Page 921 - Veterinary Immunology, 10th Edition

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γ, TNF-α, and TGF-β, as well as intraepithelial CD8 α/β T cells, are
VetBooks.ir essential for anti-coccidial immunity.
For many years it was thought that a common feature of many
protozoan infections was premunition, a term used to describe

resistance that develops after the primary infection has become
chronic and is only effective if the parasite persists in the host. It
was believed, for example, that only cattle actually infected with
Babesia were resistant to clinical disease. If all organisms were

removed from an animal, resistance was believed to wane
immediately. This is not entirely true. Cattle cured of Babesia
infection by chemotherapy are resistant to challenge with the
homologous strain of that organism for several years. Nevertheless,

the presence of infection does appear to be mandatory for
protection against heterologous strains. Babesiosis is also of interest
since splenectomy of infected animals results in clinical disease. The
spleen not only serves as a source of antibodies in this disease but

also removes infected erythrocytes.

Leishmaniasis

The importance of immunity in determining the course and nature
of a protozoan disease is well seen in canine leishmaniasis. This

disease is caused by Leishmania infantum or its New World synonym
Leishmania chagasi and transmitted by biting sandflies. When the
promastigote forms of this parasite are injected by a sandfly into the
skin of dogs, they are phagocytosed by neutrophils. When the
neutrophils undergo apoptosis, the parasites are released and are

then engulfed by macrophages and dendritic cells. In these cells, the
organisms differentiate into amastigotes. They divide within the
macrophages until the cells rupture, and neighboring cells then

phagocytose the released organisms. Depending on the degree of
host immunity, the parasites may be restricted to the skin
(cutaneous disease); alternatively, infected dendritic cells may
migrate to lymph nodes or enter the circulation and lodge in
internal organs, leading to disseminated visceral disease. Although

infection is widespread in endemic areas, most dogs are resistant to
Leishmania, and only 10% to 15% develop visceral disease.
Macrophages are the main host cells for Leishmania but are also

responsible for parasite killing. Parasites divide within the

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