Page 715 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

690 CHAPTER 3

VetBooks.ir Pseudomonas spp. Gram-negative bacteria can be the Diagnosis
A diagnosis of bacterial pneumonia is made based
sole cause of pneumonia, but they are more com-
monly isolated in combination with streptococcal
radiographic findings. On auscultation, lung sounds
pneumonias. Streptococcus equi and S. pneumoniae on history, physical examination and laboratory and
are less common causes of pneumonia. Anaerobic may include increased bronchovesicular sounds,
bacteria such as Bacteroides spp. and Clostridium crackles and/or wheezes or friction rubs in compli-
spp. are mainly found in complicated cases with cated cases involving the pleura. Thoracic percussion
pleuropneumonia. Bacterial pneumonia of neona- may reveal areas of lung consolidation or abscessa-
tal foals with bacterial sepsis is usually caused by tion, or fluid accumulation within the pleural cavity
Streptococcus spp., Escherichia coli or Actinobacillus (pleuropneumonia).
spp. Aspiration of milk is a common cause of pneu- The WBC count is usually elevated, characterised
monia for young foals that are weak or have con- by a mature neutrophilia with or without band cells.
genital abnormalities; bacterial populations are Hyperfibrinogenaemia is common and increased
mixed in these cases. In older foals, Rhodococcus equi total plasma proteins may be seen in chronic cases as
is more common, especially in foals from endemic a result of hyperglobulinaemia.
farms or areas. Thoracic radiographs are usually confirmatory
The immune defence mechanisms of the upper and are particularly helpful for assessing treatment
airways prevent most bacteria from reaching the response and as a prognostic indicator (Figs. 3.146,
lungs under normal conditions. In addition, any bac- 3.147). Thoracic ultrasonography may show areas of
teria penetrating the respiratory system to the level consolidated or atelectatic lung, and/or parenchy-
of the lungs are rapidly destroyed and removed by mal abscess(es) depending on their location in the
cellular and humoral defences of the lower respira- affected lung (Fig. 3.148).
tory tract (LRT). However, commensal bacteria of Tracheobronchial aspirates should be obtained,
the upper respiratory tract (URT) may invade the and samples submitted for bacterial culture, Gram
lungs if respiratory immune defences are damaged staining and cytological examination. Samples could
or otherwise impaired by viral respiratory infec- be collected using a bronchoscope and a protected
tion or stressors such as prolonged transport of an aspiration catheter (if there are no signs of respira-
animal. Aspiration pneumonia is also a relatively tory distress), or via transtracheal aspiration.
common sequela to episodes of choke (oesophageal
obstruction) in adult horses. Management
In response to infection of the lungs, inflammatory Ideally, therapy should be based on culture and
cells, especially neutrophils, are recruited to com- susceptibility results of samples collected from the
bat infection, but they may also contribute to tissue lower airways. Streptococcal pneumonias respond
destruction and loss of organ function. Accumulation well to therapy with penicillin, but because mixed
of cellular debris, serum exudate and fibrin within the bacterial infections are common, broad-spectrum
airways further impairs gas exchange. antimicrobials are more desirable. Penicillin/amino-
glycoside combinations are commonly used before
Clinical presentation culture results are obtained. If azotaemia or severe
Common clinical signs include fever, depression, dehydration is present, use of a fluoroquinolone can
tachypnoea, nasal discharge, coughing and exercise be considered to provide gram-negative coverage
intolerance. Fever can be intermittent. while avoiding potential nephrotoxic side-effects.
However, the use of enrofloxacin should be restricted
Differential diagnosis to adults whenever possible because of known mus-
Viral, fungal and parasitic pneumonias should culoskeletal side-effects in foals. Other antibiotics
be considered. Equine asthma (e.g. inflammatory that offer a moderately broad spectrum of activity
airway disease, heaves) and primary or second- against gram-negative and gram-positive organisms
ary pulmonary neoplasia, such as lymphosarcoma, include second- and third-generation cephalo-
may also cause similar clinical signs. sporins, ampicillin and/or trimethoprim–sulpha.