Seroprevalence of Toxoplasma gondii Infection in Police Dogs in Shenyang, Northeastern China

Toxoplasmosis is an important parasitic zoonosis caused by the protozoan Toxoplasma gondii, which is widespread in humans and animals worldwide, including dogs [1-4]. T. gondii infection may cause serious results in pregnant women and immunocompromised patients [5]. All the warm-blooded animals and intermediate hosts become infected mainly by consuming food or drink contaminated by oocysts evacuated from felids and tissue cysts from other intermediate hosts [1]. Dogs are often regarded as faithful friends and intimate companions of humans. Unfortunately, however, dogs are known to be involved in mechanical transmission of T. gondii oocysts to humans by excreting oocysts [6,7], which can pose a health problem for humans.

Worldwide seroprevalences of T. gondii in dogs are summarized by Dubey [1]; however, only limited surveys of T. gondii infection in police dogs have been reported, including China. Moreover, in China, papers were published in Chinese language in local journals and are not easily accessible to foreign scholars. Therefore, the objective of the present study was to investigate the seroprevalence of T. gondii infection in police dogs in Shenyang, northeastern China.

Shenyang is located in the southern part of northeastern China, covering an area of 12,948 km² and a population of approximately 8.2 million people. Its geographical position is at east longitude 122°25'-123°48' and north latitude 41°11'-43°2'. The area has a temperate monsoon climate, with abundant sunshine, a long winter and hot summer, with a brief spring and autumn. The average annual temperature is 8.3℃, with a mean annual rainfall of 600-800 mm. There are approximately 600 police dogs in Shenyang.

This investigation was carried out between October and December 2012 in Shenyang, and a total of 291 blood samples were collected from police dogs. Sera were separated and stored at -20℃ for analysis through the modified agglutination test (MAT). Police dog owners were asked for details of the dogs breed, age, gender, source, living conditions, and daily diet using a structured questionnaire.

The MAT test for T. gondii antibodies was performed as previously described by Dubey and Desmonts [8] using 2-fold serial dilutions from 1:25 to 1:3,200. Briefly, the harvested parasites were kept in 6% formaldehyde solution at 4℃ overnight, and suspended in the alkaline buffer at 20,000 parasites/ml. Two-fold dilutions of sera were performed using the serum diluting buffer, and agglutination was performed in U-bottom 96-well microtiter plates using a mixture of 50 µl antigen and 50 µl diluted sera. The plates were incubated at 37℃ overnight. The test was considered positive when a layer of agglutinated parasites was formed in wells at dilutions of 1:25 or higher; positive and negative controls were included in each test.

Statistical analysis of T. gondii seroprevalence between different breeds, age groups, and genders were performed using a Chi-square test with SPSS (SPSS Inc., Chicago, Illinois, USA). A P-value of <0.05 was considered statitically significant.

The results showed that the overall seroprevalence of T. gondii infection in police dogs was 30.9% (Table 1), which was far higher than that observed in Guangxi (3.3%) [9] and Guangdong (5.6%) [9] in China, and in the Czech Republic (21.7%) [10]. These differences may be due to different serological testing methods, investigation periods, and ecological and geographical factors. The MAT antibody titers were 1:25 in 45, 1:50 in 27, 1:100 in 12, 1:200 in 3, and 1:800 in 3, respectively.

Table 1.

Seroprevalence of Toxoplasma gondii infection in police dogs in Shenyang, northeastern China determined by the modified agglutination test (MAT)

The seroprevalence of T. gondii infection in males (41.5%) was higher than that in females (23.2%), and the difference was statistically significant (P<0.05). It may be interpreted that male dogs have more chance to feed on food or have contact with the surrounding environment that can be contaminated by T. gondii oocysts because in most cases police dogs, males, participate in the duty and training. There were also some differences in the seroprevalence by breed, but these data were not significant (P>0.05).

The seroprevalence of toxoplasmosis in police dogs increased progressively with age (Table 1), and the prevalence (42.9%) in older police dogs (>3-year-old) was higher than that in animals below 3-year-old (28.9%) ones, indicating that older police dogs were more likely to be seropositive than dogs under 3-year-old. However, the difference was not statistically significant among age groups (P>0.05). The results provided further evidence for the increased risk of T. gondii infection with acquisition of age through ingestion of infective oocysts from the environment or tissue cysts from intermediate hosts.

The seroprevalence (30.9%) of T. gondii infection in police dogs in this study was much higher than the results in pet dogs; 10.0% in Shenyang [3], 10.8% in Lanzhou [2], 11.2% in Xinjiang [5], 11.3% in Heilongjiang [5], 12.3% in Zhengzhou [11], 13.2% in Beijing [12], and 17.5% in Guangzhou [13]. The police dogs were often fed raw meat (chicken, pork, and beef), which may contain tissue cysts present in the meat of infected animals and lead to a higher seroprevalence in police dogs than pet dogs. Therefore, people training police dogs will be infected with T. gondii more easily, and then enhanced and integrated strategies must be implemented to prevent and control T. gondii infection in police dogs.

In this study, we used MAT for detection of T. gondii seroprevalence in police dogs because it is the major recommended test for diagnosis of T. gondii infection in several animals and man [1]. The most data on isolation of viable T. gondii are available with MAT [1]. MAT has high sensitivity and specificity among all serological methods, and it is cheaper, easier than other tests, and does not need special sophisticated equipments [1,4,14,15].