Neospora

Neospora is a single celled parasite of livestock and companion animals. It was not discovered until 1984 in Norway, where it was found in dogs. Neosporosis, the disease that affects cattle and companion animals, has a worldwide distribution. Neosporosis causes abortions in cattle and paralysis in companion animals. It is highly transmissible and some herds can have up to a 90% prevalence. Up to 33% of pregnancies can result in aborted fetuses on one dairy farm. In many countries this organism is the main cause of abortion in cattle.^[1] Neosporosis is now considered as a major cause of abortion in cattle worldwide. Many reliable diagnostic tests are commercially available. Neospora caninum does not appear to be infectious to humans. In dogs, Neospora caninum can cause neurological signs, especially in congenitally infected puppies, where it can form cysts in the central nervous system.

Genome

The genome of Neospora caninum has been sequenced.^[2] The results suggest a European origin for this parasite.

Effects of disease

Neospora caninum is a major pathogen of cattle and dogs that occasionally causes clinical infections in horses, goats, sheep, and deer as well. The domestic dog is the only known definitive host for N. caninum. In cattle, N. caninum is a major cause of bovine abortion in many countries and is one of the most efficiently transmitted parasites with up to 90% of some bovine herds infected. N. caninum causes abortion in both beef and dairy cattle. Another important factor is the gestational age and hence immunocompetence of the fetus at the time of infection.^[3] Early in gestation, N. caninum infection of the placenta and subsequently the fetus usually proves fatal, whereas infection occurring in mid to late pregnancy may result in the birth of a congenitally infected but otherwise healthy calf. Recent studies have broadened the list of known intermediate hosts to include birds. N. caninum has recently been found to infect domestic chickens and house sparrows (Passer domesticus) which may become infected after ingesting parasite oocysts from the soil. ^[4] The presence of birds in cattle pastures has been correlated to higher infection rates in cattle.^[5] Birds may be an important link in the transmission of N. caninum to other animals.

Epidemiology

The life cycle is similar to Toxoplasma. An infected dog will pass the oocysts through its feces and infect food or water. A cow or other animal will then up take the parasite. The parasite will undergo asexual reproduction in the animal's muscle until it is eaten by a dog. There, sexual reproduction will occur and oocysts will be created and passed through the feces. Dogs are often the definitive host but can act as an intermediate host as well. Cows are usually the intermediate host. No horizontal cow-to-cow transmission have been shown, although salival interactions have been suggested. Vertical transmission can occur when an infected cow gives birth to an infected calf—the calf survives the infection and grows into an adult. Vertical route is the major route of transmission in cattle and is extremely efficient as the rate of transmission is usually between 80 and 100%. ^[6] A heifer calf that is born congenitally infected is capable of transmitting the infection to the next generation when she becomes pregnant, thus maintaining the infection in the herd. Transplacental transmission in cattle is considered the major route of transmission. The life cycle is typified by three infectious stages: tachyzoites, tissue cysts, and oocysts ^[7] Tachyzoites and tissue cysts are the stages found in the intermediate hosts, and they occur intracellularly.

Detection of disease

Detection: the presence of cerebral and cardiac lesions can be seen on aborted bovine fetuses originating from a single farm. The parasite is identified in the tissues of many bovine aborted fetuses but also of stillborn calves and, rarely, of clinically affected newborn calves. The diagnosis of the infection is assisted through histopathology and immunohistochemical examination of aborted fetuses and serologic testing of cattle for evidence of infection.^[8] The abortion is the only clinical sign and can occur from the third month of pregnancy and onwards. Most of the abortions take place between the 5th and 6th months of pregnancy ^[9] The fetus is either resorbed, autolyzed, mummified, stillborn, born alive with clinical signs, or born clinically normal but chronically infected. At calving, infected calves may be clinically normal or may have neurologic signs, be underweight or unable to stand.

Prevention and control

Embryo transfer is recommended as a method of reproduction to reduce the chances of contracting the disease, as long as the disease status of the donor cow is checked. It is not recommended to rebreed heifers or cows that have this disease. Seropositive animals should be culled. To prevent horizontal transmission it is important to prevent the contamination of feed and water via the shedding of oocysts by dogs and possibly other canids like the fox. These animals should not have access to animal premises although this might be difficult to achieve. There are no drugs or vaccines available yet to prevent or control the disease.

References

O gênero Neospora apresenta como espécies importantes no âmbito médico-veterinário Neospora caninum e Neospora hughesi, sendo o último descrito como um protozoário parasita de eqüideos. Neospora caninum realiza invasão ativa do hospedeiro e multiplica-se pelo processo descrito como endogenia. Pode ser uma infecção assintomática em bovinos e causar aborto (observado primeiramente em gado leiteiro). Os canídeos, hospedeiros definitivos, infectam-se comendo tecido contaminado proveniente de hospedeiros intermediários, embora seja também possível que ocorra auto-contaminação ao comer suas próprias fezes (lembrando que também pode ocorrer a ingestão das fezes de outros cães possivelmente contaminados), sendo a via transplacentária a principal via de transmissão conhecida. Os taquizoítos encontram-se em células neurais, macrófagos, fibroblastos, endotélio vascular, monócitos, epitélio de túbulos renais e hepatócitos, além de células musculares do coração e da língua. Ao multiplicarem-se assexuadamente nessas células, o taquizoíto realiza um processo de brotamento ou endodiogenia, no qual são formados dois indivíduos na célula mãe, sendo a película da última utilizada pelas células filhas.

Ciclo Biológico

No ciclo, o hospedeiro definitivo ingere tecido contaminado com cistos teciduais. No trato digestivo, há o processo de formação do oocisto (que é liberado não esporulado, esporulando no ambiente). O hospedeiro intermediário ou mesmo o próprio hospedeiro definitivo ingere oocistos esporulados pela água ou pelo contato oral com as fezes desse. Caso bovinos tenham esse contato oral, infectam-se e há formação de cisto tecidual. Em imunodepressão (como na gravidez), bradizoítos 15 são liberados dos cistos e migram para a placenta, infectando o feto. Este pode aparentemente não estar infectado ao nascer, mas pode possuir o parasita em seu organismo. Além disso, o animal ainda pode nascer morto ou sem o protozoário em seu organismo, sendo que em muitos casos ocorre aborto ou reabsorção fetal. É importante salientar que não há tratamento para Neospora caninum em bovinos. O diagnostico pode ser feito pelo teste de Eliza e também por imunofluorescência.

ネオスポラ症はネオスポラ属（Neospora）の原虫を病原体とする感染症の総称である。イヌでは脳脊髄炎、ウシでは流産が問題となっている。日本ではウシとスイギュウのネオスポラ症が家畜伝染病予防法における届出伝染病となっている。これまでヒトで発症した例は知られていない。

病原体[編集]

ネオスポラはアピコンプレックス門に属する単細胞真核生物である。トキソプラズマと近縁であり、形態や生活環もよく似ている。2種が知られているが、N. hughesiについては不明な点が多く、以下特記なき場合はN. caninumについての知見である。

ネオスポラはイヌ属を終宿主としており、この点はネコ科動物を終宿主とするトキソプラズマと明瞭に異なる。終宿主の便とともに排出されたオーシストが成熟し、中間宿主はそれを経口摂取することで感染が成立する。急性期にはタキゾイトとして細胞内で分裂を繰り返しながら全身へ拡がるが、宿主の免疫応答に曝されると筋肉や脳でシストを作り終生慢性感染を続ける。このシストを終宿主が経口摂取することで生活環が完結するが、別の中間宿主が経口摂取した場合も同じサイクルをくり返し、また終宿主・中間宿主を問わず経胎盤感染により母から仔へ伝播することもできる。^[1]

動物別の症状[編集]

イヌ[編集]

先天感染の仔犬では上行性麻痺がよく見られる。後肢が伸び切って、筋の萎縮、拘縮、線維化などが見られる。成犬になってから症状が出ることもあり、多発性筋炎が多い。症状が出ている場合、治療しなければ死に至るが、治療した場合でも予後が悪い。^[2]

イヌの場合、主要な感染経路は食餌中のシストだと考えられている。^[3]

ウシ[編集]

先天性感染の場合、流産が主な症状であり、そのほか死産やミイラ胎仔の娩出といった症状が出る。正常に娩出された場合には不顕性のまま慢性感染に移行する場合が多いが、一部は起立困難や成長不良といった症状を示す。ウシの場合、主要な感染経路は経胎盤感染とオーシストの経口摂取だと考えられている。経胎盤感染の効率は非常に高く、初感染に限らず慢性感染の再燃でも経胎盤感染が起こる。^[3]

ウマ[編集]

N. hughesiがウマに脳脊髄炎を起こす場合があることが知られている。2種のネオスポラは抗体が交差反応するため、N. caninumがウマに感染するかどうかの知見はない。東欧から中近東にかけて10～20%のウマが抗体陽性であることが知られているが、発症した例はアメリカ合衆国に限られている。^[3]

診断[編集]

確定診断は脳脊髄液や組織中の原虫を示すことによるが、感染が立証されている動物であっても病理切片中に原虫が見付からない場合が珍しくない。症状と血清検査を合わせて推定診断とする場合が多い。トキソプラズマとの鑑別は、電子顕微鏡観察、免疫組織化学、PCRなどによって行う。^[2]

治療[編集]

イヌの場合、トリメトプリム・スルファジアジンと、ピリメタミンまたはクリンダマイシンを組み合わせて投薬する例がある。^[2]

予防[編集]

ワクチンは実用化されていない。

参考文献[編集]

1. ^ Goodswen et al. (2013). “A review of the infection, genetics, and evolution of Neospora caninum”. Infect. Genet. Evol. 13: 133-150. doi:10.1016/j.meegid.2012.08.012.
2. ^ ^{a b c} Lyon et al. (2010). “Update on the Diagnosis and Management of Neospora caninum Infections in Dogs”. Top. Companion Anim. Med. 25 (3): 170-175. doi:10.1053/j.tcam.2010.07.005.
3. ^ ^{a b c} Dubey & Schares (2011). “Neosporosis in animals--the last five years”. Vet. Parasitol. 180 (1-2): 90-108. doi:10.1016/j.vetpar.2011.05.031.

外部リンク[編集]

• ネオスポラ症（動物衛生研究所・家畜の監視伝染病）

国際獣疫事務局 リスト疾病

家畜伝染病予防法上の監視伝染病

네오스포라속(Neospora)은 정단복합체충문에 속하는 기생 원생생물 속의 하나이다. 개나 소에게 네오스포라증^[1]을 일으키는 병원균을 포함하고 있다.

각주