さ け・ま す 資 源 管 理 セ ン タ ー
研究報告 第 4 号 7-11 頁
2001 年 12 月

Bulletin of the National Salmon
Resources Center, No. 4 : 7-11December 2001

RESEARCH NOTE
Occurrence of Kabatana takedai (Microspora) in Juvenile
Masu Salmon (Oncorhynchus masou) reared at
Varying Water Temperatures in a Hatchery
Shigehiko Urawa
Research Division, National Salmon Resources Center,
2-2 Nakanoshima, Toyohira-ku, Sapporo 062-0922, Japan(urawa@salmon.affrc.go.jp)

Abstract. − Infections with the microsporidian parasite Kabatana takedai (Awakura, 1974) weremonitored for juvenile masu salmon (Oncorhynchus masou) held at various water temperaturescontrolled using river and spring waters in the Chitose Hatchery, Hokkaido, Japan. In the summerof 1999, the hatchery water became over 15oC due to unusual increase of water temperature in theChitose River. In consequence, heavy microsporidian infections occurred among juvenile masu
salmon temporally transplanted from the Shiribetsu and Shizunai rivers, discontinuing back releasesof these fish into their native rivers to prevent the parasite diffusion. These juveniles (approximately100,000 fish) were killed by the late November. The present survey also indicated that the cyst
formation of K. takedai could occur at low temperatures less than 13oC. A rearing experimentconfirmed that the parasite cysts were residual within the muscle and heart of hosts over 7 months.
Key words: microsporidian parasite, Kabatana takedai, masu salmon, control of infection, watertemperature

Introduction
Kabatana takedai (Awakura, 1974) is a highly
pathogenic microsporidian parasite infecting themuscle and heart of various salmonid fishes. Theparasite was first recorded as Plistophora sp. fromdiseased rainbow trout (Oncorhynchus mykiss)reared in the Chitose Hatchery, which used waterfrom the Chitose River (Takeda 1933). Later, theparasite was also found in Lake Akan and Tokitonuma of Hokkaido (Awakura et al. 1966; Awakura1978), and in the Taranay and Bryanka rivers on
south Sakhalin Island (Vyalova and Voronin 1987;Vyalova 1999). The distribution of K. takedai is limited in these areas of Hokkaido and Sakhalin. The
taxonomic position of this species has been altered
several times such as Glugea takedai, Nosema takeContribution A No. 23 from the National Salmon
Resources Center.
賛 2001 National Salmon Resources Centerdai, or Microsporidium takedai, and finally it is assigned to the genus Kabatana Lom, Dyková andTonguthai, 2000 by ultrastructural and molecularstudy (Lom et al. 2001).
The parasite infects only salmonids such asmasu (O. masou), pink (O. gorbuscha), chum (O.
keta), chinook (O. tshawytscha), and sockeye (O.
nerka) salmon, white-spotted char (Salvelinus leucomaenis), Dolly Varden (S. malma), and rainbow andbrown trout (Salmo trutta)(Urawa and Awakura1994). The parasite could be transmitted directly by
the ingestion of spores added to food or by spore immersion (Awakura 1974), but following similartransmission experiments were unsuccessful (Fujiyama, unpublished data). Awakura (1974) suspectedthat small aquatic organisms in river or lake watermight act as a vector for transmission of theparasite.
Awakura (1974) and Urawa (1989) found thatthe cyst formation occurred when the water temperature was above 15o C between July and September.
Because the parasite development is retarded below

8

さけ・ます資源管理センター研報 Bull. Natl. Salmon Resources Center, No. 4, 2001

this temperature (Awakura and Kurahashi 1967),
temperature manipulation has been believed as an effective method to avoid the disease outbreaks. However, critical temperature requirements in thedevelopment of K. takedai have not been determined yet. The purpose of this study is to examine
K. takedai infections in juvenile masu salmon reared
at varying water temperatures in the Chitose Hatchery, Hokkaido, Japan.

Materials and Methods
FishIn 1999, juvenile masu salmon originating fromthe Shiribetsu and Shizunai rivers were reared in theChitose Hatchery. Approximately 140,000 masu
salmon eggs were transplanted from the Mena
Hatchery along the Shiribetsu River System in southern Hokkaido. In addition, twenty thousand masu
salmon eggs were transplanted from the Shizunai

Hatchery. These Shiribetsu and Shizunai stocks
were reared in ponds supplied with spring water after hatching from January 21 and March 9, respectively. From early April, the Chitose River water
was supplied to the rearing ponds in addition tospring water with an ambient temperature of approximately 8 oC. Water temperatures in the ChitoseRiver and rearing ponds were recorded daily (Fig.
1). The number of dead fish was also recorded. Thefish were sampled on September 29, and October 7and 19, 1999 for parasite survey. To confirm the residual period of parasite cysts within hosts, two hundred masu salmon juveniles of Shizunai origin stock
were held in two closed tanks supplied with circulating parasite free water for 7 months from October 7.
The rearing water temperature was controlled at 8o Cor 13o C by chiller or heater. The fish samples werecollected from both groups 1, 4 and 7 months after
the start of trial.
In 2000, juvenile masu salmon originating from

Fig. 1. Daily changes of water temperature at rearing ponds (A-C) in the Chitose Hatcheryfor masu salmon, and in the Chitose River (R). A, B, and C indic ate water temperature s inponds where Shizunai, Shiribetsu, and Chitose stocks were reared, respectively. A dottedline indicates water temperature in a closed tank to which masu salmon were moved fromthe pond C on September 11, 2000.

Urawa − Kabatana takedai infection in masu salmon9Table 1. Prevalence and number of cysts of Kabatana takedai in juvenile masu salmon reared in the Chitose Hatchery.
Origin of stocks
Shizunai R.

Shiribetsu R.

Chitose R.

Sampling Date

No. samples

No. parasite cysts* 2

Fish fork length

Prevalence of(cm)* 1

parasite (%)

He artMusc leSeptember 29, 1999

27

11.0 ± 1.0

88.9

3.7 ± 3.1 (11)9.2 ± 12.1 (45)

Oc tober 7, 1999

21

10.1 ± 0.685.7

4.9 ± 3.9 (12)6.2 ± 9.7 (31)

Oc tober 7, 1999

10

10.0 ± 0.6

80.0

1.6 ± 2.5 (7)9.9 ± 25.7 (83)

October 19, 1999

22

10.3 ± 1.0

77.3

1.3 ± 1.7 (5)8.3 ± 15.4 (53)September 11, 200030

8.1 ± 1.3

16.7

0.1 ± 0.2 (1)0.6 ± 1.5 (5)Oc tober 3, 2000

208.2 ± 1.445.0

0.4 ± 1.1 (4)1.6 ± 3.4 (13)October 25, 200030

9.4 ± 1.1

40.0

1.8 ± 1.3 (4)1.0 ± 1.8 (7)1* Mean ± SD
* 2 Mean ± SD (max)the Chitose River were reared in the Chitose Hatchery. Approximately one hundred thousand juveniles
were reared in a pond supplied with spring and riverwaters from the Chitose River. Water temperatureand fish mortalities were recorded daily. Thirty fish
were sampled on September 11, 2000 for parasite
survey. In the same time, two hundred fish werestocked in a closed tank, where parasite free water
was circulated at about 13oC. Fish were sampled on
October 3 and 25 for parasite survey.
Parasite survey
After measuring fish fork length, the heart andmuscle were examined to detect the white cysts ofKabatana takedai under a dissected microscope.
The number of cysts were counted for each fish infected with the parasite.

Results
Parasite in hatchery fish
The prevalence of K. takedai cysts was 86-89%
in masu salmon juveniles originating from the Shizunai River (Table 1). The number of cysts was averaged at 3.7-4.9 in the heart and 6.2-9.2 in the trunkmuscle. The maximum number of cysts was 12 and45 in the heart and muscle, respectively. The watertemperature in the rearing pond increased graduallyfrom mid June, being over 15 oC in mid July (Fig.
1A). It reached near 20 oC during mid August in proportion to unusual temperature rise in river water.
The total mortality rate of fish was 43% between
June and October.
In the Shiribetsu stock, the prevalence of para-Fig. 2. Changes in the prevalence of Kabatana takedai andmean number of parasite cysts in the muscle and heart of juvenile masu salmon reared at low (8oC) and high (16 oC) water temperatures.

site cysts was 77-80% and the mean number of cysts
was 1.3-1.6 in the heart and 8.3-9.9 in the muscle(Table 1). The maximum number of cysts was 7 andさけ・ます資源管理センター研報 Bull. Natl. Salmon Resources Center, No. 4, 2001

1083 in the heart and muscle, respectively. The rearing
water temperature was below 10oC until late July,but increased to approximately 15oC in August andSeptember (Fig. 1B). The fish mortality rate was accumulated to 9% from June to October.
In 2000, Chitose masu salmon were reared below 13oC (Fig. 1C). Nevertheless, 17% of juveniles
examined in mid September were infected with K.
takedai (Table 1). The prevalence of parasite cysts
increased to 40-45% 3-5 weeks after reared in a circulating tank at 13oC.
Residual of parasite cysts
Juvenile masu salmon affected with K. takedai
were reared in two closed tanks at low (8oC) or high(16oC) temperatures for 7 months. The prevalence ofparasite cysts remained around 90% at 16oC, butslightly decreased at 8oC (Fig. 2). Mean number ofcysts in the heart decreased in both groups, but inthe muscle it decreased only in the low temperaturegroup.

Discussion
It is well reported that temperature affects the development of microsporidian parasites. Temperatures above 20oC or below 9oC interrupt the life
cycle of the gill intracellular microsporidian parasite
Loma salmonae prior to sporogony, inhibiting theproduction of xenomas (Sanchez et al. 2000).
Kabatana takedai cysts appeared in salmonid
hosts during summer when water temperature wasabove 15oC both in the Chiotose River (Urawa1989) and hatchery supplied with the river water(Awakura 1974). Thus temperature manipulation
has been used for the control of parasite infection in
the Chitose Hatchery, where the disease outbreakhas been prevented by cooling the water to less than
15oC in the rearing ponds during the summer season(Urawa and Awakura 1994).
In the summer of 1999, however, rearing water
became over 15oC due to unusual increase of watertemperature in the Chitose River. In consequence,
heavy microsporidian infections occurred in juvenile masu salmon originating from the Shiribetsu
and Shizunai rivers. The fish mortality was 43% in
Shizunai stock and 9% in Shiribetsu stock during
summer and fall, but these mortalities might becaused by other factors in addition to heavy microsporidian infection. Back releases of these fish into

their native rivers were discontinued in order to prevent the microsporidian diffusion, and the survived
juveniles (approximately 100,000 fish) were killedby the late November.
In the next year, the parasite cysts were observed
again in some masu salmon reared in the Chitose
Hatchery, although the fish pond was kept less than
13oC even in the summer. In 2001, the parasite cysts
were observed in the muscle of juvenile sockeye
salmon reared under 11oC in the Chitose Hatcheryfor 4 months (Fujiyama, unpublished data). Theseobservations suggest that K. takedai can adapt tocooler water conditions than we have expected.
Awakura and Kurahashi (1967) reported that K.
takedai could not develop after the infected rainbow
trout were transferred into cool water (8oC), but they
did not indicate how long the parasite remained in
the host. The present study confirmed that the parasite cysts prolong within hosts for more than 7months at low (8oC) or high (16oC) temperatures, although the number of cysts was slightly reduced by
host's defense responses maybe such as macrophageactivity. It suggests a difficulty in treating infectedfish by temperature manipulations. Infection experiments should be designed to determine critical watertemperatures which the parasite fails to develop
within hosts.

Acknowledgements
I thank the staffs of Chitose Branch of the National Salmon Resources Center for their kind helps
to provide fish samples and information.

References
Awakura, T. 1974. Studies on the microsporidian infection in salmonid fishes. Sci. Rep. Hokkaido
Fish Hatchery, 29: 1-95. (In Japanese with English summary.)
Awakura, T. 1978. A new epizootic of a microsporidiosis of salmonids in Hokkaido, Japan. Fish Pathol., 13: 17-18. (In Japanese with English summary.)
Awakura, T., and S. Kurahashi. 1967. Studies on thePlistophora disease of salmonid fishes-III. On prevention and control of the disease. Sci. Rep. Hokkaido Fish Hatchery, 22: 51-68. (In Japanese withEnglish summary.)
Awakura, T., S. Kurahashi, and H. Matsumoto.

Urawa − Kabatana takedai infection in masu salmon

1966. Studies on the Plistophora disease of salmonid fishes-II. Occurrence of the microsporidian
disease in a new district. Sci. Rep. Hokkaido FishHatchery, 21: 1-12. (In Japanese with English summary.)Lom, J., F. Nilsen, and S. Urawa. 2001. Redescription of Microsporidium takedai (Awakura, 1974)
as Kabatana takedai (Awakura, 1974) comb. n.
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様々な水温で飼育されたサクラマス幼魚における武田微胞子虫 Kabatana takedai の寄生
浦和茂彦

北海道の千歳事業所において河川水と湧水を混合した様々な水温下で飼育されたサクラマス幼魚における武 田微胞子虫 (Kabatana takedai) の寄生状況を1999 年と 2000年にモニタリングした．1999 年夏，千歳川の水温が異常に上昇したため，飼育水温が 15oCを超えた．その結果，一時的に飼育されていた尻別産と静内産サクラマス幼魚に武田微胞子虫が大量寄生した．未発生地域への微胞子虫の拡散を防ぐため原産地への放流は中止され，約 10 万尾の生残魚は 11月末までに処分された．また 2000 年の調査により
武田微胞子虫のシスト形成は水温 13oC 以下でも起きることが始めて確認された．形成されたシストは宿主の筋肉や心臓に 7 ヵ月以上残ることが飼育実験で示された．