Haematology, serum biochemistry, innate and adaptive immune system responses to blood pathogen infections in cattle: a review

Infection with blood pathogens, such as Anaplasma, Haemotropic Mycoplasma, and trypanosomes, has affected the livestock industry, particularly in developing countries and the tropics. Clinical signs associated with infections caused by these blood pathogens include anaemia of the haemolytic type (immune-mediated haemolytic anaemia), decreased milk yield, abortion, anorexia, cachexia, disseminated intravascular coagulopathies, conjunctivitis, ecchymoses, thrombocytopenia, leukocytosis, and death in severe cases. Immune-mediated haemolytic anaemia is more evident in anaplasmosis as the degree of anaemia is often out of proportion to the prevailing bacteriemia. Additionally, icterus, increases in serum liver enzymes and variable white blood cell count changes are observed in affected animals. Recovered cattle may become asymptomatic carriers as some apicomplexan species tend to form sequestration in the cerebrum. Asymptomatic cattle are sources of blood pathogens to competent haematophagous arthropods and consequent transfer of infection to susceptible/naive animals. Nonetheless, persistent blood infections are typically attributable to pathogens that have developed multiple strategies, including immune subversion and the dysregulation of both natural and acquired immune responses, enabling them to persist within an immunocompetent host. Therefore, this review has provided veterinary clinicians, researchers, and laboratory technicians with an overview of the haematological and serum biochemistry profiles of cattle infected with blood pathogens, prevention and control strategies, and specific innate and adaptive immune system responses, as well as the adaptive and survival properties of these blood pathogen infections in cattle.