Bit & Bridle

Equine Night Blindness (CSNB)

Equine Night Blindness (CSNB) is a hereditary visual disorder characterised by a complete inability to see in low-light conditions. Affected horses have normal daytime vision but are functionally blind in dim light or darkness. The condition is caused by a genetic mutation that disrupts the transmission of signals from the rod photoreceptors of the retina — the cells responsible for vision in dim light — to the downstream visual pathway. CSNB is non-progressive, present from birth, and cannot be treated or cured, but many affected horses lead full working lives once owners understand and accommodate the limitation.

Other Names

  • CSNB (Congenital Stationary Night Blindness)
  • Night blindness
  • Moon blindness (historical, though this term is more commonly used for Equine Recurrent Uveitis — a separate condition)
  • Nyctalopia

What Is Equine Night Blindness (CSNB)?

CSNB is a congenital (present from birth) and stationary (non-progressive) form of night blindness in horses. It is caused by a defect in the ON-bipolar cell pathway of the retina — specifically, a failure in the transmission of signals from rod photoreceptors (which detect low-intensity light) to the downstream retinal cells that carry visual information to the brain. In affected horses, daytime (photopic) vision mediated by cone photoreceptors is entirely normal. The rod-mediated (scotopic) pathway — which drives vision in dim and dark conditions — is non-functional.

The condition is caused by a specific missense mutation in the TRPM1 gene (Transient Receptor Potential Melastatin 1), which encodes a cation channel expressed in ON-bipolar cells of the retina. This mutation was identified in the Appaloosa breed and subsequently found in several other breeds.

Causes

CSNB in horses is caused by a single base-pair mutation in the TRPM1 gene. The mutation is inherited as an autosomal recessive trait — a horse must inherit two copies of the defective allele (one from each parent) to be clinically affected. Horses that carry one copy (heterozygotes) are visually normal and show no signs of night blindness.

The condition is most prevalent in the Appaloosa breed and related breeds such as the Knabstrupper and Miniature Horse, where it is strongly associated with the LP (Leopard Complex) coat patterning gene. Horses homozygous for LP (LP/LP) — those with the most extreme leopard coat patterns — are virtually always affected by CSNB. LP/LP horses are characterised by white sclera, mottled skin, and striped hooves in addition to their coat pattern.

Because LP/LP is the genotype required to produce certain high-contrast coat patterns prized by breeders, CSNB has historically been inadvertently selected for alongside these coat patterns.

Symptoms and Signs

The defining symptom is complete night blindness from birth. Because the condition is stationary (it does not worsen over time), and daytime vision is normal, many affected horses go undiagnosed for years — particularly if they are primarily worked in daylight. Signs that may prompt suspicion include:

  • Anxiety or refusal at night: The horse becomes extremely reluctant or refuses to move in low-light conditions
  • Bumping into objects in dim light that the horse navigates easily in daylight
  • Increased vigilance or startle response at dusk and dawn
  • Difficulty loading into a dark trailer or stable at night
  • Apparent anxiety in poorly lit arenas or buildings
  • Reluctance to be turned out at night or erratic behaviour once outside after dark
  • Nystagmus: Involuntary rhythmic oscillation of the eyes — present in some affected horses as a resting or strabismic nystagmus

Daytime performance is typically entirely normal. An affected horse may excel at competition during daylight hours whilst being completely unsafe to handle in darkness.

Diagnosis

Diagnosis can be confirmed through a combination of approaches:

  • Clinical assessment: Observing the horse's behaviour in progressively dimmer lighting conditions. A horse that is confident in daylight but becomes distressed and disoriented in low light is highly suspicious for CSNB.
  • Electroretinography (ERG): The definitive clinical diagnostic test. ERG measures the electrical response of the retina to light stimuli. In CSNB horses, the scotopic (dark-adapted, rod-mediated) ERG is absent or severely attenuated, while the photopic (light-adapted, cone-mediated) ERG is normal. This finding is pathognomonic.
  • Genetic testing: A DNA test for the TRPM1 mutation is available and can confirm homozygous status (LP/LP) in Appaloosas. This is the most accessible and cost-effective diagnostic option for breeders.
  • Ophthalmoscopy: The fundus appears grossly normal on examination — there are no visible retinal lesions or optic disc abnormalities, which helps differentiate CSNB from other causes of vision loss.

Treatment

There is no treatment for CSNB. The condition is caused by a fixed genetic defect and cannot be corrected pharmacologically or surgically. Management focuses entirely on adapting the horse's environment and work programme to its visual limitation:

  • Ensure all work and handling takes place in adequate light
  • Provide well-lit stabling, particularly if the horse is stabled at night
  • Use arena lighting for evening work sessions
  • Allow the horse time to adapt gradually from light to dark where possible — providing some transition time (even 10–15 minutes) can reduce anxiety
  • Avoid turning out in complete darkness without a companion horse to follow
  • Inform all riders, carers, and handlers of the horse's diagnosis
  • Use additional lighting when loading, performing veterinary examinations at night, or working in low-light conditions

Many horses with CSNB lead perfectly full working lives and compete successfully. The key is informed management.

Prevention

CSNB can be eliminated from breeding programmes through genetic testing:

  • Test breeding stock for LP genotype and the TRPM1 CSNB mutation before breeding
  • Avoid LP/LP x LP/LP matings — these always produce LP/LP offspring, all of which will have CSNB
  • LP/LP x LP/lp matings produce a 50% risk of LP/LP offspring
  • LP/lp x LP/lp matings carry a 25% risk of LP/LP offspring
  • lp/lp horses cannot produce CSNB-affected offspring regardless of their mate

Genetic testing for LP genotype is available through several equine genetics laboratories. Responsible breeding requires that the risk of producing CSNB-affected foals be weighed against the desire for extreme leopard coat patterns.

Prognosis

The prognosis for quality of life is good, provided the horse's management is appropriately adapted. CSNB does not worsen over time and does not affect daytime vision. Affected horses do not experience pain as a result of the condition. The prognosis for competitive performance depends on the discipline — equestrian sports conducted entirely in adequate lighting carry no inherent limitations for an affected horse.

When to Call the Vet

Consult your vet if:

  • Your Appaloosa or Appaloosa-type horse shows signs of visual difficulty in low light
  • You notice nystagmus (rhythmic eye movements) in a horse at rest
  • A horse is behaving unsafely in conditions that others find acceptable (e.g., being handled at dusk or in dimly lit stabling)
  • You want to establish a horse's LP/CSNB status before purchase, breeding, or competition

For related conditions affecting equine vision, see our entry on Equine Motor Neuron Disease (EMND), which also produces retinal changes. For general information about the equine eye, see our horse terminology guide on equine eye anatomy. The Appaloosa breed profile provides further context on LP genetics and coat patterns.

Frequently Asked Questions

Is CSNB the same as "moon blindness"?

No. "Moon blindness" is a historical term that has been applied to Equine Recurrent Uveitis (ERU), a painful, inflammatory eye condition that causes intermittent flare-ups and can lead to progressive vision loss and blindness over time. CSNB is a completely different condition — it is present from birth, non-painful, non-progressive, and affects only night vision. The overlapping historical terminology can cause confusion, but the two conditions are clinically and pathologically distinct.

Can a horse with CSNB be ridden safely?

Yes, in adequate light. Most CSNB horses are entirely safe to ride during the day and in well-lit conditions. They should not be ridden or worked in darkness or very low-light conditions without appropriate lighting. Riders and handlers must be informed of the horse's condition, and work environments should be assessed for adequate lighting before working an affected horse.

Does every Appaloosa with extreme markings have CSNB?

Not every Appaloosa with spotted markings has CSNB, but horses that are homozygous for LP (LP/LP) — characterised by a few-spot or near-few-spot pattern, white sclera, and mottled skin — are virtually always affected. Horses with less extreme leopard patterns may be LP/lp (heterozygous) and have normal night vision. Genetic testing is the only way to confirm LP genotype and CSNB status with certainty.

Should I disclose CSNB when selling a horse?

Yes, absolutely. CSNB is a material fact that affects a horse's management, safe use in certain conditions, and potentially its value. Failing to disclose a known diagnosis could expose a seller to legal liability. If you know or suspect your horse has CSNB, genetic testing or ERG examination before sale will give both parties clear, documented information on which to base the transaction.