The degree of pathogenicity depends on what species the host is, the virulence of the "Trypanosoma evansi" strain, and the dose received by the host. Many species such as dogs, horses and rats, have been shown to have immunological reactions to the infection, such as anemia due to decrease in erythrocytes and hemoglobin. Although not fully understood at this point in time, some theories believe that the erythrocytes could acquire trypaonosomal antigen, resulting in a negative immunological reaction.

Animals that have been infected show loss of appetite, weight loss, anaemia, odema, fever, salivations, lacrimation, and abortion. The proteases that are released during infection of T.evansi might degrade the host tissue proteins and are a huge force in the pathogenesis. That is why scientists are looking at immune targeting of these proteases to protect the infected host.

"Trypanosoma evansi", although monomorphic in most cases, can be pleomorphic in some strains. They are characterized by a long free flagellum with a narrow drawn out posterior. Kinetoplast and dyskinetoplastic forms appear. Kinetoplast is either terminal or subterminal.

Human and animal fasciolosis occurs worldwide. While animal fasciolosis is distributed in countries with high cattle and sheep production, human fasciolosis occurs, excepting Western Europe, in developing countries. Fasciolosis occurs only in areas where suitable conditions for intermediate hosts exist.

Studies carried out in recent years have shown human fasciolosis to be an important public health problem. Human fasciolosis has been reported from countries in Europe, America, Asia, Africa and Oceania. The incidence of human cases has been increasing in 51 countries of the five continents. A global analysis shows that the expected correlation between animal and human fasciolosis only appears at a basic level. High prevalences in humans are not necessarily found in areas where fasciolosis is a great veterinary problem. For instance, in South America, hyperendemics and mesoendemics are found in Bolivia and Peru where the veterinary problem is less important, while in countries such as Uruguay, Argentina and Chile, human fasciolosis is only sporadic or hypoendemic.

Mechanisms of resistance have been studied by several authors in different animal species. These studies may help to better understand the immune response to "F. hepatica" in host and are necessary in development of vaccine against the parasite. It has been established that cattle acquire resistance to challenge infection with "F. hepatica" and "F. gigantica" when they have been sensitized with primary patent or drug-abbreviated infection. Resistance to fasciolosis was also documented in rats. On the other hand, sheep and goats are not resistant to re-infection with "F. hepatica". However, there is evidence that two sheep breeds, in particular Indonesian thin tail sheep and Red maasai sheep, are resistant to "F. gigantica". No reports concerning the resistance in humans are available.