July 2026

The unwanted guests

​A blood sample collected from a clinically healthy calf was submitted for monocyte culture examination. Morphological examination under light microscopy showed elongated, flagellated organisms (Image 1).
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Thank you to Granit Qupeva, graduate student at Texas A&M and Dr. Gui Verrocai, Associate Professor at Texas A&M for contributing to this case.

​Figure 1. Stained blood smear observed under light microscopy at 100x magnification.

What's your diagnosis?

Trypanosoma theileri (Trypomastigote)

​Trypanosoma theileri is a non-pathogenic or mildly pathogenic trypanosome commonly found in cattle worldwide. It is typically transmitted by hematophagous insects such as tabanid flies. In most cases, infections are subclinical as observed in this calf and commonly detected incidentally during laboratory research.

Taxonomically, Trypanosoma theileri is classified under the subgenus Megatrypanum due to its large size compared with pathogenic trypanosomes, as well as its uniquely prominent distinct kinetoplast. In culture, Trypanosoma theileri is observed predominantly as replicative epimastigotes, with occasional differentiation into less abundant trypomastigote and amastigote‑like forms. The life cycle initiates when an infected tabanid fly deposits infective metacyclic trypomastigotes into a mammalian host during blood-feeding. Once in the bloodstream, the parasites transform into mature, extracellular trypomastigotes and multiply via binary fission. It is during this active phase that they become transiently detectable on peripheral blood smears.

​It’s very important to differentiate T. theileri from T. vivax, T. cruzi, T. congolense, T. brucei, and T. evansi because they are highly pathogenic, heavily regulated, and reportable to state veterinarians, the USDA, and international animal health organizations.

​The results from this case were molecularly confirmed by performing a PCR targeting the small subunit ribosomal RNA (SSU rRNA) gene that amplifies a ~556 bp fragment, and subsequently submitted for sequencing identification.