Background

Toxoplasma gondii is an important zoonotic intracellular protozoan parasite, which can affect all warm-blooded mammals and birds throughout the world, including humans [1, 2]. T. gondii is transmitted by ingestion of tissue cysts from undercooked or raw meat, consumption of food or drink contaminated with oocysts, or ingestion of oocysts from the environment by accident [1]. Nearly one third of the global human population has been infected with T. gondii, however, infection in healthy individuals is usually asymptomatic and only a small percentage of exposed people have obvious clinical symptoms [1, 3, 4]. Nonetheless, if T. gondii infection occurs in pregnant women it can cause severe disease such as toxoplasmic encephalitis, blindness, abortion, fetal abnormalities or even prenatal death [5]. Infection of immunocompromised patients (e.g. HIV/AIDS patients) with T. gondii can cause acute morbidity and even death [6, 7].

Pet dogs are often regarded as the faithful friends and intimate companions of humans. Unfortunately, T. gondii oocysts in pet dogs can traverse the intestinal tract and finally be excreted in the feces [8], which can pose a threat to human health, particularly in pregnant women and immunologically deficient people as described above. Investigations of the prevalence of antibodies to T. gondii in dogs have been conducted extensively in the world [9–14], however, only limited surveys of T. gondii infection in pet dogs have been reported. In recent years, there also have been various surveys of T. gondii infection in dogs in the People's Republic of China (PRC) [Table 1, [15–26]], but in Northwest China only one such investigation of pet dogs has been reported in Inner Mongolia [16].

Table 1 Prevalence of T. gondii infection in dogs in People's Republic of China
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The objectives of the present survey were to determine the seroprevalence of T. gondii infection in pet dogs in Lanzhou, the capital of Gansu province, Northwest China, and to evaluate the main associated risk factors relating to exposure to T. gondii in this region. In the present paper, we also summarise serological surveys of the prevalence of T. gondii infection in dogs in China (Table 1), because most of these surveys were originally published in local Chinese journals [16–26], which are not accessible to international readers.

Materials and methods

The study site

The study was conducted in Lanzhou Municipality, the capital of Gansu province, covering an area of 13,085 square meters in Northwest China. Lanzhou is situated in the geometric center of China's territory between east longitudes of 102°30" to 104°30" and north latitudes of 35°5" to 38°, which has an average height of 1,500 meters above sea level. The city is located between mountains to the north and south, and is crossed by the Yellow River from west to east, with a characteristic of ribbon basin geography. It has a typical temperate and monsoonal continental climate, with an annual rainfall of 360 mm mainly from June to September, an average annual temperature of 9.3°C, average yearly sunlight exposure of 2,446 hours and a frost-free season of about 180 days.

Sampling of pet dogs

A total of 259 blood samples were collected from the leg veins of pet dogs between November and December 2010 in Lanzhou. These pet dogs were admitted into pet hospitals located in four districts of Lanzhou City, including Chengguan District, Anning District, Xigu District and Qilihe District, with a variety of diseases. Pet dog owners were asked for details of the animals breed, age, sex, source and medical history using a structured questionnaire. Blood samples were kept at 37°C for 2 h and centrifuged at 2,000 g for 5 min. The resulting sera samples were stored at -20°C until further analysis.

Serological examination

Sera from pet dogs were diluted in a two-fold serial dilution from 1:20 to 1:320 and investigated for T. gondii antibodies using the modified agglutination test (MAT) as described previously [27]. MAT is a sensitive and specific technique for measuring T. gondii antibodies, which has been used extensively in experimentally and naturally infected dogs [11, 13, 28], and other animals [29]. Briefly, MAT was performed with a suspension of Toxoplasma tachyzoites fixed with formalin, serum samples which had been diluted in phosphate buffered saline (PBS, pH 7.2), positive and negative control sera, antigen diluting buffer containing bovine serum albumin (BSA), 2-mercaptoethanol [to deplete the sera of non-specific immunoglobulin (Ig) M antibody], and Evans blue dye solution. MAT titers of 1:20 or higher were considered as positive [12], and those sera with dubious results were re-tested. Positive and negative controls were incorporated in each test and tested at the same dilutions of sera samples. A negative result was returned when the base of the "U" bottom 96 well microtiter plates contained a blue pellet; conversely a clear bottom indicated a positive result.

Statistical analyses

Differences in the seroprevalence of T. gondii infected pet dogs between male and female dogs, and different age groups were analyzed using a Chi square test in Predictive for Analytics Software (PASW®) Statistics 18. The P value < 0.05 was considered statistically significant. The correlation between the rates of infection in different age groups was calculated with Excel 2003 (Microsoft®).

Results

In the present study, a total of 259 pet dogs (123 females and 136 males) from Lanzhou were examined by MAT. Out of these samples, 28 (10.81%) were seropositive for T. gondii. Among these positive pet dogs, seroprevalence varied in different age groups, ranging from 6.67% to 16.67% (Table 2). The investigation also showed that the prevalence in female animals was 8.94%, and 12.50% in male animals (Table 2). Table 3 indicates that antibodies to T. gondii were found in 28 (10.81%) of 259 pet dogs at the cut-off of 1:20, with titers of 1:20 in 14 dogs, 1:40 in nine, 1:80 in four, and 1:160 or higher in one pet dog.

Table 2 Prevalence of antibodies to Toxoplasma gondii in pet dogs by gender and age in Lanzhou, northwest China using modified agglutination test (MAT)
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Table 3 Antibody titers to Toxopalsma gondi i infection in pet dogs in Lanzhou, northwest China by modified agglutination test (MAT)
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Discussion

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 [15], 12.26% in Zhengzhou [18], and 13.21% in Beijing [25], but higher than those observed in Inner Mongolia [16], Haiko [23] and Shenzhen [17]. 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 [30]. Another study showed that dogs can vertically transmit T. gondii to their offspring by semen [31].

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.

Conclusions

The results of the present survey revealed a high prevalence of T. gondii infection in pet dogs in Lanzhou, especially in older pet dogs, which has public health significance because a previous survey of T. gondii infection in people with high risk of exposure to T. gondii in Lanzhou showed that 34.78% of pet dog owners were seropositive [32]. Therefore, it is necessary to take integrated strategies, including efficient management measures to prevent and control T. gondii infection in pet dogs, which could help to reduce T. gondii infection in other animals and humans in this area.