BLV results in reduced milk production, malaise and death in most affected cattle. When exposed to the virus, not all cows become infected. Those that do become infected remain infected for the remainder of their life. Exposure or infectiojn by the virus does not equate with clinical disease. Many cattle remain viraemic without clinical signs.
Clinically affected cattle present with general malaise, fever, lymphadenopathy and anorexia. About one-third develop persistent lymphocytosis. Only a small percentage develop visible tumours (lymphosarcoma).
Juvenile lymphosarcoma occurs most often in calves under 6 months of age, characterized by diffuse lymphoid hyperplasia with or without visceral organ involvement. Weight loss, fever, tachycardia, dyspnea, bloat, and posterior paresis have been reported. Profound lymphocytosis (>50,000/μL) often accompanies this fatal form of bovine lymphosarcoma.
Thymic lymphosarcoma usually affects older calves and young cattle under 2 years of age, with signs referable to intrathoracic (Thymic) tumours causing respiratory disease, as well as accompanying lymphocytosis.
Cutaneous lymphosarcoma presents as plaques (1 – 5 cm) on the skin. Regional lymph nodes may also be enlarged. This form of lymphosarcoma may undergo spontaneous remission; however, relapses may occur.
Lymphosarcoma is often included on the differential list for many diseases because of the wide range of clinical findings. ELISA and PCR tests are often used to test bulk-milk samples to monitor dairy herds. Serology is unreliable in calves that have ingested colostrum from BLV-positive cows due to the passive acquisition of maternal antibodies that typically wane by 4–6 mo of age. PCR testing is a sensitive and specific assay for diagnosis of BLV infection in peripheral blood.
Confirmatory diagnosis requires cytological or histopathological analysis of submitted tissue samples.
There is no treatment for viral infection or for lymphosarcoma in cattle and eradication programs have varied success in eliminating this disease from a herd.
Prevention aims at eliminating the movement of blood from infected animals to naive animals. In at-risk calves, feeding colostrum from seronegative cows is often advocated. The replacement of whole milk feeding with high-quality milk replacer may also be considered.
Cautery or other bloodless methods of dehorning should be used. Equipment used for castration, tattooing, ear tagging, or implanting should be adequately cleaned and disinfected between animals if they are expected to live past 2 yr of age.
Artificial insemination or embryo transfer (using negative recipients) may limit transmission. In beef herds, the use of a negative bull may limit transmission, but natural service is an uncommon method of viral transmission unless breeding is traumatic.
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