Congenital stationary night blindness

Why a horse with night blindness struggles to see in dim light

The retina converts light into nerve signals. Dim-light vision requires information from cells called rods to reach other retinal cells. In the forms described here, that transmission is impaired. The problem is therefore more than a horse being reluctant to enter darkness.

Congenital means night blindness is present from birth, even if recognized later. Stationary means this visual problem is not expected to worsen progressively through life. A different eye disease can still develop later, so any new visual difficulty requires its own assessment rather than being automatically blamed on the existing condition.

References [3] · [4] · [5]

Two different variants can cause night blindness: LP and CSNB2

In the first form, an extra stretch of DNA has been inserted into TRPM1. This insertion is the LP variant of leopard complex spotting. A horse that inherits one LP copy from its mother and another from its father, giving LP/LP, has this form of night blindness. A single LP copy can already affect coat pattern without causing this visual disorder. The coat and vision effects therefore do not require the same number of copies.

Another form, CSNB2, involves a GRM6 variant. This gene also contributes to retinal signal transmission. Two copies are required for this recessive form, and no coat effect has been established for this variant. A horse without leopard spotting can therefore still have CSNB2.

References [1] · [2] · [4] · [5] · [6]

What behavior can reveal a horse’s difficulty seeing at night?

An affected horse may hesitate at an unfamiliar dark entrance, struggle to locate feed or water at night, or become injured in poorly lit surroundings. Daytime vision may appear normal. Familiar surroundings can also make the difficulty less obvious to an observer.

Horses respond differently to the same limitation. Neither calm behavior nor fear directly measures night vision. Problems noticed after dark deserve investigation rather than being assigned automatically to temperament, training or a single genetic result. A horse adapting well may still have markedly limited vision in low light.

References [3] · [2]

Inherited night blindness can affect several horse breeds

LP-associated CSNB occurs in Appaloosas and other populations carrying leopard complex, including Knabstruppers and Norikers. Genetic and clinical evidence for CSNB2 has been reported in Tennessee Walking Horses, Standardbreds and Missouri Fox Trotters.

Night blindness has also been described in other populations without proof that the same tested variants explain those cases. A clinical case in a breed is not evidence by itself that CSNB2 is present. Population findings guide the choice of tests without replacing individual results or establishing that every horse of that breed has the same risk.

References [1] · [3] · [6] · [5]

Which examinations can diagnose night blindness in a horse?

The veterinarian considers the history and examines the eyes before selecting investigations. Electroretinography, or ERG, records electrical responses to light stimuli. With appropriate dark-adapted testing, it can confirm the type of retinal signaling problem involved.

Eyes that look normal externally may still have a functional problem. Conversely, impaired vision can have other causes. DNA testing investigates a particular genetic cause; ophthalmic examination and ERG assess the eye’s function. These findings complement each other rather than making one test a universal substitute for the others.

References [3] · [2] · [5] · [6]

Why a negative LP test does not rule out every cause of night blindness

LP/TRPM1 results are usually N/N, N/LP and LP/LP. GRM6 results may be N/N, N/CSNB2 and CSNB2/CSNB2. N means that the tested variant was not detected on that copy, not that the gene is absent or every eye disorder excluded.

A negative LP test does not test GRM6. Negative results for both do not guarantee normal night vision in every situation, because other causes may exist. One LP copy and one CSNB2 copy cannot be added together: each pair must be interpreted within its own gene and mechanism.

References [1] · [2] · [6]

Adapting lighting and living areas for a horse with night blindness

There is no cure for CSNB. Many horses can nevertheless live with the limitation when their surroundings account for lighting, obstacles and familiarity. Appropriate management is discussed with the veterinarian using the difficulties actually observed in the individual.

Safety matters for the horse and its handlers. A horse that cannot see an obstacle in dim light may hesitate or react in ways that seem puzzling if its visual limitation is unknown. Recognizing that difficulty helps people adapt the surroundings and what they ask the horse to do. Eye pain or a new decline in vision still requires a separate assessment: it should not automatically be blamed on CSNB, which is not inherently a progressively worsening condition.

References [3] · [2]

Assessing a foal’s night-blindness risk separately for the LP and CSNB2 variants

Assess the LP variant in TRPM1 separately from the GRM6 variant associated with CSNB2. If the mare and stallion each carry one copy of the same variant, every conception has a 25% chance of a foal inheriting it from both parents and having the corresponding night-blindness condition, a 50% chance of inheriting it from only one parent without that condition, and a 25% chance of inheriting it from neither. A foal with a single LP copy can still have an altered coat pattern. Four births are not guaranteed to follow these proportions.

A parent with two LP copies passes LP to every foal. If the other parent is confirmed N/N by the TRPM1 test, the foals will be N/LP: they will not have that night-blindness form, but their coats may be affected. Likewise, a horse with two copies of the GRM6 variant, mated to a horse confirmed not to carry that variant, produces carriers without the CSNB2 condition. These calculations cannot be pooled across the two genes. Checking both parents for one tested form does not assess every other health or breeding question.

References [1] · [2]

Questions about the condition: Congenital stationary night blindness

Scientific sources about the condition: Congenital stationary night blindness

This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.

1. 1.
   Leopard Complex, Congenital Stationary Night Blindness, and Uveitis

   UC Davis — Veterinary Genetics Laboratory · Test documentation

2. 2.
   Congenital Stationary Night Blindness (CSNB2): GRM6 testing

   UC Davis — Veterinary Genetics Laboratory · Test documentation

3. 3.
   Congenital Stationary Night Blindness: signs, diagnosis and management

   UC Davis — Center for Equine Health · University resource

4. 4.
   Bellone et al. (2013) — Evidence for a retroviral insertion in TRPM1 causing night blindness and leopard complex spotting

   PLOS ONE — University of Tampa and collaborators · Research

5. 5.
   Hack et al. (2021) — Whole-genome sequencing identifies a GRM6 variant linked to night blindness in a Tennessee Walking Horse

   Equine Veterinary Journal — UC Davis, North Carolina State and Texas Tech · Research

6. 6.
   Esdaile et al. (2024) — Additional evidence supports GRM6 p.Thr178Met as a cause of night blindness in three horse breeds

   Veterinary Ophthalmology · Research