Parasite - Kiseichu (English spelling)

Japanese: 寄生虫 - きせいちゅう（英語表記）parasite

Parasites are animals that depend for their environment and food on another type of organism called the host. In the fields of medicine and veterinary medicine, parasites are protozoa, flatworms, nematodes, hooked headed animals, and arthropods that live as parasites on vertebrates such as humans, livestock, and poultry, but there are many other animals that live as parasites, and it is no exaggeration to say that there is no class of invertebrates that does not contain parasitic species. Here we will discuss parasites as they are used in medicine and veterinary medicine.

Each type of parasite has its own host and inhabits a certain part of the host. Those that inhabit the surface of the host's body are called external parasites, and those that inhabit the inside of the host's body are called internal parasites. Mosquitoes that stay on the surface of the host's body temporarily to suck blood are also considered external parasites. As they adapt to a parasitic life, trematodes and tapeworms develop suckers and hooks to attach to the host's parasitic parts, their locomotor and sensory organs degenerate, and some tapeworms and hook-headed worms have completely lost their digestive organs. On the other hand, their reproductive organs are highly developed and occupy most of the body, and some can lay more than 100,000 eggs a day. When protozoans are expelled from the host's body and the environmental conditions deteriorate, they become resistant cysts covered with a pouch, and when the environment improves again, they become trophic, move actively, eat food, and multiply. There are also species of external parasites that have undergone significant changes to adapt to a parasitic life.

[Masaaki Machida]

Developmental cycle

Also called life cycle or life cycle. Each type of parasite has its own unique developmental cycle, but they always require at least one type of host. The host in which the parasite matures and reproduces sexually is called the definitive host, and the host in which the parasite inhabits during the larval stage is called the intermediate host. There may also be a migratory host (also called an extended intermediate host) between the intermediate host and the definitive host. This is not an essential host for the development of the parasite, but it is a potential source of infection for the definitive host.

Pinworms and whipworms use humans as their definitive host and do not require an intermediate host for their development. The eggs excreted from the body re-enter the human mouth and infect the human. Paragonimus westermani uses humans, dogs, and tigers as definitive hosts, the Japanese river snail as the first intermediate host, the Japanese mitten crab and Japanese freshwater crab as the second intermediate host, and wild boars as the mobile host. In the developmental cycle of flukes (digenean), hermaphrodite reproduction occurs within the definitive host, and complex larval reproduction occurs within the intermediate host (the first intermediate host in the case of Paragonimus). This alternation of generations in which hermaphrodite reproduction and larval reproduction alternate is called alloiogenesis. There are also known species such as fecal nematodes that alternate between hermaphrodite reproduction and parthenogenesis, i.e., heterogony.

[Masaaki Machida]

Transmission

Parasites have a certain route of entry into their definitive host. When infectious eggs or larvae enter through the mouth, it is called oral infection, and when larvae penetrate the skin, it is called percutaneous infection (malaria, Schistosoma japonicum, hookworms, heartworms, etc.). There are also placental infections that are transferred from the mother's placenta to the fetus (Toxoplasma gondii, Roundworm of dogs, etc.), and translactation infections that are transferred from breast milk to the child (Ascaris cati, Hookworm of dogs, etc.).

[Masaaki Machida]

Pathogenicity

Symptoms vary depending on the type of parasite, the number of parasites, and the location of the parasite, but in some cases there are almost no symptoms. A large number of small roundworms parasitize the cecum of horses, but non-blood-sucking types are almost non-pathogenic. However, hookworms such as the hookworm Zubini, which bite into the small intestine of humans and suck blood, cause 0.16 to 0.23 cc of blood loss per day. As the number of parasites increases, the amount of blood lost increases, and it is said that an infection of about 30 parasites can cause anemia in the host. In addition, human roundworms can migrate into the bile duct or appendix, and Paragonimus westermani can migrate into the brain and cause severe neurological symptoms. In addition, substances secreted or excreted by the parasites can cause harmful chemical damage to the host, or damage due to an allergic reaction is also possible. Blood-sucking external parasites can cause itching and dermatitis, and can also transmit viruses, rickettsia, and bacteria. It is also known that parasites that originally have animals other than humans as their definitive hosts can infect humans and cause various diseases, such as liver enlargement caused by dog roundworm larvae, stomach and abdominal pain caused by Anisakis larvae, eosinophilic meningoencephalitis caused by Angiostrongylus cantonensis larvae, and skin welts caused by Gnathostomy larvae.

[Masaaki Machida]

Control

The most effective part of each parasite's developmental cycle is to cut off. The first step is deworming. For example, if an infant is infected with pinworms, eggs are released vigorously, causing infection not only to the infant but also to others. In this case, it would be more effective to deworm the infant in a group, including the family and school, rather than just deworming one infant at a time. The second step is to kill the eggs and larvae that have been released from the body. This involves issues such as waste disposal and intermediate hosts. Since the Japanese blood fluke is distributed in the habitat of the intermediate host, the Japanese oyster, in Japan, irrigation channels have been concreted to eradicate the Japanese oyster, with great effect. In addition, it is possible to prevent infectious eggs and larvae from entering the body of the definitive host. For example, to prevent pinworms, one should cut one's nails, get into the habit of cleaning one's hands, and keep underwear and bedding clean. In addition, liver flukes and lung flukes are infected by freshwater fish and crabs, respectively, so infection can be prevented by avoiding eating these raw.

[Masaaki Machida]

Current status of parasitic diseases in Japan

Immediately after the Second World War, the parasitic infection rate in Japan exceeded 70%, and Japan was even called the "kingdom of parasites." However, due to the remarkable improvement in living conditions since then, the infection rate has now dropped to the 4% level. However, looking at the details, while roundworm and hookworm infections have certainly decreased dramatically, there is no sign of a decrease in pinworm infections. Echinococcus multilocularis, which is distributed in Hokkaido, requires caution as tapeworm eggs are often attached to the fur of red foxes. In addition, new parasitic diseases have appeared due to the diversification of dietary habits in recent years and contact with pet animals. These include "anisakiasis" which is contracted from raw mackerel or squid, "trichinosis" from eating raw bear meat, "gnathostomiasis" caused by eating loach, "toxoplasmosis" which is contracted from cat feces, and "canine roundworm disease" which originates from young dogs. Furthermore, there are increasing cases of people travelling to areas such as Southeast Asia and Africa and becoming infected with malaria parasites, amebic dysentery (amoeba dysentery), Giardia lamblia, Taenia solium, Taenia saginata, and Gnathostoma spp. Please refer to the explanations for each parasite.

[Masaaki Machida]

Major human parasites in Japan
©Shogakukan ">

Major human parasites in Japan

Developmental cycle of Paragonimus westermani (Paragonimus westermani)
©Shogakukan ">

Developmental cycle of Paragonimus westermani (Paragonimus westermani)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

寄生虫は、宿主(しゅくしゅ)とよばれる他の種類の生物に環境と食物を依存している動物である。医学・獣医学領域では、原生動物、扁形(へんけい)動物、線形動物、鉤頭(こうとう)動物、節足動物のうちでヒトや家畜・家禽(かきん)などの脊椎(せきつい)動物に寄生する動物を寄生虫というが、このほかにも寄生生活を営む動物は多数みられ、無脊椎動物の綱のうちで寄生種を含まない綱はないといっても過言ではない。ここでは医学・獣医学で扱う寄生虫について述べる。

　寄生虫は種類によってそれぞれ固有の宿主をもち、その宿主の一定の部位に寄生する。宿主の体表に寄生するものを外部寄生虫、体内に寄生するものを内部寄生虫という。吸血のために一時的に宿主の体表に止まるカなども外部寄生虫として取り扱うことがある。寄生生活に適応するにつれて、吸虫や条虫のように宿主の寄生部位に付着するために吸盤や鉤(かぎ)などが発達し、運動器官や感覚器官は退化し、条虫や鉤頭虫のように消化器官がまったく消失したものもある。一方、生殖器官は非常に発達し、体の大部分を占め、1日に10万個以上の卵を産むものもある。原生動物では、宿主体外に排出されたりして環境条件が悪化すると、ふくろ（嚢）をかぶって抵抗型の嚢子(のうし)となり、これが宿主体内に入ってふたたび環境がよくなると栄養型となり、活発に運動し食物をとって増殖する。外部寄生虫も寄生生活に適応して著しく変形を遂げた種類がみられる。

［町田昌昭］

発育環

生活史あるいは生活環ともいう。寄生虫は種類によってそれぞれ特有の発育環をもっているが、かならず1種類以上の宿主を必要とする。寄生虫がその体内で成熟して両性生殖を行う宿主を固有宿主（終宿主ともいう）といい、幼虫期に寄生する宿主を中間宿主という。中間宿主と固有宿主の間に移動宿主（延長中間宿主ともいう）が介在することもある。これは寄生虫の発育にとって不可欠の宿主ではないが、固有宿主に対して有力な感染源となる。

　蟯虫(ぎょうちゅう)や鞭虫(べんちゅう)はヒトを固有宿主とし、その発育には中間宿主を必要とせず、体外に排出された卵がふたたびヒトの口から入って感染する。ウェステルマン肺吸虫はヒト、イヌ、トラなどを固有宿主とし、カワニナを第一中間宿主、モクズガニやサワガニを第二中間宿主、イノシシを移動宿主とする。吸虫（二生類）の発育環では固有宿主体内で両性生殖、中間宿主体内（肺吸虫の場合第一中間宿主）で複雑な幼生生殖が行われる。このように両性生殖と幼生生殖とが交互する世代交代を混合生殖（アロイオゲネシスalloiogenesis）という。また糞(ふん)線虫のように両性生殖と単為生殖とが交互する世代交代、すなわち異常生殖（ヘテロゴニーheterogony）を営むものも知られている。

［町田昌昭］

感染経路

寄生虫は固有宿主に侵入するとき一定の経路をもつ。感染力を備えた卵や幼虫が口から入る場合を経口感染、幼虫が皮膚を穿通(せんつう)して入る場合を経皮感染（マラリア、日本住血吸虫、鉤虫(こうちゅう)、糸状虫など）という。このほか母親の胎盤から胎児に移行する胎盤感染（トキソプラズマ、イヌ回虫など）や、母乳から子供に移行する経乳感染（ネコ回虫、イヌ鉤虫など）もある。

［町田昌昭］

病害性

寄生虫の種類、寄生数、寄生部位などによって異なるが、ほとんど症状のないこともある。ウマの盲結腸にはおびただしい数の小形円虫類が寄生しているが、非吸血性の種類はほとんど病害性をもたない。しかしズビニ鉤虫のように、ヒトの小腸に咬着(こうちゃく)して吸血するものでは、1匹が1日に失血させる血液量は0.16～0.23ccに及ぶ。寄生数が多くなれば当然失血量も増加し、約30匹の寄生で宿主の貧血を招くという。またヒトの回虫が胆管や虫垂に迷入したり、ウェステルマン肺吸虫が脳に迷入して重篤な神経症状をおこす。このほか、寄生虫が分泌あるいは排泄(はいせつ)する物質が宿主に有害な化学的障害を与えたり、アレルギー反応による障害も考えられる。吸血性の外部寄生虫ではかゆみや皮膚炎を引き起こすほか、ウイルス、リケッチア、細菌などを媒介することもある。また本来、ヒト以外の動物を固有宿主とする寄生虫がヒトに感染していろいろな疾病の原因となることも知られている。イヌ回虫の幼虫による肝腫大(しゅだい)、アニサキスの幼虫による胃痛や腹痛、広東(かんとん)住血線虫の幼虫による好酸球性髄膜脳炎、有棘顎口(ゆうきょくがっこう)虫の幼虫による皮膚のみみずばれなどがある。

［町田昌昭］

防除

それぞれの寄生虫の発育環のなかでもっとも防除効果のある箇所を切断する。その第一は駆虫である。たとえば幼児に蟯虫が寄生していれば、卵がさかんに放出されて、本人のみならず他の人々にも感染する。この場合幼児1人だけの駆虫でなく、家族や学校で集団駆虫を行えばなおいっそう効果があろう。次に体外に排出された卵や幼虫を殺滅する。これには糞尿処理や中間宿主の問題がかかわってくる。日本住血吸虫は中間宿主であるカタヤマガイの生息地に分布するので、日本では灌漑(かんがい)水路をコンクリート化してカタヤマガイを撲滅し大きな効果をあげている。さらに感染力を備えた卵や幼虫が固有宿主体内に侵入するのを防ぐ。たとえば蟯虫の予防には爪(つめ)を切り、手指をきれいにする習慣をつけ、下着や敷布も清潔にする。また肝吸虫や肺吸虫はそれぞれ淡水産の魚やカニが感染源となっているので、これらの生食を避けることにより感染を防除できる。

［町田昌昭］

日本の寄生虫症の現状

第二次世界大戦直後の日本では、寄生虫感染率が70％を超え「寄生虫王国日本」とまでいわれたが、その後の著しい生活環境の改善によって、現在では4％台にまで低下した。しかしその内容をみると、回虫や鉤虫の感染は確かに激減したが、蟯虫の感染はいっこうに減る傾向がみられない。北海道に分布する多包条虫（エキノコックス）は、キタキツネの毛皮に条虫卵が付着していることが多いので注意を要する。また、近年の食生活の多様化やペット動物との接触から、新しい寄生虫症が登場してきた。すなわち、サバやイカの刺身から感染する「アニサキス症」、クマ肉の生食による「旋毛虫症」、ドジョウのおどり食いに原因する「顎口虫症」、ネコの糞便から感染する「トキソプラズマ症」、幼犬に由来する「イヌ回虫症」などがある。さらに、東南アジアやアフリカなどの地域に出かけ、「マラリア原虫」「アメーバ赤痢（赤痢アメーバ）」「ランブル鞭毛虫」「有鉤条虫」「無鉤条虫」「顎口虫」などに感染する例も増えている。それぞれの寄生虫項目の解説を参照されたい。

［町田昌昭］