By Fleur Whitlock

Equine infectious disease occurrences remain an ever-present threat, irrespective of the country where a horse resides. With climate change and increased international horse movements, monitoring and surveillance of infectious disease is more important than ever. But how is this conducted in our equine population?

In Northern Europe and the majority of countries worldwide, there are three infectious respiratory diseases commonly found to be circulating in horse populations (referred to as ‘endemic’). These include the viral diseases: equine influenza and equine herpes; and the bacterial disease: strangles. It is essential that horse keepers and their veterinary surgeons remain vigilant and knowledgeable around how these diseases present to ensure rapid implementation of control measures if they occur and more importantly what actions to take to prevent them in the first place.

Why is surveillance vital?

Identifying and controlling infectious diseases when they occur is important to limit both the number of infected horses on a premises and the disruptive and costly effects that disease can have on commercial enterprises, as well as avoiding the spread of infection to the wider horse population. To optimise control and prevention measures, diseases are monitored at a national and international level, through surveillance activities.

How is surveillance conducted?

There are two main ways that equine surveillance is conducted:

1. Statutory reporting of notifiable diseases

Diseases that are notifiable under veterinary or human health legislation in horses may include (but are not limited to and will have country-specific designation): African Horse Sickness (AHS), Contagious Equine Metritis (CEM), Dourine, Equine Viral Arteritis (EVA), Equine Infectious Anaemia (EIA), Glanders and West Nile fever. These diseases have been designated as notifiable, either due to their potential implications for human health, equine health or trade. Statutory reporting is required if a disease is suspected due to suspicious clinical signs or confirmed through diagnostic testing, such as those recommended by the HBLB International Codes of Practice before horses are bred each year. The specific approaches to their occurrences will be disease- and country-dependent but may include movement restrictions and testing of the in-contact population as a minimum. Information gathered from these outbreak investigations is evaluated and shared with the wider industry, through platforms such as the World Organisation for Animal Health - World Animal Health Information System (OIE-WAHIS) (https://wahis.oie.int).

2. Voluntary disease investigation and reporting of positive laboratory test results for non-notifiable diseases

If a horse is examined and undergoes confirmatory diagnostic testing through a laboratory and either the veterinary surgeon or laboratory contributes to surveillance initiatives, the diagnosis may be reported (Figure 1). Alongside this, epidemiological data relating to horse-specific factors such as the horse’s age, breed, vaccination status and specific factors such as approximate geographical location, number of resident horses on the premises and history of recent horse movements may also be available. In addition to this, to increase our understanding about pathogens and how they are changing over time, further analysis of the pathogen isolated from the infected horse(s) may be conducted to determine factors such as the particular strain of the pathogen. Information such as this can then be utilised to inform factors such as vaccination requirements. However, given the necessary voluntary steps that are required for a confirmed disease diagnosis to reach the reporting stage through this surveillance method, reported cases may not reflect the true extent of disease in a particular region or country. Also, some bias in the type of outbreaks that get reported may exist as detection and reporting may favour more severe cases, particular groups that undergo required testing or be more likely to be sampled due to subsidised testing costs existing in a particular country.

Examples of surveillance initiatives

Country-specific initiatives may be available to encourage diagnostic testing of suspect infectious cases through incentives such as subsidised laboratory fees. In the UK, equine vets can submit nasopharyngeal swab samples from horses with signs that could be indicative of equine influenza, for free PCR testing at a designated laboratory—with this scheme funded by the Horserace Betting Levy Board (HBLB) and overseen by Equine Infectious Disease Surveillance (EIDS), University of Cambridge, United Kingdom. In addition to this scheme, EIDS maintains a surveillance network of all commercial laboratories conducting equine influenza testing in the UK, encouraging the voluntary reporting of positive samples and the sharing of associated epidemiological and virological information. Both schemes enable monitoring of equine influenza in the UK, and this is essential given that equine influenza viruses naturally change and adapt, giving the potential for new strains to be more infectious or to emerge beyond the protection imparted by current vaccines. In addition to equine influenza, the UK closely monitors laboratory-confirmed occurrences of equine herpes virus-1 (EHV-1), given its ability to cause neurological signs and abortion in pregnant mares and death of newborn foals. Strangles is also under surveillance with epidemiological and bacteriological data collected and analysed to improve our understanding of this frustrating contagious disease and contribute to improving its control and prevention.

What disease reporting platforms are available?

Figure 3: Examples of the international and country-specific reporting platforms monitored by the International Collating Centre (ICC): an interim email report issued by ICC and a recent embedded disease alert.

Country-specific reporting platforms exist worldwide, and these predominantly notify stakeholders—usually through email alerts—about laboratory-confirmed disease occurrences in the reporting country. Examples in Europe include France’s Réseau D'épidémio-Surveillance En Pathologie Équine (RESPE, www.respe.net), the Netherlands Surveillance Equine Infectious Disease Netherlands (SEIN, www.seinalerts.nl), Belgium’s Equi Focus Point Belgium (EFPB, www.efpb.be) and Switzerland’s Equinella (www.equinella.ch).

Complementary to this, the International Collating Centre (ICC) is overseen by EIDS and supported by the International Thoroughbred Breeders’ Federation (ITBF) and International Equestrian Federation (FEI) and has for over 30 years collated outbreak reports from available country-specific reporting contacts and platforms worldwide. In addition, EIDS receives reports directly from veterinary surgeons and diagnostic laboratories (Figure 2). Collated reports are sent to registered subscribers on an almost daily basis through an email that contains embedded links to specific ICC outbreak alerts. A quarterly summary report is also produced and emailed to subscribers four times a year and is available in the resources and archive section of the ICC website (https://equinesurveillance.org/iccview/). Reported outbreaks are predominately made up of at least one case that has had the diagnosis confirmed through laboratory testing. It is therefore expected that those outbreaks that reach the reporting stage by the ICC will not reflect true infectious disease frequency within the international equine population; and a country with no reported outbreaks of a disease does not necessarily mean that the disease is not present in that country.

There is an interactive ICC website enabling analysis of all international infectious disease outbreaks reported through the ICC, which was launched in August 2019; and outbreak data for all of 2019 onwards is available through this platform. Through the ICC, infectious disease outbreak information is shared with stakeholders throughout the world, ensuring people remain up to date through this active communication network.

In addition to the ICC, EIDS has an equine influenza-specific platform, EquiFluNet (www.equinesurveillance.org/equiflunet), which presents influenza outbreak reports for the UK and worldwide.

A summary of the recent findings of surveillance initiatives

Equine influenza

During 2019, Europe experienced an epidemic of equine influenza with widespread welfare and economic effects, including the temporary ceasing of horseracing in the UK. During 2021, influenza occurrences in Europe reported by the ICC returned to a more expected level (Figure 3). However, the potential of viral strain changes alongside international horse movements makes monitoring and surveillance of this virus essential.

Equine herpesvirus-1 (EHV-1)

EHV-1 is endemic in Europe, and the ICC regularly reports on occurrences of EHV-1 disease. An example of an ICC report released during 2020 detailed an outbreak of EHV-1 neurological disease on a premises in Hampshire, United Kingdom, with multiple equine fatalities (Figure 4 – left panel). Another example of an ICC report included a widespread neurological EHV-1 outbreak that occurred in Spain at several international show jumping events during 2021 (Figure 4 – right panel).

Figure 4: The International Collating Centre(www.equinesurveillance.org/iccview) reports detailing outbreaks of equine herpesvirus-1 neurological disease in Hampshire, United Kingdom in January 2020 (left) and Valencia, Spain in February 2021 (right).

West Nile fever (WNF)

Figure 5: European countries reporting equine WNV outbreaks in Europe reported through the ICC from 2019, 2020 and 2021.

WNF is caused by West Nile virus (WNV) by biting mosquitoes, with birds acting as sources of the virus. It is a zoonotic disease, meaning humans can become infected if bitten by an infected mosquito. The ICC has reported equine cases across Europe over recent years (Figure 5). Given that many countries in Europe remain ‘free’ from WNV, it is still possible for horses to have neurological signs such as weakness and incoordination or even death following infection. Humans can also be affected if bitten by an infected mosquito, so monitoring and surveillance of equine WNF occurrences, alongside mosquito, bird and human surveillance are essential. By way of example, WNV was confirmed in birds and humans in the Netherlands for the first time in 2020, but as of March 2022, it has not yet been confirmed in horses.

Summary

Having an appreciation of how and why surveillance of equine infectious diseases are conducted helps to improve engagement with and encourage an increased contribution to surveillance initiatives. A well-informed view on equine infectious disease outbreaks worldwide ensures continued advancements in enhancing control and prevention measures. This in turn will help reduce the risk from disease outbreaks and ensure the industry can continue to operate to its full potential.

Sources for further information about equine infectious diseases and their control and prevention are available

More information about equine infectious diseases and prevention/control:

Horserace Betting Levy Board’s International Codes of Practice 2021: https://codes.hblb.org.uk/
Equine Infectious Disease Surveillance (EIDS) website, hosting the International Collating Centre (ICC) and Equiflunet: www.equinesurveillance.org
Sign up to receive International Collating Centre reports by contacting EIDS: equinesurveillance@gmail.com

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