How Many Animals are Infected with Chronic Wasting Disease?

Post provided by Hildegunn Viljugrein

©Alexandre Buisse

©Alexandre Buisse

The discovery of Chronic Wasting Disease (CWD) in Norway in 2016 has led to extensive measures and testing of deer in Norway. Since 2018 there have been similar measures within the EU. But how many deer need to be tested before we can be (almost) certain that a population is not infected by CWD?

In our article – ‘A method that accounts for differential detectability in mixed samples of long‐term infections with applications to the case of Chronic Wasting Disease in cervids’ – we provide important tools for estimation of prevalence and likelihood of finding infected animals in a given population. The paper is a result of a collaborative work between a multidisciplinary group of scientists from the Norwegian Veterinary Institute, Norwegian Institute for Nature Research and Prof. Atle Mysterud from Centre for Ecological and Evolutionary Synthesis at the University of Oslo.

Will the Infected Animals Test Positive?

How certain can we be that an infected individual will test positive? In a long-lasting infection, like CWD, it’s important to take into account that the probability of testing positive will increase the longer the time since infection.  The model tool in our article deals mainly with estimating test sensitivity. We discuss how this is dependent on disease progression, the age of the animal and type of tissue tested.

We Can’t Test the Whole Population

©Bjørn Christian Tørrissen

©Bjørn Christian Tørrissen

Surveillance of wildlife diseases is in general logistically difficult – it’s an ongoing challenge for those of us trying to estimate how widespread a disease is. In our case, we can’t test the whole population for CWD, as the current standard test requires samples from dead animals.

The new model tool that we’ve introduced addresses a pressing need in CWD diagnostics. It does this by modelling  the inconsistent quality of hunter-collected samples and the variability of prion deposition in target tissues with disease progression. In future, the model will be used for estimating the probability of freedom from CWD for the wild reindeer herds closest to where CWD was first detected.

To find out more, read the Methods in Ecology and Evolution article ‘A method that accounts for differential detectability in mixed samples of long-term infections with applications to the case of Chronic Wasting Disease in cervids

This post was first published on the Norwegian Veterinary Institute website.

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