房総半島の野生イノシシのトキソプラズマに対する血清疫学研究

誌名誌名 家畜衛生学雑誌 = The Japanese journal of animal hygiene

ISSNISSN 13476602

巻/号巻/号 423

掲載ページ掲載ページ p. 105-109

発行年月発行年月 2016年12月

農林水産省 農林水産技術会議事務局筑波産学連携支援センターTsukuba Business-Academia Cooperation Support Center, Agriculture, Forestry and Fisheries Research CouncilSecretariat

I Original report I 105

Seroepidemiological study on Toxoplasma gondii in the wild boar

(Sus scrota leucomystax) in the Boso Peninsula, Japan

Haruka Nomura I), Masaaki Oi 1>, Mitsuru Kurachi 2>, Hiroshi Sentsui I) and Sadao Nogami 1>*

( 1l Department of Veterinary Medicine, College of Bioresource Sciences, Nihon University,

1866 Kameino, Fujisawa, Kanagawa 252-0880, Japan 2> The Nambu Livestock Hygiene Service Office, Chiba Prefecture,

52 Y airo, Kamogawa, Chiba 296-0033, Japan

*Corresponding Author: Sadao Nogami (snogami@brs.nihon-u.ac.jp))

(Received 3. Sep. 2015/ Accepted 3. Sep. 2016)

Summary

Recently, the size of the wild boar population in Japan has increased, and their habitat has expanded. However,

information on the prevalence of Toxoplasma gondii in wild boars in Japan is limited. Sera of 385 wild boars were

collected from the Boso Peninsula, Japan, from March 2009 to February 2012, and anti-T. gondii antibody was

measured by latex agglutination test. Antibody titers at 1:64 or greater were considered seropositive. The

overall seroprevalence rate was 15.1 % (58/385). High titers predominated among the 58 positive individuals. The

seroprevalence rates were 14.8% (34/229) in males and 15.8% (23/146) in females. The seroprevalence rates were

12.5 % (7 /56) in the northern area and 15.5 % (511329) in the southern area.

Key words : Seroprevalence, Sus scrofa, Toxoplasma gondii, wild boar

Introduction

Toxoplasma gondii is a zoonotic protozoon that

parasitizes Felidae as the final host and various

homeothermic animals as intermediate hosts. Humans

are infected by T. gondii through consumption of raw

or undercooked meat containing cysts, ingestion of

water or vegetables contaminated with oocysts

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Jpn. J. Anim. Hyg. 42, 105~ 109 (2016)

excreted by cats, or by transplacental transmission of

tachyzoites from a mother to her fetus 5). As T. gondii

infection is usually asymptomatic, it has attracted little

attention from the public. However, when women

acquire a primary infection during pregnancy, abortion,

hydrocephalus, or retinochoroiditis of the fetus can

occur 18). Human T. gondii infection affects an estimated

one third of the world's population12l and 10.3% of

pregnant Japanese women16>. Therefore, human T.

gondii infection is one of the most important zoonotic

protozoa! diseases in Japan. As T. gondii cysts

containing bradyzoites can survive within the tissue of

the intermediate hosts for a long time, T. gondii-infected

106 Jpn.]. Anim.Hyg. Vol. 42, No.3 (2016)

livestock are strictly regulated by the Slaughterhouse

Act in Japan to prevent human T. gondii infection

through contaminated meat.

Recently, the size of the wild boar (Sus scrofa

leucomystax) population in Japan has increased, and

their habitat has expanded, causing significant

economic losses for numerous agricultural products 15!.

Consequently, some local governments recommend

using boar meat as a regional resource. In addition,

there is increasing consumption of boar meat as

"gibier" meat among the general public uninformed of

the zoonotic risks. Toxoplasma infection in wild boars

have been reported in several countries 1 -

3·

6 -

9l. and

they may serve as sources of human infection. Indeed,

human toxoplasmosis associated with ingestion of boar

meat has been reported in South Korea 4l_ Therefore,

to ensure food safety, the prevalence of T. gondii in

wild boars in Japan must be determined. However,

information on the prevalence of T. gondii in wild boars

in Japan is limited 10· 11·

13' 1

4' 1

71, and the seroprevalence of

T. gondii in wild animals varies depending on the

sampling conditions, such as the region, species,

sampling time, age, and sex.

In consideration of the above-mentioned

backgrounds. this study was performed to investigate

the seroprevalence of T. gondii in wild boars in

southern Boso Peninsula, Japan, and to characterize

the seroepidemiological trends in the wild boar

population.

Materials and Methods

Blood samples were collected from 385 wild boars

(229 males, 146 females, and 10 of unknown sex) hunted

in southern Boso Peninsula, Japan, between March

2009 and February 2012. The samples were collected

by the Nambu Livestock Hygiene Service Office, Chiba

Prefecture, primarily for use in an epidemiological

survey of livestock health. Sera were stored at -20°C

until use. The presence of anti-T. gondii antibody was

measured using a commercially available latex

agglutination antigen (Toxocheck-MT; Eiken-Kagaku,

Tokyo, Japan) as reported previously ll. 13· 17). Briefly,

each serum sample was diluted 16-fold using a diluting

buffer, and 25 µL of the test sample was further diluted

with an equal volume of diluting buffer to generate

serial 2-fold dilutions from 1:32 to 1:1.024 on a 96-well

U-bottomed microplate. Each well was added 25 µL of

antigen-coated latex reagent. and the plate was incubated

overnight at room temperature. Agglutination was

detected using a reading mirror, and the antibody

titers were measured in duplicate. Antibody titers at

1:64 or greater were considered seropositive 11.13

· 17

l.

The boars were classified into three groups by body

weight: ::;;10 kg, 11-50 kg, and 2':51 kg. Survey areas

were classified into two groups: the northern area (the

towns of Onjuku and Otaki, and the cities of Isumi,

Katsuura, Kimitsu. Kisarazu, and Sodegaura) and the

southern area (the town of Kyonan. and the cities of

Futtsu, Kamogawa, Minamiboso, and Tateyama).

These two areas ware separated topographically with

livers and plane.

The seroprevalence of T. gondii in wild boars was

statistically analyzed according to gender and area

using the x 2 test. In all analyses. P < 0.05 was taken

to indicate statistical significance.

Results

The seroprevalence of T. gondii and distribution of

the antibody titers are summarized in Table 1. Of the

Table 1. Seroprevalence and distribution of anti-Toxoplasma gondii titers in wild boars in Boso Peninsula, Japan

No. of boars Anti-T. gondii titer 1: Sex

positive/examined(%) 64 128 256 512 1,024

Male 34/229 (14.8) 10 13 7 4 0

Female 23/146 (15.8) 10 6 3 4 0

No data 1110 (10.0) 0 0 1 0 0

Total 58/385 (15.1) 20 19 11 8 0

Nomura et al. : Seroplevalence of T. gondii in wild boars 107

Saitama

~ Kanagawa ~

~? u.

Northern area 12.5% (7/56)

Southern area 15.5% (51 /329)

Figure 1. Survey a r eas and Toxoplasma gondii seropreva lence (numb er of positive individuals/ number of examined) in wild boars in Boso Peninsula, Japan.

385 samples, 58 (15 .1 %) were seropositive. The

seroprevalence rates were 14.8% (34/ 229) in males and

15.8% (23/ 146) in fema les. Among t he 58 positive

individuals, anti-T. gondii antibody titers were 1:64 in

34.5% , 1:128 in 32.8%, 1:256 in 19.0%, and 1:512 111

13.8%.

T he seroprevalence rates according to location are

shown in F igure 1. The seroprevalence rates were

12.5% (7 / 56) in the northern area and 15.5% (51/ 329) in

the southern area.

The seroprevalence from March 2009 to March 2010

was 13.5 % (211156), from April 2010 to March 2011

was 15.8% (19/ 120), and from September 2011 to

February 2012 was 16.5% (18/ 109).

The seropreva lence according to body weight is

shown in F igure 2. The seroprevalence rates were

8.6% (3/ 35) in boars weighing ~ 10 kg, 15.3% (34/222)

in the 11-50 kg group, and 16.9% (20/ 118) in the ~51

kg group.

Discussion

The seroprevalence in wild boars was 15.1 % (58/ 385)

in this survey, which was higher than in previous

reports; 6.3% (1 11175) in Gunma Prefecture 111, 0%

(%)

20

15

r.11 512

1::11256 10

Ell 1128

01 64

5

0 ::::10 11-50 <'.5 1 (kg)

Figure 2. Relationship between an ti-Toxoplasma gondii t iters an d body weight in positive wild boars in Boso Peninsula, Japan.

(0/ 115) in Shikoku 141, and 1.1 % (1190) in Amak usa

Island m

There was no significant difference in seroprevalence

between northern and southern animals, suggesting

that the parasite is widespread across the survey area.

In addition, the seroprevalence in feral dogs housed in

a public animal shelter in the survey area was 41.7%

(15/ 36; unpublished data) when antibody titers at 1:64

or greater were cons ide r ed seropositive, which

suggests that other animals in the survey area are at

high risk of infectio n.

In the previous reports, the seroprevalence of anti-T.

gondii antibody showed a tendency toward higher

prevalence among females compared to males s. 61•

Differ ences in behav ior between sexes have been

considered as the cause of difference in seroprevalence

between the sexes. However, there was no difference

in seroprevalence between the sexes in this survey.

This further indic ates w id esp r ea d T. gondii

contamination as both males and females have an equal

risk of infection in the study area.

In contrast to previous studies indicating relatively

few boars with high anti-T. gondii titers 10·

13·

14·

171, the

seropositive animals in the present study showed high

108 Jpn. J. Anim.Hyg. Vol. 42, No.3 (2016)

titers. These high titers suggest recent infection,

reinfection, or heavy infection. Consequently, T. gondii

infection appears to be endemic in this study area.

Although the trend was not significant among three

weight groups, the seroprevalence in 11 - 50 kg and

~ 51 kg increased in the present population. Weight

gain is a reflection of aging, suggesting that

seroprevalence increases with advanced age. Several

investigators have reported a higher risk of exposure

with increasing age in animals 1. 4

· 5

l_ Interestingly,

Antolova et al. tl reported that no wild boars younger

than 1 year old were positive for T. gondii. The s 10

kg group may still include animals with maternal

antibodies. Our results also indicated that the parasite

is mainly transmitted horizontally in the survey area.

Consequently, we suspect that infection occurs

primarily in juvenile boars after weaning.

This study comprised a large sample size with many

positive animals compared with previous studies;

therefore, we were able to analyze the characteristics

of the infection in boars in detail.

The authors thank the staff of the Nambu Livestock

Hygiene Service Office, Chiba Prefecture, for their help

in obtaining the samples included in this study. This

work was financially supported in part by the

Academic Frontier Project for Private Universities

from the Ministry of Education, Culture, Sports,

Science, and Technology of I apan.

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