Signalment (JPC Accession # 1913179): Bull, age and breed unspecified

HISTORY: This bull was inoculated with an infectious agent. On day 8 post-inoculation, there was a slight febrile reaction. By day 18, the peripheral lymph nodes were enlarged. The animal became progressively anorectic and emaciated and was euthanized and necropsied on day 25.

HISTOPATHOLOGIC DESCRIPTION: Slide A: Lymph node: Diffusely, greater than 90% of the cortical and medullary architecture is expanded and obscured by sheets of large, monomorphic “lymphoblastic cells” that multifocally infiltrate the capsule. Lymphoblasts have variably distinct cell borders, scant amphophilic cytoplasm, round to oval vesiculate nuclei, and 1-2 prominent nucleoli. Mitotic figures average 4 per 40X HPF. Rarely lymphoblasts contain 1-2 um diameter, round, basophilic, intracytoplasmic protozoal schizonts (Koch’s bodies). There is multifocal lymphocytolysis in both the cortex and paracortical areas. Cortical germinal centers are often replaced by small foci of fibrin and eosinophilic and karyorrhectic debris (necrosis). Small numbers of tingible body macrophages are randomly scattered within the cortex and medulla. Multifocally, cortical and medullary sinuses contain mild, draining hemorrhage admixed with low numbers of hemosiderin laden macrophages. The lymph node capsule is thickened up to 2x normal and expanded by low numbers of lymphocytes, plasma cells, fewer macrophages and hemorrhage.

MORPHOLOGIC DIAGNOSIS: Lymph node: Hyperplasia, lymphoblastic, diffuse, severe, with multifocal necrosis and intralymphoblastic protozoal schizonts, breed not specified, bovine.

HISTOPATHOLOGIC DESCRIPTION: Slide B: Cerebrum: There are multifocal to coalescing areas of necrosis within the cortex and extending into the medulla. Affected areas are often well demarcated and characterized by loss of grey and white matter and replacement by necrotic debris containing many gitter cells mixed with fewer lymphocytes and plasma cells, and moderate amounts of hemorrhage, fibrin, and proteinaceous fluid (infarct). Adjacent cortical and meningeal vessels are often partially occluded by fibrin thrombi and/or distended by numerous lymphoblastic cells. Lymphoblasts have distinct cell borders, scant amphophilic cytoplasm, and large, irregularly round, vesiculate nuclei with 1-2 prominent nucleoli. Mitotic figures among lymphoblasts average 1 per 40X HPF; and occasionally, lymphoblasts contain 1-2 um diameter, round, intracytoplasmic protozoal schizonts (Koch’s bodies). Diffusely, blood vessels within the neuropil and meninges are lined by hypertrophied, reactive endothelium; and rarely, vessel walls are disrupted by low numbers of neutrophils, macrophages, fibrin, edema and necrotic debris (vasculitis). Multifocally, Virchow-Robin space and the meninges are expanded up to 3x normal by moderate numbers of lymphocytes, plasma cells, and fewer macrophages and lymphoblasts. Diffusely, there is vacuolation of the gray matter (spongiosis), mild gliosis consisting of low to moderate numbers of astrocytes, fewer gemistocytic astroctytes, and rod-shaped microglia. Multifocally within the white matter there are dilated myelin sheaths and swollen, eosinophilic axons (spheroids).

MORPHOLOGIC DIAGNOSIS: Cerebrum: Encephalomalacia, acute, multifocal to coalescing, marked with multifocal thrombosis (infarct), moderate nonsuppurative meningoencephalitis, many intra and perivascular lymphoblasts, and intralymphoblastic protozoal schizonts, breed not specified, bovine.

ETIOLOGIC DIAGNOSIS: Lymph nodal and cerebral theileriosis

CAUSE: Theileria parva parva

SYNONYMS: East Coast Fever; turning sickness

GENERAL DISCUSSION:

Members of the genus Theileria are obligate intracellular hemoprotozoan parasites belonging to the phylum Apicomplexa, order Piroplasmida, and the family Theileriidae
Theileria infection reported in numerous bovid species (African buffalo, blue wildebeest, tsessebe waterbuck, Grant’s gazelle, klipspringer, blesbok, reedbuck, bushbuck, nyala, common eland, sable, roan, kudu, gray duiker, and American bison)
Bovine theileriosis is an OIE listed and reportable disease
East Coast Fever (ECF) is a tick-transmitted protozoal disease of cattle characterized by high fever and lymphadenopathy
The disease is confined to areas of eastern, central, and southern Africa where the principle vector, Rhipicephalus appendiculatus, occurs
Within endemic areas, mortality in susceptible cattle may approach 90-100%, especially in non-indigenous or naïve indigenous species

PATHOGENESIS:

Ticks feeding on the host transmit sporozoites within their saliva (3-4 days after feeding starts) > sporozoites enter lymphocytes within local lymph nodes and macrophages > macroschizonts are formed during schizogony which transform infected lymphocytes into lymphoblastic cells (leukocytic or tissue phase) > lymphoblastic cells proliferate, dividing synchronously with macroschizonts to infect daughter cells (clonal expansion) > 10 days post-infection, macroschizonts undergo merogony to produce merozoites (microschizonts) > merozoites are released and invade erythrocytes (erythrocytic phase) to form piroplasms which are infective to ticks
- Lympholysis and progressive anemia also occur during the acute phase when lymphocytes contain schizonts
In infected B-cells, T. parva induces c-Myc and activates anti-apoptotic protein Mcl-1; overexpression of c-Myc is important in certain B-cell lymphomas
Transformation of parasitized lymphocytes is reversible; upon drug-induced parasite death, parasitized lymphocytes stop proliferating and revert to a resting state or undergo apoptosis
Morbidity and mortality depend on the magnitude of the infected tick challenge, host susceptibility, and virulence of the infecting strain of parasite

TYPICAL CLINICAL FINDINGS:

Severity of clinical signs is dose-dependent and depends on the virulence of the infecting strain
- Acute: massive lympholysis and progressive anemia
- After 2-week incubation period, there is swelling of the parotid lymph node (the ear is the preferred feeding site for ticks), followed by high fever, anorexia, decreased milk production, and depression; in later stages, there is generalized enlargement of the superficial lymph nodes, nasal and ocular discharge, corneal opacity, and splenomegaly with diarrhea, dyspnea, recumbency, and coma in terminal stages
- Progressive leukopenia and anemia
- Abortions are common
- Turning sickness: Fatal condition in which parasitized lymphocytes accumulate and form thrombi within cerebrospinal vessels; characterized by circling in affected cattle

TYPICAL GROSS FINDINGS:

There is enlargement of lymphoid tissues, including Peyer’s patches; on cut section, lymph nodes have a red-brown cortex containing focal hemorrhages, and a dark red-brown medullary area

Cerebrum- acute: Multifocal infarcts, meningocerebral congestion, hemorrhage, and thrombosis, yellowish discoloration of brain, intraventricular hemorrhage
Cerebrum -chronic: Resorption of necrotic tissue results in intracerebral cystic spaces
- Splenomegaly in acute cases; shrunken spleen seen often with chronicity
- Ulcerative abomasitis; erosive or catarrhal enteritis
- Small gray-white patches (“pseudo-infarcts”) on surface of liver and kidney that bulge slightly (proliferative foci of perivascular lymphocytes)
- Severe pulmonary edema and congestion; copious froth in airways; hydrothorax and hydropericardium

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Lymphoid organs: In early stages, there is diffuse lymphoid hyperplasia (in part due to production of IL-2 by parasitized lymphocytes); with progression, there is a general loss of small lymphocytes which are replaced by numerous parasitized lymphoblasts containing intracytoplasmic schizonts (Koch’s blue bodies); in terminal stages, there is widespread, prominent lymphocytolysis, especially within germinal centers that is accompanied by hemorrhage and fibrinous exudate within the cortex of lymph nodes
Lymphocytic infiltrates in liver (periacinar and to a lesser extent, periportal) and kidney (perivascular and periglomerular)
Lung: Severe pulmonary edema and lymphoid infiltration of the alveolar septa
Bone marrow: In terminal stages, marrow is hypoplastic, and the remaining cells consist of parasitized, blastic lymphocytes and atypical erythroblasts
Turning sickness: Hemorrhagic infarcts in brain and spinal cord due to thrombi containing parasitized lymphocytes

ADDITIONAL DIAGNOSTIC TESTS:

Peripheral blood or lymph node smears stained with Giemsa may show 2-16 um diameter, macroschizonts (Koch’s blue bodies) which are considered diagnostic to identify Theileria spp.
With Romanowsky-type stains (including Giemsa), schizonts and piroplasms have pale blue cytoplasm with red chromatin granules, and in piroplasms only the chromatin occupies a single pole
Serology: ELISA, indirect immunofluorescent antibody test (IFAT), complement fixation, indirect hemagglutination
PCR required for species differentiation

DIFFERENTIAL DIAGNOSIS:

Gross and histological lesions resemble malignant catarrhal fever (MCF) (gammaherpesvirus; D-V15, S-V01, U-V02); presence of Koch’s blue bodies support ECF
- Hallmark features of MCF include fibrinoid necrotizing vasculitis in addition to lymphocytic and lymphoblastic proliferation (especially T-lymphocytes)
Jembrana disease: A lentiviral disease of Bali cattle and buffalo that causes lymphoblastoid and histiocytic infiltration of many organs
Heartwater (Ehrlichia ruminantium) – differential for pulmonary edema
Trypanosomiasis – differential for anemia, edema, and lymphadenopathy
Babesiosis/ anaplasmosis – differentials for anemia and fever
Bovine viral diarrhea (pestivirus)/ rinderpest (morbillivirus) – differentials for fever, diarrhea, and lymphoid necrosis
Lymphoma

COMPARATIVE PATHOLOGY:

T. parva lawrenci causes corridor disease of calves in southern Africa; endemic in buffalo
- T. lestoquardi: Most virulent species; occurs in Asia, the Middle East, and parts of Africa and Europe
  - T. parva bovis causes January disease in cattle in east and central Africa; infection is more acute than classical ECF; only occurs from January to March
  - T. mutans causes Tzaneen disease in cattle, which is mild with few clinical signs
  - T. annulata causes Mediterranean or tropical theileriosis across north Africa, central Asia, and southern Europe; usually benign; transmitted by Hyalomma spp vector; the erythrocytic and lymphatic forms are both important for T. annulata
  - T. hirci causes disease similar to ECF in sheep and goats in north Africa, the Middle East, and southern Eurasia
  - T. ovis and T. separate: is the small ruminant equivalent of T. mutans and is fairly benign; same geographic distribution as T. hirci
  - T. cervi infects white tailed deer in the US; does not cause significant disease; transmitted by lone star tick (Amblyomma americanum) which serves as both vector and definitive host
  - T. buffeli: Missouri, Texas, North Carolina, and Michigan; also widely distributed in East Asia and is relatively benign; piroplasms present in clinically ill cows and rare in subclinical cases; anemia
  - T. equi: severe regenerative hemolytic anemia and jaundice in acute cases; immunocompromised animals and foals are most susceptible; histologically there is renal tubular necrosis and disseminated hemosiderin-laden macrophages; transplacental infection may cause abortion; initial infection invades mononuclear cells and merozoites are released to infect erythrocytes; there are sporadic outbreaks in Florida and Texas
  - T. annae: Reported in red fox in Croatia and Canada; previously classified as Babesia gibsoni which is now broken into 5 different taxa; in Spain 21 dogs were infected and were anemic with signs of protein-losing glomerulopathy
  - Theileria sp (sable) or provisionally Theileria sp. (sable-like): causes infection +/- disease in hippotraginid antelope (roan and sable antelope); causes parasitized and nonparasitized mononuclear leukocyte infiltration (mostly histiocytes, monocytes, and lymphocytes) in the liver, lung, kidney, adrenal gland, and heart (less frequently in the spleen and lymph nodes; may cause Theileria sp. (sable)-associated calf mortalities; IHC is most useful in diagnosing these species as PCR cannot determine significance in disease
  - T. orientalis Ikeda: newly identified agent of bovine infectious anemia in the United States; commonly coinfected with Anaplasma marginale; identified in Virginia and suspected to be transmitted by ticks other than those that transmit A. marginale
  - Cytauxzoon felis affects felids in the United States, reproduces by fission in red cells and by schizogony in macrophages; whereas Theileria sp. are found in red cells and reproduce by schizogony in lymphocytes
  - Case report of Theileria in giraffe (Oosthuizen et al. 2009)

References:

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Clift SJ, Martí-Garcia B, Lawrence JA, Mitchell EP, Fehrsen J, Martínez J, Williams JH, Steyl JCA. Theileriosis in naturally infected roan antelope (Hippotragus equinus). Vet Pathol. 2022 Nov;59(6):1031-1046.

Clift SJ, Collins NE, Oosthuizen MC, Steyl JCA, Lawrence JA, Mitchell EP. The pathology of pathogenic theileriosis in African wild artiodactyls.Vet Pathol. 2020:57(1):24-48.

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