Equine Grass Sickness Surveillance Scheme
| Equine Grass Sickness Surveillance Scheme Introduction |  |
EGS has been recognised for almost 100 years since the first reports of cases in Forfar, Scotland at the beginning of the 20th Century. Ever since then Great Britain has had the highest frequency of cases in the world, with disease now occurring in Scotland, England and Wales. The disease also occurs in several northern European countries and an identical condition called mal seco (literally translated as ‘dry sickness') has been recognised in Argentina, the Falklands, Colombia and Chile. Interestingly, EGS is rare in the rest of the world including the U.S.A., Africa, Australia and Asia. Notably, despite extensive horse movements between mainland Great Britain and Ireland, EGS remains virtually unknown in the Emerald Isle.
What are the signs of EGS?
EGS is classified into 3 broad types, based on the length of survival from the onset of clinical signs:
- Acute Grass Sickness (AGS) cases survive 1-2 days
- Subacute Grass Sickness (SAGS) cases survive 2-7 days
- Chronic Grass Sickness (CGS) cases survive more than 7 days
The most severely affected body system is the GIT producing clinical signs of intestinal paralysis of varying degree (Figure 1) or colic (Figure 2). Other signs are also often seen (Table 1).
| Figure 1: Clinical result of intestinal paralysis | Figure 2: Colic in AGS |
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| Kindly supplied by Professor B. McGorum | Kindly supplied by Professor C. Proudman |
Table 1: Specific clinical signs associated with acute/subacute and chronic EGS (medical terms commonly used by veterinary surgeons are included in parentheses)
| Acute EGS - signs are severe and appear suddenly Subacute EGS - signs similar to AGS but usually less severe | Chronic EGS - more gradual onset of clinical signs |
| Depression Absence of gut sounds (ileus) High heart rate (tachycardia) Salivation Reflux of gut contents through the nose (gastric reflux) Muscle tremors (fasciculation) Drooping eyelids (ptosis) Patchy or generalised sweating Difficulty swallowing (dysphagia) Abdominal discomfort (colic) Constipation (impaction) | Rapid and severe weight loss leading to emaciation (Figure 3) Markedly tucked up abdomen Base narrow stance Dry, crusty nose (rhinitis sicca) (Figure 4) Drooping eyelids (ptosis) Slightly elevated heart rate (tachycardia) Muscle tremors (fasciculation) Patchy sweating Reduced appetite (inappetance) Slight difficulty swallowing (dysphagia) |
| Figure 3: Emaciation in CGS | Figure 4: Rhinitis sicca common in CGS |
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| Kindly supplied by Professor C. Proudman | Kindly supplied by Dr. E. Milne |
All cases of AGS and SAGS are incurable and affected animals are therefore put down on humane grounds. Recovery from the chronic form may be possible based on careful case selection, and only with tremendous amounts of supportive care.
How is EGS diagnosed?
Equine grass sickness may be a diagnostic challenge as the clinical features are common to other diseases. Diagnosis is often based on historical data, the pattern of clinical signs and details of the horse. A thorough physical examination by a veterinary surgeon is essential to obtain as much information as possible. Definitive diagnosis of EGS is identification of specific lesions in the nerve cells. This technique requires general anaesthesia and abdominal surgery, which unfortunately are not indicated in horses considered suitable for treatment due to the high risks involved. Other diagnostic tests which sometimes help reach a diagnosis include: x-ray imaging and application of eyedrops which work to reverse any eyelid drooping.
Can EGS be treated?
Treatment should only be attempted in less severely affected chronic cases. For these cases treatment is symptomatic and is based on intensive nursing care. Important features include keeping the horse warm and comfortable, grooming, walking and providing fresh, palatable, easy-to-eat succulent feedstuffs which provide adequate nutritional support for the convalescent horse.
What causes EGS?
Studies have to date failed to identify definitively a specific cause of EGS. Previous investigations have examined the role of various agents including: poisonous plants, bacterial toxins, insects and viruses. The current favoured theory, due to the nature of damage to the nervous system and the clinical signs, is that EGS is a toxico-infectious form of botulism caused by C botulinum type C, with the disease occurring when the neurotoxin is produced locally in the intestine (Figures 5 and 6). This is in contrast to classical botulism in which the preformed neurotoxin is eaten in contaminated feed such as spoiled silage. While further research is required to prove the causal link between toxico-infection with Clostridium botulinum and EGS, if this is proved, then prevention of this invariably fatal disease by vaccination is theoretically possible in the same way that tetanus (caused by another clostridial toxin) has been controlled very effectively by vaccination with a simple inactivated toxin vaccine (a so-called ‘toxoid').
| Figure 5: The site of damage: the equine intestines | Figure 6: Nerve damage from ingested toxins |
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| Kindly supplied by Dr. E. Milne | Kindly supplied by Dr. E. Milne |
What are the risk factors for developing EGS?
Various risk factors have been identified through observational epidemiological studies including:
- Grazing
Grazing is the main risk, with only a couple of isolated reports of EGS in stabled horses. - Season
There is a peak of cases in May, although cases may occur in any month of the year. - Recent movement to new premises or pastures
Most cases are associated with exposure to new pasture within the last few weeks. - Age
All ages of horses are susceptible but most frequently cases are 2-7 years old. - Previous occurrence of EGS on the premises
Premises that, particularly recently, have had a case of EGS provide increased risk. - Pasture disturbance
Several recent studies have identified increased risk associated with pasture disturbance, possibly through soil ingestion.
Interestingly horses present on pastures when EGS occurred previously were found to have a reduced risk of the disease implying that exposure to the aetiological agent generates a protective immune response.
How common is EGS?
Although the phenomenon of EGS has been know to veterinary science for around 100 years, and many aspects of its clinical and pathological significance have been studied, there are no accurate records of how frequently it occurs in the United Kingdom in the 21st Century.
Addressing the need for surveillance of EGS
To address this shortcoming the Animal Health Trust is currently in the process of developing and implementing the first nationwide system for the surveillance of EGS to collect details of clinical cases and produce an accurate representation of the true frequency of the disease in Britain. As well as quantifying the true impact of EGS on the welfare of horses, such frequency estimates will also be important in demonstrating trends in occurrence of the disease over time and in different areas of the UK. These estimates will also be used to calculate accurate sample size requirements for any future vaccine development and will, therefore, subsequently allow the welfare benefits arising from vaccination to be rapidly and accurately assessed
The main steps in the development of this surveillance project include:
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This project is funded by The Horse Trust, in collaboration with the Equine Grass Sickness Fund, the University of Edinburgh and the University of Liverpool. It is intended that following the initial development of the surveillance scheme results will be included in the AHT/DEFRA/BEVA quarterly equine disease surveillance report, now published on the AHT website at http://www.aht.org.uk/equine_disease.html and regularly in The Veterinary Record.
How can you help?
In order to facilitate equine grass sickness surveillance, two questionnaires have been developed to give more information on the incidence of the disease. The first questionnaire has been designed to gather information for every new case of equine grass sickness which occurs, while the second questionnaire focuses on the history of affected premises since the year 2000. For the most accurate surveillance picture we hope to obtain details on all the cases of equine grass sickness which have occurred in Great Britain between the year 2000 and present, as well as gather information on all new cases which occur in the future.
The success of the Animal Health Trust surveillance scheme is dependent on publicity and awareness. It is important that all cases of EGS are reported to Claire Wylie at the Animal Health Trust, Newmarket: 01638 552993 Ext: 1241 or claire.wylie@aht.org.uk. More information on the scheme can be found at the dedicated website: www.equinegrasssickness.co.uk
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This information has been reproduced with kind permission of the Animal Health Trust
Added on: 29/01/08. Views: 1446
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