JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2019265): 3-year-old post-partum Suffolk ewe
HISTORY: This ewe delivered 2 lambs estimated to be 10 days premature and dead in utero for about one day. There were no gross lesions in the fetuses. The cotyledons had multiple 1-2 mm white nodules. The intercotyledonary placenta was normal.
HISTOPATHOLOGIC DESCRIPTION: Placenta, cotyledon: Affecting 70% of this section, there is both coagulative and lytic necrosis of the cotyledon characterized by complete loss of villar trophoblasts and replacement by eosinophilic cellular and karyorrhectic debris, aggregates of a deeply basophilic material (mineral), scattered hemorrhage and fibrin. Multifocally, low numbers of trophoblasts adjacent to necrotic foci are expanded by a parsitophorous vacuole containing clusters of approximately 2x3 um oval, pale basophilic, apicomplexan tachyzoites. Diffusely, less affected villi are expanded by edema and low numbers of lymphocytes, fewer plasma cells and macrophages and occasional degenerate neutrophils. Multifocally, the chorioallantoic connective tissue is expanded by fibrin and low numbers of similar inflammatory cells.
MORPHOLOGIC DIAGNOSIS: Placenta, cotyledon: Placentitis, necrotizing, subacute, multifocal to coalescing, moderate, with intratrophoblastic apicomplexan tachyzoites, Suffolk ewe, ovine.
ETIOLOGIC DIAGNOSIS: Placental toxoplasmosis
CAUSE: Toxoplasma gondii
- Apicomplexan, obligate intracellular, parasite that causes disseminated disease, central nervous system infections and abortions in many species, except cattle
- Felids are definitive hosts; all mammals and birds can be intermediate hosts
- Sporozoites and tachyzoites are readily destroyed; tissue cysts (bradyzoites) are resistant and perpetuate the disease
- Tachyzoites (trophozoites) are rapidly dividing forms and occur in free groups
- Bradyzoites multiply slowly and are found in tissue cysts
- Oocysts contain infective sporozoites after being shed
- Contamination of feed by cat feces is a common transmission route
- Cats are infected by eating meat containing tissue cysts
- Sexual replication occurs in feline intestinal epithelium, especially the ileum
- Unsporulated oocysts pass in feces and sporulate in 1-5 days
- Sporulated oocysts are infectious to intermediate hosts and cats
- After ingestion, sporozoites excyst, multiply intracellularly in the intestines and related lymph nodes by endodyogeny and form tachyzoites that spread to other tissues via blood and lymph.
- Parasitemia develops in 3-4 days
- Tachyzoites actively penetrate host cell plasmalemma and reside within a parasitophorous vacuole derived from host and parasite
- Cell to cell spread continues until T-cell mediated immunity develops
- Intracellular tissue cysts form after 1-2 weeks and persist for months to years
- Ingestion of contaminated feces by the ewe results in parasitemia; the organism passes to the caruncle, then the trophoblast and then the fetus; it replicates in the caruncular septa, producing foci of necrosis
- gondii is able to infect all types of cells because it binds laminin (an extracellular matrix protein) and then binds to laminin receptors on the target cell
- Secretory organelles, called rhoptries, are important for cellular invasion; rhoptry membranes fuse with the anterior limiting membrane and a lytic product is secreted from fusion rosettes and facilitates invasion into the cell
- gondii has both catalase and glutathione peroxidase activity and is resistant to hydrogen peroxide, but is susceptible to hydroxyl radical and singlet oxygen (respiratory burst of inflammatory cells)
- In parasitophorous vacuoles the protozoan initiates the production of the anti-inflammatory cytokines IL-10 and transforming growth factor-β (TGF-β), which inhibit the production of proinflammatory cytokines IL-12 and TNF-α
- A recent study (see reference one below) suggests based on lesions found within the placentomes, different mechanisms are involved in the pathogenesis between ‘classical’ and ‘acute phase” ovine toxoplasmosis abortion
- Classical is associated with necrosis with variable inflammation whereas acute phase infection may involve infarction and thrombosis of maternal vessels
TYPICAL CLINICAL FINDINGS:
- Effect on pregnancy depends on the stage of gestation when the infection occurs
- Infection in early or mid-gestation results in fetal death with resorption or mummification
- Occasionally lambs infected in mid-pregnancy survive to term but are stillborn or are weak and die shortly after birth
- A fetus infected in late pregnancy develops an immune response and is born live, infected, and immune
- Infected ewes rarely show clinical signs and do not abort in subsequent pregnancies
- Small percentage of fetuses develop cerebral leukoencephalomalacia secondary to the anoxia from placentitis
TYPICAL GROSS FINDINGS:
- Aborted fetuses with no gross lesions
- Placental lesion usually confined to cotyledon
- Characteristic appearance: bright to dark red cotyledons may contain numerous 1-3 mm diameter, soft white nodules or flecks of mineral on villi.
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Foci of necrosis with mineralization and mild mononuclear cell infiltration in trophoblastic epithelium, villous stroma and endometrium; sloughing of trophoblasts
- 2-4um fusiform basophilic tachyzoites may be free in trophoblasts or in cysts; encysted zoites may also be found in endometrium
- 95% of infected fetuses have mild nonsuppurative encephalitis with few scattered foci of necrosis and glial and mononuclear infiltrates
- Focal areas of leukoencephalomalacia without cellular infiltration and toxoplasma cysts at the periphery of lesions
- Multifocal necrotizing nonsuppurative hepatitis, pneumonia, myositis, myocarditis and nephritis
- Zoites reside within a parasitophorous vacuole
- Pellicle (outer membrane) consists of 3 membranes: A plasmalemma and two closely applied membranes that form an inner membrane complex.
- At the anterior surface is a cylindrical cone (conoid) consisting of microtubules wound like a spring and is used to probe host cell surface prior to entry
- Rhoptries are club-shaped excretory organelles between the anterior tip and nucleus that secrete a proteolytic enzyme used in host cell penetration
- Micronemes are rod-shaped structures found at the anterior end
ADDITIONAL DIAGNOSTIC TESTS:
- Fluorescent antibody or immunoperoxidase on tissue sections
- Antibody tests are of limited value.
For abortion in sheep:
- Neospora caninum: Foci of necrosis in cotyledons with normal intercotyledonary regions; prominent multifocal encephalitis with gliosis and necrosis in fetus
- Chlamydophila abortus: Necrotizing placentitis with vasculitis affecting intercotyledonary areas; leathery thickening of chorioallantois
- Coxiella burnetii: Gross lesions similar to Chlamydophila abortus, usually without vasculitis
- Brucella ovis: Leathery thick intercotyledonary regions with thick brown exudate that cover chorionic surface; vasculitis
- Campylobacter fetus: Nonspecific edematous changes in the fetus with distinctive targetoid hepatic necrosis up to 2 cm in diameter; exudate covering placenta
- Listeria monocytogenes: Necrotizing and suppurative placentitis of cotyledons and intercotyledonary areas; gram-positive bacilli in trophoblasts.
- Pigs: Sporadic outbreaks; embryonic death; sows with fever and mild illness; hepatitis and lymphadenitis in congenitally infected piglets
- Alpaca: Single case report of toxoplasmosis-associated abortion
- Lemur: Single case report of localized infection causing placentitis, stillbirths and disseminated fetal infection
- Cattle are resistant and rarely abort or show signs of disease; usual cause of abortion is due to Neospora caninum with no useful identifying gross lesions
- Benavides J, Fernandez M, Castano P, Ferreras MC, Ortega-Mora L, Perez V. Ovine Toxoplasmosis: A New Look at its Pathogenesis. J Comp Path. 2017 Jul:157(1):34-38.
- Dubey JP, Johnson JE, Hanson MA, Pierce V. Toxoplasmosis-associated abortion in an alpaca (Vicugna pacos) fetus. J Zoo Wildl Med. 2014;45(2):461-464.
- Foster RA. Female reproductive system and mammae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th St. Louis, MO: Elsevier; 2017:1183.
- Gutierrez J, O’Donovan J, Proctor A, et al. Application of quantitative real-time polymerase chain reaction for the diagnosis of toxoplasmosis and enzootic abortion of ewes. J Vet Diagn Invest. 2012;24(5):846-854.
- Hazlett MJ, McDowall R, DeLay J, et al. A prospective study of sheep and goat abortion using real-time polymerase chain reaction and cut point estimation shows Coxiella burnetii and Chlamydophila abortus infection concurrently with other major pathogens. J Vet Diagn Invest. 2013;25(3):359-368.
- Juan-Salles C, Mainez M, Marco A, Sanchis AM. Localized toxoplasmosis in a ring-tailed lemur (Lemur catta) causing placentitis, stillbirths, and disseminated fetal infection. J Vet Diagn Invest. 2011;23(5):1041-1045.
- Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis: Elsevier; 2016:420-421.
- Zachary JF. Mechanisms of microbial infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th St. Louis, MO: Elsevier; 2017:238-239.