A total of 3215 cases of ophthalmic examination were investigated. A total of 102 domestic dogs infected with thelaziosis were found. 62 cases were binocular infection and 40 cases were monocular infection. The total infection rate was 3.17%. At the same time, 1277 T. callipaeda were collected. The average number of nematodes carried by each domestic dog was 7.80 ± 8.51 in the oculus sinister and 8.00 ± 9.08 in the oculus dexter. Further exploring the relationship between the prevalence rate and various risk factors, it was found that the epidemiological characteristics of thelaziosis were significantly correlated with regional distribution, seasonal distribution, field travel history and anthelmintic history, but not with individual factors (age, sex, breed and body type), living environment, diet and immunization history (Table 1).
Due to the relative lack of epidemiological survey data of cases of domestic dogs thelaziosis in China, compared with the survey results abroad, the total prevalence rate obtained in this test is low. Although it is similar to the prevalence rate (3.8%) and carrying capacity (8.08 ± 9.49) of pet dogs in Portugal reported by Maia , it is quite different from the prevalence rate (26.1%, 33.1% and 68.0%) of dogs in Spain reported by Marino . The difference is large, and the main reason may be the differences in sampling areas and sampling objects. The sampling site of this test is Beijing, and the sampling objects are mainly domestic pet dogs. The sampling area of Marino  is in the same latitude range as China, but there are many wild animals in this area. Among them, the collection objects with the highest prevalence rate (68.0%) are hounds with high prevalence risk, and there are fruit plantations around the sampling site. The results of this experiment show that regional distribution and field travel history have a significant impact on domestic dogs infected with thelaziosis. The prevalence rate of domestic dogs in the rural area of Beijing is significantly higher than that in the urban area. The prevalence rate of domestic dogs with field travel history is significantly higher than that without travel, and the proportion of domestic dogs living in the rural with field travel history is higher. It shows that the influence of field travel history on the prevalence rate is of great significance, which may be related to the activities of wild animals, the continuous reproduction of intermediate hosts, ecological environment and other factors.
Thelaziosis can affect a variety of mammals, including wild animals and even economic animals in addition to domestic pets. Therefore, its prevalence rate will be relatively high in areas with wild animal activities . According to the survey data of European countries, in areas with high prevalence rate of dogs, there are more cases of other hosts such as cats and foxes [24, 27]. In European countries that were not endemic before, the geographical distribution of thelaziosis is attributed to the distribution of several wild species, such as wild predators (such as red fox, wolf and badger) and rabbits [28, 29]. In Korea, according to Seo , the infection rate of military dogs is as high as 33.5%, 12.4 times that of farm dogs, mainly because military dogs are raised in mountainous areas. At the same time, there are many fruit planting parks around the city, which are suitable for the reproduction and development of intermediate host. There is a report of a dog infection case , and there are many strawberry plantations in its living area. Combined with the results of this test, it can be speculated that the increase of the activities of wild hosts and the reproductive activities of intermediate hosts can promote the infection and transmission of thelaziosis. Domestic dogs often exposed to the wild environment are more likely to contact other definitive host and transmission vector, so the prevalence rate is relatively high.
The results showed that the seasonal distribution had a significant effect on the infection of thelaziosis in domestic dogs. The prevalence rate in summer and autumn was significantly higher than that in spring and winter. This result is related to the reproductive law of P. okadai. The reproduction, density and biological activity of P. okadai vary seasonally according to climatic conditions . Therefore, thelaziosis may occur seasonally. According to the investigation of P. okadai in Spain by Marino , the number of P. okadai captured is the largest when the daily average temperature is 24.5 -26.0 ℃, but no P. okadai are captured when the daily average temperature is 8.8 ℃. The density and number of P. okadai increase with the increase of temperature, and the proportion of male P. okadai (79.65%) is significantly higher than that of female P. okadai (20.35%). Generally, only male P. okadai feed on animal and human ocular secretions. Therefore, it can be seen that the number of male P. okadai increases significantly in summer with high temperature, and the transmission capacity is the largest at this time, so the prevalence rate is the highest in summer and autumn . In addition, the survey data of Marino  also show that the proportion of male P. okadai is positively correlated with the temperature at the time of capture, that is, with the decrease of temperature, the proportion of female P. okadai will gradually increase. Because mature female T. callipaeda will continuously release larvae (L1) during the active period, so as to maintain their reproductive cycle, the infected P. okadai in the previous year can move again in early spring after overwintering. Therefore, there is still a certain risk of infection in spring and winter.
In this experiment, the prevalence rate of nondeworming domestic dogs was significantly higher than that in deworming domestic dogs. It can be seen that anthelmintic has a certain effect on the prevention of this disease. Different kinds of anthelmintic target different types of parasites, and animal owners have different awareness of regular internal and external anthelmintic. Therefore, the results of this test do not mean that any kind of anthelmintic has preventive effect. Because the investigation of anthelmintic history is largely affected by the subjective judgment of animal owners, the effective components and use frequency of anthelmintic drugs are not counted in this test, and the data with more guiding significance for treatment and prevention need to be further studied.
In this study, the number of monocular T. callipaeda and ocular clinical symptoms of 102 cases of domestic dogs thelaziosis were classified and counted respectively. The cases with moderate symptoms (increased ocular secretion and purulent secretion with conjunctivitis) accounted for the largest proportion. Therefore, there was no significant correlation between clinical symptoms and infection intensity (Table 2). In general, because the movement of T. callipaeda will directly cause damage to conjunctival sac and other parts, it is reasonable to guess that ocular clinical symptoms will increase with the number of monocular T. callipaeda, but the results of this test deny this view. According to the report of hodžić , the severity of ocular clinical symptoms has nothing to do with the number of infected nematodes, which is the same as the results of this test. The results of Maia  showed that the prevalence rate of dogs with ocular clinical symptoms was significantly higher than that of asymptomatic dogs. However, it is also emphasized that in the early stage of infection, animals usually do not show obvious ocular clinical symptoms, so they may be ignored by owners and veterinarians.