JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1374690): Cat, age and breed unspecified
HISTOPATHOLOGIC DESCRIPTION: Lung: Diffusely, alveolar septa and, to a lesser extent, peribronchial and peribronchiolar interstitium are thickened up to 10 times normal by numerous macrophages, fewer neutrophils and lymphocytes, fibrin, increased clear space (edema), and moderate vascular congestion. Multifocally, alveolar septa are discontinuous with replacement by eosinophilic and karyorrhectic debris, fibrin, edema and similar inflammatory cells (alveolar septal necrosis). There is moderate type II pneumocyte hyperplasia. Alveolar lumina, and to a lesser extent the bronchial and bronchiolar lumina, frequently contain numerous degenerate and nondegenerate neutrophils, macrophages, fewer lymphocytes, erythrocytes (hemorrhage), rare multinucleate giant cell macrophages, eosinophilic cellular and karyorrhectic debris (necrosis), eosinophilic fibrillar beaded material (fibrin), and edema. Occasionally, alveolar macrophages and type II pneumocytes are distended up to 3 times normal by an intracytoplasmic parasitophorous vacuole containing numerous 2-4 um, round to fusiform, basophilic tachyzoites. Tachyzoites are rarely free within alveolar lumina adjacent to disrupted alveolar macrophages. Multifocally, there are perivascular and peribronchial/bronchiolar aggregates of lymphocytes and plasma cells (mild BALT hyperplasia), as well as perivascular edema, and multifocal mesothelial cell hypertrophy (reactive mesothelium).
MORPHOLOGIC DIAGNOSIS: Lung: Pneumonia, interstitial, necrotizing, diffuse, severe, with type II pneumocyte hyperplasia, edema, and intrahistiocytic and intraepithelial tachyzoites, etiology consistent with Toxoplasma gondii, breed unspecified, feline.
ETIOLOGIC DIAGNOSIS: Pulmonary toxoplasmosis
CAUSE: Toxoplasma gondii
- Apicomplexan obligate intracellular coccidian parasite that causes disseminated disease, central nervous system infections, and abortions in a wide range of intermediate hosts
- One of the most ubiquitous organisms; like Neospora caninum, it is unique among protists in its ability to parasitize a wide range of hosts and tissues, and its geographic distribution is increasing due to climate change
- Felids are the only definitive hosts (intraintestinal life cycle); whereas many animals (mammals, fish, amphibians, reptiles, birds) can serve as intermediate hosts (extraintestinal life cycle)
- Felids are unique in that they can serve both as definitive and intermediate host
- Clinical disease is seen most commonly in young, immunologically immature animals and in immunocompromised animals
- Three forms identified in young dogs: Generalized, CNS, and radiculoneuritic
- Definitive host, intestinal/enteroepithelial/coccidian cycle (felids):
- Ingestion of infected prey tissue (tachyzoites or cysts containing bradyzoites), or ingesting sporulated oocysts in contaminated food or water
- Sporozoites released from oocyst or bradyzoites infiltrate intestinal epithelial cells and undergo five recognized asexual stages (schizont cycle) and eventually form a male microgamont or female macrogamont
- In the sexual phase (gamont cycle), microgamonts are released and fertilize a macrogamont to form an oocyst
- Oocysts (unsporulated, noninfective) are shed in feces 3-10 days after ingesting bradyzoites, 13 days after ingesting tachyzoites, and 18 days after ingesting oocysts
- Sporulation (development of infective sporozoites inside the oocyst) takes 1 to 5 days, depending on environmental conditions
- Intermediate hosts, extraintestinal cycle (many mammals and birds, including felids):
- Transmission: Ingestion of infected prey tissue (tachyzoites, or cysts containing bradyzoites), ingestion of sporulated oocysts in contaminated food or water, or transplacental (common in sheep and goats)
- Sporozoites released from oocyst or bradyzoites released from tissue cyst within the lumen of the small intestine invade enterocytes and divide asexually (endodyogeny) within a parasitophorous vacuole to form tachyzoites (rapidly dividing; form associated with acute disease)
- Tachyzoites lyse the enterocyte and spread locally to adjacent enterocytes, the lamina propria, and monocytes-macrophages, and to local and regional lymphoid tissues (e.g. Peyer’s patches, mesenteric lymph nodes)
- Tachyzoites spread systemically via leukocyte trafficking (lymphocytes, macrophages, and granulocytes) or free in plasma via blood and lymphatics, and can multiply and survive intracellularly in almost any tissue in the body (e.g. lungs, lymphoid system, liver, heart, skeletal muscle, pancreas, intestine, eyes, nervous system) within a parasitophorous vacuole, destroying the host cell with cell-to-cell transmission à focal necrosis
- With chronicity and the development of an antibody response, tachyzoites transform into slow-growing/replicating bradyzoites within tissues; may proliferate for an indefinite number of generations
- Transplacental infection can occur when previously uninfected hosts are exposed and infected during pregnancy; the organism multiplies in the placenta and spreads to fetal tissues (mechanism of spread is unknown)
- Transmission efficacy varies based on life stage ingested: cats can become infected after ingestion of one bradyzoite, but it takes up to 1,000 oocysts
- T. gondii tachyzoites express glycosylphosphatidylinositol-linked surface proteins (SAGs, e.g. SAG1 and SAG3) that function in target cell attachment and immune modulation, and may also cause direct injury to enterocytes; SAG1 binds to enterocyte laminin, lectin, and SAG receptor proteins
- Rhoptries in apex of tachyzoites produce lytic substances that facilitate cell invasion and form a parasitophorous vacuole,
- Parasitophorous vacuole: A “hybrid” membrane within the host cell that helps the organism avoid detection by the immune system, avoids fusion with macrophage lysosomes, and promotes the production of anti-inflammatory cytokines IL-10 and TGF-β (which inhibit production of IL-12, IFN-γ, and TNF-α)
- Low levels of IFN-γ à decreased macrophage activation à predisposition to systemic toxoplasmosis and other diseases (e.g. canine distemper, ehrlichiosis, and lymphosarcoma in dogs)
- Bradyzoites have been shown to revert to tachyzoites in the absence of IFN-γ
- Necrosis due to direct cell damage from organism multiplication and inflammatory cytokine-mediated destruction
- T. gondii can cross barrier systems (intestinal mucosa, blood-brain barrier, blood-retinal barrier, and placenta) using parasite motility (linear myosin, F-actin filaments, and gliding-associated proteins) and infect endothelial cells > vasculitis > ischemic necrosis
- Humoral and cell mediated immunity cause formation of tissue cysts within the CNS, skeletal and cardiac muscle
- Type III hypersensitivity may be involved in vascular injury and type IV hypersensitivity may be involved in tissue necrosis
- Immunosuppression of latently infected hosts allows cysts to rupture with reactivation of acute disease
- CNS infection occurs hematogenously; neurons and astrocytes are the eventual targets
TYPICAL CLINICAL FINDINGS
- Enteroepithelial cycle (cat): Self-limiting diarrhea or asymptomatic
- Extraintestinal cycle: Infection is widespread (determined by serologic studies), but disease is sporadic and rare unless immunocompromised (exception: an important cause of abortion in sheep and goats); clinical signs depend on organs affected:
- CNS infection: Neurologic signs vary with age, species, and lesion localization; may include seizures, paresis, weakness, tremors, depression, circling, blindness, and ataxia
- Radiculoneuritis: Affects puppies less than 3 months old; acute paraparesis with rigid extension of hind limbs, loss of patellar and withdrawal reflexes, pain, and muscle wasting
- Disseminated disease: Fever, lethargy, anorexia, ocular and nasal discharges, and respiratory distress
- Abortion: common in sheep and goats; outcome depends on maternal and fetal immune response and stage of gestation: early gestation results in abortion, late infection results in vertical transmission
TYPICAL GROSS FINDINGS
- Hallmarks: Interstitial pneumonia, focal hepatic necrosis, lymphadenitis, myocarditis, and nonsuppurative meningoencephalitis
- Multifocal necrosis is present in multiple organs (lung, liver, kidneys, intestine, pancreas, placenta, skeletal muscle, heart) and thought to be related to the rapid replication of tachyzoites
- Lung: Most commonly identified in kittens; pinpoint white foci scattered throughout the lung or diffuse interstitial pneumonia
- Splenomegaly and lymphadenomegaly
- Eye: granulomatous chorioretinitis
- Aborted fetus: T. gondii produces pathognomonic lesions in the placenta of sheep and goats, consisting of multifocal necrosis and mineralization of cotyledonary villi; individual cotyledons contain 1-2 mm yellow to white foci of necrosis anywhere on the villus from the tip to the base; intercotyledonary chorioallantois may be edematous or unaffected
TYPICAL LIGHT MICROSCOPIC FINDINGS
- The finding of tachyzoites and/or cysts in association with areas of coagulative necrosis in one or more organs is highly suggestive of toxoplasmosis
- Tachyzoites usually evident in alveolar macrophages, and may be in bronchiolar epithelium and vessel walls
- Diffuse interstitial pneumonia: Alveolar septa thickened by predominantly mononuclear inflammatory cells (fewer neutrophils, eosinophils), alveoli filled with macrophages and fibrinous to fibrinonecrotic exudate +/- occasional hyaline membranes, scattered foci of necrosis of alveolar septa, bronchiolar epithelium, and blood vessels
- Subacute: Abundant type II pneumocyte hyperplasia, or “epithelialization of alveoli”; may be sufficiently abundant to appear adenomatous
- Other tissues (liver, lymph nodes, spleen, pancreas, heart, skeletal muscle): Intracellular tachyzoites, variably sized necrotic foci, with variable presence of inflammation (predominantly mononuclear)
- CNS: Nonsuppurative meningoencephalitis of gray and white matter (especially perivascular), vasculitis, hemorrhage, necrosis, tissue cysts, leptomeningitis; gliosis and tissue cysts in chronic cases
- Radiculoneuritis: Lymphoplasmacytic and histiocytic inflammation, most severe in the roots of the lumbosacral spinal cord
- Eye: Granulomatous chorioretinitis
- Placenta: Multifocal cotyledonary necrosis +/- mineralization with rare organisms within trophoblasts
- Aborted fetus: 95% of fetuses will have a mild, nonsuppurative encephalitis; occasional leukoencephalomalacia due to anoxia
- Tachyzoites present within a parasitophorous vacuole, 4-6 um diameter, contain an anterior apical complex with a polar ring, conoid, micronemes, and rhoptries
- Also contain a nucleus, amylopectin granules, microtubules, and dense bodies
ADDITIONAL DIAGNOSTIC TESTS
- Cytology: tachyzoites may be detected in the acute phase
- Electron microscopy
- Immunohistochemistry- cross reactivity with Neospora antibodies possible
- Serology utility is limited
- Neospora caninum: Requires immunohistochemical techniques or electron microscopy to differentiate from Toxoplasma organisms; may or may not have a parasitophorous vacuole; N. caninum cysts are found only in the CNS
- Neospora hughesi: Specific to horses, whereas T. gondii is usually not clinical in horses
- Encephalitozoon cuniculi: Pseudocyst is larger (up to 120um, Toxoplasma is 60um), spores are acid fast (Toxoplasma is not), gram positive (Toxoplasma is gram negative), necrosis is not a common finding
- Leishmania and Trypanosoma spp.: Tachyzoites with a kinetoplast and a distinct nucleus, often seen together as two basophilic dots
- Histoplasma sp.: Organisms are about the same size as Toxoplasma but have a PAS-positive and GMS-positive cell wall, and found only within macrophages/histiocytes; Toxoplasma is PAS-positive only
- Pneumocystis carinii: Much smaller and almost always found in pulmonary alveoli
- Nosema spp.: Especially in the brain, may resemble bradyzoites of Toxoplasma but will have gram positive cyst walls (Toxoplasma is gram negative)
- Sarcocystis sp.: Requires Immunohistochemical techniques or electron microscopy to differentiate from Toxoplasma organisms; ultrastructurally, zoites of Sarcocystis are not within a parasitophorous vacuole, and merozoites lack rhoptries; associated with limited tissue necrosis
- Hammondia hammondi: Has an entero-epithelial cycle in cats, but is not found in the lamina propria and does not cause extraepithelial infection in cats
- Toxoplasma gondii can infect a wide variety of animals as intermediate hosts (extraintestinal life cycle) including fish, amphibians, reptiles, birds, human beings and many other mammals
- Dogs: Most frequently seen in puppies; characterized by neurologic signs, gastrointestinal disease with diarrhea or pneumonia; disease is triggered by immunosuppression, such as with canine distemper virus infection
- Sheep and Goats: Important cause of abortion
- Swine: Pneumonia, encephalitis, abortion
- New World monkeys and Australian marsupials are highly susceptible and acute disease can be fatal
- Old World monkeys, rats, cattle and horses seem highly resistant
- Wildlife: Infection has been described in many wildlife species including Atlantic bottle‑nosed and Spinner dolphins, and is implicated as an important threat in some endangered species
- Humans: Evidence linking chronic infection with several psychiatric disorders
- Zebrafish: A new in vivo model of T. gondii infection when fish are adapted to 37 degrees C; disease is similar to mammals, with parasites developing in the somatic muscle, heart, liver, spleen, kidney, and brain
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