Bovine Leukemia

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Bovine Leukemia

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1)Enzootic bovine leukosis (EBL)
2) Caused by retrovirus called bovine leukemia virus (BLV)
3) Asymptomatic
4) Persistent Lymphocytosis (PL)
5) Lethal form of BLV-induced leukemia
6) Sporadic Bovine Leukosis
7) Not transmissible

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Enzootic bovine leukosis: Etiology

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1) enveloped RNA virus
2) Retroviridae
3) subfamily oncovirinae

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EBL: pathogenesis

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1) Viral envelope glycoprotein 51 (gp 51) mediates attachment to B lymphocytes
2) Reverse transcription
3) Proviral integration into host cell DNA
4) Molecula mechanisms resulting in B lymphocyte transformation not fully understood
5) X region of the env gene encodes Tax, a regulatory protein
6) Tax increases transcription rate by activating a promoter in the LTR region

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EBL: Transmission

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1) BLV does not persist outside of the host
2) Transmission by infected B lymphocytes in blood or colostrum
3) Practices leading to transmission: crowding, dehorning, tagging, tattooing, surgical instrument contamination
4) in utero (10% of calves born to seropositive dams)
5) Ingestion of colostrum containing infected lymphocytes
6) no insect transmission under natural conditions

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EBL: Clinical signs

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1) Life-long infection
2) 95% of infected animals do not develop clinical signs
3) 30% develop persistent lymphocytosis (PL)
4) 5% develop a fatal B cell lymphoproliferative disease, starting 3-10 yrs after infection
5) Lymphoproliferative disease usually manifests as lymphoma, occasionally (5-10%) as lymphocytic leukemia
6) enlarged lymph nodes
7) retrobulbar lymphosarcoma--> bilateral exophtahalmos

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EBL: Diagnosis

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1) clinical signs, gross path, histopath
2) blood smears to detect malignant lymphoma
3) cytology of lymph node aspirates
4) detection of antiviral antibodies: a)agar gell immunodiffusion (AGID), b) ELISA
7) PCR

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Persistent lymphcytosis

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Absolute lymphosytosis in 2 consecutive blood counts obtained 3 mos. apart.

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Control

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1) Cull cattle w/ persistent lymphcytosis
2) separate sero+ dams from calves prior to colostrum intake
3) one animal, one needle
4) one animal, one obstetric sleeve
5) wash and disinfect bloody intruments
6) use electocautery in dehorning
7) place calving cows in individual pens
8) change/disinfect bedding btwn calvings
9) take pre-colostrum serum samples form calves born to sero+ cows.
10) use colostrum from BLV- cows in low prevalence herds
11) feed colostrum that was previously frozen form sero+ cow in high prevalence herd to destroy infected lymphocytes
12) separate sero+ and sero- cows (if practical)
13) examine/treat sero- cows first

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Sporadic Bovine Leukosis: Calfhood Multicentric Lymphosarcoma

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a) gross enlargement of prescapular lymph node, b) symmetrical enlargement of other lymph nodes also, c) smooth, painless, freely moveable, d) not involving the skin

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Sporadic Bovine Leukosis: Thymic lymphosarcoma

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1) most cases seen in 6-24 mos age group
2) Generalized lymphadenopathy absent
3) No granulomas

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Sporadic Bovine Leukosis: Skin lymphosarcoma

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1) Rare
2) 6-24 mos. of age
3) Generalized lymphadenopathy
4) Large ulcerated lesions, limited to the head