On strangleholds and tipping points

Summary

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The infectious agents Streptococcus equi subspecies equi (S. equi, the cause of strangles) and EHV-1 (a cause of mild cold-like symptoms, but also of abortion and neurological disease) occur frequently in horses in the Netherlands, as in most of the rest of the world. Research into the best control strategies for these diseases does not currently include models of transmission. This thesis contains several building blocks which will help apply such models to the control effort of these diseases.

The basic reproduction number (R0) for S. equi was estimated from real-world outbreaks, and for EHV-1, we attempted to demonstrate an effect of herd vaccination on transmission potential of the virus, but were unable to show a significant effect. By comparing model outputs to real-world epidemiological data on S. equi, we were able to demonstrate that a long-held belief that one in four horses do not mount lasting immunity after strangles may be incorrect.

We surveyed apparently healthy horses and ponies from throughout the Netherlands and found that around 1 in 26 of these were strangles carriers. Combining this information with 2022 competition records and disease surveillance reports, simulations showed that carriers and susceptible horses very frequently meet at competitions (hundreds of thousands of times per year), but that very few of these contacts lead to infection and disease in the susceptible horses.

A questionnaire survey revealed that very few equine premises in the Netherlands implement effective bio-security measures, which is similar to reports from other countries. Approximately 4/5 of surveyed premises reported horses temporarily leaving the premises to participate in competitions or for training.

Network analysis using sports and racing competition records revealed that the "social network" of sport horses and of racehorses are each highly connected. The Dutch sport horse network has five degrees of separation, meaning there is a connection between any two horses (that participate in sports competitions) of no more than five steps of horses that have attended the same competitive event at some time during the year.

Horses are group housed like livestock, but travel and socialise like humans. Therefore, the control of equine infectious diseases poses challenges unique to the species. The information gathered in this thesis can be carried forward into data--driven models to evaluate the most (cost-)efficient control strategies for EHV-1 and S. equi.

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