JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-P01

 

Signalment (JPC #2209517): Dog

HISTORY: This dog had muscle weakness.

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle: Multifocally effacing and replacing myofibers are discrete, up to175µm diameter aggregates of neutrophils, macrophages, and fewer lymphocytes and plasma cells (pyogranulomas) with frequent intrahistiocytic, 3-4µm, spherical protozoal merozoites. Multifocally separating and compressing myocytes are multilamellar mucopolysaccharide cysts (“onion skin cysts”) that are up to 200 µm in diameter, composed of concentric lamellations of amphophilic, mucinous material further surrounded by 1-3 layers of spindle cells, all centered on a macrophage with abundant foamy to granular cytoplasm, a single nucleus with a prominent nucleolus, and rarely an intracytoplasmic, round to oval, 6-10µm, eosinophilic protozoal trophozoite. Rarely within the center of the cyst, there is a 75µm meront containing numerous 4-5 µm diameter merozoites. Diffusely there is marked loss of myofibers; remaining myofibers are either shrunken (atrophy), have pale, swollen, vacuolated sarcoplasm (degeneration), have hypereosinophilic, fragmented sarcoplasm with loss of cross striations and nuclear pyknosis or karyolysis (necrosis) and occasional granular basophilic sarcoplasm (mineralization), have increased sarcoplasmic basophilia with multiple central nuclei (regeneration), or are rarely multinucleated, misshapen, regenerative myocytes (muscle giant cells, abortive regeneration). The perimysium and endomysium are diffusely expanded up to 50 µm by abundant fibrosis and edema with few scattered macrophages, neutrophils, lymphocytes, and plasma cells.

MORPHOLOGIC DIAGNOSIS: Skeletal muscle: Myositis, pyogranulomatous, chronic, multifocal to coalescing, moderate, with myocyte degeneration, necrosis, regeneration, and atrophy, fibrosis, and numerous mucopolysaccharide (onion skin) protozoal cysts and merozoites, breed unspecified, canine.

ETIOLOGIC DIAGNOSIS: Hepatozoon myositis

CAUSE: Hepatozoon americanum

CONDITION: American canine hepatozoonosis

GENERAL DISCUSSION:

• Hepatozoon americanum is a tick-borne apicomplexan protozoan (family Hemogregarinidae)
• First recognized in the U.S. in 1978 on the Gulf of Mexico coast; this organism is now diagnosed in dogs throughout much of the southeastern U.S and is spreading to other states, as the range of its tick vector spreads
• Clinical signs, laboratory abnormalities, pathological lesions, tissue tropism, parasite morphology, and tick vectors differ from disease associated with canis; H. americanum can cause debilitating disease even without concurrent immunosuppression (unlike H. canis)
• Prominent features of disease: Skeletal and cardiac myositis; disseminated periosteal bone proliferation; massive neutrophilia
• Parasitized cells are of leukocyte origin; when in muscle, parasites are in host cells (monocytes/macrophages) between muscle fibers and are induced to produce mucopolysaccaride which forms “onion skin” lamellations

LIFE CYCLE

• Two hosts required for life cycle:
• Definitive host: Gulf Coast tick (Amblyomma maculatum)
• Intermediate host: Dogs and other mammals, birds, reptiles
• Immature ticks feed on infected intermediate host > ingest leukocytes containing gamonts > sexual reproduction in tick > oocysts (contain numerous sporocysts, each with 10-26 sporozoites) > tick molts into adult > dog ingests infected tick > sporozoites penetrate intestinal wall > enter circulation and move to muscle and target tissues > parasite develops in macrophages between myocytes > layers of mucopolysaccharides laid down around host cell, forming "onion-skin" cyst > asexual reproduction > when mature, cyst ruptures > merozoites released > severe inflammation (neutrophils and monocytes) > infected inflammatory cells return to circulation > spread to distant sites or become gamonts > ingested by ticks

PATHOGENESIS

• Transmitted via ingestion of the definitive host (Amblyomma maculatum), predation with ingestion of cystozoites from infected intermediate host tissues, and probably via intrauterine transmission; NOT transmitted via tick bite
• Pyogranulomas/pyogranulomatous myositis stimulated by rupture of meronts and release of merozoites; single-celled cyst does not stimulate immune response
• Humoral factors, rather than local, are suggested by widespread, symmetrical distribution of periosteal new bone growth
• Chronic infection > persistent antigenic stimulation > vasculitis, proliferative glomerulonephritis, amyloid deposition, uveitis
• Cysts can lie dormant for long periods of time; cycles of asexual reproduction continue when activated (waxing and waning of disease)

TYPICAL CLINICAL FINDINGS

• H. americanum causes a debilitating and usually fatal disease if untreated; chronic infection may have waxing and waning or progressively worsening clinical signs
• Clinical signs due to myositis and periosteal bone formation; may mimic those of meningitis or discospondylitis; gait abnormalities; pain; weight loss; muscle wasting (may be severe); depression; mucopurulent ocular discharge; pyrexia
• Clinical pathology: Leukocytosis (may be severe or leukemoid response)– ranges from 20,000 to 200,000; characterized by mature neutrophilia +/- left shift); CK may be normal or mildly elevated

TYPICAL GROSS FINDINGS

• Cachexia, muscular atrophy
• Disseminated, symmetric, periosteal new bone formation that most frequently and most severely involves the diaphysis of long bones (femur, humerus, radius, ulna), vertebral column, and pelvis
• Pyogranulomas: 1-2 mm diameter white/tan foci scattered throughout muscle and other tissue

TYPICAL LIGHT MICROSCOPIC FINDINGS

• Muscle biopsy: Identification of cysts, meronts, and pyogranulomas with zoite-containing leukocytes (macrophages) is diagnostic
• Myositis with muscle atrophy, necrosis, inflammatory cell infiltrates
• "Onion-skin cysts" in skeletal and cardiac muscle: Round to oval with single round basophilic nucleus surrounded by concentric layers of light blue staining laminar membranes (nonsulfated acid mucopolysaccharide); 250 to 500 µm in diameter; may have developing meront inside
• Bone: Periosteal new bone growth primarily at the proximal diaphyseal regions of proximal long bones, as well as vertebrae and ilium; characterized by woven bone forming trabeculae perpendicular to the cortex
• Similar to hypertrophic osteodystrophy microscopically; differs in distribution of lesions
• Kidneys: Glomerular amyloidosis; may lead to progressive renal insufficiency and proteinuria (nephrotic syndrome)

ULTRASTRUCTURE

• Apicomplexan protozoa: Conoid, rhoptries, micronemes, micropore, pellicle with subpellicular microtubules
• Unique to Hemogregarinidae: Rhoptries both anterior and posterior to nucleus

ADDITIONAL DIAGNOSTIC TESTS

• Radiographs: Characteristic appearance of periosteal proliferation at margins of affected bones
• Blood smears: Rarely detected in peripheral blood smears, buffy coat examination may increase likelihood of detection; oval to elliptical, pale blue, 9 x 4µm gamonts (gametocytes) within leukocytes (monocytes and neutrophils) may peripheralize the nucleus and distort the cell border
• Serology, IFA, PCR, IHC

DIFFERENTIAL DIAGNOSIS:

• Other protozoan parasites:
• Sarcocystosis, Toxoplasma gondii, Neospora caninum: Muscle lesions develop from cysts with a myriad of zoites but no “onion-skin cysts”
• Hepatozoon canis: Carnivorous mammals in Africa, southern Europe, Asia (including Middle East), islands of Pacific and Indian Oceans
• No muscle lesions; form classic “spoke wheel” schizonts in hemapoietic tissue
• Infects hemolymphatic tissues and can cause a mild anemia and lethargy
• Neutrophil is the favored host cell and zoites can be seen histologically
• Rhipicephalus sanguineus (brown dog tick) and other ticks are the invertebrate host
• Trypanosoma cruzi: pseudocysts with amastigotes containing a parallel kinetoplast, in cardiac muscle, no “onion-skin cysts”
• Other causes of periosteal proliferation:
• Hypertrophic osteopathy (M-N05): Very similar histologically and grossly; lesion distribution can help differentiate; hypertrophic osteopathy typically affects radius, ulna, tibia, metacarpals and metatarsals (as opposed to proximal limb and axial bones)

COMPARATIVE PATHOLOGY:

Hepatozoon spp. in other veterinary species:

• Wild canids: Hepatozoon spp infections reported in black-backed jackals, African wild dogs, golden jackals, crab-eating foxes, red foxes, Pampas gray foxes, coyotes
• Cats: Hepatozoonosis reported but species not determined
• Rats: muris
• Domestic rabbits: cuniculi; gametocytes in peripheral blood, schizonts in spleen; arthropod host suspected to be flea; minor pathologic significance
• Kiwi: kiwii, gametocytes are present within monocytes, Ixodes anatis (kiwi tick) is thought to be the vector, but this has not been verified
• Bats: Hepatozoon spp have been reported in free ranging bats, no associated clinical disease
• americanum or a similar organism has also been described in coyotes, bobcats, and ocelots in the southeastern U.S.
• Hepatozoon also reported in hyenas, albatrosses, storm petrels, reptiles (including experimental infection in Easter fenced lizards, Great Plains skinks, and crevice spiny lizards), amphibians (anurans, caudate, and caecilians)

REFERENCES:

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