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Grass Sickness - The Silent Killer
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Grass Sickness - The Silent Killer

In August 2011, I lost my beloved horse to a disease I had only vaguely heard of and knew very little about. The cause of the disease is unknown. There is no known cure. Victims are taken at random; from a multi-million dollar racehorse, (Dubai Millennium, 2001), to a happy hacker, a child's Shetland pony or even a humble companion donkey. There have even been cases recorded in a captive zebra and Przewalski's horse. 

Age and sex are not a defining factor, although age two to seven years shows the highest number of affected animals.  Interestingly, foals are rarely affected and older animals seem less likely to succumb, possibly because they have built up some resistance to the causative agent and thus built up a resistance to the disease.

The disease is called Grass Sickness and is so named because it occurs almost exclusively in animals with access to grazing. The disease first appeared in army remount horses in 1907.

During the 1920s so many draught horses died that the introduction of the tractor was artificially accelerated as a direct result. Currently, one in 200 horses dies every year from Grass Sickness in some parts of the UK.  Cases of Grass Sickness have been seen world-wide but the highest concentration is seen in the UK, particularly in eastern counties and Scotland and in Northern parts of Europe.

Grass Sickness can occur at any time of the year but most cases are seen between April and July, peaking in May.

Risk Factors

Most horses affected, although not all, are kept at grass throughout the day. Certain premises, or even particular fields within a single yard, are associated with Grass Sickness outbreaks. Animals which have only been on affected yards for less than two months are more likely to develop the disease. Only one horse at a time is generally affected although more cases may appear over a period of a few weeks. Type of grazing does not seem to be a factor, although evidence suggests that high nitrogen content, soil disturbance and excessive amounts of clover are present in a high number of pastures where Grass Sickness cases occur.

Animals in good to fat condition are most commonly affected as are those who have been stressed, for example moving to a new yard, long distance travel or even being newly backed. Statistics and owner opinion suggest that prevailing weather conditions, cool and dry with a temperature from 70 to 110, may be a trigger for outbreaks.

Possible causes

Despite 100 years of exhaustive investigation, the exact cause of Grass Sickness is still unknown. Poisonous plants, chemicals, bacteria, viruses, insects, and mineral deficiencies have all been suggested and ruled out as causative.  The disease is not contagious.

Recent research suggests the likely involvement of the soil-associated bacterium, Clostridium botulinum.


There are three forms of Grass Sickness; acute, sub-acute and chronic.

The main symptoms relate to partial or complete paralysis of the digestive tract from the gullet downwards.  In the acute form, the symptoms are severe, sudden and the horse will die within two days of the onset. Signs of classic, severe impacted colic together with difficulty in swallowing and drooling of saliva occur. Patchy sweating may be seen together with fine muscle tremors.

The sub-acute form of the disease presents similar, less severe symptoms. The horse may consume small amounts of food although with difficulty and very rapid weight loss will occur. The horse will be constipated and have difficulty passing droppings due to the lack of hindgut function. As the disease progresses, breathing difficulties may be experienced. The horse will adopt a typical tucked-up, roach-backed stance and will spend increasing amounts of time lying down.

Horses diagnosed with acute or sub-acute Grass Sickness should be humanely put to sleep without delay as they will not recover and death is inevitable.

In cases of chronic Grass Sickness less severe symptoms are exhibited. One of the major symptoms is rapid and severe weight loss, leading ultimately to emaciation. These horses may partly recover but will need intensive and dedicated nursing over a very long period of time and most are ultimately unable to return to full work.


It has recently been announced that vets have at last developed a potential vaccine for Grass Sickness and are proposing a clinical trial of this in 2014. This will truly be a wonderful breakthrough if it is successful.

I remember that lovely July evening when I arrived at the yard to bring my beautiful six year old baby in for the grooming session he so adored before setting off for a ride through the fields. I found him lying down in his paddock, apparently with a touch of colic. I called the vet straight away. Two hours later he was taken to the veterinary hospital as an emergency admission for colic surgery. Two days later he was diagnosed with sub-acute Grass Sickness. One week later he was put to sleep; and a part of me went with him.

I just hope and pray that this vaccine is effective and can be offered to all horse owners. It has come too late for Spencer, but I would not want anyone else's horse to have to go through what he did.

I have written a book in Spencer's memory; royalties to go towards the fundraising effort for the vaccine trial.  "If I Won A Pony" by Eve Paege is published on Amazon.

Disclaimer: Of Horse! and sponsors do not endorse nor validate the accuracy of a blog post. Each article is the opinion of the blogger.

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  1. PonyGirl
    I am so sorry for your loss. Grass sickness sounds truly terrible. I have never heard of it before. Thankfully, (according to what I was able to find online) cases are extremely rare in the U.S. I hope they find a cure soon. I'm in the off season of my work, but once I begin to make a little more money, I will certainly buy a copy of your book. I know someone who would really enjoy it. :D
    1. autumnap
      Thank you, that's very kind. I like to think my little guy could help other horses in some way. x
  2. naturegirl
    I've never heard of this. Aren't horses supposed to feed on grass? I read that grass is their main diet when they are wild. Voted.
    1. autumnap
      Yes I know, it seems pretty odd. I don't know what the incidence is in wild horses but they think that the bacterium most likely responsible is found in soil and that incidences are more frequently observed where pasture has been harrowed or cleared with an automatic poo picking machine. The soil is disturbed so horses are more likely to ingest it whilst grazing. Wild horses would not experience this environment as they move from sparse grazing to better - I guess. Let's hope they can finally find an effective vaccine. x
  3. Rene Wright
    Rene Wright
    Voted... I'm sorry for your loss. I don't know that we have Grass Sickness here in the U.S. but I truly hope they find the cause & cure quick fashion as it seems to come on & take over rather quickly.

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