DEPRIVING LIGHT AND LOW ILLUMINANCE LEVELS CAUSES MYOPIA IN CHICKS
Low concentrations of retinadopamine, a light-driven neurohormone, has previously been associated with myopia. The latest studies show that the refractive development of the chicken eye also depends on light color (Schaeffel et al.1), light period and light intensity (Cohen et al.2).
Chicks were divided into 6 lighting groups and exposed to illuminance levels of 50, 500 or 10,000 Lux warm white light. Three of the six groups were illuminated with a lighting period of 12 hours per day, the remaining groups 24 hours per day. After 90 days, the chicks developed the least refractive myopia under continuous light and the chicks reared under light with a low light intensity <50 Lux at 12 hours per day, developed the most refractive myopia. After 90 days, by far the majority of chicks from the latter group developed refractive myopia of an average of -2.41 ± 1.23.
This suggests that the vast majority of chicks reared under warm or neutral fluorescent light with low light intensities are already short-sighted before they even go into production. This possibly explains the enormous number of keel-bone fractures in laying hens. Chickens already have more trouble estimating depth at a distance. If during the rearing period the hens also developed an average myopia of -2.41, there is a chance that they will misjudge the distance when they jump from stick to stick and break their keel-bone. Practical research should show whether chicks in a well-lit rearing house develop less myopia and suffer less keel-bone fractures later in life than chicks reared under fluorescent lighting with low illuminance.
THE EFFECT OF LIGHT COLOR ON REFRACTION DEVELOPMENT AND RETINAL DOPAMINE
Schaeffel et al. (2018) 1 examined the dopamine DA release, refraction development and growth of chicken eyes, which were exposed to light with different colors in clear view, no view or were covered with a mat diffuser to induce myopia.
The chicks developed significantly less myopia when kept under UV-A and blue light compared to under red and white light. The eyes with clear vision became the least myopic in blue and UV-A light. Chickens have UV-A and blue-sensitive melanopsin and neuropsin pigment in their retina, both of which can control dopamine release. The inhibitory effect of UV-A light on myopia could therefore possibly be explained by the stimulatory effect on dopamine release and refractive development.
In addition to the fact that light color influences the biological sleep-wake rhythms, (re)production and behavior of a chicken, light color and light intensity also influence the refractive development of the eye. Once the eye has grown in a myopic direction, the eye will always remain myopic. The spectrum and the light intensity during the rearing period are therefore very important for the development of the eye.
1Schaeffel et al. (2018)
2Cohen et al. (2012)
Author: Remco van Dijk | Haitzled Poultry Light