Chagas disease in a Texan horse with neurologic deficits

Abstract

A 10-year-old Quarter Horse gelding presented to the Texas A&M University Veterinary Teaching Hospital with a six month-history of ataxia and lameness in the hind limbs. The horse was treated presumptively for equine protozoal myeloencephalitis (EPM) based on clinical signs but was ultimately euthanized after its condition worsened. Gross lesions were limited to a small area of reddening in the gray matter of the thoracic spinal cord. Histologically, trypanosome amastigotes morphologically similar to Trypanosoma cruzi, the agent of Chagas disease in humans and dogs, were sporadically detected within segments of the thoracic spinal cord surrounded by mild lymphoplasmacytic inflammation. Ancillary testing for Sarcocystis neurona, Neospora spp., Toxoplasma gondii and Leishmania spp. was negative. Conventional and real time polymerase chain reaction (PCR) of affected paraffin embedded spinal cord were positive for T. cruzi, and sequencing of the amplified T. cruzi satellite DNA PCR fragment from the horse was homologous with various clones of T. cruzi in GenBank. While canine Chagas disease cases have been widely reported in southern Texas, this is the first report of clinical T. cruzi infection in an equid with demonstrable amastigotes in the spinal cord. In contrast to previous instances of Chagas disease in the central nervous system (CNS) of dogs and humans, no inflammation or T. cruzi amastigotes were detected in the heart of the horse. Based on clinical signs, there is a potential for misdiagnosis of Chagas disease with other infectious diseases that affect the equine CNS. T. cruzi should be considered as a differential diagnosis in horses with neurologic clinical signs and histologic evidence of meningomyelitis that originate in areas where Chagas disease is present. The prevalence of T. cruzi in horses and the role of equids in the parasite life cycle require further study.

Keywords: Amastigotes; Chagas disease; Equine; Formalin fixed paraffin embedded tissue; Polymerase chain reaction; Trypanosomiasis.

Fig. 1.

Gross and histologic lesions within the spinal cord of a 10-year-old Quarter Horse gelding from Texas. (a) Longitudinal and transverse sections of the thoracic spinal cord reveal multifocal areas of congestion and mild hemorrhage within the dorsal and ventral horn gray matter. (b) Histologically, clusters of 2–3 μm in diameter, protozoan amastigotes surrounded by a few plasma cells are in the hyperemic areas of spinal cord identified in panel a. In the insert, amastigotes have a thin outer membrane, contain a 1 μm basophilic nucleus and have a bar-shaped kinetoplast (arrow) that is often oriented parallel to the nucleus; HE, 1000×, Bar = 10 μm. (c) Large clusters of amastigotes are negative with Sarcocystis spp.—specific immunohistochemical staining, which would stain organisms bright red if positive; IgG antibody conjugated with horseradish peroxidase, 1000×.

Fig. 2.

DNA gel electrophoresis of PCR amplicons from the horse and control samples using the T. cruzi TCZ1/TCZ2 and Tc121/Tc122 PCR primer sets, 2% agarose gel, 100 V for 30 min. Each band of the DNA ladder (L) represents 100 bp. From left to right for the TCZ1/TCZ2 primer set: formalin-fixed paraffin embedded (FFPE) heart from a canine patient with IFA confirmed Chagas disease; FFPE horse spinal cord (BiOstic® extraction) and PCR reaction blank. For the Tc121/Tc122 primer set: T. cruzi Sylvio X10 strain positive control; FFPE horse spinal cord (E.Z.N.A® extraction) and PCR reaction blank.

Fig. 3.

A comparison of selected protozoal agents that affect the CNS. The details of the cases in panels b and c have been previously published (Rodrigues et al., 2009), and the cases were re-photographed for this report. (a) Trypanosoma cruzi amastigotes (arrow) within the cerebrum of a dog; HE, 1000×, Bar = 10 μm. (b) Trypanosoma evansi trypomastigotes in the peripheral blood of a horse; Dif-quick, 1000×. (c) Immunohistochemistry identifies a Trypanosoma evansi trypomastigote in the brain of a horse; Avidin-biotin-peroxidase complex, 1000×, Bar = 10 μm. (d) Sarcocystis neurona merozoites within the brain of a horse; HE, 1000×, Bar = 10 μm. (e) Neospora spp. tissue cyst in the brain of a dog; HE, 1000×, Bar = 10 μm. (f) Toxoplasma gondii tissue cyst within the brain of a cat; HE, 1000×, Bar = 10 μm.