small ruminant lentivirus ( srlv ) is a group of retroviruses ( family retroviridae , genus lentivirus ) that infect sheep , goats and are responsible for causing maedi - visna disease in sheep and caprine arthritis - encephalitis ( cae ) in goats . although srlv mainly spreads vertically through the ingestion of contaminated colostrum and milk , it is evident that horizontal transmission plays an important role in the spread of cae regardless of a goat 's age [ 1 , 2 ] . in herds where kids are fed only with virus - free milk , it was found that the within - herd incidence of srlv - infection increased , indicating horizontal transmission [ 35 ] . the two main horizontal routes of transmission are via respiratory secretions   and milk [ 7 , 8 ] as both the lungs and the udder are target organs for srlv infection . there are no reports of infection of does by srlv - positive bucks through natural mating , but infection of does by artificial insemination with srlv positive semen has been confirmed . srlv nucleic acid has been identified in the seminal fluid , non - spermatic cells and preputial swabs of bucks [ 1013 ] . two studies imply that srlv - infected bucks play an important role in horizontal srlv transmission . demonstrated not only rna and proviral dna of srlv but also expression of viral antigens in most of the organs of the reproductive tract of naturally infected bucks . .   showed that goat uterine epithelial cells were susceptible to srlv infection in vivo . a cross - sectional study was carried out to investigate the relationship between presence of srlv - seropositive bucks and seroprevalence of srlv infection in does in herds . the sampling frame included 76 goat herds which tested positive for srlv infection between 1996 - 2014 . the size of the herds ranged from 10 to 600 adult females with a median of 60 ( interquartile range 30 to 115 ) . a simple random sample of adult female goats from each herd was selected to allow detection of at least one seropositive animal at the expected within - herd seroprevalence of 10% and 95% confidence level . sample size ranged from 7 to 68 individuals with a median of 18 ( interquartile range 11 - 30 ) . the animals were screened with one of three commercial immunoenzymatic tests : indirect elisa checkit caev / mvv ( dr . bommeli ag , bern , switzerland ) between 1996 - 2014 , indirect pourquier elisa maedi - visna/ caev serum verification ( institut pourquier , france ) in years 2007 - 2010 , and i d screen mvv - caev indirect screening elisa ( idvet innovative diagnostics , france ) between 2011 - 2012 . to obtain true seroprevalence apparent seroprevalence was adjusted by the sensitivity and specificity of the elisa test used . they were 84% and 100% , respectively , for both the bommeli and pourquier elisas , and 91.7% and 98.9% , respectively , for the i d screen elisa . standardized questionnaires were used to gather knowledge of 3 hypothesized herd - level confounding factors : number of years for which a herd had existed until testing ( herd age ) , goat replacement from other herds in poland and use of machine milking . hierarchical linear regression model was performed in three - level fashion : first , the herd age was forced into the model to control for the role of time as cae is a slowly- spreading contagious disease and is likely to be more prevalent in older herds . then , two potential confounding factors were entered into the model as dichotomous variables ( goat replacement from other herds in poland and machine milking ) . at the last level presence of srlv - seropositive males factors were considered statistically significant if their p - value was < 0.05 . adjusted coefficient of multiple correlations ( r ) was a goodness - of - fit measure for the final model . within - herd seroprevalence of srlv infection ranged from 9.7% to 100% in the herds where seropositive males were present ( median of 60.1% , interquartile range from 35.7% to 87.9% ) and from 3.4% to 100% in the herds were only seronegative males were kept ( median of 35.8% , interquartile range from 10.1% to 49.6% ) ( fig . seroprevalence of srlv infection with respect to the presence of srlv - seropositive bucks in a herd controlling for the herd age , machine milking and goat replacement from other herds in poland , presence of srlv - seropositive bucks proved to be an independent factor linked to the higher within - herd seroprevalence of srlv ( p = 0.001 ) ( table 1 ) . srlv infection of bucks accounted for 12% of the total variability of the within - herd seroprevalence ( table 2 ) . the final model had an adjusted- r of 0.28 ( f(4,71 ) = 8.18 , random error = 693.57 , p < 0.001 ) . results of the hierarchical linear regression analysis evaluating the relationship between the presence of seropositive males in a herd and within - herd seroprevalence adjusted by possible confounding factors variables retained in the final model as potential confounders change of coefficient of multiple correlations ( r change ) associated with addition of subsequent variables to the hierarchical model includes only intercept and herd age machine milking and goat replacement from other herds in poland added presence of srl v - positive males in a herd added the study revealed that if all other confounding factors had remained constant the presence of seropositive males in a herd would have been associated with 22-fold higher within - herd seroprevalence on average . the buck effect was difficult to evaluate quantitatively as the range varied from a 10-fold to 35-fold increase in within - herd seroprevalence . this may be because the time bucks remain in direct contact with female goats in polish herds is usually restricted to the mating season and varies from 2 to more than 10 weeks . however , given that between - animal contacts are intensive and frequent in this time it is likely sufficient for srlv transmission , although route of this transmission remains questionable . two out of three potential confounding factors which were forced into the model turned out to be insignificant  machine milking and the time for which a herd had existed until testing . however , there is important rationale for retaining each of them in the model . the role of milking practices in the spread of srlv infection in a herd has been described   as well as a variety of possible mechanisms accounting for this route of transmission , including shared milking machines and backflow of milk into the teat during machine milking were proposed . even if machine milking had not proven statistically significant in our situation it was reasonable from the epidemiological standpoint to control for this variable in the model . the longer a herd exists the more likely the infection is to spill over into and spread within this herd . this conclusion holds only unless a cae control program is being implemented which is indeed the case in poland . much better would be to take into account the time for which a herd had been infected until testing . however , no such data are available since no constant monitoring of this disease has ever been carried out . the time for which a herd had existed until testing ( the herd age ) , may be considered as the best available estimate of the herd infection span . the most influential confounder was goat replacement from other herds in poland which resulted in 20-fold increase of within - herd seroprevalence on average . this result was justified by the fact that three consecutive serosurveys carried out for last 20 years has shown that between - herd seroprevalence is high and tends to rise rather than decline . given that horizontal route plays an important role in srlv transmission between animals purchase of infected goats precipitates infection spread and increases within- herd prevalence . nonetheless whether several weeks of direct contact are sufficient for respiratory srlv transmission remains doubtful as all available studies indicate that horizontal spread of srlv is a fairly slow process [ 3 , 7 ] . the study is also not able to determine the direction of the transmission  higher seroprevalence may result from the contact with infected males ; on the other hand males may be seropositive due to contact with many infected does . however , sexual transmission of lentiviral infections is believed to occur much easier from males to females than vice versa , mainly due to higher risk of abrasions of female genital tract during sexual intercourse . furthermore , although presence of srlv - infected cells was demonstrated in the uterus and oviducts   no data on infection of the vagina or external genital organs are available . concluding , the study indicates that seropositive bucks may facilitate the spread of srlv infection in goat herds and therefore their presence should be considered as a risk factor .
caprine arthritis - encephalitis is an economically important disease of goats . 
 it is evident that horizontal transmission through respiratory secretions and milk plays an important part in the disease spread whereas the role of sexual transmission remains questionable . 
 the cross - sectional study was carried out to investigate the relationship between presence of small ruminant lentivirus ( srl v)-seropositive bucks and seroprevalence of srl v infection in does in herds . 
 the analysis included 76 goat herds seropositive for srl v infection . 
 a sample of adult female goats from each herd was selected in a simple random fashion . 
 all males present in a herd were also enrolled in the study . 
 the animals were screened with commercial serological immunoenzymatic tests . 
 standardized questionnaires were used to gather knowledge of 3 hypothesized herd - level confounding factors : number of years for which a herd had existed until testing , goat replacement from other herds in poland and use of machine milking . 
 three - level hierarchical linear regression model was developed to evaluate the relationship (  = 0.05 ) . 
 median ( interquartile range ) within - herd seroprevalence of srl v was 60.1% ( 35.7% to 87.9% ) and 35.8% ( 10.1% to 49.6% ) in herds where seropositive males were present and absent , respectively . 
 controlling for possible confounders presence of srl v - seropositive bucks proved to be an independent factor linked to the higher within - herd seroprevalence of srl v ( p = 0.001 ) . 
 the study indicates that seropositive bucks may facilitate the spread of srl v infection in goat herds and therefore their presence should be considered as a risk factor .