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Trypanosoma equinum and T. evansi are synonymous: T. equinum is neither a separate species nor a sub‐species of T. evansi. However, there are morphological differences between T. evansi and T. equinum: T. evansi expresses both kinetoplastic and dyskinetoplastic forms (i.e., ones retaining only fragments of kinetoplastid DNA) but T. equinum lacks a kinetoplast (i.e., it is ‘akinetoplastic’). This was one of the reasons for separating them, but molecular studies indicate that genetically they are the same.

Despite the molecular evidence, we will follow the older parasitology textbooks in dealing with them as though they are two distinct species. This is because the literature often provides different accounts of them in terms of their host range and pathology. Trypanosoma equinum only occurs in various South American countries where it causes a disease commonly known as ‘mal de caderas’ in Spanish and ‘mal de cadeiras’ in Portuguese – which translates as illness of the hips. Trypanosoma equinum is principally a parasite of horses. It infects other equids and a variety of domestic and wild animals (e.g., capybara), but these do not tend to suffer severe disease. It is mechanically spread by biting flies, such as tabanids, and in swampy areas where these flies are most numerous, T. equinum is a particular problem.

Unlike surra, mal de caderas normally causes a chronic disease in horses. The condition develops over a period of months but usually has a fatal outcome. Symptoms begin with a fever and loss of weight and then the hindquarters become progressively weaker (hence the name ‘mal de caderas’) resulting in staggering and then an inability to walk. Horses can also exhibit conjunctivitis, the eyelids become oedematous (filled with fluid), and transient plaques form on the neck and flanks. In addition, the kidneys, brain, and spinal cord show signs of inflammation and necrosis.