ISRAEL JOURNAL OF
VETERINARY MEDICINE
Vol. 56 (4) 2001
DE NOVO-INFECTION OF A LARGE ACCREDITED
BLV-FREE DAIRY HERD.
S. Garazi1, G. Hoida2, Z. Trainin3, H. Ungar-Waron3, and J. Brenner3
1. Israeli Veterinary Services, Rehovot, 2. Israeli Veterinary Services, Hadera, 3. Department of
Immunology, Kimron Veterinary Institute, 50 250 Bet Dagan, Israel.
Summary
The diagnosis of bovine lymphosarcoma (LS) in a slaughtered dairy cow from an accredited BLVfree herd initiated this study. On visiting the farm we learned that 75 pregnant heifers had been
introduced into the herd 2.5 years before the LS case was reported. Of these heifers from various
sources 22 came from a BLV-infected herd including the cow with LS.
In an attempt to trace the source of infection we first randomly tested 199 out of a total of 280
milking cows aged 2 years and older in the farm. Eight of the 199 tested belonged to the BLV-infected
group of 22 heifers. A total of 7/199 (3.5%) BLV seropositive cows was found. Four reactors (2.1%)
belonged to 191 local non-imported cows. The other 3 of 8 cows (37.5%) belonged to the BLVinfected importation. Comparison of the two proportions of sero-positive reactors revealed a highly
significant difference of 35.4%± SE 7% (p=0.0014). The odds of the introduced cow (‘foreign”) being
BLV sero-positive was 28 times greater (95% CI 5, 160) than that of a local one.
This episode illustrates the facilities with which even a single example of failure in surveillance can
cause severe economic losses.
Introduction
Enzootic bovine leukosis (EBL) is an infectious disease caused by bovine leukemia virus (BLV).
This is an oncogenic c-type retrovirus that is responsible for persistent lymphocytosis (PL) in the
infected animal leading in some cases to lymphosarcoma (LS) (1).
The relevance of BLV-infection to production, reproductive capacity and economic life span of the
bovine was demonstrated (2,3,4,5). Furthermore, BLV-infection may cause significant economic losses
associated with the costs of control, eradication, trade restriction of animals and animal products at the
individual herd, national and international levels (see review 6).
In order to overcome the economic problems posed by BLV-infection and by the import/export
restrictions, programs to control and eradicate BLV were introduced in several localities, such as
Denmark, Sweden, Germany and North America, at the national or regional levels (5,7,8,9).
Most BLV infections arise from transmission of BLV-infected lymphocytes among cattle (10), while
age-specific BLV rates reveal that horizontal transmission is responsible for most of the infections (11).
Large Israeli dairy farms practice a regime of management where the young stock is maintained
separately till they reach 36 months. At this point sero- BLV positivity levels within a herd may change
from approximately 3% to about 50% (12,13).
The aim of this report is to describe a breakdown of BLV in a large accredited BLV-free dairy farm.
Materials and Methods
Herd history
In the accredited herd a high culling rate from sub-clinical staphylococcal mastitis was followed by a
high turnover rate. To maintain the milk yield quota, an introduction of 75 foreign pregnant heifers
took place in February 1997. Amongst them were 22 pregnant heifers from a herd known to be BLVinfected (personal communication).
In October 2000, serological tests were carried out on 199 milking cows 24 months of age or older
from a total of 280 milking cows in the herd. The 22 pregnant heifers imported from BLV-infected
herd were introduced to the farm on two occasions (February 1997 and February 1998, in groups of 11,
respectively). Eight of these 22 cows were amongst the tested animals. Twelve additional cows
belonging to this group were not tested because they were slaughtered before blood sampling, the LS
case however was one of these.
Serological test
A commercial AGID BLV kit was used and the tests were performed according to the
manufacturer’s instructions (Sero Diagnostic IDG-Leucose Bovine, Institut Pourqier-Montpellier).
Statistics
To compare the proportions of “local” and imported BLV sero-positive cows we calculated the z
value (Table 1). We also calculated the difference ± standard error (SE) and the 95% confidence
interval (CI).
The odds of a cow being infected was measured by calculating the odds ratio (OR) and 95% CI from
a contingency 2x2 table.
To compare the groups daily average milk yield (imported vs. age-mate “local” cows) the Student’s t
test was used while the Gehan’s test to compare the survival rate of 12 foreign cows and 13 age-mate
local counterparts was used.
A p value of 5% or less was considered statistically significant for all tests.
Results
In September 2000, LS was diagnosed by histo-pathological examination of a lymph node
originating from a local BLV-free accredited dairy herd. No additional diagnostic procedures such as
PCR and/or serological tests were carried out to verify whether BLV infection was or was not involved
in this process (namely, connected with BLV-infection or the sporadic form of disease).
Statistic
Seven (3.5%) of the 199 milking cows tested for BLV reacted sero-positively (Table 1).
The comparison of the two proportions (local vs. foreign) of sero-positive reactors revealed a highly
significant difference (p=0.001). In fact, 3 (37.5%) out of 8 foreign cows were sero-positive while only
4 (2.1%) of 191 local cows which were tested were BLV sero-positive.
The odds of a foreign cow being BLV sero-positive was 28 time (95% CI 4.9, 160) higher as of a
local cow.
The comparisons of the two survival rates namely, 12 foreign cows vs. 13 local age matched local
counterparts (Gehan’s test) and the comparison of groups average daily milk production (Student’s
tests) did not meet the desired critical point of 5%.
Table 1:
Comparison of BLV sero-reactivity of the introduced cows from infected herd
(foreign cows) and the remaining population (local cows).
BLV sero-positive
BLV
negative
sero-
Number
animals
Foreign
3 (37.5%)
5 (62.5%)
8
Local
4 (2.1%)
187 (97.9%)
191
Total
7 (3.5%)
192 (96.5%)
199
of
Discussion
Hereby we described an episode of BLV re-infection of a BLV-free accredited dairy cattle farm.
By tracing back the infection we were able to identify the BLV infectious focus from which the
primary BLV-infection was introduced, and propagated among the BLV- free population.
All the 22 pregnant heifers on which our study was focused, were introduced to the BLV-free herd
from a herd that was heavily infected with BLV. However, the farm from which the 22 heifers
originated presents a unique unusual epidemiological feature. In this infected herd, the age (stratified)
infection rate is almost the same throughout all age groups (personal communication). This unique
situation is in great contrast to the general trend reported elsewhere and in Israel (11,12 and review 15).
It is logically assumed that the introduced infection propagated from the population with the higher
BLV sero-prevalence rate to the lower one (prevalence rate of 37.5% and 2.1%, respectively). In our
case the direction of infection was from the “foreign” to the “local” population, as the foreign cows
were 28 times more likely being sero-reactors than their counterparts.
It is documented that a retroviral iatrogenic transmission can be easily contained and even abolished
if the veterinary control and management are corrected (9) and maintained.
Once a farm gains its BLV-free status, no infectious focus within the herd’s population exists to
generate BLV re-infection. However, when an infected nucleus is introduced and established within a
farm’s premises, like in this case by infected animals, the iatrogenic means of transmission are
“activated” and propagate BLV into the naive stock of animals. This happened in Australia where
BLV-free farms were vaccinated against anaplasmosis and babesiosis, with BLV contaminated blood
originating from one uncontrolled donor sire (15).
Although no direct evidence can connect the cow, which was microscopically diagnosed with LS to
BLV infection, the probability of being so is very high. It relies on the high BLV prevalence rate of the
pregnant heifers introduced to the BLV-free farm from an infected farm.
Different authors assessed economic damage due to BLV infection. The sole unanimous conclusion
among them was the augmented culling rate of the infected animals (2,4,6). Other parameters such as
milk yield and reproductive capacity are of no concordance (2,6). Analyzing this minor case, no
conclusion could be drawn with regard to the above parameters. However, this herd is about to execute
another process of BLV eradication that will provide an additional economic burden to the damage
already done.
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