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What is Hairy Vetch Poisoning?

Hairy vetch, also known by its biological name of Vicia Villosa, is a forage plant which, as noted above, when ingested by your horse can cause systemic granulomatous disease which is a disease which targets the immune system of the horse.  Systemic granulomatous disease is also known as equine idiopathic systemic granulomatous disease, sarcoidosis, generalized or granulomatous disease, equine histiocytic disease or equine histiocytic dermatitis and can be deadly for your horse depending on the severity of the disease and how far it has progressed.

Hairy vetch is a forage plant found in pastures, which when ingested by your horse, can cause systemic granulomatous disease which can have long-term and sometimes permanent involvement.

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Symptoms of Hairy Vetch Poisoning in Horses

Hairy vetch poisoning (ultimately causing systemic granulomatous disease) is not a common disease in horses, being more commonly found in cattle.  Here are some of the symptoms you may see:

  • Welts on the skin with hair loss
  • Thickening of the skin 
  • Itching and rubbing of areas affected by welts
  • Peeling skin generally found around nose and eyes
  • Ulcers of mucous membranes
  • Cough
  • Weakness
  • Conjunctivitis
  • Weight loss
  • Diarrhea

These are some, but not all, of the symptoms that, if noted, should be considered reasons for medical evaluation, as progression of the disease can include liver, heart and lung involvement.

Types  

Three basic forms of hairy vetch poisoning have been noted, each affecting different areas and systems of the body:

  • First form usually presents with mucous membrane involvement, hair loss, weakness and loss of appetite
  • Second form usually presents with acute neurological involvement
  • Third form usually presents with pruritic dermatitis with hair loss, diarrhea, weight loss, decreased milk yield and sporadic abortions in mares and urine with a red tinge in it

Causes of Hairy Vetch Poisoning in Horses

While hairy vetch poisoning is not common to horses but is more common in cattle, it tends to be more of a problem when the hairy vetch is a primary source of forage or when the plant is in its maturing stage, (mid to late spring). The identity of the exact toxin felt to be the cause of systemic granulomatous disease is not known and the disease has been seen in animals who have not been exposed to hairy vetch conditions.  Some of the symptoms and signs noted in horses are also similar to some of other diseases and vets have come to refer to those other diseases as “vetch-like diseases” when the exposure to hairy vetch is not a component.  

Some researchers believe that the seeds of hairy vetch might actually contain cyanide.  The infiltrates from the disease have been microscopically noted in the skin, heart, liver and other organs and can, accordingly, result in a high morbidity rate for equine afflicted with this poisoning.  The ultimate cause of the infiltrates are from the immune system itself, since this disease attacks the immune system of the host.

Diagnosis of Hairy Vetch Poisoning in Horses

The diagnosis of hairy vetch poisoning will require a complete history from you, a thorough physical examination by your veterinary professional as well as laboratory testing of blood and tissue samples.  The lab and your vet will be looking for increased white blood cells (which reveal presence of inflammation and infection) and excessive proteins in the blood.  The pathological review of the tissue samples will likely reveal the infiltrates consistent with systemic granulomatous disease, specifically lymphocytes (white blood cells), monocytes (a particular type of white blood cell that emanates from the bone marrow) and multinucleated giant cells (the smallest unit of living structure which can live independently).  

The results of these testing modalities will provide necessary information to determine if the infection is immune-mediated (a disease process resulting from the activities of the immune system).  Thoracic radiology may be obtained as well as ultrasonography of various parts of the animal to detect abnormalities which may be present. Once this determination has been made, an appropriate treatment plan can be developed and initiated.

Treatment of Hairy Vetch Poisoning in Horses

This disease is fairly rare in horses and, accordingly, the history of treatment options is limited.  The administration of corticosteroids is generally the preferred course of treatment, with prednisolone over prednisone being the steroid of choice for horses.  The course of this treatment option may be needed over a period of several months, tapering off slowly once the clinical reactions have subsided.  Some horses, historically, have had a spontaneous healing but this is also rare.  

This treatment works best when it is administered early in the disease process.  It is noted that horses having only cutaneous involvement respond better to treatment and have a better prognosis.  Having said that, it is important to note that more aggressive immunosuppressive treatment may be required but advantages and disadvantages to this aggressive treatment will need to be weighed against the risks of such long-term treatment.  This will also be weighed against the possibility that more conservative treatment may be the better option for those cases which are only cutaneously affected.

Recovery of Hairy Vetch Poisoning in Horses

The cases of this disease presenting in horses is rare and treatment options are more limited because of this rarity.  The prognosis for your horse will be dependent upon the severity of the infection and the degree to which the disease has progressed  afflicting your horse.  It is important to understand that, due to the involvement of the immune system and the degree to which the disease process can affect various major organs, the morbidity rate is high.  It is, therefore, vital that you get your veterinary professional involved as early in the disease process as possible to avoid euthanasia of your horse.  It will, or course, be a primary concern for removal of the remaining horses in your herd from the hairy vetch exposure if it is a component of the affliction.