The impact of maedi-visna on breeding flocks (PhD)

Summary

Small Ruminant lentiviruses (SRLV) including Maedi Visna (MV) and Caprine encephaltitis and arthritis (CAE) are the cause of chronic progressive disease in sheep and goats. The most common clinical signs include lung disease, wasting and mastitis with severe impacts on sheep health and productivity. The disease has a long latent period with clinical signs sometimes not detected until years after the initial infection. There are no vaccines or treatments available, leaving testing and culling of infected animals as the only realistic control option for MV in sheep flocks.

We do not know what the national prevalence rate of the disease is as there is no systematic monitoring and the control programmes (the MV accredited free flock scheme run by Scotlands rural college, and the MV Monitored free scheme run by Axiom laboratories) in place are voluntary. However what data there is suggests the number of infected flocks has risen steeply in recent years. This project examined a number of aspects of MV transmission and impact in UK sheep flocks with a long term goal of improving testing and control strategies.

The first set of experiments examined the risk of sexual transmission of the virus from infected rams to niave ewes using intravaginal insemination as a proxy for natural mating. This 7 week trial demonstrated that several of the 13 naturally infected rams did indeed harbour low levels of virus in their reproductive tracts but that this did not go on to cause a detectable infection in the 12 ewes. This indicates that the risk of transmission from sexual infection is low and that genetic rescue of high value rams via semen collection and AI before culling would be feasible.

Longitudinal monitoring records for these rams and 15 others from the same cohort demonstrated that the one animal that was heterozygous for the MV resistant allele of the TMEM154 gene apparently cleared the infection 16 months after initial diagnosis, testing negative via serology and qPCR in all tissues. Nasal swabs collected at post mortem from 13 of the affected rams demonstrated viral RNA detection at quite high loads indicating that this might be a viable route for diagnostic sampling that does not require blood collection. Balancing that, we do not know what the current variability in MV viral sequence is in the UK and whether one qPCR test would detect them all. Multiple published PCR tests for MV failed to detect the virus strain in these rams and deep sequencing was required to identify the virus strain and develop the qPCR tests used in this study.

The final set of work in this project used milk data from a 319 head East Friesian x Lacunae sheep diary herd that had undergone a breakdown in MV control in multiviariable regression models to determine the impact of MV infection on milk yield and quality. This demonstrated a 6.60 % drop in milk yield and a paradoxical decrease in SCC, demonstrating that loss in milk yield is a significant factor in production losses due to MV.

Sector:

Beef & Lamb

Project code:

61110040

Date:

01 October 2016 - 30 September 2019

Funders:

AHDB Beef & Lamb

AHDB sector cost:

£69,327

Total project value:

£69,327

Project leader:

University of Nottingham

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About this project

The Challenge

Prevalence of maedi-visna (MV), a chronic wasting disease of sheep and goats with no cure or treatment is currently on the rise within the United Kingdom (UK). The current diagnostic tests do not pick up all strains of the virus due to the high variability seen between strains of maedi-visna virus (MVV), a lentivirus and causative agent of MV.

The Project

Therefore one of the aims of this project is to produce a more reliable and cost effective diagnostic test for the detection of MVV infection. This study could aid in the improvement of the current control scheme within the UK, the MV accreditation scheme

A further aim of the study is to quantify the long term impact of MV in a cohort of 28 naturally infected rams. From these rams, blood collected over multiple time points and tissue collected at time of euthanasia/death will be used in a longitudinal case study of morbidity and mortality due to the disease.

The final aim of the study is to evaluate the risk of sexual transmission of MV from rams to ewes inseminated intravaginally with semen from known infected rams. Data collected in this study will allow assessment of the risk associated with using semen from infected rams to salvage valuable genetics that may otherwise be lost.

Student

Scott Jones