Cyst of Giardia sp. in a fecal preparation stained with iodine. Cysts of Giardia sp. are 9-13 x 7-9 µm and typically ovoid. Note the 4 nuclei, 2 median bodies, and axostyles within the cyst shown.

Trophozoite of Giardia sp. in a direct fecal smear from an infected dog. Trophozoites are 12-17 X 7-10 µm and are tear drop shaped with 8 flagella, 2 nuclei, and 2 median bodies.

Cysts of Giardia duodenalis recovered on fecal float from a dog. This image was taken with the 40X objective allowing more details to be seen. Giardia sp. cysts are 9-13 x 7-9 µm and typically ovoid. When intact, each cyst can be seen to contain axostyles, median bodies, and 2 to 4 nuclei.

Cysts of Giardia duodenalis recovered on fecal float from a dog. This image was taken with the 20X objective and shows numerous collapsed cysts and a few that have not yet collapsed.

Histologic preparation showing amoeboid stage of Histomonas meleagridis, causative agent of blackhead disease in turkeys.

Lesions on turkey liver due to histomoniasis and secondary bacterial infection. Characteristic multifocal white pinpoint lesions can be seen around multiple dark, slightly depressed areas on a generally enlarged liver. Without secondary bacterial infection, Histomonas infections are only mildly pathogenic. In turkey flocks not on prophylactic antibiotics, histomoniasis mortality rates may reach 100%.

Amastigotes of Leishmania infantum within macrophages in a Romanowsky stained spleen impression smear from a dog. Note darkly staining kinetoplasts and nuclei within the ovoid amastigote bodies.

Cutaneous leishmaniasis lesion in a brown rat. ​

Trypomastigotes of Trypanosoma cruzi in a Romanowsky stained blood smear. Note the darkly staining kinetoplasts and nuclei and the undulating membranes.

A collection of Trypanosoma cruzi amastigotes seen within a cardiac muscle cell. Replication of Trypanosoma cruzi takes place as intracellular binary fission of amastigotes, within the cells of the reticuloendothelial system and striated muscle.

Trypanosoma lewisi trypomastigotes in a blood smear from a rat. Transmitted by the rat flea Nosopsyllus fasciatus, Trypanosoma lewisi is generally non-pathogenic.

Trypanosoma theileri trypomastigote, a hemoflagellate of cattle, transmitted and hosted by tabanid flies. Trypanosoma theileri trypamastigotes are among the largest, ranging from 60 - 120 μm in length, with a well developed undulating membrane and a well defined free flagellum.

​Trichrome stained Tritrichomonas foetus trophozoites. Note the undulating membrane indicated by arrow.

Lesions of trichomoniasis, commonly referred to as 'canker' in pigeons or 'frounce' in raptors, consisting of diffuse multifocal areas of caseous necrosis in the esophagus and oropharynx of a dove. Lesions of trichomoniasis are usually found in the mouth, pharynx, esophagus, and crop, and are rarely seen posterior to the proventriculus.

Entamoeba coli
Cyst of Entamoeba coli stained with iodine from an infected olive baboon, Papio anubis. Mature cysts can have up to 8 nuclei.

Entamoeba histolytica / dispar
Entamoeba histolytica / dispar cysts stained with iodine from an infected olive baboon, Papio anubis. The cysts of ​Entamoeba histolytica and Entamoeba ​dispar are considered morphologically indistinguishable. Mature cysts are spherical, have 4 nucleii, and measure 12 to 18 µm in diameter. ​

Entamoeba invadens
Entamoeba invadens trophozoite from tissue of a tortoise. An important pathogen of captive reptiles, Entamoeba invadens is the causative agent of amoebiasis in reptiles. Trophozoites measure 10-38 μm by 9-30 μm and contain one to four nuclei.

Lesion of Entamoeba invadens in the stomach of a tortoise. Lesions typically occur in the intestinal tract and liver, creating ulcers characterized by hyperemia, inflammation, and an infiltration of neutrophils. Amebae are found at the periphery of the ulcer, leaving the cavity filled with necrotic tissue

Feeding trophozoites in a section of brain tissue. Naegleria fowleri causes primary amebic meningoencephalitis (PAM) in humans.

Balantidium coli trophozoite (stained). Note the large bean-shaped macronucleus, fine layer of cilia surrounding the organism, and mouth-like structure. Trophozoites are typically seen in direct fecal smears.

​Balantidium coli cyst (stained). Note the large macronucleus.

​Balantidium coli cyst (not stained). In non-stained specimens the large, bean-shaped macronucleus is not easily observed.

Cyst of Buxtonella sulcata. A ciliate found in the colon of cattle, Buxtonella sulcata exists in cyst form outside the host. The ciliated form resembles Balantidium coli in swine. Pathogenesis is minimal and the presence of cysts in feces is considered an incidental finding.

Ichthyophthirius multifiliis trophont cross section in fish gill. Ich has two basic phases to its life cycle, the free-living stage which includes the infective tomites and cystic tomocysts, and the parasitic phase, consisting of feeding and reproducing trophonts. Trophonts are the stage found on skin scrapings of affected fish.

Trophont of Ichthyophthirius multifiliis from the skin of a fish. Trophonts can range greatly in size, but have a charcteristic horseshoe shaped macronucleus, seen as a pale structure in this image, a cilliated buccal cavity near the anterior end, and an oval to round body.

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.