Page 712 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.4 Medical conditions of the lower respir atory tr act 687

VetBooks.ir 3.4 Medical conditions of the lower respiratory tract

INFECTIOUS CONDITIONS

RHODOCOCCUS EQUI PNEUMONIA and prefers light or sandy soils. Following inges-
tion, the organism replicates in the intestinal tract
Definition/overview and, especially in foals, large numbers of bacteria
Rhodococcus equi is a primary respiratory pathogen are shed in faeces. Densely stocked mare and foal
that causes suppurative bronchopneumonia and paddocks can thus harbour a large quantity of bac-
lung abscesses in foals between 1 and 6 months old teria. Pulmonary infection occurs from inhalation
(Figs. 3.142, 3.143). R. equi also causes extrapul- of bacteria-laden dust. Intestinal tract and abdomi-
monary abscesses, mainly in the abdomen and in nal infection can occur following ingestion of a
bones of affected foals. R. equi pneumonia in older sufficient infectious dose from soil or secondary to
foals and adults is rare. The bacteria replicates in pulmonary disease due to swallowing of bacteria-
the soil and the gastrointestinal (GI) tract of foals; laden pus. Abscesses can develop at other sites by
high bacterial numbers are also exhaled in the breath haematogenous dissemination from the lung or gut.
from infected foals. Most foals with milder pulmo- Abscess formation occurs when R. equi establishes
nary disease recover and progress to useful athletic intracellular infection and persists within macro-
careers. The prognosis for abdominal and bone phages. The ability to survive within macrophages
abscesses is guarded. There is currently no effective is conferred by a plasmid encoding a group of ‘vir-
R. equi vaccine. ulence-associated proteins’ (Vap proteins A–G)
that act to prevent phagosome–lysosome fusion and
Aetiology/pathophysiology hence prevent respiratory burst killing of infected
R. equi is a gram-positive pleomorphic coccobacil- cells. Virulent strains in foals are Vap A positive. In
lus that may appear cytologically as a rod or cocci. the lung, R. equi causes suppurative bronchopneu-
It is a soil-living, environmental organism, which monia, lung abscesses and mediastinal and tracheo-
replicates efficiently in hot, dry, dusty conditions bronchial lymph-node abscessation. Lung abscesses

3.142 3.143

Fig. 3.142 Dorsoventral view of the lungs of a foal Fig. 3.143 Cut surface of the lung of the foal in
with R. equi infection as seen at necropsy. Multifocal 3.142 at necropsy, showing multifocal abscesses
abscesses (arrows) are visible in the lung parenchyma. containing purulent material.