Increasing Bovine leukemia virus (BLV) proviral load is a risk factor for progression of Enzootic bovine leucosis: A prospective study in Japan
Introduction
Bovine leukemia virus (BLV), of the genus Deltaretrovirus in the family Retroviridae, is etiologically associated with Enzootic bovine leukosis (EBL). BLV was initially isolated from cultured lymphocytes of cattle in 1969 (Miller et al., 1969). While more than 20 countries have successfully eliminated BLV through control programs, BLV remains common worldwide, with prevalence of up to 90% of herds in endemic areas such as Eastern Europe, South America and several Asian countries (Polat et al., 2017). In Japan, the proportion of farms that possesses more than one seropositive animal is 68.1%, and overall seropositivity is 28.7% and 40.7% in beef breeding and dairy cattle, respectively (Murakami et al., 2011, 2013).
Only approximately 5%–10% of infected cattle develop EBL, while the remaining majorities remain asymptomatic throughout their productive lives (Burny et al., 1988). Several studies have provided evidence on how the disease progresses through various cellular signaling dysregulation induced by the integrated BLV genome, the expression of BLV mRNAs or BLV proteins such as tax, BLV miRNA, or antisense RNA (Willems et al., 1990; Kerkhofs et al., 1998; Lefebvre et al., 2002; Szynal et al., 2003; Klener et al., 2006; Kincaid et al., 2012; Durkin et al., 2016; Gillet et al., 2016; Rosewick et al., 2017). In addition, the host immune system may be involved in the leukemogenesis of EBL (Konnai et al., 2017). Epidemiological studies have also reported possible links between genetic characteristics and EBL development (Rodriguez et al., 2011). However, there are no clear determinants that separate those that develop EBL from those that remain asymptomatic.
Recently, whole blood proviral load (PVL) levels have been used as an indicator of disease progression in BLV-infected cattle. Previous cross-sectional studies have reported that whole blood PVL levels of persistent lymphocytosis (PL) cattle and EBL cattle were significantly higher than those of asymptomatic BLV-infected cattle (Panei et al., 2013; Somura et al., 2014). However, asymptomatic BLV-infected cattle exhibited a very wide range of PVL levels (Panei et al., 2013). Moreover, because the long subclinical infection period makes it difficult to follow cattle until EBL onset in cohort studies, there have been no longitudinal studies to evaluate whether or not high PVL levels increase the risk of developing EBL.
In 2003, the Cattle Traceability Law was enacted in Japan. All cattle are thereby required to be ear-tagged with an identification number (ID) and registered to a national database that retains extensive information, including the date of birth, ID of maternal parent, breed and all transaction history from birth to slaughter or death (Sugiura and Onodera, 2008). Here, we conducted a prospective analysis of 866 cattle by the whole-herds ELISA test in a city within Kanagawa prefecture, Japan. Among the 866 cattle, we characterized the PVL levels of 407 seropositive cattle, and followed all enrolled cattle for possible EBL progression until the time of slaughter in a local abattoir or death at the farm. Here, we report the characterization of seropositivity and distribution of baseline PVL levels of asymptomatic BLV-infected cattle. We have also evaluated possible associations between PVL, longevity, and progression to EBL.
Section snippets
Study area
The study was conducted in the city of Shonan, located in Kanagawa prefecture, near the capital of Tokyo in the central Japan. This area was selected for long history of dairy farming and the presence of many small scale farms, with an average herd size of 32 cows per farm. These farms have been supplying milk to major urban areas of the country since the early 20th century. The mean herd turnover rate in this city is 21.6% (SD 5.8%) per year.
Sampling strategy
A convenience sample of 27 herds out of the 30 herds
Age difference in BLV seroprevalence
During the period from August 2015 until December 2015, a total of 866 cattle (850 Holstein, 13 Jersey and 3 Japanese Black Cattle) from 27 dairy herds were enrolled in this study. On the first day of the enrollment, blood was withdrawn and tested for anti BLV-antibody, followed by PVL assessments for antibody-positive cattle. 407 out of 866 cattle were seropositive to the BLV antibody. Age difference in BLV seroprevalence in dairy cattle is detailed in Table 1. The proportion of
Discussion
In this study, we prospectively evaluated 866 cattle which include 407 BLV antibody-positive infected cattle enrolled in our epidemiological survey between August 2015 and December 2015, followed by continuous observations for approximately three years until November 2018. Based on our results, at the time of enrollment, median PVL levels of EBL cattle were significantly higher than those of cattle that did not progress to EBL (Table 2). Because there exists the possibility that very small
Conclusions
In conclusion, here we demonstrated that high PVL levels and increased age are the significant risk factors useful to identify high-risk cattle for EBL development in the herds, and we encourage producers to measure PVL copy numbers of BLV-positive cattle. Our findings further support the idea that measuring PVL copy numbers is a useful strategy for eradicating EBL from herds. This knowledge might be useful to design a strategy for decreasing economic loss by EBL. Further molecular studies are
Declarations of interest
None to declare.
Acknowledgements
We thank the staff of Kanagawa Shonan Livestock Hygiene Service Center for blood sample collection. We also thank the staff of Meat Inspection Station, Kanagawa Prefectural Government for providing information of EBL cattle. This work was supported in part by Livestock Promotional Funds of Japan Racing Association (JRA).
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