Diagnostic Exercise From The Davis-Thompson Foundation*

Case #: 121 Month: May Year: 2019

Answer Sheet

Title: Enzootic bovine leucosis (EBL) in a cow

Contributor: Erin K. Morris, DVM, Diplomate ACVP, Contributor for the International Veterinary Pathology

Coalition (IVPC); WRAIR-NMRC, Forest-Glen Annex, MD; Virginia Pierce, VMD, Maryland Department of

Agriculture.

Clinical History: This 7-year-old Angus cow weighed 1605 lbs on February 18, 2018. On April 1, 2018, the

cow was moving slowly and appeared “sucked up”. The weight had dropped to 1490 lbs and the cow

developed watery diarrhea. On April 11, 2018, the cow weighed 1177 lbs. Laboratory tests for Clostridium

spp., Rotavirus, Giardia, Cryptosporidium, Johne’s disease, and fecal ova and parasites were all negative.

Necropsy Findings: The diaphragm, pericardial sac, and abomasum had varisized, multifocal to coalescing,

raised, tan areas.

Gross and Microscopic Images:

Figure 1. Diaphragm. Multifocal to coalescing raised tan lesions.

Figure 2. Pericardium. Marked thickening by neoplastic cells.

Figure 3. Abomasum. Diffuse thickening by neoplastic cells.

Figure 4. Rumen. Neoplastic lymphocytes infiltrate the submucosa and muscular tunics below the

mucosal surface. 20X, H&E.

Figure 5: Rumen. Neoplastic cells extend beyond the muscular layer into the surrounding omentum

effacing the normal architecture. 20X, H&E.

Figure 6: Neoplastic cells are composed of monomorphic lymphocytes with few mitotic figures. 400X,

H&E.

Morphologic Diagnosis (based on the pictures shown):

Diaphragm, pericardium, digestive tract: Lymphoma.

Possible Cause(s):

1. Bovine Leukemia Virus (C Type Retrovirus)

2. Sporadic (Non-Viral): includes calf type, juvenile (thymic) type, and skin type

a. Calf type: Affects calves less than six months of age; diffuse lymphadenopathy

b. Juvenile type: Thymic lymphoma in yearlings

c. Skin type: Affects 2-3 year old cattle with ulcerated plaques on the skin; indolent

Comments: This cow tested positive for Bovine Leukemia Virus (BLV). The condition affecting adult cattle

is called enzootic bovine leukosis (EBL), which is the most common neoplastic disease in cattle.5,7 Calves are

believed to be infected by the virus in their first year of life. Vertical transmission may occur in utero or

during delivery, and about 10% of calves born to BLV-infected dams are already infected at birth. The most

vulnerable time of transmission is through the ingestion of infected colostrum and/or raw milk either

naturally or artificially. Calves infected during the first week of life could play an active role in early

propagation of BLV to susceptible animals.6 Semen has also been discussed as a possible route of

transmission, but a recent study by Benitez et al. found that BLV infected bulls that are healthy and aleukemic

may not represent a significant risk of BLV transmission during a defined breeding season.2,3,7 Viremia occurs

in a small window after infection and prior to antibody formation.7 Approximately 30% of cattle will

experience persistent lymphocytosis as their immune system, specifically the bovine major histocompatibility

complex, keeps the virus in check. However, in 3% of cattle, malignant transformation occurs with the peak

incidence at 6-8 years of age, as in this case. The cause of malignant transformation is multifactorial, but in

50% of infected cattle, p53 tumor suppressor dysregulation is involved, suggesting a genetic component. As

the viral load increases, so does the level of detectable antibody formation. Like most retroviruses, the virus

hides in the bone marrow. Dairy cattle are more commonly infected than beef cattle, possibly due to the

fact that direct contact and infected colostrum is believed to be the primary route of infection and is increased

during summer months.7 In North America, an epidemiological study of BLV prevalence in U.S. dairy cattle

conducted by the Department of Agriculture’s National Animal Health Monitoring System demonstrated that

83.9% of dairy cattle were BLV-positive at herd level, and 39% of beef herds had at least one BLV-infected

animal.5

Most neoplastic cells are naïve B cells with rearranged immunoglobulin genes.7 BLV-induced tumors usually

arise from the CD5+ IgM+ B-cell subpopulation.5 There are different topographic forms of the BLV associated

lymphoma, which include lymph nodes (including but not limited to submandibular, prescapular,

supramammary, pelvic nodes); gastroenteric (abomasum); spinal (involving primarily the sublumbar lymph

nodes and epidural fat of the lumbar spinal cord); cardiac; uterine; hepatic; splenic; and renal.7

There are ten different BLV genotypes worldwide, with each genotype encoding specific amino acid

substitutions in both structural and non-structural gene regions.5 Three genotypes of BLV, namely genotype-

1, genotype-4 and genotype-6, are the main across the world. Genotype-1 is the most dominant genotype

of BLV and is distributed across almost all continents, including Europe, America, Asia, and Australia.

Genotype-1 in particular spread to South and North America, and these continents still have a high

prevalence of BLV infection. In the U.S., genotypes 1, 3, and 4 have been detected. The genotypes are spread

via animal trade and intercontinental breeding programs.5

BLV is significant because cattle with EBL invariably die within months and are not approved for human

consumption in some countries.4 Currently there are no vaccines or treatment, so the only way to control it

is to prevent infection. The 30% of cattle that are carriers, referred to as having persistent lymphocytosis, are

a constant source of transmission within a herd.7 Aggressive monitoring of persistent lymphocytosis is a less

expensive way to identify carriers than PCR testing; however, little is known about how immune cell reference

intervals differ among cattle breeds.4 Lymphocyte counts in the BLV-free and infected Japanese Black cattle

were significantly lower than those in the Holstein cattle, for instance, requiring different diagnostic criteria

when assessing lymphocytosis.4 Of equal concern is evidence that there is a possible causal role of BLV in

human breast cancer.1 BLV has also been found in domestic yaks in China, signifying viral spread between

species.8

References:

1. Baltzell KA, Shen HM, Krishnamurthy S, et al. Bovine leukemia virus linked to breast cancer but not

coinfection with human papillomavirus: Case-control study of women in Texas. Cancer. 2018; 124(7):

1342-1349.

2. Benitez OJ, Roberts JN, Norby B, et al. Lack of bovine leukemia virus transmission in natural breeding

of cattle. Theriogenology. 2018; 126: 187-190.

3. Givens MD. Review: Risks of disease transmission through semen in cattle. Animal. 2018; 12(s1):

s165-s171.

4. Mekata H, Yamamoto M, Kirino Y, et al. New hematological key for bovine leukemia virus-infected

Japanese Black cattle. J Vet Med Sci. 2018; 80(2): 316–319.

5. Polat M, Takeshima SN, Aida Y. Epidemiology and genetic diversity of bovine leukemia virus. Virol J.

2017; 14(1): 209.

6. Ruiz V, Porta NG, Lomonaco M, et al. Bovine leukemia virus infection in neonatal calves. Risk factors

and control measures. Front Vet Sci. 2018; 5: 267.

7. Valli VEO. Hematopoietic system. In: Maxie G, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic

Animals. Vol 3. 5th ed. St. Louis, MO: Elsevier; 2007: 199-201.

8. Wang M, Wang Y, Baloch AR, et al. Molecular epidemiology and characterization of bovine leukemia

virus in domestic yaks (Bos grunniens) on the Qinghai-Tibet Plateau, China. Arch Virol. 2018; 163(3):

659-670.

The material has been reviewed by the Walter Reed Army Institute of Research. There is no objection

to its presentation and/or publication. The opinions or assertions contained herein are the private

views of the author, and are not to be construed as official, or as reflecting true views of the

Department of the Army or the Department of Defense.

*The Diagnostic Exercises are an initiative of the Latin Comparative Pathology Group (LCPG), the

Latin American subdivision of The Davis-Thompson Foundation. These exercises are contributed by

members and non-members from any country of residence. Consider submitting an exercise! A final

document containing this material with answers and a brief discussion will be posted on the CL

Davis website (http://www.cldavis.org/diagnostic_exercises.html).

Editor-in-chief: Vinicius Carreira

Associate Editor for this Diagnostic Exercise: Ingeborg Langohr