Infectious agents

Anaplasma phagocytophilum in a dog

Infections with organisms can be detected by examination of peripheral blood smears. Some are important as causes of disease in the host animal, while others are essentially non-pathogenic under most circumstances. Some infectious agents cause hematologic changes but are not identified on the blood smear. Infectious agents (or byproducts of their presence) can be seen in all cells in blood as well as in the background plasma.  In this section, we provide information on the following:

  • Infection mimics: Things in smears (artifacts or otherwise) that mimic infectious agents. Don’t get caught out!
  • Anaplasma: Bacteria of the order Ricketsiales, family Erhlichiaceae and genus Anaplasma. Some are seen in leukocytes (e.g. Anapasma phagocytophilum in horses and dogs), whereas others are typically erythroparasites and cause a hemolytic anemia (extravascular hemolysis, e.g. Anaplasma bovis). One species infects platelets of dogs (Anaplasma platys).
  • Babesia/Theileria: Apicomplexan protozoal parasite of the order Pirosplasmida in two different families (Babesiidae and Theileriidae). These organisms typically infect RBCs and both cause a hemolytic anemia, which can have an intravascular component with some species of Babesia (e.g. Babesia canis). Although they are found in different families of the order, they can be difficult to distinguish on light microscopy, hence they are grouped together for the purposes of this site. Separating the two organisms relies on specific PCR tests or genome sequencing (e.g. 16S RNA).
  • Cytauxzoon: Apicomplexan protozoan parasite of the order Achromatorida. This organism primarily infects cats in southern USA states, e.g. Oklahoma, and intra-erythrocytic merozoites (piroplasms) and schizont-filled macrophages may be seen in blood or tissue aspirates (schizonts). The disease typically is of acute onset and results in pancytopenia. Once thought to be universally fatal, it is now known that infected cats can recover and become persistent carriers. With newer anti-protozoal drug combinations, many cats cats can survive acute infection.
  • Ehrlichia: Bacteria of the order Ricketsiales of the family Anaplasmataceae and genus of Ehrlichia. The organisms are typically seen in leukocytes, primarily granulocytes (e.g. Ehrlichia ewingii  in dogs). Even though some species can infect mononuclear cells (e.g. Ehrlichia canis), they are rarely seen in these cells in blood.
  • Mycoplasma: These are small coccobacilli that lack a cell wall. Certain species infect red blood cells in animals and can cause anemia (e.g. Mycoplasma haemofelis). The species that infect RBCs are distinguished by the prefix “haemo” in front of their specific name.
  • Viruses: Although viruses can cause various hematologic changes, this section is restricted to those we can “see” within hematologic cells, such as canine distemper.
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