A 1-year-old female domestic long hair (DLH) cat (Felis domesticus)The cat was 5-6 weeks pregnant and died after partial abortion (aborting three of six fetuses). Fixed tissues in 10% buffered formalin and blood smears were received at Tifton Veterinary Diagnostic and Investigational Laboratory.

Histopathologic Description:

Within sections of kidney, liver and spleen, veins, arterioles and arteries contained few to several foamy macrophages which occasionally partly occluded the vascular lumens (Fig. 2-1). The macrophages are enlarged up to twice normal size and contained cytoplasmic schizonts with numerous 1-2 micron in diameter round to oval bluish organisms (merozoites). Similar macrophages containing protozoan schizonts were seen within several glomerular capillaries (Fig. 2-2). The schizont-laden macrophages were seen attached to endothelium, at times occluding blood vessels, and were numerous especially in sections from the spleen and liver (Figs. 2-3, 2-4). 

Additionally, tissues (slides not submitted) from heart, lung, stomach, small intestine, ovary and urinary bladder and gall bladder were examined. In almost all examined tissues, there were similar large schizont-laden macrophages in the blood vessels. These were numerous in the sections of liver, kidney, spleen and heart. The other findings were within normal limits.

Morphologic Diagnosis:  

Kidney, spleen and liver, blood vessels (veins, arteries and arterioles): Histiocytosis, diffuse, moderate with intrahistiocytic intracytoplasmic schizonts, morphology most consistent with Cytauxzoon felis (a Theilerial parasite)


Cytauxzoon felis

Contributor Comment:  

Cytauxzoon felis is a protozoan parasite classified in family Theileriidae and infects wild ungulates species in Africa including the kudu, eland and giraffe, as well as domestic and wild Felidae in North America.(3) The organism is believed to be transmitted from bobcat, the reservoir host, to domestic cats via a tick vector.(1) The clinical disease in domestic cat is most prevalent in the early and late summer, corresponding with the activity of its tick vector, Dermacenter variabilis.(4)

The infection is associated with both tissue (schizogonous) phases and an intraerythrocytic phase that correlates with the clinical phases of severe circulatory impairment and hemolytic anemia, respectively.(1) The clinical signs in domestic cats include depression, anorexia, pyrexia, dehydration, pallor, icterus, dark urine and occasionally dyspnea.(1,3)

The cat in the current case presented to the veterinarian, after partial abortion (aborting three of six fetuses) the previous night, in a near comatose stage with lethargy and mild gingival icterus. Initially, the cat had a reduced heart rate, which progressed to seizures, then death. A blood smear was collected immediately postmortem by cardiac puncture. Numerous schizont-laden macrophages and small round ring-shaped piroplasms in several erythrocytes were observed in the Wright-Giemsa stained smears. Generally, occasional schizont-laden macrophages may be observed on the feathered edge of blood smears in infected cats, but numbers similar to those in this case are unusual. Large numbers of schizont-laden macrophages in blood smear in the current case could be attributed to collection of blood from the heart. Because of their large size these cells are less likely to be found in peripheral circulation and are not normally found in blood smears from infected cats.(6) As in the current case, it was previously reported that high percentage of parasites are identified histologically from the spleen, liver, or lungs, suggesting sampling from these organs as the most appropriate sites for organism identification.(4) Because the tissue phase occurs prior to the erythrocytic phase, some cats can be severely ill but not have detectable parasites in their red blood cells.(1)

Three remaining fetuses were recovered at necropsy. Formalin fixed tissues from the cat and some fetal tissues (skeletal muscle, developing bone and placenta) were examined. Macrophages or piroplasms associated with the organism were not seen in fetal tissues. Whether Cytauxzoon infection in this cat would be incriminated for the partial abortion observed could not be ruled out.(6) The tissue schizonts are the phase that is responsible for clinical manifestation of cytauxzoonosis.(4) Clinical Cytauxzoon felis infection is usually fatal in domestic cats (1,4), although some infected cats survive.(1) Because of this, domestic cats (Felis domesticus) are regarded as accidental hosts.(6) Bobcats, thought to be the reservoir hosts, are persistently parasitemic, yet they rarely manifest marked clinical disease(4). Rare fatal cases of cytauxzoonosis in free-ranging bobcats has been reported.(5)

JPC Diagnosis:  

Kidney, liver, and spleen: Histiocytosis, intravascular, diffuse, moderate with intrahistiocytic schizonts, etiology consistent with Cytauxzoon felis

Conference Comment:  

The contributor gave a good overview of Cytauxzoon felis and its importance in the domestic cat. During the conference other blood parasites were discussed. With erythrocytic parasites, it is important to determine the exact location of the parasite within the erythrocyte in order to accurately identify the parasite and thus make a correct diagnosis. Included below is a non-comprehensive list of blood parasites that are important in veterinary medicine.
Intracellular parasites
(within erythrocytes)
Hemoproteus spp.
Leukocytozoon spp.
Plasmodium spp.

Cytauxzoon felis
Babesia cati
Babesia felis

Anaplasma marginale
Anaplasma centrale

Babesia bovis
Babesia bigemina

Theileria mutans
Theileria annulata

Theileria cervi

Babesia canis
Babesia gibsoni

Babesia equi
Babesia caballi

Babesia ovis
Babesia motasi






Deer, Elk



Epicellular parasites
(On membrane surface of
depression of erythrocytes)
Trypanosoma johnbakeri

Hemobartonella felis
(Mycoplasma haemofelis)

Hemobartonella canis
(Mycoplasma haemocanis)

Eperythrozoon suis
(Mycoplasma haemosuis)

Eperythrozoon wenyoni

Eperythrozoon sp.





Extracellular parasites
(within the plasma)
Dipetalonema reconditum

Dirofilaria immitis

Setaria spp.

Trypanosoma theileri
Trypanosoma congolense
Trypanosoma vivax

Trypanosoma cruzi

Trypanosoma brucei
Trypanosoma evansi

Dogs (sometimes cats)






1. Birkenheuer AJ, Le JA, Valenzisi AM, Tucker MD, Levy MG, Breitschwerdt EB: Cytauxzoon felis infection in cats in the Mid-Atlantic States: 34 cases (1998-2004). J Am Vet Med Assoc 228(4):568-71, 2006
2. Brockus CW, Andreasen CB: Erythrocytes. In: Duncan & Prasses Veterinary laboratory Medicine, Clinical Pathology, eds. Latimer KS, Mahaffey EA, Prasse KW, 4th ed., pp. 19-21. Blackwell Publishing, Ames, IA, 2003
3. Maxie G: Cytauxzoonosis. In: Jubb, Kennedy and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol 3, pp. 152-158. Elsevier Saunders, Philadelphia, PA, 2007
4. Meinkoth J, Kocan AA, Whitworth L, Murphy G, Fox JC, Woods JP: Cats surviving natural infection with Cytauxzoon felis: 18 cases (1997-1998). J Vet Intern Med (5):521-5, 2000
5. Nietfeld JC, Pollock C: Fatal cytauxzoonosis in a free-ranging bobcat (Lynx rufus). J Wildl Dis 38(3): 607-10, 2002
6. Weismann, JL, Woldemeskel, M, Smith, KD, Merill, A, Miller, D: Blood smear from a pregnant cat that died after partial abortion. Vet Clinc Pathol 36:(2), 209-211, 2007

Click the slide to view.

2-1. Cat, spleen.

2-2. Cat, kidney.

2-3. Cat, liver.

2-4. Cat, liver

Back | VP Home | Contact Us |