Epidemiology, infection dynamics and effective control of Bovine Leukemia virus within dairy herds of Argentina: a quantitative approach

Summary

Bovine Leukemia Virus (BLV) is a retrovirus that causes lymphomas (leucosis) and other disorders in cattle and it has a large economical impact on the livestock sector of many countries around the world. BovineLeukemia has been reported inArgentinafor the first time in 1968. Recently, a prevalence study in the whole country was performed showing an animal level prevalence of 33 % while 84 % of the sampled herds were positive.Given that the disease is more prevalent in dairy operations, the main objectives of this thesis were to providean analytical framework to quantify several aspects related with the epidemiology and control of BLV in dairy herds fromArgentina. Understanding and quantifying the transmission of BLV in dairy cattle is essential to the design of effective control strategies and BLV prevention in general.Two types of BLV isolates - Australian and Argentine - were present in dairy herds from different areas ofCentral Argentinaand thephylogeneticaltree clearly shows that Argentine isolates represent a separate and homogeneous group compared to other clusters. The low rate of non-synonymous substitutions compared to synonymous substitutions found in the analysis supports the hypothesis of purifying selection ofenv, gag andpolgenes and several subunits; consequently, molecular evolution occurs under some functional constraint. It appears that for theTransmembraneHydrophobic Region within theenvgene - at least for the Argentine isolate - the host seems to drive the selective pressure andsubtle natural variation in the structure resulted from host-pathogen interactions; hence it is a site that might be a good potential candidate for future functional studies. Simulation modeling showed that after introduction of only one infectious animal (either a heifer or adult cow) in a naive herd there is a high probability that an outbreak will occur and most likely the infection will then become endemic. Hence, if farmers want to eradicate BLV from their herds or keep their herds free of BLV, they should be extremely cautious in the addition of new animals to their herds. It was shown that although in larger herds it is possible to eradicate BLV, it takes longer and it demands more efforts in comparison with smaller herds. Simulation further showed that the time to achieve eradication was not significantly affected by a sampling frequency being either 6 or 3 months, resulting in lower costs for the farmer. The decision to adopt a specific BLV eradication strategy (combination of control measures, sampling frequency and test) thatoptimisesthe probability to achieve eradication and the time till eradication has been achieved depends heavily on the herd size. Also, early detection of infected calves dramatically reduces the time needed to achieve eradication. Our estimation of the properties (Sensitivity and Specificity) of diagnostic tests commonly used and available for surveillance showed that they are sufficiently high for being used in eradication strategies as simulated. Although several types of ELISA recently have been officially approved for surveillance purposes it does not incorporate the ELISA 108 in milk which can be advantageously used for routine screening.