BLV is found within lymphocytes, which is one type of white blood cell. Most transmission to a new uninfected cow is thought to occur when intact lymphocytes from an infected cow enter the blood stream of an uninfected cow. Lymphocytes may be found in almost all body fluids, including blood, semen, colostrum and saliva. Any contact with these bodily fluids from an infected animal could transfer the disease, but only if the infected lymphocytes can get through the skin or mucus membrane barriers and enter into the new host’s blood stream. This is called hematogenous or blood borne transmission. The transmission of HIV/AIDS is the most well-known example of hematogenous transmission. BLV is most commonly transmitted among herdmates, i.e. horizontal transmission. Iatrogenic transmission (transmission associated with medical examination or treatment) is probably responsible for a large proportion of new infections. This can occur by reusing equipment contaminated with BLV-infected blood, such as reproductive examination sleeves, hypodermic needles, tattoo pliers, dehorners, etc. BLV can also be transmitted vertically across the placenta from a cow to a calf. This occurs in 4 – 18% of cases where the cow is infected with BLV. Infection can also occur by blood exchange during birthing and by ingestion of contaminated colostrum or milk.
Methods of BLV Transmission
- Reproductive exam and artificial insemination sleeves contaminated with BLV-infected blood
- Multiple-use hypodermic needles
- Gouge type dehorning and tattooing
- Colostrum and milk
- Biting flies
- Blood exchange during natural breeding
- Placental transmission and transmission by blood exchange during birthing
In a study conducted in Michigan15, the BLV prevalence within each of 113 Michigan dairy herds was determined by means of the BLV milk ELISA, and the managers of those herds were interviewed regarding herd management practices. Results of a multivariable analysis identified several management practices positively associated with within-herd BLV prevalence. Those practices included reuse of unsterilized hypodermic needles, lack of fly control, gouge-type dehorning, increasing number of reproductive examinations (almost always performed without changing reproductive examination sleeves between cows), and increasing number of injections to adult cows during the dry period15. Additionally, exclusive use of artificial insemination to breed heifers was associated with a decreased BLV prevalence15.
Although intervention studies are needed before causal relationships can be hypothesized, multivariable analysis helps to reduce confounding to reveal those factors that are independently associated with anti-BLV antibody prevalence, and therefore identify associations that are more likely to be causal in nature.
Internationally, segregation and culling of cows that test positive for antibodies against BLV are practices commonly used to control and eradicate BLV from cattle populations16,17. At the national level, successful control of BLV has involved programs that emphasize various combinations of three approaches: 1) management interventions with ongoing monitoring, 2) test and segregate positive cattle, and 3) test and slaughter positive cattle16,18,19. These methods have been employed in those countries that have eradicated BLV.
In a study conducted in a dairy herd with a high within-herd BLV prevalence19, implementation of single-use hypodermic needles and reproductive examination sleeves, disinfection of tattoo equipment, use of cautery-type dehorners, and feeding of milk replacer and heat-treated colostrum resulted in a decrease in the prevalence of BLV-infected heifers from 44% to 17% in 2 years. Management practices also associated with an increase in the risk of hematogenous BLV transmission include gouge-type dehorning and not changing hypodermic needles or disinfecting tattoo pliers between cattle17,20-26.
Failure to change bloodied examination sleeves between cows following artificial insemination or reproductive examination has also been identified as a risk factor for the hematogenous transmission of BLV. In humans, a frequent route of transmission for HIV/AIDS is the exchange of blood through rectal bleeding; therefore, it seems reasonable to hypothesize that this route may also be important for the transmission of other retroviruses such as BLV. In cows, rectal examination and artificial insemination involve the insertion of a sleeve-covered arm into the rectum, and it is common for rectal bleeding to be associated with these procedures. In the Michigan management study15, reproductive examination sleeves were routinely changed between each cow on only 5 of 113 herds. BLV was not detected in 2 of those 5 herds and the remaining 3 herds had a low within-herd BLV prevalence (mean, 5.8%), whereas the mean within-herd BLV prevalence for all 113 herds was 33%. Results of that study also indicate that the number of reproductive examinations per cow was positively associated (P= 0.03) with BLV prevalence15. These findings suggest that the use of a clean reproductive examination sleeve for each cow during artificial insemination or rectal examination could reduce BLV transmission.
Another finding in the Michigan study15 was that the use of natural service to breed heifers and cows was associated with increased BLV prevalence. Other researchers have also reported this association22,27,28. During natural service, penile or vaginal trauma is most likely responsible for the exchange of blood. Use of artificial insemination to avoid natural breeding might reduce BLV transmission in a herd. If bull breeding is necessary, only BLV-negative bulls should be used.
Results of multiple studies15,26,29 report that lack of a fly-control program is a risk factor for BLV infection. Biting flies are known to be a possible route of transmission, but it is not known what percentage of new infections occur in this manner.