By Will Gratwick
Will Gratwick qualified from the University of Liverpool in 2009. He then spent 4 years working in private practice in the UK, initially in a rural mixed practice in North Wales and then in a large, mainly dairy practice in Leicestershire. Eager for a new challenge, Will moved to South Africa in 2013 and is currently a clinical assistant (resident ) at Onderstepoort, studying for his MMedVet in Bovine Health and Production (Dairy herd health). His thesis is looking into the prevalence of EBL in South Africa.
Surveillance for Enzootic Bovine Leukosis (EBL), which has been carried out only on a very limited scale in South Africa, indicates along with anecdotal reports that the disease is highly prevalent within the national dairy herd. One of the unique features of the disease is the life-long persistence of the infection, making the infected animal a permanent source of infection to the rest of the herd.
What is EBL?
EBL is an oncogenic retroviral disease of cattle which is caused by bovine leukaemia virus (BLV). Following infection with the virus there is a transient viraemia and subsequent seroconversion, with both intralymphocytic BLV and humoral antibodies present for the rest of the animal’s life. Up to 30 % of infected cattle develop a persistent lymphocytosis (PL). The extent to which individual animals contribute to the spread of the virus within the herd is very variable and is affected by their proviral load as well as by the presence or absence of PL.
After an extended period, often more than 3 years, a variable but relatively small proportion (0.1-10%) of infected cattle go on to develop clinical EBL. This is characterised by the development of multiple solid lymphoid tumours of various organ systems and is ultimately fatal. Clinical signs are often non-specific and include reduced production and weight loss, with many cows being culled before a specific diagnosis is made. In the later stages of the disease external and internal lymph nodes will usually be hard and enlarged. Specific clinical signs such as exophthalmos, hind limb paresis and signs of heart failure may be detected depending on the location of the tumours.
EBL can cause significant losses to dairy producers, primarily due to reduced milk production, lower reproductive efficiency, increased culling and a reduction in the immunity of the cattle to other diseases. A large scale study carried out in the USA reported that being seropositive for the disease was associated with a 3% reduction in milk production and the total annual cost of the disease being present in a herd was estimated at $59 per cow. Outbreaks of clinical disease with significant mortality have occurred in South Africa.
Natural transmission requires transfer of blood containing infected lymphocytes between individual animals. This can occur during various routine procedures such as dehorning, injections, tattooing, blood sampling, branding, ear tagging and rectal examinations, particularly where poor hygiene is present. Transmission has been shown to be possible via both natural and artificial insemination and via biting insects but these routes are likely to be less significant under natural conditions.
Transplacental infection usually occurs in around 3-6% of cases, although this can be higher in heavily infected herds. While transmission can occur through colostrum feeding, in most cases this risk is outweighed by the protective effect of the antibodies in the colostrum against any infection which occurs during the calving process.
Treatment and Prevention
There is no commercial vaccine, treatment or cure available for EBL. To prevent the introduction of the infection into clean herds it is necessary to quarantine purchased animals and test them for antibodies to BLV on two occasions at least two months apart, to account for the delay between infection and seroconversion. Good general biosecurity measures must also be taken.
Eradication from infected herds is possible and can be achieved by limiting the spread of infection within the herd by adhering to strict hygiene and management procedures. This must be carried out alongside regular testing with marking of infected animals and selective culling or separation into dirty and clean herds. Eradication becomes more difficult in herds with a high prevalence (>60%) due to an increased chance of significant spread within the herd occurring despite all reasonable control measures being taken.
There is no evidence that BLV infection leads to clinical disease in humans and adequate pasteurisation of raw milk effectively denatures the virus and prevents infection from milk. BLV is, however, closely related to human T-cell leukaemia virus (HTLV) 1, which has been implicated in several human diseases including adult T-cell lymphoma. Antibodies to BLV have been identified in serum samples from healthy humans and further work identified that BLV was present in human breast tissue, which indicates that humans can be infected with the virus. Evidence was also found that the virus was replicating in a small proportion of those infected. As such, there is potential for consumer concern regarding the disease.
Complete eradication of BLV, as has been achieved in some European countries. While this is not considered to be possible in South Africa due to the likely high prevalence of the disease and the lack of identification and traceability within the cattle population as a whole, there are advantages to the individual farmer in determining and monitoring the presence /prevalence of this costly disease within their own herd. Disease free herds can protect their status through effective biosecurity while infected herds can implement a management plan to either eradicate or control the virus.
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