The overall prevalence of CCVD in the present study was lower compared to the previous studies such as in Addis Ababa (Ethiopia), 4.52%, by Abebe and Wondimkun [24], in Saudi Arabia, 3.36%, by Oriowo and Alotaibi [12], in Manipur (India), 5.28 %, by Shah et al [13], in Shekhan City/Kurdistan region (Iraq), 3.28%, by Abdulrahman [14], in Welkite town (central Ethiopia), 4.10%, by Woldeamanuel and Geta [15] but higher than a report in Bhopal (India), 2.50% by Gupta et al. [16]. The lower prevalence of CCVD in the present study relative to other studies conducted in Ethiopia [24, 25], could be due to the larger sample size of the current study. In this study, the majority of students were unaware about colour vision deficiency disease, except students from Health Science and Medicine.
The prevalence of CCVD recorded in males in the present study (3.75%) was similar to the reports in western Nepal (Pokhara, 3.80%) [31] and in Rajasthan (India, 3.20%) [32]. Yet the frequency of CCVD recorded amongst males in this study was low relative to investigations in Tehran (8.20%) [33], in Denmark (8.67%) [34], in Nepal (5.58%) [17], in Erbil City (Iraq, 8.47%) [18] and in Bhopal (India, 4.20 %) [16]. However, the prevalence of the CCVD (males) in the present study was higher than that of India (2.30%) [35] and in Qazvin (Iran, 2.56%) [20].
Several studies indicated that the prevalence of CCVD in females is usually lower as compared to that of males (e.g., [12, 13, 23, 31]). The prevalence of CCVD amongst females in the present study was higher than in Italy (0.10%) [36], in northern Ethiopia (0.20%) [23], but comparable to that of Saudi Arabia (0.75%) [12] and in Qazvin(Iran, 0.93%) [20]. A relatively higher frequency of CCVD in females is reported by couples of studies, for examples, in the Punjab city of India, (1.10%) [37] and Erbil City (Iraq, 1.37 %) [18]. However, studies conducted in Libya [35], in western Nepal (Pokhara) [31], in Rajasthan (India) [32] reported no cases of CCVD amongst female subjects. On the contrary, studies done in Faisalabad (Pakistan) [38] and in Kolkata (West Bengal/India) [39] reported a higher prevalence of CCVD in females compared to males. The higher prevalence amongst males as compared to females indicates the genetic causation of the disorder [4,5,6,7,8,9,10]. Congenital colour vision deficiency is genetically determined by X-linked recessive inheritance and thus occurs in males but is transmitted via females and about 8.0% of all women are carriers [40].
The prevalence of achromacy in this study was comparable with Abeshge district (central Ethiopia, 0.19%) [25] and in Ugep (Nigeria, 0.20%) [41]. On the other hand, the prevalence of deutan in males (2.89%) was lower compared to the northwest Ethiopia (3.20 %) [23], but similar to Abeshge district (2.89%) [25].
The prevalence of CCVD amongst students of Amhara (3.04%) and Oromo (3.06%) ethnic was lower compared to some ethnic groups in Nepal (e.g. 5.00% in Brahman, 5.10% in Gurung, 9.10% in Newar and 14.30% in Darji) but higher relative to other ethnic groups (e.g. 2.8% in Chhetri and 2.1% in Magar) [31]. Furthermore, the prevalence of CCVD in males (3.75%) and females (0.68%) in the present study was lower compared to various ethnic groups in Iraq (e.g. 8.45%/1.20% (males/females) in Kurd, 9.44%/2.24% in Arabs, 8.52%/1.56% in Turkman, and 7.40%/1.05% in Kldan) [18]. The lower frequencies of various types of CCVD in the present study compared to Iraq [18] and India [13], could be due to the common practices of consanguineous marriage in countries in the middle east and southeast Asia [12, 13, 18, 31, 35]. In this regards, in Ethiopia consanguineous marriage is rare as it is not promoted because of cultural, religious and legal factors. However, a couple other factors contribute for the difference in the prevalence of CCVD between populations and geographic regions, in addition to the degree of consanguineous marriage (e.g. population movements, the molecular structure of gene on the X chromosome, natural selection). For example, increasing migration of people within a country and across countries (e.g. in Saudi Arabia [12], in India [13]), might indirectly lead to increase in the rate of exogamous marriages that could contribute increase in the prevalence of CCVD. It is also suggested that increase incidence of colour blindness amongst the Caucasians may be due to difference in the molecular pattern of X chromosome of colour vision genes. Furthermore, it was observed that overall frequency of colour vision defects has been observed quite low amongst scheduled tribe groups (traditionally food gatherers and hunters and later occupied in shifting cultivation and as agricultural laborers) from all the zones in India followed by scheduled caste groups (about 90% of scheduled castes are agricultural laborers) which is followed by caste groups [13].
Average heterozygosity is a measure of genetic diversity at the population scale and indicates the average proportion of individuals that are heterozygous for a given trait. The frequencies of CCVD alleles found in the present study was lower compared to those of the frequencies reported in five populations in Manipur (India) [13] and for science students in the ASC Rahuri College (India) [42]. Furthermore, the level of heterozygosity found in the present study amongst female students of Amhara (8%) and Oromo (8%) ethnic origin was low compared to that of the population in India (e.g. Meitie (26.61%), Sheikh (24.45%) [13]. In addition, the heterozygosity documented in the present study for female students was almost half of that of the prevalence in a general population (14.70%) [43]. Similarly, the frequency of double heterozygosity found in the present study for female subjects was one fifth of the prevalence in the general population [43]. Females who had a recessive allele for deutan on one X chromosome, but a recessive allele for protan on the second X chromosome are called ‘double carriers’ (or compound heterozygote) [43]. Carroll [43] has also examined a female subject who had one deutan son and a protan son, which indicated her identity of double heterozygote/double carrier. Furthermore, the identity of this subject as both deutan and protan was also confirmed by genetic analysis. In the present analysis, we did not explicitly find double carrier subjects, but we do not rule out the possibility of getting such subjects.
In some jobs such as doctors, educational trainers and drivers, colour recognition is essential and hence detection of colour vision deficiency at an early age is useful to avoid certain occupational hazards [2, 13, 21, 22]. However, in Ethiopia, early testing of colour vision deficiency is not common when students join university to study fields like engineering, soil sciences, chemistry, electricity and electronics, etc. Since colour vision deficiency is not a deadly disease, an individual can be a colour vision deficiency without noticing the scene. Unnoticed deficiency may lead to professional inefficiency and some risks. A study conducted by Abebe and Wondmikun [24] on defective colour perception amongst licensed car drivers in Addis Ababa revealed a prevalence of 4.50% of colour vision impairment. Furthermore, the results showed that 31.8% of colour blind subjects had road traffic accidents the past three years prior to the study. They have also found that an examination of colour vision in driving license seekers in Ethiopia does not screen out colour-blind individuals. The likelihood that colour-blind drivers would encounter an accident is about twice as high as amongst non-colour-blind drivers [24]. Therefore, in today’s automated world, when every livelihood is connected with modern technology, analysis of colour vision deficiency is indispensable to mitigate the potential impact of colour vision deficiency [2, 13, 21, 24].
Limitations
The overall prevalence of CCVD found in the present study was lower compared to the previous studies done in Ethiopia. The prevalence of CCVD was highest amongst males, but relatively low amongst female subjects. There was a clear difference in the prevalence of colour vision deficiency amongst students of various ethnic groups. Many students were unaware of colour vision deficiency itself, except students from health science and medicine. Therefore, proper screening, education and counseling are needed to minimize the influences of colour vision deficiency in the country and could be beneficial to the affected subject in tackling difficulties in everyday work and for proper choice of future profession.