Epidemiological data regarding the distribution of animal chlamydiosis in China are scarce, and most of them are on food animals such as sheep, cattle, and pigs [14–16]. Companion animals, such as pet cats and dogs, are considered to be faithful friends of humans; however, cats and dogs could be important sources of Chlamydia infection in humans. Therefore, the present study aimed to estimate the chlamydial seroprevalence in cats and pet dogs in Lanzhou, northwest China. Several studies have reported the C. felis prevalence in cats in various regions. For example, Halánová M et al. used direct immunofluorescence and detected an overall 45.16% prevalence of C. felis among cats in Slovak, Millán, and Rodríguez  detected a 27% seroprevalence in serum samples from European wildcats using enzyme-linked immunosorbent assay. The overall seroprevalence of C. felis exposure in cats in Lanzhou was 5.9%, which is lower than those in the above mentioned studies but higher than that observed in cats in Dongguan (2.38%), southern China, using the same commercial IHA kit . The differences in the seroprevalence of C. felis exposure in cats could be related to differences in ecological and geographical factors such as temperature, rainfall, or landscape differences; serologic tests used; feeding; and animal welfare protocols for cats.
Our results indicate that the seroprevalence of C. felis exposure in stray cats (14.3%) was significantly higher than that observed in household cats (3.9%) (P < 0.05), which supports the results obtained by Halánová . These differences are probably attributed to lifestyle and animal welfare protocols. Stray cats have more opportunities to come into contact with infected birds or other animals, and they suffer from poor nutrition and possibly compromised immune systems that may contribute to increased exposure to infectious pathogens. This may explain the higher C. felis prevalence in stray cats than in pet cats. One study reported that a large number of cats and dogs become unowned each year in the UK, which may have considerable implications for their welfare .
Studies using both culture and polymerase chain reaction (PCR) indicated that cats less than 1 year of age were the most likely to be infected with Chlamydia, and cats older than 5 years were the least likely to be infected . However, although the differences were not statistically significant, we found a higher prevalence of antibodies to C. felis in older animals in this study. The C. felis seroprevalence in male and female cats were different, but statistical analysis showed no significant difference. This suggests that C. felis exposure in the cats in this study had no sex predilection, consistent with previous studies.
In addition to in cats, a high prevalence of Chlamydia has been found in growing pigs with or without clinical diseases , and chlamydial infections occur frequently in German sheep flocks, even in the absence of elevated abortion rates . Reports of canine chlamydiosis are not common, possibly because C. felis is rarely considered to be a disease-causing pathogen in dogs. However, a few studies found chlamydial infections in clinically normal dogs [24, 25]. For example, Pantchev et al. used species-specific real-time PCR assays and revealed that four of five dogs were infected by C. felis. The data of that study suggested that this species is highly adapted to cats and that despite the high prevalence of infection, C. felis-positive dogs, unlike cats, seldom suffer from conjunctivitis .
There is limited knowledge about chlamydiosis in dogs in China. Hence, we reported the prevalence of C. felis exposure in pet dogs in northwest China for the first time. In the present study, antibodies to C. felis were found in 32 (12.1%) of 264 pet dogs, which is a higher prevalence than that detected in Dongguan (2.87%), southern China . Compared with other age groups, a higher prevalence of C. felis exposure was found in 2-year-old cats; however, the difference was not statistically significant among different age groups (P > 0.05).
The results of this study revealed that C. felis is highly prevalent among the cats and pet dogs in Lanzhou, China, which suggests that this pathogen could be a significant cause of ocular diseases in these animals. Chlamydia felis is zoonotic pathogen, and maintenance of hygienic conditions and prompt treatment of affected cats and dogs is recommended to prevent human disease. The samples in the present study were collected between November 2010 and July 2011, which is a short period of time; thus, the obtained result may not reflect the actual situation of C. felis infection during longer periods. However, our results provide useful information for future studies. A number of factors may contribute to the varying seroprevalence, such as geographical conditions, diagnostic methods, feeding and living styles, and stress. In future studies, it would be interesting to test environmental samples such as soil, water, air, and ventilation systems to assess the contamination level of premises with C. felis.