, gastrointestinal disease, lameness, and mortality represented 95% of all

J. Christensen et al. / Preventive Veterinary Medicine 20 (1994) 47-61

55

Table 3

(J~273) o f disease a m o n g sows and non-weaned piglets in the D a n i s h Health and Production Surveillance system for swine production during the study period. T h e average incidence density (~'=A273/273) and for comparison the e s t i m a t e d incidence rates from the National Swine Survey ( N S S ) ( U S D A : A P H I N : V S , 1992) are also given. S t a n d a r d errors are given in parentheses T h e incidence density

Disease classification

3273

,~

NSS £

(per 1000 sow-days) Disease at farrowing Gastrointestinal disorders Death Disease at service Lameness O t h e r diseases Respiratory disease Systemic disease Injury

421.3 ( 138.9 ) 22.6 (16.9) 44.0 43.6 17.1 19.3 22.6 6.3

(26.2) (33.1) (13.7) (12.0) (11.3) (6.1)

f~273

"~

NSS £

(per 100 piglet-days)

1.5 0.1

-a 0.1

210.4 (23.4)

0.8

0.3

0.2 0.2 0.1 0.1 0.1 0.0

-a 0.2 _b 0.0 _b _b

92.7 (15.9) 36.2 (9.7) 12.6 (4.9) _ _

0.3 0.1 0.0 _ _

0.6 0.0 0.2 _ _

aThe incidence o f total reproductive disease in the D a n i s h Health a n d P r o d u c t i o n Surveillance system is 1.7 per 1000 sow-days (0.2 + 1.5 ). In NSS, the incidence of'non-farrowing reproductive problems' a n d 'milking p r o b l e m s ' is 1.5 per 1000 sow-days. bin HEPS, the incidence o f disease except for respiratory disease, gastrointestinal disorders, lameness, a n d reproductive disease grouped together is 0.2 per 1000 sow-days. In NSS, the incidence o f the similar group o f diseases is 0.4 per 1000 sow-clays.

registered treatments (Fig. 5 ). The weight class distribution of treatments for the four most common diseases is shown in Fig. 6.

4. Discussion In order to assess the validity of the disease surveillance, the sampling method in the pilot project needs to be considered. First, willingness to participate in the pilot phase was regarded as more important than having a random sample. Second, the target population was Danish swine herds using the Efficiency Control System program. The Efficiency Control System covers mainly larger herds and accordingly the average herd size in the HEPS pilot project of 153 sows and 780 fatteners is considerably larger than the corresponding national averages of 49 sows and 263 fatteners (Danske Slagterier, 1991 ). In this study the preweaning mortality was 4.3% of piglets born alive. The corresponding figure (_+SE) for 1591 herds using the Efficiency Control System from October 1991 to October 1992 was 11.4_+ 3.6% (M. Holm, personal communication, 1993). Therefore, generalizations from the results of the pilot project to the internal and external population may not be valid. However, HEPS is projected to expand gradually to a national surveillance scheme based on a random sample of herds

56

J. Christensen et al. / Preventive Veterinary Medicine 20 (1994) 47-61 2.1F

2,0t

--

--

•':

1.9

~'~ "",,,

1.6

''"'

~I.~

\'t

]

tl

//-- "",

",

// kf.~

I

/

-

,-. SOWS,DISEASEATFARROWING

""~'//~\\ /

'

,

,....

""

'" "" \

\

-.

-.

.

,

.

/

~l,I'C ~0.~ LU

zO0.~

lo,e

."

o,5

.

J ,""

... "~

o,4 o,3 0.2

SOWS, O'IHER DISEASES

0.1 0.(; Oql'~) NOV90 DEC90 ~

FEB~ ~

~R91 MAY~ JUN~ JUI.~

DATE

Fig. 3. The smoothed estimates of incidence density of disease at farrowing (dashed line) and other diseases (solid line) among sows in the Danish Health and Production Surveillance system. Ninetyfive percent confidence limits are indicated by the dotted lines. The band width is set to b = 21 days. The horizontal axis indicates the first day of the month•

using the Efficiency Control System. This will improve the validity of inferences made from the results of HEPS. Data quality is another important issue in observational studies and the producer-recorded data in particular must be regarded with caution. All producers in the HEPS pilot project were accustomed to the recording of production data for the Efficiency Control System and only a few additional data were required to establish disease surveillance as well. Also, from the producer's point of view, participating in HEPS made in-herd health monitoring possible and improved the basis for veterinary advice. Some producers claimed that an immediate benefit of HEPS was the use of recording lists. They were regarded as valuable tools in the day-to-day management, especially when there were several people employed in the herd, and they were an important incentive to record disease. The quality of denominator data (animals-at-risk per day) for calculating incidence density is high. The stock is counted at 3 month intervals and subsequently, the producer is confronted with the accuracy of his records of entries and exits from the herd. The disease coding system ensures that almost all swine diseases occurring in

J. Christensen et al. /Preventive Veterinary Medicine 20 (I 994) 4 7-61

57

0.9 j

G~TRO -INI~TINAL

~0.8 I 0.7 IL

! ~0.~

o z DEATH

02 LA~IENESS

0.1 0.0 i

OCTgO

i

i

i

i

i

r

i

~L91

DAtE Fig. 4. The smoothed estimates of preweaning incidence density of gastrointestinal disorders, death, lameness, and other diseases among piglets in the Danish Health and Production Surveillance system (band width b=21 days). Only for gastrointestinal disorders are 95% confidence limits indicated. The horizontal axis indicates the first day in the month.

Denmark are recordable if all three digits are used. If only clinical signs are recorded in relation to treatment or death, the interpretation becomes relatively simple. In our experience, the grouping of disease events into the categories 'death', 'coughing/hyperventilation', 'diarrhea', 'lameness', disease 'at farrowing' or 'at service', 'injury', 'systemic disease', or 'other/unknown' was easily interpreted. The recordings were standardized by recommendations of how to record diseases but the disease recordings were not validated owing to the lack of a gold standard for these recordings. Although these first-digit recordings have not yet been validated, the relative sensitivity of producer-recorded causes of preweaning mortality (a second digit recording) was evaluated in a separate study. Generally, the relative sensitivities of mortality causes were low (Christensen and Svensmark, 1992 ), which underlines the fact that the validity of the crude disease classification should not be over-emphasized (Vaillancourt et al., 1990, 1992 ). For disease at farrowing, one could argue that cumulative incidence (CI) was a more appropriate measurement than incidence density because the risk of disease is closely related to the time of farrowing, Nevertheless, incidence density was preferred over cumulative incidence for the following three reasons. ( 1 ) The

58

J. ('hristensen el al, / Preventive l'eterinary Medicine 20 (1994) 47-6 l PEP,e.,EIqf

50

40

30

20

10

0 :

~ RESPIRATORY DIARRHOEA LAMENESS

DEATH

INJURY

OTHER

DISEASE~RCATION

Fig. 5. The distribution of 22 503 cases by disease code in fatteners (representing 5 460 000 fattencrdays) in the Danish Health and Production Surveillance system during the period from 1 October 1990 to 1 July 1991.

aim was to study the frequency of disease over calendar time. (2) The proportion of sows at high risk (namely, the nursing sows) relative to all sows was almost constant (mean+SE, 0.18+_0.009; minimum, 0.15; maximum, 0.20). Therefore, sows-at-risk is a good estimate for population at risk and (3) although not closely related to farrowing, abortion was classified as a disease at farrowing. An apparent change in incidence for other diseases among sows was seen in November 1990, January 1991, and April 1991 (Fig. 3) but care has to be exercised in the interpretation of the confidence limits (Andersen et al., 1993 ). However, closer examination of the data revealed that the treatment frequency for respiratory disease increased in four herds during November 1990. The peaks in January and from April were due to treatments for systemic disease in one herd and for disease at service in another. Likewise, disease at farrowing increased in November, January and February (Fig. 3 ) which could be due to misclassification because producers tend to record any disease that occurs at the time of farrowing as disease at farrowing. The only comparison that was made was with the National Swine Survey (NSS) (US Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services, 1992), to the knowledge of the authors these are the only comparable data published. The preweaning mortality in HEPS appears to be

J. Christensen et al. /Preventive Veterinary Medicine 20 (1994) 47-61

59

FREffOB~CY 4000~

~00

2O0O

D

~ y

d~oas8

[]

Gasto- ~ n a l

dsora~

1000

20-24

25-34 35-44 45-54 55-64 66-74 75-84 W~n CaSS~

85-

Fig. 6. The distribution of disease recordings in fatteners by weight class in the Danish Health and Production Surveillance system during the period from 1 October 1990 to 1 July 1991.

half the mortality in NSS but the preweaning morbidity appears to be considerably higher in HEPS (Table 3). These differences may be due to a commonly used treatment strategy in HEPS. Often all piglets in a litter are treated if only one or two develop diarrhea. The difference in mortality may be explained by the fact that the HEPS herds were better than Danish herds on average.

5. Conclusion The HEPS pilot project has met the overall objective to develop a prototype and to evaluate the feasibility of an integrated system for monitoring health and production in Danish swine herds. Estimates of disease frequency in sow herds are available from the preliminary results presented here but estimates for sow mortality and mortality and morbidity among fatteners will be presented later. The data from this study will be utilized to describe variation among herds, detection of changes in disease frequency and analytical studies.

Acknowledgments We thank all of the participating pig producers, the local veterinarians, the local advisory services, and the Danish Agricultural Advisory Centre for their cooperation.

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