The association of cattle husbandry practices, environmental factors and farmer characteristics with the occurence of chronic bovine tuberculosis in dairy herds in the Republic of Ireland

Preventive Veterinary Medicine, 17 ( 1993 ) 145-160

145

Elsevier Science Publishers B.V., Amsterdam

The association of cattle husbandry practices, environmental factors and farmer characteristics with the occurrence of chronic bovine tuberculosis in dairy herds in the Republic of Ireland

J o h n M . G r i f f i n a, T o m H a h e s y a, K e v i n L y n c h a, M . D . S a l m a n *'b'~, J o h n M c C a r t h y a, T o m H u r l e y a

aTuberculosis Investigation Unit, University College Dublin, Veterinary College, Ballsbridge, Dublin 4, Ireland bCollege of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523, USA (Accepted 5 May 1993)

Abstract

A matched case-control study was undertaken to provide information on the role of farm management practices, environmental factors and farmer characteristics in the epidemiology of bovine tuberculosis. Eighty dairy herds with chronic tuberculosis were compared with the same number of herds which had been free of the disease for many years. A standardized questionnaire was used to obtain information from the farmers. The study was conducted from August to October 1990, in Counties Cork and Kilkenny, Republic of Ireland. Factors which were identified as possibly contributing to recurrent outbreaks of tuberculosis outbreaks included nutritional factors, cattle purchases (especially bulls), the presence of badgers, and the spreading of slurry. Overall, the findings suggested that intensively managed dairy herds were at greater risk of bovine tuberculosis outbreaks than were other herds. The study did not support some of the hypotheses which traditionally have been put forward as contributing to tuberculosis outbreaks. These included contact with neighboring cattle owing to movements to and from fragments or poor boundary fencing, presence of sub-standard cattle housing, movement of equipment or vehicles onto farms, and exposure to water supplies from rivers or streams. In the light of these findings, and in view of the lack of evidence in the scientific literature to support these hypotheses, we suggest that a general re-evaluation of their role in chronic tuberculosis is needed. *Corresponding author. ~Author to w h o m reprint requests should be sent.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-5877/93/$06.00

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Introduction

The need for information on the epidemiology of tuberculosis has assumed increasing importance in Ireland because of the relative lack of progress of the eradication programs in recent years (O'Connor, 1986; Downey, 1990). The program began in 1954 and was based on a 'test and slaughter' policy using the single intradermal comparative cervical test as the method of diagnosis. Rapid progress was made between 1954 and 1965 when the herd incidence fell from 80 to 2.8% (Watchorn, 1965 ). However, this progress was not maintained and the herd incidence in 1988 was only slightly lower than in 1965 (O'Connor and O'Malley, 1989). A feature of the disease is its chronic nature in certain herds. Farm management and environmental factors such as feeding and nutrition, standards of fencing at farm boundaries, slurry disposal methods, cattle trading practices, farm hygiene and, in more recent years, the presence of wildlife have been put forward as an explanation for these recurrent breakdowns. However, until now a designed study has not been undertaken in Ireland to determine the relative importance of these factors. A recent study of the characteristics of farms which broke down with tuberculosis in Waikato in New Zealand indicated that the purchase of cattle was the most important factor (Pfeiffer et al., 1991 ). A literature search revealed that, apart from this study, little work has been done in this area to date. The objective of the case-control study reported here was to identify risk factors which were associated with the presence of chronic tuberculosis in dairy herds in Counties Cork and Kilkenny. The specific areas that were examined were: ( 1 ) farm characteristics, including fragmentation of holdings, herd demography, farm enterprises, the presence of badger colonies, and ecological features such as soil type and the presence of woodland which might influence the size of the badger population; (2) management practices, including feeding, summer grazing and wintering systems, slurry and manure disposal, fencing of farm boundaries, stock replacement and cattle movement; (3) selected personal farmer data such as age and marital status. Materials and methods

A matched case-control design was used (Breslow and Day, 1980). The cases consisted of dairy herds which had a history of recurrent tuberculosis between 1986 and the beginning of the study in August 1990. In Ireland, all herds in which tuberculosis is detected by means of tuberculin testing or during routine post mortem examination of carcasses in the abattoir are placed under movement restriction. These herds are tuberculin tested at 60 day in-

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147

tervals and the restrictions remain in place until the herd has two clear tests. In this study, herds were considered to have had chronic disease if they were placed under m o v e m e n t control twice between the beginning of 1986 and August 1990 or were restricted continually for a period of more than 12 months during that period. To improve the chances of finding associated risk factors, herds were only included as cases if one of the movement restrictions occurred in 1990. All such herds were included in the sample on a chronological basis, beginning on 1 January 1990, until the appropriate number of cases, i.e. 80 herds were obtained. Herds were included in the control group if they had been free of tuberculosis since 1982 and dairying was the main farming enterprise. The controls were matched with the cases on the basis of the following criteria. ( 1 ) Herd size. Herds were divided into three categories based on the total number of cattle present in the herd: 75 cattle or less; 76-150 cattle; 151 cattle or more. (2) Geographical proximity. After meeting the other criteria, a control was assigned to each case on the basis of closest geographical proximity. Two counties (Cork and Kilkenny) were included in the study. The sample size for the study was 80 cases and an equal number of controls. This sample size was chosen restrictively on the availability of resources rather than on the statistical calculation. The field investigations were carried out during August-October 1990. Only two selected case herds and three selected control herds were unwilling to co-operate and these were replaced by alternates. A standardized 21 page questionnaire (available on request from the senior author) was designed to obtain information directly from the farmers in the study. The questionnaire was pre-tested on eight of the dairy farms included in the study. These farmers were revisited following the revision of the form. Each question was read to the farmer and the interviewer selected the appropriate answer category based on the response. During the course of the interview, the locations of farm fragments were outlined on 6 in. ordinance survey maps to facilitate the response to the survey in terms of topographical features of the farms. The survey was undertaken by eight Teagasc (Agriculture and Food Development Authority) surveyors under the supervision of members of the Tuberculosis Investigation Unit in co-operation with the local District Veterinary Offices. A list of the most important variables used in the analyses is given in Table 1. The outcome for this study was whether or not the farm was classified as a chronic tuberculosis farm. The unit of observation for the primary analysis was the whole farm. In addition, comparisons between the home farms (i.e. the farm fragments surrounding the homestead) only, were undertaken for a

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Table 1 The names, descriptions, distributions a n d results of screening of categorical risk factors for bovine tuberculosis in 80 case and 80 control dairy herds Variable

Description

No. of herds

Cases (%)

Controls (%)

F a r m characteristics

ALTITUDE

BADGHOME ~

BADGHMAR ~

BEEF~

COWCHNGE ~

DRANHOME

EWES

HORSES

PIGS

POULTRY

ROUGH ~

ROCK l

SHELHOME

Home farm situated in upland area (altitude > 500 ft.) Less than 500 ft. More than 500 ft. Presence of badger colony on home farm No Yes Presence of badger colony within 1 mile of home farm No Yes Presence of beef enterprise No Yes Increase in cow numbers in last 10 years No change/decrease Increase Soil drainage on home farm Drainage poor or moderate Well drained Presence of ewes on farm No Yes Presence of horses on farm No Yes Presence of pigs on farm No Yes Presence of poultry on farm No Yes Presence of area of rough grazing on the home farm No Yes Presence of rocky outcrops on the home farm No Yes Presence of shelter belt on home farm No Yes

125 35

48 57

52 43

99 61

42 62

58 38

40 120

60 53

40 47

89 71

42 61

58 39

102 58

55 41

45 59

57 103

49 50

51 50

122 38

50 50

50 50

136 24

49 54

51 46

148 12

50 50

50 50

102 58

47 55

53 45

54 106

63 43

37 57

132 28

48 71

52 29

106 54

54 43

46 57

149

J.M. Griffin et aL / Preventive Veterinary Medicine 17(1993) 145-160

Variable

Description

SLOPE

Home farm had a sloping topography (slope > 3°C) Non-sloping Sloping Presence of store enterprise No Yes Presence of suckler enterprise No Yes Soil texture on home farm Heavy Medium Light Presence of woodland on or bordering the home farm No Yes

STORE

SUCKLER

TEXTHOME

TREEHOME

No. of herds

Cases (%)

Controls (%)

159 1

50 100

50 0

30 130

53 49

47 51

119 41

51 46

49 54

48 86 26

48 51 50

52 49 50

880 80

51 49

49 51

139 21

51 43

49 57

44 116

41 53

59 47

121 39

49 54

51 46

117 40

54 60

46 40

67 93

58 44

42 56

47 113

57 47

43 53

69 113

52 47

48 53

149 11

50 45

50 55

General management practices

COWBYRE

CUBICLE ~

FTROUGH

HAYPRIN z

HSECLEAN t

HSEDISIN

HSESUIT

LAKE

Use of cow byres for cattle No Yes Use of cattle house with cubicle standings No Yes Presence of unraised feeding trough (s) at pasture All raised All not raised Feeding of hay as the principle winter feed for one or more of the cattle groups No Yes Some cattle houses on the farm not cleaned annually No Yes Some cattle houses on the farm not disinfected annually No Yes Suitability of cattle houses for disinfection All suitable Some or none suitable Access of animals to lake/pond No Yes

J.M. GriJ]in et al. / Preventive Veterinary Medicine 17 (1993) 145- 160

150 Table 1 ( c o n t i n u e d ) Variable

Description

MAINWATR

Use of piped water supply for animals on farm No Yes Awareness of farmer of mineral deficiency on farm No Yes Mineral/salt licks provided to the herd Yes No Some cattle on the farm outwintered No Yes Access of animals on farm to river/ stream No Yes Use of rotational grazing system on farm No Yes Use of set stocking grazing system on farm No Yes Feeding of silage as the principle winter feed for one or more of the cattle groups No Yes Use of cattle house with slatted floor No Yes Use of straw bedded houses for cattle No Yes Presence of unraised water troughs at pasture All raised All not raised

MINDEFIC

NOMINLICK 1

OUTPRACT

RIVER

ROTGRAZ

SETSTOCK

SILPR1N 2

SLATS

STRAWBED

WTROUGH 1

No. of herds

Cases (%)

Controls (%)

12 148

58 49

42 51

121 39

51 46

49 54

103 57

67 41

33 59

113 47

49 53

51 47

74 86

53 48

47 52

23 137

43 51

57 49

47 113

55 48

45 52

11 149

36 51

64 49

98 62

50 50

50 50

13 147

46 50

54 50

50 110

38 55

62 45

J.M. Griffin et al. /Preventive Veterinary Medicine 17 (1993) 145-160

Variable

Description

Slurry manure disposal CTLMNOWN Cattle manure produced on farm No Yes CTLSLOWN ~ Cattle slurry produced on farm No Yes MANRSK Risk from solid on the basis of where it is spread Low Medium MNSTORE Manure spread without being stored Stored Not stored PIGSLOWN Pig slurry produced on farm No Yes PIGMNOWN Pig manure produced on farm No Yes PLTMNOWN Poultry manure produced on farm No Yes SLRSK Risk from slurry on the basis of where it is spread Low Medium High SPREDEQ Equipment hired or borrowed to spread farm waste Low risk High risk SLSTORE ~ Slurry spread without being stored Stored Not stored M o v e m e n t o f cattle a n d contact with neighboring herds (a) Standard of fencing and movement of cattle ADQCTFNC Fencing with contiguous herds inadequate Adequate Not adequate DOUBLECT Double fence at boundary with contiguous herds No Yes FENCEIMP 1 Fencing improved in last year No Yes

151

No. of herds

Cases (%)

Controls (%)

6 154

50 50

50 50

44 116

39 54

61 46

110 50

54 42

46 58

132 28

51 46

49 54

157 3

50 33

50 67

153 7

50 43

50 57

148 12

50 50

50 50

81 24 55

46 58 53

54 42 47

80 80

49 51

51 49

91 69

43 59

57 41

57 103

47 51

53 49

82 78

46 54

54 46

63 97

40 57

60 43

152

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Table 1 ( c o n t i n u e d ) Variable

Description

HOMEONLY

Some animals kept on home farm for entire grazing season No Yes Some animals moved between home farm and outfarm(s) Neighbor's cattle break in herd owner's herd in past year No Yes Neighbor's cattle move on adjacent roads No Yes Own cattle broke into neighboring herd in past year No Yes Own cattle move frequently on roads No Yes

MOVEBET NBRCATTL

NBRROADS

OWNCATTL

ROADMOVE

(b) Trading policy BULLPRCH

CALFPRCH

COWPRCH 1

DEALPRCH

DIRPRCH

FREQPRCH

HEIFPRCH

Purchase of bulls in the last 5 years No Yes Purchase of calves in the last 5 years No Yes Purchase of cows in the last 5 years No Yes Purchase of cattle from a dealer No Yes Purchased cattle directly from another farm No Yes Frequency of purchase of cattle Self-contained or buy bull only Buy infrequently Buy each year Purchase of heifers in the last 5 years No Yes

No. of herds

Cases (%)

Controls (%)

13 147 104 56

38 51 51 48

62 49 49 52

81 79

53 47

47 53

67 93

45 54

55 46

96 64

51 48

49 52

86 74

55 45

45 55

76 84

41 58

59 42

116 44

52 45

48 55

132 28

47 64

53 36

140 20

51 40

49 60

99 61

47 54

53 46

81 37 42

46 62 48

54 38 52

13 29

47 62

53 38

153

J.M. Griffin et al. /Preventive Veterinary Medicine 17 (1993) 145-160

Variable

Description

MARTPRCH

Purchase of cattle from market No Yes Purchase of stores in the last 5 years No Yes

STORPRCH

(c) Other contact FACSHR

FARMERCT

FARMRELF

FARMWORK

NEIGHBOR

RELATION 1

TRANSMED

Cattle crush or other facilities shared None Cattle crush/other Self in contact with cattle in other herds No Yes Farm relief worker (s) involved in farm work No Yes Farm worker (s) involved in farm work No Yes Neighbor (s) involved in farm work No Yes Relative(s) involved in farm work No Yes Use of neighbor's or contractor's transport for moving cattle None/own Others

No. of herds

Cases (%)

Controls (%)

89 71

45 56

55 44

137 23

49 57

51 43

141 19

49 58

51 42

123 35

51 49

49 51

139 21

48 62

52 38

132 28

5 46

49 54

125 35

49 54

51 46

125 35

46 66

54 34

42 118

52 49

48 51

63 97

46 53

54 47

23 137

48 50

52 50

122 38

48 55

52 45

Farmer data and attitudes

ADVCSERV

MARSTAT

STDMANAG

Use of agricultural advisory service Used in 1 year Not used in last year Farmer's marital status Single Married Standard of management average or less in opinion of surveyor Excellent/very good/good Fair/poor/bad

~P< 0.15; offered to multiple logistic regression model. 2Not necessarily collinear. In Ireland the cattle herd is divided into a number of groups for management purposes. HAYPRIN could be 'yes' if hay was fed as a principle feed to any of these groups. In some cases the cow group may have been fed hay as the principle feed and the younger cattle may have been fed silage.

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J.M. Griffin et al. / Preventive ~\eterinary Medicine 17 (I993) 145-160

number of variables. The descriptive statistics were generated using software of the Statistical Analysis Systems Institute (PROC FREQ and UNIVARIATE; SAS, 1985 ). The data were analyzed in two stages. In the first stage, the measure of association between each factor and the outcome was calculated using an unadjusted odds ratio (OR) (Breslow and Day, 1980). These were tested for statistical significance by a X2 test for independence, using a McNemar test for paired data (Remington and Schork, 1977). Means and standard deviations for non-categorical variables were tested for statistical significance by a paired t test (Remington and Schork, 1977 ). In the second stage, factors associated (P~<0.15) with the outcome at the initial screening were considered for multivariable modeling. Since a matched case-control design was used, conditional logistic regression (in which factors in the model were adjusted for the matched characteristics) was the appropriate model (Breslow and Day, 1980; Kleinbaum et al., 1982). The epidemiologic, graphics, estimation, and testing computing package (EGRET; Statistics and Epidemiology Research Corporation, 1991 ) was used to carry out the conditional logistic regression analysis. The conditional logistic regression model was constructed using a forward stepwise algorithm that assesses multicollinearity and interaction at each step of the algorithm, utilizing the likelihood ratio chi-square statistic (LRCS) (Breslow and Day, 1980). Results

Univariate analysis Overall, 22 risk factors were associated with chronic tuberculosis herds (e<~0.15). At the preliminary analysis stage, there was a significant difference in herd size between the case and control group (Table 2) even though they were matched for herd size. This was due to the fact that the herd size ranges, especially the 151 + range, were sufficiently large to allow the differences that existed between the cases and controls in the population to appear in the sample.

Multivariate analysis Of the 22 variables which passed screening stage 1, 16 were offered to the modeling and six provided a statistically significant contribution to explaining variation in tuberculosis levels and were included in the final logistic regression model. The logistic regression coefficients and their associated standard errors obtained from the fitted model were used to calculate ad-

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15 5

Table 2 Summary of non-cateforical variables in the study of risk factors associated with bovine tuberculosis outbreaks in Counties Cork and Kilkenny, August-October 1990, based on paired analysis of 80 case and 80 control dairy herds. Values are means (standard deviation in parentheses) followed by the range Variable

Description

Cases

Controls

ALLGRASS 1

Total amount of grassland (acres)

FRAGNUM

Total number of fragments

TOTAL 1

Total number of cattle on farm

STOCKRTE

Stocking rate (livestock units acre- ~)

HAYLU l

Quantity (acres) of hay saved/purchased per livestock unit in past year Quantity (acres) of silage saved?purchased per livestock unit in past year Quantity (tons) of concentrates fed per livestock unit in past year Farmer's age (years)

124 ( 68 ) 25 -407 2.2 ( 1.5 ) 1 -7 132 ( 86 ) 23 -526 0.94 (0.19) 0.5-1.4 0.1 (0.1) 0 -0.6 0.7 (0.3) 0 -1.4 0.5 (0.2) 0.1-1.3 48 (13) 26 -66

110 ( 57 ) 28 -360 2.4 ( 1.5 ) 1 -7 117 ( 62 ) 22 -394 0.93 (0.22) 0.5-1.5 0.1 (0.2) 0 -0.7 0.6 (0.3) 0 -1.3 0.4 (0.2) 0.1-0.9 46 (12) 21 -66

F a r m characteristics

SILAGELU 1 MEALSLU 1 AGE IP_<0.15.

justed odds ratios with 95% confidence intervals for the risk factors associated with having a chronic tuberculosis problem (Table 3 ). Four of the factors (BEEF, WTROUGH, FENCEIMP and COWCHNGE) which passed screening were not examined in the multivariate analysis because it was felt that the differences between the case and control groups occurred as a consequence of tuberculosis breakdowns in the case group rather than as an antecedent event. Beef enterprises, for example, are sometimes adopted in herds with chronic tuberculosis because of the loss of dairy cows due to tuberculosis or because of the difficulties in selling stores on the open market owing to movement restrictions. These were the likely explanations for the fact that a higher proportion of case herds had beef enterprises. A likely explanation for raised water troughs and improved fencing on a higher proportion of case farms is that these measures were undertaken by farmers following a breakdown to prevent access by badgers and to prevent contact between their cattle and cattle in neighboring herds. It is likely that a smaller proportion of case farmers had increased their cow numbers in the last 10 years because of restrictions imposed by the tuberculosis breakdowns and by losses of animals. It was not possible to continue with the variable MENLSLU (the quantity of concentrates fed for livestock unit) because of 19

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Table 3 The final logistic regression model of risk factors associated with herds having recurrent bovine tuberculosis outbreaks in Counties Cork and Kilkenny, August-October 1990 (paired analysis of 80 case and 80 control dairy herds) Risk factors

NOMINLICK No Yes BULLPRCH Yes No CUBICLE Yes No ROUGH No Yes BADGHOME Yes No HAYPRIN Yes No NOMINLICKX ROUGH No Yes

Adjusted odds ratio

95%CI

1 (2.7)

(0.55)

1 3.9

1.2

12.4

1 8.1

1.7

39.4

1 (1.7)

(0.43)

1 3.9

1.3

1 0.3

0.09

1 29.5

1.46

13.78)

(6.44)

12.2

0.98

594.4

missing values. The remaining variable omitted from the multivariate analysis was TOTAL (the total number of cattle on the farms). This was omitted because herd size was one o f the matching criteria used in the study design. The factors significantly associated ( P ~ 0.05 ) with an increased risk of a herd having a recurrent tuberculosis problem were the use of mineral lick supplementation for cattle, purchase o f a bull, the presence of cubicle housing, rough grazing areas and badgers on the farm. The use of hay as a principal feed for one or more groups of animals during the winter was associated with a decreased risk. Addition to the model o f all two-way interactions of the variables already in the model showed that the interaction between the use of mineral licks and the presence of rough grazing was statistically significant. Examination o f the reactor group on its own showed that the reactor rate in bulls was higher than in other types o f cattle. Twelve (35%) of the bulls that were housed during the winter prior to the survey were deemed reactor. Discussion This was an exploratory study. It provided a way of examining the influence of factors which might be associated with the persistence or recurrence

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157

of tuberculosis in dairy herds that could not readily be examined by other methods. The purpose of the study was to identify factors that could be further examined in other ways or that might be amenable to manipulation for the purposes of disease prevention. The difference observed in relation to the provision of mineral licks in this study may be related to a lower natural immunity in the case group of cattle. It was noted, however, that there was no difference in farmers' impressions of mineral deficiencies between the two groups. This was one of the few areas where the level of husbandry practice could have been perceived as being of a lower standard in the reactor group than in the clear group. There is some evidence that an animal's resistance to tuberculosis is reduced by a shortage of feed and/or an imbalanced diet. Reduced resistance to tuberculosis in mice and in cattle was attributed to food deprivation (Dubos, 1955; Tadeusz and Bouazza, 1984). A reduction in resistance to tuberculosis was attributed to a deficiency of protein and an increase of fatty acids in the diet of mice (Hedgecock, 1955 ), to a deficiency of protein in the diet of hamsters (Ratcliffe and Merrick, 1957 ), a deficiency of vitamins A, B, C, D, cobalt, copper, iron, iodine, and manganese in animal rations (Udris, 1983 ) and a deficiency of protein, minerals and vitamins in the diet of humans, cattle, guinea pigs and pigs (Kabrt, 1962 ). There was a strong interaction between the mineral lick and rough grazing variables in the model. The relative odds of having a chronic tuberculosis problem associated with not having access to mineral licks was 2.75 for herds which were comparable on all other factors in the model. The equivalent figure for rough grazing was 1.7. The relative odds of having a chronic tuberculosis problem increased substantially to 30 on farms where both of these factors were present, i.e. farms which had areas of rough grazing and which did not feed mineral licks. A possible explanation for this interaction is that areas of rough grazing, reflecting poor quality soil, are likely to be deficient in minerals and that a herd grazing such an area will be at an increased risk of having a chronic tuberculosis breakdown if they are not given mineral supplements. The maps that were used to record areas of rough grazing were published at the beginning of this century. It is likely that these areas have been reclaimed on many of the farms and are presently in pasture. A higher proportion of farmers in the case group had purchased bulls in the previous 5 years and the purchase of bulls was significant in the model. On the basis of these findings, it is suggested that there is a need for a more detailed study on the importance of the introduction of disease to a herd through the purchase of bulls. The study did not support the hypothesis that the level of herd breakdowns was related to the level of contact with neighboring herds. It is widely perceived, in Ireland, that bad fencing and movement of cattle along roads, to and from fragments, facilitated the transmission of disease

15 8

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(O'Connor, 1986). While the present study indicated that there was a high potential for contact between cattle in neighboring herds, it also showed that this applied to both case and control herds, indicating that these variables were not significant risk factors for chronic tuberculosis. There was an association between recurrent tuberculosis and the presence of cubicle housing. It is possible that this variable is a proxy for some other aspect of intensive management. However, it is also possible that this form of housing may be more stressful for cattle than other types of housing and that this factor plays a direct role in determining the susceptibility of cattle to tuberculosis. Cattle in cubicle houses will often have access to open yards and therefore they may be subjected to greater variations in weather than cattle kept in other types of housing. Such a housing system is also commonly associated with a self-feed silage system where the cattle feed directly at the silage face, and this may also cause more stress to an animal than the easy-feed system, where the silage is delivered to a feeding trough. The role of badgers in the epidemiology of bovine tuberculosis has assumed increasing importance in recent years (O'Connor and O'Malley, 1989). The present study provides further evidence that this wildlife species is making an important contribution to the level of tuberculosis found in cattle in Ireland. These findings must be treated with caution, however, because of the possibility that farmers in the reactor group were more aware of the presence of badgers on their land or in their area. The presence of badgers on the farm was used as a proxy for the presence of tuberculous badgers on the farm, the latter information being unavailable. The survey indicated that badgers were present not only in the areas where the case herds were located but also in areas in which the control herds were located. The widespread presence of badgers in the countryside may provide an explanation for the continuous geographical m o v e m e n t of tuberculosis in cattle in recent years. It may be that this spatial m o v e m e n t of disease in the cattle population is a reflection of a similar type of m o v e m e n t of the disease within the badger population. There is no obvious explanation as to why a greater emphasis on the feeding of silage and concentrates rather than hay should increase the risk of contracting tuberculosis. A higher proportion of case than control herds were fed 'straight' concentrate feeds as distinct from c o m p o u n d feedstuffs. Detailed studies are needed to determine whether deficiencies in such diets make animals more susceptible to infection when they are exposed to the Mycobacterium bovis organism. In the univariate analysis, it was found that farms which produced and spread slurry on pasture without prior storage had a higher probability of having tuberculosis in the cattle herd than farms which produced other types of manure or which stored the slurry prior to spreading. It is possible that these differences were also a proxy for intensive management. No difference was

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159

observed in relation to the risk to cattle from the spreading of slurry. Cattle were considered to be at high risk if they were grazed on land on which slurry had been spread in the previous 2 months. However, cattle may become infected from contaminated slurry in other ways, such as inhalation ofM. bovis organisms during the spreading process. Hahesy et al. ( 1992 ) recovered bacterial contaminants of slurry for distances of up to 300 m using conventional slurry spreading methods in windy conditions. The possibility, therefore, of a direct association between the production of slurry and the incidence of tuberculosis cannot be ruled out. The general impression gained from the study was that more intensively managed herds were at a higher risk of bovine tuberculosis outbreaks. Manifestations of extensive production included the use of cubicle rather than straw-bedded houses, the purchase of a stock bull, lesser use of hay and the production of slurry rather than solid manure. A possible explanation for an association between intensive production and tuberculosis outbreaks is that intensively managed herds are under greater stress, making them more likely to succumb to the disease. The study did not support other hypotheses which have been put forward as contributing to the recurrence of tuberculosis breakdowns in dairy herds. These included movement of equipment or vehicles onto farms, and exposure to water supplies from rivers or streams. A literature review revealed that there is little scientific evidence available on the role of these management factors in the epidemiology of bovine tuberculosis. Many of the beliefs in relation to herd management were developed when the level of tuberculosis in the cattle population was much higher than at present. In the light of the findings of this study and of the many changes that have occurred in the bovine tuberculosis program in recent years, there is a need to re-evaluate whether these traditional beliefs are still appropriate.

Acknowledgments We are grateful to the staff of the District Veterinary Offices in Kilkenny and Cork South West for their assistance with this study, especially Jim Walsh, S.V.I. and Harry Cummins, S.V.I. We are also grateful to Dr. Dick Power and his farm survey staff at Teagasc for their assistance with the field work. We are also pleased to acknowledge the assistance of members of the Tuberculosis Investigation Unit, especially Professor J.D. Collins, who read earlier drafts of the paper, Kathy Christiansen, Dr. Bob Hammond and Leonard A. Dolan, V.I., who assisted with the analysis and Sharon McAuley who typed the paper. The cooperation of the herd owners, without which the study would not have been possible, is also gratefully acknowledged.

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