Epidemiology of Bovine viral Diarrhoea in Tanzania small holder dairy sector

Bovine viral diarrhoea, caused by the bovine viral diarrhoea virus (BVDV) in the Pestivirus genus of the Flaviviridae, is one of the most important diseases of cattle worldwide causing poor reproductive performance in adult cattle and mucosal disease in calves. In addition, it causes immunosuppression and increased susceptibility to other infections, the impact of which is uncertain, particularly in sub-Saharan Africa where animals are exposed to a much wider range and higher intensity of infections compared to Europe. There are no previous estimates of the seroprevalence of BVDV in cattle in Tanzania.

We are currently collecting samples from small holder dairy cattle populations in 7 clusters across Tanzania as part of a BMGF/DFID funded project under the CTLGH and ADGG programmes. Abortion and early calf mortality rates are ~ 6.7% (95% CI 4.1-10.7) in Arusha Region (the first we have examined) and a third of these are live calf births that do not survive more than a month. There is little published data on BVD in SSA but a previous study in Cameroon found herd-level and within herd seroprevalences of 92% and 30% respectively and 16.5% of herds were classed as having a persistently infected calf (PI) in the herd within the last year based on the ''spot'' test approach [1]. Another study in Kenyan small holder cattle found seroprevalence of ~20% in year old calves (that had survived to this age) and that BVD was strongly associated with other respiratory viruses in the calf respiratory syndrome [2].

The small holder dairy system is a relatively new farming system in Tanzania and has developed out of programmes of cattle distribution of high performing breeds such as Friesian and Ayrshire that have been crossed in a haphazard way with local breeds and little is know of their susceptibility to BVD or the impact it may have in this system.

The proposed project would screen the banked sera and blood samples stored at ILRI and NM-AIST for antibodies to BVD virus and viral sequences. Using the associated individual animal data on age, location, calving history etc they would develop statistical and dynamic models to identify risk factors and estimate transmission parameters and economic impact of the disease in this system. In addition there is Illumina 50K SNP data for each animal allowing some basic genetic association studies in this population [3].

This project would provide the student with a range of training experience include wet lab diagnostics, epidemiology and modelling and genetic analysis. There would also be the potential for a small pilot study to collect samples from calves in a limited geographical area in the current study setting and use metagenomics approaches to better understand pathogens present in early mortality situations.

[1] Handel et al. (2011) Seroepidemiology of Bovine Viral Diarrhoea Virus (BVDV) in the Adamawa Region of Cameroon and Use of the SPOT Test to Identify Herds with PI Calves. PLoS ONE 6(7): e21620. doi:10.1371/journal.pone.0021620
[2] R. Callaby et al. (2016) Research in Veterinary Science 108: 120-124
[3] Murray et al. (2013) Genetic susceptibility to infectious disease in East African Shorthorn Zebu: a genome-wide analysis of the effect of heterozygosity and exotic introgression. BMC Evolutionary Biology 13:246.