Oorsig/Review
number of infected animals in a herd has reached
a tipping point, it is very difficult to control & the
losses may be significant.
Comparative Prevalence
USA - Has a prevalence of about 44% of dairy cattle
(The Merck Veterinary Manual, 2012, http://www.
merckmanuals.com/vet/generalized_conditions/
bovine_leukosis_bovine_lymphosarcoma_leukemia_
malignant_lymphoma/overview_of_bovine_
leukosis.html). During the palpation of > 500,000
cows & > 2,000 LDA surgeries the occurrence of
the abomasul & uterine forms were rare (< 100,
personal experience, 1994 to 2008).
Finland - Had a very low prevalence: We conclude
that herd-level prevalence of EBL/BLV infection
never exceeded 5% (Eradication of enzootic
bovine leukosis from Finland, Nuotio L, Rusanen H,
Sihvonen L, Neuvonen E, Prev Vet Med 2003, http://
www.ncbi.nlm.nih.gov/pubmed/12719016).
South Africa - Enzootic Bovine Leukosis was
widely present in the KZN province at generally
low prevalences, except in the central region
where the highest prevalence at district level
was recorded to be 70% (A serological preference
study of important infectious diseases of cattle in
rural areas of Kwa Zulu Natal, South Africa, Uta
Hesterberg, MSc Thesis, UP, 2007, http://upetd.
up.ac.za/thesis/available/etd-05062008-081645/
unrestricted/dissertation.pdf ).
Control/Eradication
USA - There is no national eradication program
because of the poor cost:benefit. Purebred &
small herds that elect to eradicate or control do
so by 1. test & slaughter, 2. reducing spread by
objects (needles, palpation sleeves, dehorners,
etc.) & 3. by testing purchased cattle. Large herds
commonly have no control program. In spite
of this, many such herds milk more than 5,000
cows & produce as much as 35 kg/cow/d. The
cost:benefit ratio is not viable since there are too
many cows. Making it impractical to use a single
needle &/or palpation sleeve per cow. Strict &
aggressive culling policies (with high replacement
heifer pressure) & cows usually being replaced
before clinical signs develop (usually from 4 to 8
yrs. of age) self-limit the disease.
Finland - It nevertheless took 30 years
to eradicate the disease and the infection.
(Eradication of enzootic bovine leukosis from
Finland, Nuotio L, Rusanen H, Sihvonen L, Neuvonen
E, Prev Vet Med. 2003, http://www.ncbi.nlm.nih.
26
gov/pubmed/12719016). This was starting at a
prevalence of 5%.
South Africa - A herd based control/eradication
program would include 1. test & slaughter, 2.
reducing the spread by objects (needles, palpation
sleeves, dehorners, etc.), 3. testing of purchased
cattle, 4. the aggressive culling of underperforming
cattle & 5) a higher herd replacement rate
facilitated by replacement heifer pressure (i.e.
an excellent reproductive program).
Large
herds with few clinical cases & sufficient culling
pressure, may elect not to manage the disease.
A national eradication program would require
huge resources & a public/private partnership.
The benefit:cost will almost certainly be negative,
leaving individual producers to manage the
disease within the constraints of their own herds.
Example herd:
We don’t know the herd size, nor the prevalence,
but let’s consider the following:
500 cow herd, 30 kg/cow average production =
15,000 kg/d
Assume 50% prevalence = 250 infected cows
30% of infected cows develop lymphocytosis = 75
cows that may show decreased milk production.
If the loss in milk production is 3% for all 75 cows
= 67.5 kg/d loss = 0.45% milk loss/d for the herd
5% (max) of infected cows develop
lymphosarcoma & will die = 13
It would be highly unusual for these cows to die
within a short period of time. The cost of a control/
eradication program needs to be evaluated in the
context of the possible milk loss & the number
of replacements needed. A test & slaughter
program might not be viable in a high prevalence
herd. Instead, infected cattle would be replaced
naturally, whilst there is strict compliance with
limiting horizontal transmission. The strategy
considered would closely approximate the
thought process & strategies evaluated to control/
eradicate Staph aureus mastitis.
Author List
AF – Alan Fisher
AH – Andy Hentzen
Al - Andy Lund
AvZ - Albert van Zyl
CN - Cizelle Naude
DH -Dietmar Holm
DM - Dave Midgley
EdP – E du Preez
FM - Faffa Malan
GB - Geoff Brown
GK – Gavin King
JG – John Grewar
LS - Luis Schwalbach
MJ - Martin Jansen
ML -Mike Lowry
MvdL - Martin van der Leek
PI – Peter Irons
PT - Peter Thompson
R – Rob (Underberg)
RM – Riaan Mulder
SM - Sello Maboe
TC – Tod Collins
TS – Thys Snyman
WG – Will Gratwick
WS – Willem Schultheiss