Seroprevalence of Dirofilaria immitis in Cats from Liaoning Province, Northeastern China

Hou, Honglie; Cao, Lili; Ren, Wenzhi; Wang, Dansheng; Ding, He; You, Juan; Yao, Xinhua; Dong, Hang; Guo, Yanbing; Yuan, Shuxian; Zhang, Xichen; Gong, Pengtao

3,500 g for 10 min, and sera were stored at -20˚C in airtight vials until tested. All samples were tested to detect D. immitis using a commercial antigen detection kit (SNAP Heartworm RT, IDEXX Laboratories, Westbrook, Maine, USA) according to the manufacturer's instructions. For the statistical analysis, the outcome variable was the prevalence of D. immitis, and the independent variables were gender (male, female), age (< 3 years, ≥ 3 years), and rearing condition (domestic or feral). Seroprevalence with the odds ratio (OR), 95% confidence intervals (CI) and P-value (P) were calculated. The prevalence is 4.45%, and 29/651 tested samples were antigen positive samples. Since the presence of as few as 2 adult heartworms can cause cardiac enlargement and a severe acute respiratory syndrome in felines [24] , an infected cat may have low production of heartworm antigens and antibodies. Therefore, the actual rate of infection may be higher than the test results indicated. The association of D. immitis prevalence and risk factors (gender, age and rearing condition) from positive cats were analyzed. There was no significant difference between the male and female cats (15/14, 4 .7%/4.2%, P > 0.05, OR 1.12, 95% CI 0.53-2.35). The prevalence of D. immitis was obviously difference between cats aged ≥ 3 years and< 3 years (23/6, 8.0%/1.6%, P < 0.01, OR 0.19, 95% CI 0.08-0.44), and the prevalence of D. immitis in feral cats was significantly higher than in domestic cats (24/5, 8 .4%/1.4%, P < 0.01, OR 0.15, 95% CI 0.06-0.36). One possible reason is that domestic cats spent more time indoors as pets than feral cats, so feral cats could be exposed for a longer time to mosquitoes, with a consequently higher chance of being bitten by the mosquitoes [25, 26] . The prevalence of D. immitis in different rearing conditions by age and gender group between domestic cats and feral cats is shown in Table 1 . No significant difference in D. immitis prevalence was found in male and female cats (4.7% vs 4.2%, P > 0.05), which is in agreement with other studies, in stray and pet cats (3.3% vs 2.7%, P > 0.05) in northwestern China [21] , in stray cats (2.6% vs 2.6%, P > 0.05) in south Korea [27] , and in with or without outdoor access cats (0.4% vs 0.4%) in the United States and Canada [28] , but this result differs from Kramer and Genchi, in which the male cat prevalence was significantly higher than that in female cats (19.0% vs 13.0%, P < 0.01) [29] . Domestic cats aged ≥ 3 years and< 3 years showed no significant differences in prevalence (P > 0.05). However, the prevalence of D. immitis in older feral cats was significantly higher compared to that in younger feral cats (P < 0.01). There is one existing report that older dogs were more likely to be infected with D. immitis [30] , possibly because the older dogs were more likely to be exposed to mosquitoes for a longer time, with a consequently higher chance of being bitten, which might be the same reason for cats.