4-year-old, castrated male, crioulo horse (Equus caballus).
During the summer of 2003, approximately 200 horses on 3 bordering farms, as well as 10 mares from a fourth farm that were sent to one of the 3 farms for breeding, were maintained near the banks of a river. Farm employees reported that capybaras (Hydro-choerus hydrochaeris) were affected by a similar disease and died at the time horses were affected but we were unable to confirm this information. However, we could confirm that several horses in one of the farms had been treated with a common needle early in the outbreak. Approximately one month after their arrival in the farm, many of the horses presented with marked weight loss, muscle atrophy, and incoordination. Within 2 to 3 months, and extending through the rest of the year and into 2004, approximately 100 horses died. We did evaluate clinically and necropsied 15 of these horses, 22 of which presented a chronic wasting form of the disease. The horse of this report is one of a group in which the onset of disease was insidious and characterized by progressive weight loss (despite voracious appetite), lethargy, incoordination, gait instability (wobbling) involving the pelvic limbs (mal das cadeiras = disease of the hip), atrophy of heavy muscles of the hindquarters, difficulty in rising, muscle weakness, pallor of mucous membranes, subcutaneous edema of the ventral portions of the trunk and limbs. Additionally to above described signs that ran a course of 14 days, encephalic neurologic clinical signs including marked ataxia (Fig. 1), blindness, circling, hyperexcitability, somnolence, proprioceptive deficits, head tilt, and paddling movements were observed and ran a course of 20 days.
There were marked hindquarter muscle atrophy, splenomegaly, and lymphadenomegaly. Gross lesions in the brain included asymmetric swelling of the cerebral hemispheres with flattening of gyri. The white matter of the parietal, temporal, and frontal lobes was unilaterally yellow, gelatinous, and friable due to severe edema and malacia. Similar lesions were observed in the basal nuclei, thalamus, and mesencephalon.
Histologically, lymphoid organs had marked follicular hyperplasia, erythrophagocytosis, and hemosi-derosis. In the liver there was moderate lympho-plasmacytic periportal hepatitis, Kupffer cell hypertrophy, and hemosiderosis.
The brain lesions affected mainly the white matter and were characterized by moderate to severe perivascular lymphoplasmacytic menin-goencephalitis, edema and necrosis. Lympho-cytes and plasma cells, often with intracy-toplasmic Russell bodies (Mott cells), greatly expanded the Virchow-Robin spaces and extended into the surrounding neuropil. Lesions in the spinal cord tended to wane from cranial to caudal. Organisms of T. evansi were not detected in the HE stained sections of brain, but numerous T. evansi organisms or fragments of these organisms were detected in formalin-fixed, paraffin-embedded sections of the brain by immunohistochemistry through anavidin-biotin-peroxidase complex immunoperoxidase method using rabbit anti-T. evansi, diluted 1:1000 as the primary antibody (the immunohistochemistry tests were performed at the Prairie Diagnostic Service, University of Saskatchewan, Western College of Veterinary Medicine). The parasites were observed in the perivascular spaces an in the neuropil.
Lymphoplasmacytic encephalitis, moderate to severe, 4-year-old, castrated male, crioulo, Equus caballus, horse.
Etiologic diagnosis: Protozoal encephalitis
Etiology: Trypanosoma evansi
Name of the condition: Trypanosomiasis
Hematologic findings included normocytic normochromic anemia, with leukocytosis due to lymphocytosis. Erythrophagocytosis and protozoa identified as T. evansi were seen in the peripheral blood. High titers against T. evansi (optical density, 1,422) were detected in the serum of this horse.
Trypanosoma evansi is a flagellate protozoan parasite that causes disease in several mammalian species. The hind limb weakness of equine trypanosomiasis gave rise to denomination mal das cadeiras or mal de caderas respectively in in Brazil and in many of Spanish speaking countries of South America. Other denominations for the same disease are murrina (Panama) and surra (Asia). In South America, capybaras (Hydrochoerus hydro-chaeris) coatis (Nasua nasua) small marsupials (e.g., Monodelphis spp.), and armadillos (Dasypus spp.) are possible reservoirs for T. evansi. Unlike the African Trypanosoma species that cause nagana in animals and -Ç-ÿ-Ç-ÿsleeping sickness in human beings, T. evansi does not require development stages within its vector, and is mechanically transmitted through the bite of insects, especially tabanids and stomoxids, by the South American vampire bat (Desmodus rotundus) and, possibly, by ticks.(3,6)
Trypanosomiasis in horses is characterized by intermittent fever, anemia, progressive weakness, loss of body condition, and unstable gait.(21) Neurologic signs have been described occasionally in horses in the terminal phase of natural infection by T. evansi.(21) Trypanosomiasis by T. evansi is enzootic in horses from the Midwestern Brazil, mainly the Pantanal region.(3,4,6,9,22, 23)
T. evansi induces a wasting disease with a protracted clinical course(28) associated with anemia and instability of pelvic limbs in horses, camels, and dogs.(4) Natural infection by T. evansi rarely is recognized as a cause of encephalitis in horses.(21) Neurologic signs have been reported in cattle(27) and hog deer(26) naturally infected by T. evansi; however, the histopathologic changes in the brain of these species were not fully described. Mild lymphoplasmacytic meningo-encephalitis was reported in horses, donkeys, dogs, goats,(2) coatis (Nasua nasua),(9) and buffalo(25) experimentally infected by T. evansi. The horse of this report and the other necropsied horses in the current outbreak had a severe lympho-plasmacytic meningoencephalitis with marked edema and variable necrosis. The lesions in the brain of these horses resembled those described in horses infected by Trypanosoma brucei brucei,(14,17) which causes a disease known as nagana, and those described for human trypano-somiasis caused by Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense. (13,16)
T. evansi antigen was detected by immunohistochemistry in the brain of 8 of 9 horses affected in the outbreak, including the brain of the horse of this report. The protozoa were detected within blood vessels, in perivascular spaces, and in the brain parenchyma. This suggests that trypanosomes invaded the brain parenchyma and were responsible for the lesions observed here. The pathogenesis of cerebral necrosis was not elucidated since thrombosis was absent. Neither the mechanism by which trypanosomes entered the neuroparenchyma was determined. The introduction of T. evansi into a na+â-»ve population, as was the case here, could explain the development of fatal trypanosomiasis with severe encephalitis. The reason for the sudden appearance of the disease in this region of Brazil, where it was previously unreported, is unknown, but it could have been introduced by transportation of infected horses or migration of capybaras from regions where trypanosomiasis is enzootic in this country. A factor that may have contributed to the development of severe encephalitis was that these affected horses were treated with subtherapeutic doses of diminazene aceturate and other antitrypanosomal drugs.(19,20) Several studies have demonstrated that the use of subtherapeutic doses of diminazene aceturate may prolong survival of horses experimentally infected by T. brucei spp. however this faulty procedure results in subsequent invasion of the central nervous system by the organisms which induce necrotizing encephalitis.(8,10,11,15,18,24) Diminazene aceturate clears trypanosomes from tissues except those localized on the central nervous system,(19) because the drug does not cross the blood-brain barrier(11,17) allowing trypanosomes harbored in the central nervous system to survive antitrypanosomal therapy; a change in these organisms surface glycoproteins favor recurrent parasitemias by variants of the organism.(20)
How trypanosomes penetrate the blood-brain barrier is not clear, but several mechanisms have been proposed: (i) entrance through sites where the blood-brain barrier is incomplete, such as sensory ganglia and circumventricular organs;(16), (ii) increase in vascular permeability due deposition of immune complexes in the choroid plexus; and (iii) opening of intercellular tight junctions of the ependymal lining of the ventricular system by toxins released by the parasite.(12) It is believed that invasion of the central nervous system by trypanosomes occurs where the blood-brain barrier has been disrupted, either directly by the parasites or by release of chemical mediators, such as cytokines and proteases.(7,13,15) An interaction between the trypanosomes and the host response could promote tissue damage and facilitate the entry of parasites into the central nervous system.(13) This scenario is supported by the histopathologic changes and immunohistochemistry observe in the horse of this report. Trypanosomiasis due to T. evansi should be considered in the differential diagnosis of encephalitis in horses in regions where the disease is enzootic.
T. evansi should be considered in the differential diagnosis of encephalitis in horses in regions where the disease is enzootic. The list of differentials should include equine herpesvirus type 1 myeloencephalopathy, Eastern equine encephalitis, Western equine encephalitis, Venezuelan equine encephalitis, equine protozoal myeloencephalitis, West Nile virus infection, and rabies. The nature and distribution of lesions in the central nervous system of horses with naturally occurring T. evansi infection should help to distinguish trypanosomiasis from viral or other protozoal infections.
Telencephalon: Meningo-encephalitis, lymphoplasmacytic and histiocytic, diffuse, severe with vasculitis, spongiosus and gliosis.
Conference participants were struck by the marked degree of perivascular cuffing and described the presence of edema, lymphocytes, and macrophages (Gitter cells) expanding Virchow-Robin spaces. They also described lymphocytic infiltration, gliosis, gemistocytes and neuronal degeneration and necrosis in the surrounding neuropil. The meninges were also expanded by a lymphoplasmacytic infiltrate and edema. Some sections contained areas of vascular damage with the presence of cellular debris, protein and hemorrhage in vessel walls and therefore some participants also described vasculitis. Multi-focally, there are hypertrophied endothelial cells in the less affected vessels.
T. evansi is the causative agent of surra, which originated in Africa, but is also present across the Middle East, Asia, in Central and South America and has also been reported in Russia. It is thought to derive from genetic modification (a deletion) in T. brucei. It has a wide host range, indeed the widest among salvarian trypanosomes, including camels, dogs, horses, deer, llamas, cattle, sheep, goats, cats, pigs, and elephants among others. Disease is particularly devastating in camels and horses, often proving fatal in the absence of treatment. Clinical signs can vary dramatically between species and even between individuals of the same species. Infection in camels can occur acutely as in horses, but often has a more protracted course with signs such as intermittent fever, weakness, anorexia, weight loss, abortion, anemia, edema and petechial and ecchymotic hemorrhages. All age groups are affected and the urine of infected camels apparently has a specific odor, which may allow diagnosis of the disease. Dogs are also highly susceptible to infection and death may result at 1-4 weeks post infection.
Trypanosomes have developed mechanisms which allow them to both evade the immune system and cause immunosuppression. This includes antigenic variation in main membrane surface glycoproteins, variant surface glycoprotein (VSG), which forces the host to constantly redevelop their humoral response. The immunosuppressive effects of T. evansi infection are not fully understood but may involve modulation of macrophage activity, decreased responsiveness of lymphocytes, and/or changes in the CD4:CD8 lymphocyte ratio; they may even be capable of eliminating memory B cells. The complex immunosuppressive or immunomodulatory mechanisms of T. evansi have created significant barriers in developing effective vaccines and maintaining effective treatments.(5)
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