Prevalence of Dirofilaria immitis among shelter dogs in Tokyo, Japan, after a decade: comparison of 1999–2001 and 2009–2011

Changes in the seroprevalence of Dirofilaria immitis infection among shelter dogs between a decade ago and the present were evaluated. Serum samples were collected from 200 adult dogs in urban and suburban areas in Tokyo, Japan, during two 2-year periods (April 1999 to March 2001 and April 2009 to March 2011). Sera were tested for the presence of D. immitis antigen using a specific commercialized kit. The seroprevalence of D. immitis infection was 46% in 1999–2001 and 23% in 2009–2011. A decrease was observed in the prevalence of infection between 1999–2001 and 2009–2011; in particular, the prevalence in urban areas decreased significantly compared with that in suburban areas (P < 0.01). There was no significant difference in prevalence between the sexes in each period, but there was a significant difference between mixed-breed and purebred dogs (P < 0.01). The decrease in prevalence of canine heartworm disease in urban areas could be related to better veterinary care.


Introduction
Cardiopulmonary dirofilariosis is a parasitic disease with a worldwide distribution caused by Dirofilaria immitis Leidy, 1856 [9], a nematode that is transmitted by mosquitoes. The adult worms are commonly found in the pulmonary arteries and right ventricle of dogs, and result in pathological damage, such as edema, ascites, arrhythmia, tachyarrhythmia, and even death of infected dogs [10]. The increasing temperatures caused by climate change and global warming can affect the distribution and density of mosquitoes, and consequently the transmission and spread of mosquito-borne diseases, such as dirofilariosis caused by D. immitis [3,5]. Predictive models in Europe and Argentina have demonstrated an actual risk of spread of Dirofilaria spp. infections into areas previously free from the disease [4,5,11,14].
In Japan, D. immitis infection of dogs has been considered to be prevalent since the earliest report on canine dirofilariosis in 1880 [1]. In the latest study, Nogami and Sato [12] reported a prevalence rate of 46.8% in shelter dogs tested at necropsy from 1989 to 1995. However, current epidemiological information on the prevalence of D. immitis infection is limited. The present study was conducted to determine the prevalence of D. immitis infection among shelter dogs in Tokyo, Japan, in order to evaluate the modifications in the prevalence between the present and a decade ago, and between urban and suburban areas.

Materials and methods
The present study was carried out in Tokyo, Japan A questionnaire including sex, estimated age, breed, and housing area (urban or suburban) was collected for each dog. Serum was stored at À30°C until assay. All samples were tested for circulating D. immitis antigen using a specific commercialized immunochromatography kit (Solo Step Ò CH; Heska Co., Loveland, CO, USA) according to the manufacturer's instructions.
The data were analyzed to evaluate the differences between proportions of dogs infected with D. immitis using the chi-square test. In all analyses, P < 0.05 was taken to indicate statistical significance.

Results and discussion
The research area, sex and breed of distribution in dogs with D. immitis infection are shown in Table 1 In the present study, a significant decrease from 46.0% to 23.0% was observed in the overall prevalence of D. immitis infection in Tokyo over the past decade. During the past decade, various types of macrocyclic lactones for D. immitis prevention have come into use worldwide, and the increased use of chemoprophylaxis is probably the major factor responsible for the decrease in prevalence of D. immitis infection in Tokyo. However, there are contrasting results in terms of breeds. The prevalence of infection in purebreds decreased by half over the past decade (22.9% in 1999-2001 and 10.3% in 2009-2011), while the infection in mixed breeds was still highly prevalent (58.5% in 1999-2001 and 50.0% in 2009-2011). This could reflect a higher rate of chemoprophylaxis in the purebred canine population than that in the mixed-breed canine population in Tokyo.
Decreases in prevalence of D. immitis infection were observed in both urban and suburban areas. These findings suggest that the prevalence of infection is markedly affected by veterinary care. The decreased prevalence, particularly in urban areas, has been conspicuous compared with that in suburban areas. This is because the rate of purebred populations in the urban areas increased and most of these dogs probably received chemoprophylaxis. Alternatively, urban areas with less parks may be more susceptible to changes in the environment than suburban areas.
The results of the present study were compared with those of other studies performed in different countries on approximately the same latitude as Japan. The prevalence of D. immitis infection in dogs in these countries varied markedly: 6.9%-20.9% in Korea [7,13], 0.33%-3.33% in China [15], 3.6%-8.9% in Portugal [2], 34.13% in Greece [6], and 1%-12.5%   in the United States [8]. The present study demonstrated that D. immitis infection is endemic in Japan. The differences in prevalence among countries may be due to the differences in density of vectors, density of the parasite reservoir, and rate of prevention in dogs. This is the first study to evaluate changes in the seroprevalence of D. immitis infection in a metropolitan city over the past decade.