Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of paratuberculosis Johne’s disease (JD), a chronic granulomatous enteric disease of domestic and wild ruminants. JD has become a widespread infectious disease problem for cattle herds in developed countries ( Manning & Collins, 2001 ).

JD causes serious economic losses in dairy farming, mainly as a result of reduced milk production (Kudalh et al ., 2004; Beaudeau et al ., 2007 ; Aly et al ., 2010 ), increased susceptibility to other diseases, especially mammary infections ( Tiwari et al ., 2005 ; Villarino & Jordan, 2005 ), loss of body weight ( Johnson-Ifearulundu et al ., 2001 ), and consequential premature culling and increased replacement costs. Paratuberculosis has also been related to reduced fertility rates ( Tiwari et al ., 2005 ; Dieguez et al ., 2008 ; Elzo et al ., 2009 ). Raizman et al . (2007) indicated that MAP was also associated with pneumonia.

The disease has a long incubation period and a slow course; the clinical signs typically appear only after two to five years. The main signs include progressive weight loss and chronic watery diarrhea in the most advanced stages; MAP bacteria slowly damage the intestines of infected animals and therefore signs get worse with time because the digestive tissue becomes increasingly injured. Enteritis causes malabsorption of protein and essential nutrients across the intestinal epithelium and in the later stages results in net protein loss.

As a consequence, the infection with MAP has been related to important losses of slaughter value ( Benedictus et al ., 1987 ; Chi et al ., 2002 ). More recently, Kudahl & Nielsen (2009) demonstrated weight losses of up to 10% and slaughter value losses up to 17% in seropositive cows. In a study performed in Spain, Vazquez et al . (2012) estimated that mean weight losses ranged from 3.7% for cases identified by the presence of microscopic specific JD inflammatory lesions to 12.4% for cases with positive results in the antibody ELISA test. Isolation of high bacterial loads in tissues and occurrence of diffuse granulomatous enteritis were associated with the highest weight loses, 22.2% and 26.0% respectively.

A study on immunization against MAP of adult dairy cattle suggested that vaccination have a positive effect on carcass weights of the animals with severe histopathological lesions ( Alonso-Hearn et al ., 2012 ).

In economic terms, the reduction in mean weight of culled cows attributable to paratuberculosis represented a loss of approximately 1,150 US$ paratuberculosis ( Johnson-Ifearulundu et al ., 1999 ).

Another previous study showed that there was no association between body condition score and MAP infection status, with the majority of the culture-positive cows being in good condition; although in this case no carcass data were assessed ( McKenna et al ., 2004 ).

However, there are fewer studies that consider the main carcass quality traits altogether in a multivariate approach: weight and conformation and fat cover scores. Providing accurate estimates on the economic impact of MAP, along with information regarding the incidence and epidemiology of the infection, an economic analysis could be conducted to measure the financial losses in dairy industry. This would provide a solid base to assess the feasibility of control programs.

The aim of the present paper was to assess the influence of MAP infection status at culling (as measured by antibody ELISA) on carcass traits: weight and conformation and fat cover scores.

Descriptive analysis of data showed that the mean observed carcass weight was 296.41 kg for seronegative animals and 231.28 for seropositive ones; the difference was statistically significant according to the t-test ( p <0.001) ( Table 1 ). Besides, 17 out 32 seronegative animals (53.1%) were scored O, while the remaining 15 (46.9%) were scored P; otherwise, 9 out of 32 seropositive animals (28.1%) were scored O while the remaining 23 (71.9%) were scored P. Regarding the fat cover score, among the seronegative animals, 13 out of 32 animals (40.6%) were scored 1, 4 animals (12.5%) were scored 2, 5 cows (15.6%) were scored 3 and 10 (31.3%) were scored 4. Among the seropositives, 22 out of 32 (68.8%) were scored 1, 7 (21.9%) were scored 2, 2 (6.3%) were scored 3 and 1 (3.1%) were scored 4. The chi-squared test and Fisher`s test also indicated significant differences in the distribution of both conformation and fat cover scores according to the animal´s serological status ( p =0.039 and p =0.014, respectively) ( Table 1 ).

Subsequently, the multivariate linear model showed that seropositive animals had, on average, carcass weights 58.45 kg lower than the seronegative ones ( p <0.001; Table 2 ). Cows culled at any stage of lactation also had lower weights than those slaughtered during the dry period ( p =0.006, p =0.003 and p =0.007 when comparing cows culled in early, mild and late lactation to those culled during the dry period; Table 2 ). The interaction term between seropositive and lactating at culling was significant when added ( p <0.001; Table 2 ). Age at culling did not seem to have an effect on carcass weight and was removed from the model.

As regards conformation score, the logistic model indicated that the odds of having P instead of O conformation was 3.85 times higher in seropositive cows ( p <0.003; Table 3 ). The odds was also higher when comparing cows in mid or late lactation with those culled during the dry period ( p =0.013 and p =0.033; Table 3 ); all the animals culled in early lactation (both seropositives and seronegatives) were scored P. The interaction term between seropositive and lactating at culling was again significant ( p =0.007; Table 3 ). As in the previous case, no association between conformation score and age at culling could be demonstrated.

Finally, ordinal regression indicated that the fat cover score was lower in seropositive animals ( Table 4 ) or in those culled during the lactation period. This score was not related to the age at culling. Thereby, the odds of seropositive animals achieving a higher score was approximately five times lower than the odds for seronegatives (Odds ratio=0.21, p =0.013, Table 4 ). The lowest scores were also observed most frequently in early ( p =0.032) and mid lactation (although not significant) ( p =0.057), followed by late lactation ( p =0.022) when compared to the reference category (dry cows) ( Table 4 ).

Each animal affected by JD falls into one of the four stages of disease ( Whitlock, 1992 ). Stage one is silent infection. Stage two is the pre-clinical stage, which consist of unspecific clinical signs (such as increased susceptibility to other diseases, reduced milk production or mild weigh loss), which may develop into stages 3 and 4, characterized by severe weight loss, intermittent diarrhea that can become permanent and possibly edema as a consequence of hypoproteinemia. The present paper aims to assess the effect of the infection on carcass traits before the appearance of specific signs (emaciation, diarrhea or edema).

The results observed in the present paper imply a reduction of slaughter value as a result of lower carcass weight and worse conformation and fat cover scores, which indicates that the cows suffered some weight loss, although not severe according to clinical observation; therefore none of the studied animals was eliminated for this reason. The decrease in weight and the worsening conformation and fat cover were different expressions of poor condition due to MAP infection, but considering that the carcasses’ value is dependent upon the three aforementioned traits, evaluating them independently would provide a more accurate estimation of the potential economic losses.

A possible limitation of the study is the use of antibody ELISA as the indicator of MAP infection. ELISA sensitivity ( Se ) is a direct function of the distribution of the infection stages in the test population ( Collins & Sockett, 1993 ). False negative results are more likely to occur in the initial stage of the infection; as the infection progresses the Se of the ELISA increases ( Sweeney et al ., 2006 ).

Nielsen & Toft (2008) indicated that the specificity of several commercial ELISAs ranged between 91% and 98%. The proportion of false positive results is expected to be low considering that the samples were pre-absorbed with Mycobacterium phlei. In any case, in all the studied farms, the presence of bacteria had been confirmed by means of fecal culture at least in one cow. Moreover only animals with at least two positive tests were included in the present study.

The sale of animals for slaughter represents, in the studied population, 7-8% of revenue. Owing to the falling prices of milk in recent times, Spanish farmers have been gradually increasing the sale of their least productive cows for slaughter, as a way to cope with the lack of financial liquidity. According to the reference prices in Spain, the value of a carcass with the average weight, conformation and fat score of a seronegative cow, as determined in the present paper, was €490 €350 in the case of an average seropositive cow carcass, which means 28.6% less ( Lonja Binefar, 2015 ).

Although most of the loss (77%) attributable to paratuberculosis was categorized as loss of future income as a result of suboptimal culling, 12% of the loss resulted from a lower slaughter value of infected culled cattle and treatment costs (that mainly consist of veterinary visits for diagnosis and extra farm labor cost due to disease); meanwhile, lower milk production accounted for 11% of the total loss attributable to paratuberculosis ( Groenendaal & Galligan, 2003 ). This underlines the importance of including carcass traits in the estimation of the economic losses incurred to the farmer.

Previous studies have differed in terms of the reduced value of the animal depending on the parameters considered (mainly carcass weight, and less often conformation score), the test used to detect MAP and the area of the study. Benedictus et al . (1987) found that slaughter value of infected cows was 30% lower than usual slaughter value, but they included animals that were culled because they had specific clinical signs of paratuberculosis. More recently, Chi et al . (2002) , assumed a 25% decrease in slaughter value for a cow affected with JD (also, by antibody ELISA) whereas Kudahl & Nielsen (2009) indicated that, compared with presumably unaffected cows, slaughter value was reduced by 17% in cows with positive ELISA at slaughter. Thereby, the reduction of slaughter value observed in the present paper was higher than that observed in other studies also based on antibody test.

Weight at slaughter was not only affected by MAP infection but also by lactation stage. After controlling by lactation stage, the present paper found an average reduction of more than 58 kg in seropositive cows, which means a 19.6% reduction compared to seronegative ones. Previously, Johnson-Ifearulundu et al . (1999) found that a 10% increase in the proportion of seropositive cows was associated with a 33.4 kg decrease in the mean weight of culled cows at herd level. At individual level, Kudahl & Nielsen (2009) indicated that the mean slaughter weight was reduced by 10%, in cows with positive ELISA at slaughter which is less than what has been observed in the present study. Another previous study, also carried out in Spain, estimated mean weight losses from 3.7% for cases identified by the presence of microscopic specific JD inflammatory lesions to 12.4% for cases with positive results in the paratuberculosis antibody ELISA test ( Vazquez et al ., 2012 ). Unlike most previous studies, McKenna et al . (2004) , using body condition scores, found no relationship between this trait and isolation of MAP in the intestines or lymph nodes.

A smaller number of studies have assessed the influence of MAP on conformation and fat cover scores. Kudhal & Nielsen (2009) used conformation scores to estimate the influence of MAP on the overall reduction of slaughter value. The present paper aims to measure the probability of having worse conformations or fat scores in MAP positive cows.

Recently, Whist et al . (2014) have studied several variables to assess whether a herd should be incorporated in a surveillance program for MAP. They concluded that the identification of slaughtered animals >3yr of age that had been classified as having low conformation and no or little fat could be considered a risk indicator for MAP.

The multivariate models indicated that detectable infection with MAP (as measured by antibody ELISA) resulted in less carcass weight, worse conformation score and thinner fat covers after controlling by lactation stage at culling and considering herd-cluster effects.

The estimated effects provide comprehensive information on losses related to lower slaughter value that could be useful for its consideration when performing cost-benefit analysis of a paratuberculosis control program.

The authors thank AFRICOR Lugo for its invaluable help with this study and F. Camino Preciado for the language correction of this manuscript.