Preventive Veterinao' Medicine, 4 ( 1987 ) 471-484 471 Elsevier Science Publishers B.V.. Amsterdam - - Printed in The Netherlands

The Ef fec t s of M a e d i - V i s n a Virus In fec t ion on P r o d u c t i v i t y in E w e s

I.R. DOHO0", D.P. HEANEY ~-. R.G. STEVENSON :~, B.S. SAMAGH 4 and C.S. RHODES s

'Animal Pathology Division, Agriculture Canada, 2255 Carling Avenue, Ottawa, Ontario, K I A OY9 (Canada) "-'Animal Research Centre, Agriculture Canada, Ottawa, Ontario, KI A 0C6 (Canada) :~Animal Pathology Laboratory, Agriculture Canada, Sackville, New Brunswick (Canada) ~Animal Disease Research Institute, Agriculture Canada, Ottawa, Ontario (Canada) "Department of Herd Medicine and Therlogenology, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan (Canada)

(Accepted for publication 10 March 1987)

ABSTRACT

Dohoo, I.R., Heaney. D.P. Stevenson, R.G., Samagh, B.S. and Rhodes, C.S., 1987. The effects of maedi-visna virus infection on productivity in ewes. Prey. Vet. Meal, 4: 471-484.

A total of five data sets obtained from four separate sheep flocks were used to evaluate the effects of subclinical maedi-visna virus infection (as determined by serological procedures) on produc- tivity in ewes. In general, infection with maedi-visna virus had a detrimental effect on productiv- ity. Infection resulted in a reduction in conception rates, with the odds of an infected ewe conceiving being only approximately 0.67 times those of a non-infected ewe. For ewes which did conceive, there was no detrimental effect of infection on the number of lambs born. The birth weights of lambs born to middle-aged (3-4 years) infected ewes were 3-6% lower than the weights of lambs born to non-infected ewes of the same age. There was very little apparent effect of infection in younger (I-2 years) and older (5 +years) ewes: some factors which may have biased these results are discussed. Lamb weights at 25-50 days of age appeared to be reduced by maedi-visna infection in the ewe but the results were not statistically significant.

INTRODUCTION

Ever since 1933, when maedi-visna (MV) was first introduced into Iceland, resulting in a major epidemic with mortality rates of up to 20-30%, the disease has been considered a potentially serious threat to the sheep industry {Daw- son, 1980). In the United Kingdom, where the disease has been recently intro- duced, it is a source of considerable concern. (Anon., 1984).

*Current address: Atlantic Veterinary College, University of P.E.I., 550 University Avenue, Char- lottetown, Prince Edward Island, C1A 4P3 Canada.

0167-5877/87/$03.50 © 1987 Elsevier Science Publishers B.V.

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There is ample evidence that the MV virus is widespread in North America. A survey of culled ewes in western U.S.A. revealed serological prevalence pro- portions that ranged from 1 to 68%, with four states having rates over 35% (Cutlip et al., 1977a). In Canada, a serological survey in Quebec revealed an overall prevalence of 36%, with rates for various regions of the province rang- ing from 29 to 68% ( Lamontagne et al., 1983 ).

Methods of eradication of the disease from a sheep flock have been devel- oped. These include eradication by removal of lambs from their dams at birth and keeping them completely segregated from potentially infected animals (Houwers et al., 1983) or by repeated serologic testing of the ewes and the culling of reactors (Houwers et al., 1984). Both of these methods are time- consuming and expensive for the producer. Before a logical decision whether or not to eradicate the disease from a flock can be made, it is important to know the prevalence of the infection in the flock and its economic impact. The economic impact of the disease depends on (1) mortality rate; (2) the inci- dence rate of clinically evident cases and the costs associated with those cases; (3) the effects of subclinical infections on measures of productivity and (4) the effect of the condition on the longevity of ewes.

Unfortunately, there is little known about the overall impact of clinically evident MV virus infection. The clinical signs associated with cases of MV have been well described (Dawson, 1980; Stevenson and Bouffard, 1984) but the incidence rate of clinical disease is not well documented. In the United Kingdom, a number of flocks which were apparently free of the disease until the early 1970s have now been shown to have widespread infection with MV virus ( Markson et al., 1983; Pritchard et al., 1984). However, clinical cases of MV have not been a prominent feature in these flocks. In fact, in one flock which probably became infected in 1973 and which had a serological preva- lence of > 50% by 1980, there had not been any clinical cases of MV up to 1981 (Markeson et al., 1983). In Quebec, despite the relatively high serological prevalence of MV, only 5.6% of all cases of pneumonia diagnosed at provincial and veterinary college diagnostic laboratories are attributed to MV (Dohoo et al., 1985).

While there is little known about the incidence of clinical MV, there is even less known about the subclinical effects of MV virus infection on measures of productivity in ewes. Similarly, there is very little information about the effect of MV virus infection on the longevity of ewes.

The purpose of this study was to determine if subclinical infection with MV virus has a detrimental effect on fecundity in ewes or on lamb's weights at birth or at 25-50 days of age.

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MATERIALS AND METHODS

The data

A total of five data sets (designated A-E) , arising from four flocks, were used in this study. The data contained in each of these data sets are described below.

(1) Data set A contained data from an eastern Ontario commercial flock containing both purebred and crossbred Clun Forest ewes. Data from 117 ewes which lambed in the spring of 1983 were available. The ewes were pastured in the summer, housed in semi-confinement in the winter and bred once a year (to lamb in the spring).

(2) Data set B contained data from the Agriculture Canada research flock at Nappan, Nova Scotia, containing 88 crossbred ewes which lambed in the spring of 1983. Management of this flock was similar to that used in (1), with the exception that hormonal synchronization of breeding was employed during the annual breeding period. Ewes were on a nutrition trial during the period in which these data were obtained.

(3) Data set C contained data from the University of Saskatchewan research flock at Saskatoon, Saskatchewan. Data from 270 ewes of various breeds which lambed in 1979 were available. The ewes were kept in small paddocks during the summer and housed in semi-confinement during the winter. The ewes were bred once a year.

(4) Data set D contained data from the same flock described in (3) but from the year 1983. Data were available from 267 ewes, of which 221 lambed. The management of the flock was essentially the same except that ewes were on an accelerated lambing schedule aimed at producing three lamb-crops every 2 years. Data from two lambing periods were used.

(5) Data set E contained data from the Agriculture Canada, Animal Research Centre research flock at Ottawa, Ontario. Data on 1097 ewes of various breeds were available and of these 763 lambed during 1979. The flock is housed in total confinement year-round. The lambs are artificially reared after removal from the ewes at 6-24 h of age (after ingestion of colostrum) and the flock is on an accelerated lambing schedule. The ewes were on a number of reproduc- tive trials dealing with factors which influence synchronized breeding such as light control, hormonal dose rates and method of breeding (natural service vs. artificial insemination). Data from two lambing periods were used. Approxi- mately 20% of the ewes were bred at 6.5-7 months of age for each lambing period.

The MV status of ewes in data sets C and E was determined by a complement fixation (CF) test (Girard et al., 1978). All other ewes were tested using an agar-gel immunodiffusion (AGID) test as described by Cutlip et al. (1977b) but modified to a micro-version.

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TABLE I

A listing of the variables ( and their acronyms) used in a study on the effects of maedi-visna virus infection on productivity in ewes

Acronym Description

CONCP

NBORN TOTB W TOTLW BREED AGE

EXP

MV PMA L E

BRSD

DAYSLW

Conception - a dichotomous variable representing whether or not a ewe conceived during a specific breeding period Number of lambs born per ewe lambing Total birth weight of all lambs born per ewe lambing Total weight of lambs raised by the ewes at time of performance testing A categorical variable representing the breed of the ewe A categorical variable representing the age of the ewe (categories were 1-2, 3-4 and 5 + years ) A single or a series of categorical variables representing the experimental group (s) that the ewe was in if they were cocurrently undergoing any experimental field trials. The farm identifier was included in EXP for those analyses in which data was pooled across farms A dichotomous variable representing serological positivity for maedi-visna A continuous variable representing the proportion of the lambs born which were male A categorical variable representing the number of lambs born and the number raised to a minimum of 28 days. ( Categories were: born and raised single; born twin and raised single; born twin and raised twin; born triplet and raised single, twin or triplet2) A continuous variable representing the actual lam~ ; age, in days, at the time TOTLW was determined

"Data from ewes with more than three lambs were excluded from the analyses.

Statistical analyses

All analyses .zere performed using either the Statistical Analysis System (SAS Institute, 1982a, b) or the Biomedical Computer Programs (BMDP, 1979). A description of the variables and their acronyms used in the analyses is presented in Table I. The variables CONCP, NBORN, TOTB W and TOTLW were dependent variables for statistical models. All other variables (BREED, AGE, EXP, MV, PMALE, BRSD, DA YSL W and NBORN) were used as inde- pendent (or predictor) variables. The variable EXP represented one or more separate categorical variables. It described the experimental make-up of the animals used in the analysis if any of them were on other trials. For data sets which pooled data from several farms or over several lambing periods, EXP also contained a categorical variable representing the lambing period and farm of origin. The actual number of variables contained in EXP varied depending on the complexity of the experimental design in the study flock (s). Data from data sets A-D were frequently combined in analyses. Due to marked differ- ences in the management practices employed in Flock E, and the much more

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TABLE II

Descriptive statistics of the data sets used in the evaluation of the effect of maedi-visna virus infection on productivity in ewes

Data n Serological NBORN TOTB W DA YS L W TOTL W set prevalence (kg) (kg)

of MV+ (%}

A 117 9.4 1.69 6.78 n.d. ~ n.d. B 83 33.7 1.48 6.39 n.d. n.d. C 261 7.7 1.80 7.71 29.9 18.0 D 221 24.4 1.69 7.25 47.2 27.0 A-D 682 16.6 1.71 7.25 35.9 20.9 E 763 65.8 1.80 6.40 n.d. n.d.

~No data.

complex experimental designs used in this flock, these data were always ana- lyzed separately.

Conception (CONCP) was modelled using stepwise logistic regression ( B M D P - L R ) with the terms BREED, AGE, MV and EXP forced into the model. The interaction term M V x A G E was made available for selection based on a P-value to enter of 0.05. Only data sets D and E contained data on breed- ings and lambings and hence were the only ones used in the analysis of CONCP. Logistic regression coefficients were converted to odds ratios according to the method described by Lemeshow and Hosmer (1984).

NBORN, TOTBW and TOTLW were all modelled using a general linear model based on the method of least squares (SAS-GLM) . NBORN was modelled as a function of BREED, AGE, MV, M V x A G E and EXP. A similar model was used for the dependent variable TOTB W, except that the indepen- dent variab!es NBORN and PMALE were included. TOTLW was modelled as a function of BREED, AGE, MV, MVxAGE, BRSD, BRSDxAGE and DAYSLW. Only data sets C and D contained lamb weights based on results from an official testing program {Record of Performance) and hence were the only ones used in the analysis of TOTLW. Models were only fit if the data set contained a minimum of 10 MV-positive and i0 MV-negative ewes. All tests of significance were carried out at P~< 0.05.

RESULTS

In total, data from 1445 ewes were used. The prevalence of MV in the flocks varied widely from 7.7 to 65.8% (Table II) . There was also considerable vari- ation in some of the productivity variables among the flocks. The number of lambs born per ewe lambing ranged from 1.48 to 1.80 and the total birth weight

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TABLE III

Proport ion of ewes positive for maedi-visna in various age groups

Data Age (years) set

1-2 3 4 5 +

A-D 2.8 18.2 25.7 32.5 E 59.0 77.2 76.0 74.0

of those lambs ranged from 6.39 to 7.71 kg. Since the lambs in data sets C and D were weighed at different ages, it is not possible to compare the lamb weights between those two data sets.

In the combined data sets A-D, the prevalence of serological reactions to MV increased with the age of the ewes (Table III). In data set E, all ages had a high serological prevalence but it appeared to increase up to the age of 3 years and then remain relatively constant.

Unconditional associations between MV status and the various productivity variables are presented in Table IV. The effect on conception rates was vari- able with an apparent decrease in data set A-D and an increase in data set E. For all the remaining variables (including the number of lambs born, their birth weight and the weights at testing) it appeared that infection with MV virus had a beneficial effect on productivity.

When the effect of MV virus infection on conception was examined in more detail by the use of logistic regression (Table V), it was found that it reduced the probability of a ewe conceiving. In data set E the reduction was statistically significant. In data set D the reduction, although similar in magnitude, was not statistically significant. The odds ratios shown in Table V indicate that

TABLE IV

Values of dependent variables for MV-negat ive and MV-posit ive ewes with no adjus tment for potentially confounding factors

Variable Data MV - MV + set

CONCP (%) D 89.2 86.1 E I 64.6 71.4

NBORN A-D 1.80 2.00 E 1.68 1.87

TOTBW (kg) A-D 7.15 7.75 E 6.18 6.51

TOTLW (kg) C + D 20.2 24.9

tApproximately 20% of the ewes in this data set were bred at 6.5-7 months of age.

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T A B L E V

Effect of maedi-visna virus infection on conception rates in ewes as determined by logistic regression

Data n Conception M V t e r m Odds set rate {%) ratio

Coefficient Significance

D 267 88.4 - 0 . 2 3 n.s. L 0.64 E 1097 69.1 - 0 . 1 8 0.05 0.69

'No t s ign i f i can t ( P > 0.05).

the odds of a MV-positive ewe conceiving were only approximately 0.67 times those for a negative ewe.

There was no evidence of any association between MV status and the num- ber of lambs born to a ewe that lambed (Table VI). The coefficients for the M V terms in both analyses were very small and non-significant. The interac- tion terms between M V and AGE were also non-significant, indicating that the effect of M V was not dependent on the age of the ewe. The R 2 values for these linear models were quite low (28.3 and 22.9% ).

The main effect of MV on birth weights ( TOTB W) was variable in that in one data set (A-D) it was positive and in the other (E) it was negative (Table VI). In neither case was it statistically significant. However, in both analyses the interaction term between M V and AGE was highly significant indicating that the effect of MV was dependent on the age of the ewe. Consequently, the data were subdivided according to the ewe's age (Table VII) and separate models fit for each age group. There was only a sufficient number of 1- and 2- year-old, MV-positive ewes in data set E to carry out an analysis. The results suggested that infection with MV virus resulted in a very small (0.08 kg) increase in the total birth weight of the lambs born. In ewes aged 3-4 years,

TABLE VI

Effects of maedi-visna virus infection on the number of lambs born to a ewe, the total b i r th weight of those lambs and the total lamb weight at testing, as determined by general least-squares models

Data n M V t e r m M V × A G E R 2 set term ( % )

Coefficient Significance Significance

N B O R N A-D 682 0.14 n.s. l n.s. 28.3 E 758 0 n.s. n.s. 22.9

T O T B W A-D 668 0.20 n.s. 0.001 67.8 E 758 - 0 . 1 1 n.s. 0.001 66.2

T O T L W C + D 347 - 0.57 n.s. 0.02 69.2

~Not significant (P> 0.05).

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TABLE VII

Effects of maedi-visna virus infection on the total b i r th weight of lambs born to ewes within specific age categories, as determined by a general least-squares model

Age Data n Serological M V term R e (years) set prevalence of ( % )

MV + Coefficient Significance (%)

I -2 A-D 246 2.8 i.d. ~ i.d. i.d. E 711 55.8 + 0.08 0.001 61.5

3-4 A-D 308 21.4 - 0 . 4 2 0.001 63.1 E 263 78.3 - 0.20 0.01 62.3

5 + A-D 117 32.5 + 0.04 0.01 67.8 E 133 75.2 0 n.s. 2 70.4

~Insufficient data. Th i s age-group conta ined only seven ewes positive for maedi-visna. ~Not significant ( P > 0.05 ).

there was a reduction in birth weights of 0.20-0.42 kg (data sets E and A-D, respectively). In ewes aged >t 5 years there was either a very small increase (0.04 kg in data set A-D) or no effect (data set E) . The R 2 values for the models with TOTB W as a dependent variable all ranged from 61 to 71%, indi- cating that the models explained a reasonable amount of the variation in the dependent variable.

It appeared that infection with MV virus resulted in a decrease of 0.57 kg in the weight of lambs at time of testing (Table VI) but this effect was not sta- tistically significant. In addition, the M V and AGE interaction term was sig- nificant so the data were subdivided according to age group and separate models fit (Table VIII ). No significant effects were observed in these separate models. There were insufficient data to construct a model for ewes aged 1-2 years and in the older age groups the effects of MV were negative but not statistically

TABLE VIII

Effects of maedi-visna virus infection on the total weight of lambs from ewes within specific age categories, as de termined by a general least-squares model

Age Data n Serological M V term R 2 (years) set prevalence of ( % )

MV ÷ Coefficient Significance (%)

I -2 C + D 114 2.6 i.d.' i.d. i.d. 3-4 C + D 172 17.0 - 0 . 5 5 n.s. 2 64.0 5 + C + D 83 29.6 - 0 . 3 2 n.s. 68.1

' Insufficient data. Th i s age group conta ined only three ewes positive for maedi-visna. :Not significant ( P > 0.05).

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significant. The R 2 values for the models with TOTLWas a dependent variable all ranged from 64 to 70%, indicating that the models explained a reasonable amount of the variation in the dependent variable.

DISCUSSION

The data

The data used in this study came from a variety of flocks having a wide range of serological prevalence proportions of MV. The flocks which made up the combined data set A-D had generally low to moderate prevalence proportions (range of 7.7-33.7% ) while Flock E had a very high prevalence ( 65.8% ). The flocks in data set A-D were also generally managed in a more traditional man- ner (based on summer pastures or paddocks and semi-confinement in the win- ter) compared to Flock E which was very intensively managed in a total confinement system. Since the effects which were being determined in this study were relatively small, it was necessary to combine the smaller data sets (A-D) in order to have a sufficiently large sample size.

One of the problems of a retrospective study based on existing data is that it is necessary to control for potentially confounding factors. The fact that the ewes in two data sets (B and E) were undergoing concurrent experiments meant that the experimental effects had to be taken into account during the analyses. This was particularly true for data set E, in which the ewes were undergoing reproductive trials since reproductive variables were being inves- tigated in this study. Despite this, it is unlikely that the experimental proce- dures have unduly affected the results of this study for two reasons: (1) the ewes were assigned to the experimental groups without any knowledge as to their MV status (equivalent to random assignment); (2) inclusion of terms for experimental group in the logistic and general linear models will have adjusted for the main effects of those t reatments .

The objective of the analyses in this study was to determine the effect of MV on measures of productivity. Other independent variables (BREED, AGE, PMALE, BRSD, DAYSLW, EXP) were included in the statistical models in order to control for their effects. It was not the purpose of this study to inves- tigate their effects on productivity and, therefore, for the sake of brevity and clarity their coefficients and significance are not presented in this paper. The effects of various variables on the weaning weights of lambs have recently been reported (Morris et al., 1984) and it was on the basis of those results that the interaction term between BRSD and AGE was included in the model for TOTLW.

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Serologic testing

The MV virus causes a persistent lifelong infection resulting in permanent seroconversion, so the use of serological criteria to identify infected ewes is reasonable. This is borne out by the fact that it is possible to eliminate MV virus infection from flocks by the use of a serological testing and culling pro- gram ( Houwers et al., 1984). However, the disease has a long incubation period and seroconversion does not occur until some months after infection ( Martin and Stamp, 1980; Dawson, 1980; Martin, 1983). Consequently, animals in the early stages of infection will be incorrectly classified as negative on the basis of a serological test. One may hypothesize that, given the slow rate at which MV produces lesions, these early infections may be of limited consequence to the animal.

A potentially more serious limitation to the use of serological tests to identify infected animals is the limited sensitivity of the tests involved. Both the CFT and AGID tests have been shown to have relatively high specificity (DeBoer et al., 1979 and Molitor et al., 1979) but the sensitivity of the tests is more suspect. Using the CFT, Lamontagne et al. {1983) found one region of the province of Quebec, Canada, to have a serologic prevalence of 68% and the same prevalence was reported by Cutlip et al. (1977a) for culled sheep from one state in the U.S.A. Consequently, the minimum possible sensitivities for those tests must approach 68%. Dawson et al. (1979) reported that the AGID was able to identify more than twice as many ewes as positive than CFT ( 56.8 vs. 23.8%) out of a sample of 185 sheep of unknown infection status. Both tests have been shown to be less sensitive than the more recently developed ELISA with relative sensitivities of 46 and 55% being reported for the CFT and AGID, respectively, by Houwers et al. (1982).

Given the apparently high specificity but low sensitivity of the tests used, it is likely that some infected animals were incorrectly classified as negative. Quade et al. (1980) have shown that misclassification of individuals can bias the estimation of a rate as well as increase the standard errors of the estimate and reduce the power of a study to detect a difference between two populations. In this study, any misclassification would have had the effect of reducing any real differences between the MV + and M V - groups and hence all the esti- mates of effects are likely to err on the side of conservatism.

Age distribution of MV

The increasing prevalence of MV with age that was observed in data set A-D has been reported previously (Gates et al., 1978; Light et al., 1979; Dawson, 1980). It suggests that horizontal transmission of the infection among adult ewes is an important mechanism of spread. The fact that the prevalence in Flock E peaked at approximately 3 years of age and then remained relatively

481

constant suggests three possibilities: ( 1 ) there may be a proportion of the flock which is either not susceptible to infection or does not seroconvert upon infec- tion; (2) the sensitivity of the test used was such that it was unable to detect all positive sheep; ( 3 ) premature culling at tr ibutable to MV offset any increase in prevalence.

Effects o[ M V on productivity

The unconditional associations between MV and various productivity vari- ables generally suggest that infection with MV virus enhances productivity. Huffman et al. (1981) reported a higher number of lambs born to M V + ewes and higher birth weights for those lambs but pointed out that age was a poten- tially confounding factor. Age has been identified as an important confounding factor in other studies of the effects of disease on productivity (Dohoo and Martin, 1984) and probably biased the effects reported in Table IV. Gates et al. (1978) reported no significant associations between MV s t a t u s and pro- ductivity but is is unclear whether or not the effects reported were adjusted for factors such as age and breed.

The results of the logistic regression with conception as the dependent vari- able (Table V) suggest that infection with MV virus has a detrimental effect on conception rates. The magnitude of the effect was very similar in both anal- yses (odds ratios of 0.64 and 0.69) but only the result obtained from the larger data set (E) was statistically significant ( P < 0.05). An odds ratio of 0.67 for conception is equivalent to an odds ratio of 1.5 for failure to conceive. This suggests that the odds of an MV + ewe failing to conceive were 1.5 times those of an M V - ewe. If a group of M V - ewes had a conception rate of 80% then the expected conception rate in M V + ewes in the same situation would be approximately 73%.

There is no evidence at all from this study that MV virus infection has any effect at all on the number of lambs born to a ewe that does conceive. The low R 2 values for the general linear models with NBORN as the dependent variable indicate that factors other than the ones contained in the model explain a large portion of the variability in the number of lambs born to a ewe.

However, there was an effect of MV on the birth weights of lambs. The fact that the interaction term between M V and AGE was highly significant in the analyses for both data sets A-D and E indicates that the effects of MV vary depending on the age of the ewe. When the data were subdivided into groups based on the ewe's age it appeared that there may be a small positive effect of MV on birth weights in young ewes (1-2 years) . The possibility exists that this small effect is in fact a spurious one due to the confounding effect of age. The age range 1-2 years would include some ewes lambing for the first time and some for the second. Ewes lambing for the second time would be expected to produce heavier lambs and would also tend to be at the upper end of the age

482

range and hence more likely to be sero-positive for MV. There were insufficient data in this study to refine the age variable further in order to fully investigate this possibility.

In 3-4-year-old ewes there was a significant reduction in the birth weight of lambs that amounted to approximately 3-6% of the total birth weight. In older ewes there was a very small beneficial effect in data set A-D and no effect at all in data set E. Given the slow but progressive nature of MV, it would be reasonable to expect the greatest impact in this age group. However, there may be a selection bias involved in thisstudy. If MV virus infection increases the risk that an animal will be culled from a flock, then only high producing MV -t- ewes may remain past 5 years of age. This selection procedure would tend to mask any adverse effects attributable to MV.

There were limited data with which to evaluate the effects of MV on lamb weights at the time of testing and the results are inconclusive. The interaction term between MV and AGE was highly significant in this analysis, but when the ewes were subdivided into age groups there were insufficient data to eval- uate the effects of MV in 1- and 2-year-old ewes. In older ewes, there was a trend that suggested that MV had a detrimental effect of approximately 0.3-0.5 kg. However, this effect was not statistically significant and will require con- firmation from studies with larger numbers of animals.

A study of this nature does not provide any information as to the mechanism whereby the MV virus exerts its adverse effect on productivity. It may be that subclinically infected ewes do have some impairment of respiratory function although given the low incidence rate of clinically evident respiratory disease, this seems unlikely. There have been a number of reports recently that link MV-virus infection to lesions other than those found in the respiratory tract. They include arthritis (Cutlip et al., 1985a), vasculitis (Cutlip et al., 1985b) and interstitial lymphocytic mastitis (Anderson et al., 1985; Cutlip et al., 1985c; van der Molen et al., 1985 ). This last finding may be of particular relevance to the possibility of MV reducing weight gains in lambs if the mastitis limits the ewe's milk producing ability.

ACKNOWLEDGMENTS

The contribution of valuable data by Mr. F. Calder of the Agriculture Can- ada Experimental Farm in Nappan New Brunswick is gratefully acknowl- edged. Christine Yamazaki is thanked for her assistance with the manipulation and analysis of the data.

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