The present investigation showed that the overall seropositivity for T. gondii exposure was 10.81% in pet dogs in Lanzhou, which was lower than the values of 17.5% in dogs in a study performed in Guangzhou , 12.26% in Zhengzhou , and 13.21% in Beijing , but higher than those observed in Inner Mongolia , Haiko  and Shenzhen . Among these regions, the difference in T. gondii seroprevalence may be due to ecological and geographical factors, as well as feeding and animal welfare conditions for dogs in these areas.
It is known that the average annual rainfall of Lanzhou is only 360 mm, and the average annual temperature is 9.3°C, with a typical continental monsoon climate. Dry and cold circumstances may be a challenge for the survival of T. gondii oocysts, and unfavorable for epidemics of toxoplasmosis.
In comparison with other age groups of dogs, a higher prevalence of infection was detected in the group of dogs >3 years old. Although the difference was not statistically significant among age groups (p > 0.05), there is a general tendency for older animals to have had more exposure to T. gondii. Older animals have had more opportunities to come into contact with felids, so they may be more likely to acquire T. gondii infection by ingesting food contaminated with oocysts which have been shed and excreted in feces by cats. There were limited data on seroprevalence of T. gondii infection in cats in Lanzhou, our preliminary survey showed that the seroprevalence of T. gondii infection in pet cats in Lanzhou was 22.83% (unpublished data), indicating a high risk as a source of T. gondii infection in dogs and other animals and humans.
In addition to being infected by T. gondii oocysts shed and excreted in feces by infected cats, dogs may also become infected through ingestion of raw or uncooked meat/flesh contaminated with T. gondii cysts. Moreover, congential T. gondii infection in dogs is considered an important factor, the fetus may acquire initially T. gondii infection during pregnancy in female canines. A study found that reinfected pregnant female canines could transplacentally transmit T. gondii to their neonates . Another study showed that dogs can vertically transmit T. gondii to their offspring by semen .
Statistical analysis showed that differences in T. gondii infection between female and male pet dogs were not significant (p > 0.05), suggesting that gender of the host is not a crucial factor for T. gondii infection. In this study, we found a low (8.06%) prevalence of T. gondii infection in pet dogs in Anning District, compared with a seropositive rate of 9.52% in Qilihe District, 10.11% in Chengguan District and 13.79% in Xigu District, but statistical analyses showed that these differences were not significant (p > 0.05).
In this study, we investigated the seroprevalence of T. gondii infection in 259 pet dogs from four districts in Lanzhou, northwest China between November to December, 2010. The number of pet dogs sampled was large enough to be representative, with the potential limitation that the results of the present investigation may not reflect the actual seroprevalence in other seasons, and in other districts of Lanzhou. Therefore, further comprehensive surveys of T. gondii infection in pet dogs in Lanzhou are warranted to sample more pet dogs during each climatic season and from all of the 8 districts and counties of Lanzhou. Also, T. gondii infection in stray dogs in Lanzhou will be considered in further investigations.