Page 740 - Veterinary Immunology, 10th Edition

VetBooks.ir Box 23.3

Rhodococcus equi

Rhodococcus equi is a ubiquitous bacterium found in soil and animal
bedding. Newborn foals are exposed to R. equi soon after birth.
Most foals then mount a protective immune response, but a few
develop severe pneumonia at a time when maternal antibodies are

waning.
Multiple defects have been identified in the immune systems of
neonatal foals. Foal neutrophils have a phagocytic ability similar to
that in adult horses, but their serum opsonic activity is low. Their

killing activity is also low since they mount a weak inflammatory
burst. Their dendritic cell function is defective, owing to decreased
expression of MHC class II molecules and of CD1b.
Because this disease is associated with declining maternal

antibody levels, it is possible that antibodies are critical in
determining resistance or susceptibility to R. equi. Foals generally
lack mucosal IgA for the first 28 days of life. While production of
IL-8, IL-10, IL-12, and IL-23 in newborns is comparable to that in

older foals, interferon-γ (IFN-γ) and IL-6 production are impaired.
On stimulation with R. equi, neonatal foal mononuclear cells
increase their production of IFN-γ, IL-6, and IL-23, but much less
so than cells from older foals. R. equi growth is greater in 3-month-

old foals than in macrophages from younger foals or from adult
horses. Expression of IL-4 mRNA is higher in monocytes, while
expression of IL-6, IL-18 and TNF-α is higher in bronchoalveolar
macrophages. Induction of IL-1β, IL-10, IL-12 p40 and IL-8 mRNA

is highest in cells from 1- to 3-day-old foals. Thus the persistence of
this organism within macrophages probably plays a role in its
tissue tropism and in determining age susceptibility.
In prospective studies, foals destined to develop R. equi

pneumonia were born with fewer leukocytes, fewer segmented
+
neutrophils, a lower proportion of CD4 T cells, and a lower
CD4/CD8 ratio than normal foals. It has long been accepted that a
Th1 response is essential for protection against R. equi. Since it is an

intracellular organism, activation of phagocytic cells by IFN-γ is
required for its elimination. Unfortunately, foals younger than 3 to
4 months have reduced IFN-γ expression compared with adults. It

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