What is Lasix?

  • Lasix is a diuretic drug which means that when administered to a horse it causes excretion of extra urine, i.e. the horse gets rid of extra fluids from its system.
  • Lasix is under the trade name of Dimazon in Ireland and its active ingredient is fureosemide.

    • Is Lasix a licensed drug?

  • Yes, it is fully licensed for use in horses here under the name Dimazon. It is a prescription only medicine (POM) and can only be given intravenously.

    • What are the veterinary uses of Lasix?

  • It is commonly used in veterinary treatments for the following conditions:
  • Filling in horses’ legs
  • Oedema or fluid along the lower abdomen, for example, in mares in late pregnancy
  • Filling of the sheath, in colts or geldings
  • Filling of tendon sheaths, etc
  • Lasix is often used in conjunction with antibiotics for treating an inflamed leg

    • What is the main use of Lasix in racing?

  • The main use in race horses is to help to prevent or reduce the likelihood of a lung bleed in a horse that has a susceptibility to ‘bleeds’.
  • These horses are referred to as ‘bleeders’. The veterinary term for the condition is Exercise Induced Pulmonary Haemorrhage or EIPH.

    • Would a lung bleed/EIPH prevent a horse from winning a race?

  • Yes, it is an important cause of disappointing race performance. A horse that has a lung bleed under pressure in extreme racing conditions could lose a race irrespective of the animal’s ability, fitness level or form.
  • The effect of EIPH on performance is thought to be related to the volume of haemorrhage. A small amount of pulmonary haemorrhage may or may not significantly reduce performance but a larger amount of bleeding will be likely to impact on it.
  • Blood in the lung can cause lung inflammation for at least three weeks post bleed episode.

    • What causes horses to bleed from the lungs/EIPH?

  • Although EIPH is common, its causes are not fully understood. Currently, the most accepted theory is that exercise-induced high pressure in the lungs results in capillary damage. When galloping, racehorses have a very high heart output that pushes blood through the lungs at very high pressure which may rupture capillaries there, causing release of blood into the lung. Blood in the lung can impair the exchange of oxygen and carbon dioxide so the horse is forced to slow up.
  • Other causes of lung bleeds include pre-existing scarring within the lung tissues from previous bleeds, airway disease, i.e inflammation caused by viral infection or allergy, the inhalation of stable dust and other foreign particles, etc.
  • Any upper respiratory tract obstruction (such as laryngeal hemiplegia/‘whistling’) may potentially contribute further to EIPH.
  • The prevalence of bleeds tends to increase with age in horses that have repetitive lung bleed episodes, as fibrosis and chronic inflammation can develop contributing to further haemorrhage.

    • Can Lasix permanently prevent recurrence of lung bleeds in a horse?

  • No, there are currently no methods or treatments to permanently prevent the occurrence of bleeders.
  • However, certain treatments and management strategies can assist in reducing the severity of haemorrhage and therefore reduce the ongoing repetitive lung damage.
  • Ideally, treatment includes management of the damage already present in the lung by providing the lungs with sufficient time to clear blood and to heal.
  • Approximately one month is likely to be needed to recover from a single acute episode of EIPH (bleeding).
  • A longer period is required in many cases where chronic lesions are present.
  • But unfortunately despite months of rest and treatment, some horses continue to bleed when faster exercise resumes.

    • How does Lasix work when used on a potential bleeder?

  • Lasix/Furosemide decreases plasma volume, heart output and therefore blood pressure in the lungs. It is said to reduce the incidence and severity of EIPH by up to 50% under certain circumstances.
  • It is administered to the horse approximately three hours before performance or racing, i.e. where lasix is allowed under Rules.
  • The horse then passes large amounts of urine/fluids which causes a dramatic drop in blood pressure and in the horse’s weight.
  • The horse passes up to 15 litres of urine in the space of an hour post Lasix administration, i.e. about the same amount that he would normally pass in a day without Lasix. Consequently the small blood vessels in the lungs are less likely to burst when the horse comes under pressure or duress in the final furlongs of the race.

    • Is this drop in the horse’s blood pressure and body weight likely to affect performance negatively?

  • No. In fact it is acknowledged as a performance-enhancing drug and the drop in a horse’s weight can result in a horse running a faster race.