Review/Oorsig Volume 22, Issue 05 - Page 26

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