West Nile virus is a mosquito-borne pathogen of global public health importance. With the rapid global spread of WNV and the endemic state, WNV might spread in new geographical areas in Egypt. To the best of our knowledge, this is the first large-scale study investigated the WNV seroprevalence between horses and the associated risk factors in Egypt. Therefore, the results of this study provide updated information about the WNV seroprevalence and the associated risk factors that can be used as a guide for effective prevention strategies. The major advantages of the current study are: 1) broad context of many provinces, placing it amongst the few studies that have examined the seroprevalence of WNV infections in horses across the most densely equine populated areas in Egypt; and 2) large number of participating horses that reflects on the robustness of the association between variables. The main finding of this study is that the male mixed breed horses with ages ≥ 15 y are more susceptible to WNV infection in Egypt.
West Nile virus circulating in Egypt since 1951s 9. In Egypt, most of the earlier studies were focused on the investigation of WNV epidemiology in humans 9,16,17. Recently, the epidemiological status of WNV infection among horses is receiving a growing attention in Egypt. Overall, the antibodies against WNV were detected in 156 out of 930 (16.7%) examined horses. The reported seroprevalence rate concur with other rates reported in different countries, 15.08% in Poland 18, and 15% in Portugal 19. However, this rate is lower than that reported in surroundings, where the prevalence of WNV infection among horses in Israel is 39% 20, 24.9% in Jordan 21, 26.8% in Algeria 22, 31.1% in Morocco 23, 31.6% in Turkey 24, and 68.7% in Senegal 22. In USA, the prevalence of WNV infection was 56.4% in 2003 25. In Europe, the European Centre for Disease Prevention and Control (ECDC) and the World Organization for Animal Health (OIE) declared a total of 86 human cases and 14 cases in horses 26. The lower horse-level seroprevalence of WNV that reported in this study is might be due to low number of horse farms and most of the horses enrolled in this study are individual raising horses. This may decrease the chance of horse exposure to virus. Additionally, one of the main factors affecting the spreading of WNV infection is the environmental factors 16. The higher temperature above 30 °C as in the Nile Delta of Egypt particularly in summer, and low rainfall during winter and spring were associated with reduction in the mosquito activity and decrease in its growth. Consequently, the spread of WNV among horses will be decreased because the propagation of WNV in mosquitoes is temperature dependent 27. A negative association has been reported between the WNV infection in mosquitoes and WNV epidemics and the previous year’s precipitation 20,28−30. Other factors such as humidity, presence of local and migratory birds’ settlements, and vegetation index have also significant contributions to the propagation of WNV 31. Furthermore, that most of sample collected early summer and most of WNV infection occurs usually in late summer and fall 21.
The results of this study didn’t show difference in the seroprevalence of WNV infection between studied geographic regions in the Nile Delta of Egypt in agreement with earlier studies 16. This is a sensible result because the similar geographic nature of the land in the five studied provinces that is characterized by flat low-lying areas. Consequently, the insect population density, the main transmission vector for BLV, are approximately similar in these provinces. Another possible reason for the non-significant difference in the seroprevalence of WNV infection among the studied geographical areas is that the equine trading between provinces is not common in Egypt.
The results of the current study showed marked increase in the risk of WNV infection with age suggests recurrent circulation of WNV. This result is consistent with earlier studies 12,32,33. However, other studies didn’t show any association between the seropositivity of WNV infection and age 10,18,21. It has been reported that the older affected horses have been more likely to die from the WNV infection 34–36. In humans, the median age of affected human is 63 y, suggesting the older people are more prone to acquire WNV infection and approximately 20-fold increased risk at age > 60 y for susceptibility to neurological involvement and retinopathy associated with severe disease 32,33. The reason behind this positive relationship between seropositivity of WNV infection and age is unknown but the older horses > 10 years are might be immunocompromised animals, where aging is associated with a dysregulation of the immune system 37. With age, alterations occur to adaptive immunity compounds and reduction in innate recognition and signaling resulting in increased susceptibility to WNV in elderly horse 38,39.
The second risk factor for WNV infection reported in this study was the gender. Our findings come in accordance with Epp, et al. 34 that found a higher seroprevalence of WNV infection among stallions than mare or gelding. This interesting finding has been reported in human where they reported a higher seroprevalence of WNV in male than female 40. Additionally, a higher rate of hospitalization and a higher incidence of neuroinvasive disease was shown in WNV-infected males than WNV-infected females 41. The reason behind this result is unclear but several human studies reported gender deference in the seroprevalence of WNV 40,42. They suggest that gender may play an important role in sustaining WNV immunity. A marked difference was found in immune response against WNV infection between males and females as the infection progressed, with WNV-infected males exhibiting an altered cytokine response 42. The difference in immune response to viral infection between males and females may be partly attributed to the sex hormones estrogen and androgen 43. It has been reported that stallions have poor immune response in comparison with mare or gelding because the testosterone hormone impairs immune system functioning 44. Furthermore, due to culture believe, the male horses are more preferable for pulling large wagon and horse-drawn carriage that make them more vulnerable to mosquitoes and WNV infection.
Intriguingly, the mixed breed horses showed higher seroprevalence of WNV than the Arabian and Thoroughbred horses in agreement with other earlier study 21. However, different study reported no effect of breed on the seroprevalence of WNV infection in horses 18. The reasons for the higher WNV seroprevalence found in our study among mixed breed horses, as compared to other breeds, is unknown but it might be due to different preference of mosquitoes to different breeds of horses, due to variables such as density of hair and differences in sweat composition20.
The limitation of the present study warrant mention is that this study is a cross-sectional study that is require a larger sample size and more geographic representation than was the case in this study and therefore to verify the associations obtained in this study, longitudinal studies are required.