Apicomplexa

Coccidia

Cryptosporidium sp.
Oocysts of Cryptosporidium sp. in a sugar fecal flotation (arrows). Depending on species, oocysts range from 3.5-7 µm in diameter. Cryptosporidium spp. are now considered gregarines but we have retained them here for historic reasons.

Cryptosporidium sp. oocysts from a dog (arrows) a Trichuris vulpis egg is visible out of focus on the right. Transmission is via ingestion of the oocyst, which is immediately infective when passed and contains four sporozoites. Oocysts are viable for months unless exposed to extreme temperatures (below freezing or greater than 65°C) or desiccation.

Cryptosporidium sp.
Modified Kinyoun's acid fast stain of Cryptosporidium sp. oocysts (arrows). Cryptosporidium spp. are now considered gregarines but we have retained them here for historic reasons.

Cystoisospora spp.
Oocysts of Cystoisospora spp., commonly called coccidia, in a canine fecal float. Note the two different sizes and that several are sporulated.

Cystoisospora felis and Cystoisospora rivolta
Two species of coccidia in cats include Cystoisospora felis (larger of the two, ovoid in shape, sizes range from 38-51 μm by 27-29 μm, oocyst wall is smooth with no micropyle, sporulation time is 72 hrs) and Cystoisospora rivolta (smaller of the two, ellipsoidal in shape, sizes range from 21-28 μm by 18-23 μm, micropyle is present at narrower end, sporulation time is 4 days). Both species are usually non-pathogenic.

Eimeria sp.
Oocysts of Eimeria sp. in a pig fecal float.

Eimeria bukidnonensis and Eimeria auburnensis
Two Eimeria species of cattle. Eimeria bukidnonensis (pear-shaped to oval, yellowish brown to dark brown in color, average size of 44 μm by 31 μm, micropyle present at narrower end, sporulation time 72-96 hrs) is at the top of field of view and Eimeria auburnensis (one of the most common coccidia in cattle, oocyst shape varies from ellipsoidal to tapering, sizes range from 32-46 μm by 20-25 μm, sporulation time is 48-72 hrs) is at the bottom of the field of view.

Eimeria zuernii
The major pathogenic coccidium of cattle, oocysts are spherical to ellipsoidal in shape, with sizes ranging from 15-22 μm by 13-18 μm. The wall of the oocyst is thin and smooth, being clear to pale-yellow in color. There is no visible micropyle and sporulation occurs in 23-24 hours. Disease is characterized by hemorrhagic diarrhea.

Eimeria arloingi
Oocysts of Eimeria arloingi in a goat fecal float. Note the micropyle on each oocyst.

Eimeria leuckarti
Eimeria leuckarti is a parasite of horses and donkeys. Eggs are large for the genus, measuring 80-88 μm x 55-59 μm and may be found on both sedimentation and flotation procedures. Infections have little clinical significance in horses and are usually seen in foals.

Eimeria macusaniensis
An oocyst of Eimeria macusaniensis, a parasite of cammelids. This Eimeria species can be quite pathogenic.

Neospora caninum
Neospora caninum encysted bradyzoites in a histologic preparation of calf cerebrum. Neospora caninum is an important cause of abortion in cattle and also causes ascending paralysis in dogs.

Neospora caninum or Hammondia heydorni oocyst
Oocysts, both sporulated and nonsporulated, found in canine feces, measuring 11 μm in diameter, viewed at 1000X. Oocysts of Neospora caninum and Hammondia heydorni are almost identical in morphology, with Hammondia heydorni sometimes described as approximately 1 μm larger on average.​ Oocysts of Neospora caninum are only rarely found in canine fecal preparations.

Sarcocystis sp. Sarcocystis sp. cyst in a histologic muscle preparation from a cow. Sarcocysts are usually large, well demarcated structures found within cardiac and skeletal muscle fibers of intermediate hosts and often contain interior septa.

Sarcocystis cruzi sporocyst
Sporocyst of Sarcocystis cruzi found in dog feces, measuring 14.5 X 9 μm. Sarcocystis spp. usually shed sporocysts in the feces of their definitive host.

Sarcocystis felis and Sarcocystis neurona sarcocysts
Gross pathology of the tissue cysts of Sarcocystis felis (yellow arrow) and Sarcocystis neurona (blue arrow) (~5 mm) at 10X magnification in the muscle of a skunk. Cysts are often septate and contain both metrocytes and bradyzoites.

Toxoplasma gondii
Toxoplasma gondii tachyzoite. Tachyzoites are the rapidly dividing, motile stage of the parasite that disseminates throughout host tissues.

Toxoplasma gondii
Toxoplasma gondii encysted bradyzoites in a histologic section of mouse brain. Bradyzoites are the slowly dividing, semi-dormant stage of the parasite. These cysts can form in any organ but are predominantly found in the brain, striated muscle, and liver.

Toxoplasma gondii unsporulated oocyst
Unsporulated oocyst from feline feces measuring 10 μm X 10 μm, viewed at 1000X.

Besnoitia besnoiti
Histologic section of bovine dermis showing 'pearl' cysts. Cattle serve as an intermediate host for this ampicomplexan, which forms spherical bradyzoite packed cysts which may be found in the dermis, subcutaneous tissues, fascia, and mucosa of affected animals. Visible cysts in the scleral conjunctiva are the most characteristic clinical sign of infection.

Besnoitia darlingi
Besnoitia darlingi tissue cysts in the dermis, subcutaneous tissues, and fascia of an opossum. Cats have been shown to be the definitive host of Besnoitia darlingi and opossums (Didelphis virginiana) the intermediate host. Opossums acquire infection through the ingestion of infected tissues or water contaminated with oocysts from cat feces.

Hemosporidians, Malarias, and Hepatozoon

Babesia bigemina
Piroplasms of Babesia bigemina in erythrocyte on Romanowsky stained blood smear from an infected cow. Piroplasms of Babesia bigemina are 4.5 x 2.5 µm, teardrop-shaped, and typically paired.

Babesia canis
Piroplasms of Babesia canis in erythrocytes on a Romanowsky stained blood film from an infected dog. Piroplasms of Babesia canis are 4-5 µm, pear-shaped, and typically paired.

Babesia gibsoni
Piroplasms of Babesia gibsoni in an erythrocyte on a Romanowsky stained blood film from an infected dog. Piroplasms of Babesia gibsoni are 1-3 µm and signet ring shaped.

Cytauxzoon felis
​Macrophage containing Cytauxzoon felis schizonts. Romanowsky stained lung impression smear.

Cytauxzoon felis
Romanowsky stained blood film with numerous erythrocytes containing Cytauxzoon felis piroplasms, which are 1-3 µm and signet ring shaped.

Cytauxzoon felis
A naturally infected bobcat found febrile and moribund. Bobcats serve as a wild reservoir for Cytauxzoon felis, giving the disease its common name "bobcat fever." Domestic cats can also serve as a reservoir for this parasite.

Haemoproteus sp.
Gamonts of Haemoproteus sp. in a Romanowsky stained blood film. Gamonts are found within erythrocytes and are approximately 7 µm.

Hepatozoon americanum
Gamont of Hepatozoon americanum in a leukocyte on a Romanowsky stained blood film from an infected dog. Gamonts are only occasionally seen on blood films, as parasitemia is typically extremely low.

Hepatozoon americanum
Characteristic "onion skin" cyst of Hepatozoon americanum in a histologic muscle preparation from an infected dog. These cysts are associated with parasite schizogony within leukocytes situated between striated muscle fibers, and are pathognomonic for infection with this parasite. Here, the parasite has invaded the host cell and induced cyst formation, but has not yet begun to undergo schizogony

Leucocytozoon sp.
Gamonts of Leucocytozoon sp. in a Romanowsky stained blood film from an owl. Note the host cell nucleus pushed to one side and the cytoplasm extending beyond the parasite, giving the cell a "horned" appearance.

Plasmodium circumflexum
Schizonts of Plasmodium circumflexum in the red blood cells of a Pintail duck seen on blood smear. Known as avian malaria, Plasmodium circumflexum is transmitted by Culex mosquitoes

Plasmodium falciparum
A gametocyte of Plasmodium falciparum seen in the red blood cell of a human. Plasmodium falciparum is one of six species that commonly infect humans and is considered the most pathogenic. This blood parasite is vectored by the female Anopheles mosquito.

Plasmodium mexicanum
Gamont of Plasmodium mexicanum seen in the red blood cell of a Sagebrush lizard. Plasmodium mexicanum infects lizards of the Sceloporus genus and is transmitted by Lutzomyia sandflies.

Theileria equi
Romanowsky stained blood film with numerous erythrocytes containing Theileria equi piroplasms. Equine piroplasmosis is endemic in parts of Asia, Africa, the Middle East, and Central and South America, but has been introduced into the United States on occasion.

Theileria cervi
Blood smear from a white-tailed deer (Odocoileus virginanus) containing numerous Theileria cervi piroplasms. Theileria cervi is usually non-pathogenic in deer and not known to be transmitted to cattle or sheep.