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²[[Category: To Do - Steph]]
²[[Category: To Do - Steph]]
[[File:BLV.jpeg|thumb|200px|right|Conjunctival prolapse in cow with bovine viral leucosis (sourced from Wikimedia Commons)]]
[[File:BLV.jpeg|thumb|200px|right|Conjunctival prolapse in cow with bovine viral leucosis (sourced from Wikimedia Commons)]]
'''''Also Known As''''' — Enzootic Bovine Leucosis — Bovine leukosis — EBL — BLV — Bovine leukemia — lymphosarcoma — Sporadic Bovine Leukosis — Bovine Malignant Lymphoma
=Introduction=
=Introduction=
Bovine leukaemia virus is a retrovirus causing two specific diseases: Bovine lymphosarcom and Persistent lymphocytosis.
Bovine lymphosarcoma is fatal while persistent lymphocytosis is not usually so.
Bovine leukosis is not transmissible to humans.
This disease is notifiable to the World Organisation for Animal Health ([http://www.oie.int/ OIE])
=Signalment=
=Signalment=
Cattle are thought to be the only species naturally susceptible, and prevalence rates are higher in dairy breeds than beef.
The majority of infected animals are more than 2 years of age, with younger animals developing sporadic bovine leukosis which is thought to be unrelated – see below.
Sheep are susceptible to experimental infection which appears more pathogenic in this species.
Manifestation of the fatal neoplastic lymphosarcoma form of disease also appears better represented in dairy cattle.
Sporadic bovine leucosis is seen in young animals and causes thymic and cutaneous lymphosarcome. These animals are usually seronegative for EBL and so the pathogenesis is thought to be different to enzootic bovine leukosis.
=Distribution=
=Distribution=
BLV is globally distributed, but prevalence widely varies.
While 89% dairy operations in the USA showed seropositive results and 43.5% of all USA dairy cattle were seropositive <ref> USDA/APHIS/Veterinary Services, 1997. High prevalence of BLV in US dairy herds. Fort Collins, CO, USA: USDA: APHIS:VS Centers For Epidemiology and Animal Health.</ref>, seroprevalence in the EU rarely exceeds 0.5-1.5%. <ref>Lorenz, R. J., Straub, O. C. (1987) The epidemiology of enzootic bovine leukosis. In: Burny A, Mammerick, M, eds. Enzootic bovine leukosis and bovine leukemia virus. Boston, USA: Martinus Nijhoff Publishing, 51-68</ref>
=Clinical Signs=
=Clinical Signs=
===Transmission===
Transmission of bovine leukaemia virus is mainly horizontal. Iatrogenic procedures are an important route as they facilitate transfer of contaminated blood between individuals, e.g. vaccinations, dehorning, rectal examinations and surgical procedures. Natural transmission by direct contact requires contact with infected blood, exudates or tissues which enter through broken skin.
Vertical transmission is most likely from colostrums/milk and is less important, although it does provide a method for control.
===Lymphosarcoma===
The most common presentation is that of enlarged lymph nodes which may cause multiple palpable swellings in the pharynx, flanks and limbs.
This is often an early indicator of disease.
5-20% cases are peracute and present as sudden death. This is thought to be due to sequelae such as adrenal gland involvement or rupture of abomasal ulcers etc.
Other signs are multicentric and non-specific:
Cardiovascular – Dysrhythmmias, jugular distension, tachycardia, murmurs
Gastrointestinal – Anorexia, hepatomegaly, dysphagia, constipation or diarrhoea, abomasal tympany, hypomotility
Retrobulbar tumours sometimes cause blindness and other ocular signs.
Neurological signs and lameness also sometimes occur due to local tumour growth.
Animals are usually afebrile.
Infected animals become permanent carriers.
===Persistent Lumphocytosis===
Persistent lymphocytosis is defined by an increase in total lymphocyte count by 3 times the normal standard deviation above normal, persisting for at least three months with no clinical signs of neoplastic lesions. This is thought to represent 1/3 of all BLV infected cattle.
=Diagnosis=
=Diagnosis=