Page 714 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.4 Medical conditions of the lower respir atory tr act 689

VetBooks.ir (PCR) assays based on detecting the VapA gene pro- idea that not all foals require treatment. Increasing
numbers of resistant isolates have been identified
vide increased sensitivity over culture but should be
interpreted in light of clinical signs and imaging, as
recommendations have shifted away from treating
they may yield positive results in foals without clini- with the advent of antimicrobial use. Consequently,
cal disease. Similarly, serology and faecal culture are every foal with mild thoracic ultrasound changes but
unreliable because seroconversion and GI tract colo- with no clinical signs. Instead, it is recommended
nisation in the absence of disease are common. that foals with minor changes be monitored more
frequently, but only be treated if disease becomes
Management clinical.
The intracellular persistence of R. equi makes anti- Practical measures to reduce exposure to R. equi
microbial treatment difficult. Treatment with mac- include maintaining young foals (<4 weeks old)
rolide antimicrobials, which are concentrated in on clean grass paddocks and avoiding dusty, over-
macrophages and thus target the site of bacterial crowded dirt paddocks with little grass. This reduces
persistence, is recommended in combination with the environmental burden and hence the risk of
rifampin. The historical treatment is long-term infection of the foal in its first weeks of life. To limit
(1–2 months) oral combination therapy with eryth- propagation of the disease at the farm level, affected
romycin (estolate 25 mg/kg p/o q6 h, phosphate foals may be removed from pasture, since they
37.5 mg/kg p/o q12 h) and rifampin (5–10 mg/kg shed high numbers of bacteria in faeces. In high-
p/o q12–24 h). A newer alternative to erythromycin risk facilities, hyperimmune equine plasma can be
is azithromycin, which allows reduced-frequency administered to newborn foals at birth and again at
dosing (10 mg/kg p/o q24 h for 5 days, then q48 h) 3–4 weeks of age to reduce the risk of respiratory
and possibly reduced side-effects. Clarithromycin infection early in life.
(7.5 mg/kg p/o q12 h) in combination with rifampin
is also widely used and has been shown to have supe- Prognosis
rior clinical efficacy when compared with treatment The prognosis is poor for foals with severe pulmo-
with erythromycin and rifampin. Combination nary disease, even if treated aggressively. For foals
therapy is always recommended to reduce chances with less severe disease the prognosis is moderate
of resistant isolate development. Treatment can be to good. Foals that recover do progress into train-
expensive and adverse reactions, including fatal ing and have an effective athletic life. Racing per-
clostridial diarrhoea or hyperthermia, may occur. formance in Thoroughbreds does not seem to be
Hyperthermia, most common with use of erythro- affected. The prognosis for foals with extrapulmo-
mycin, occurs because foals lose the ability to sweat nary abscesses is generally poor.
properly; care should be taken to provide adequate
shade or fans during periods of increased environ- BACTERIAL PNEUMONIA
mental temperature and humidity. Foals with severe
pulmonary signs may require hospitalisation, oxy- Definition/overview
gen therapy, inhalation and systemic therapy with Bacterial pneumonia is characterised by inflamma-
bronchodilators and fluid therapy. tion of the lungs that occurs because of bacterial
Close surveillance of foals for signs of respira- colonisation of the pulmonary parenchyma. Most
tory disease, including regular haematology screens cases are secondary to viral respiratory infection,
to identify foals with raised total white blood cells although primary bacterial infections can occur.
(WBCs), neutrophilia and raised fibrinogen, allows
early identification of affected foals. On enzootically Aetiology/pathophysiology
infected farms, serial monitoring of thoracic ultra- Common pathogens isolated from bacterial pneu-
sounds can also help detect foals in the early stages monia in adult horses include Streptococcus zooepi-
of the disease. However, many foals with ultraso- demicus, beta-haemolytic Streptococcus spp. and
nographic changes will clear the infection without gram-negative organisms such as Pasteurella spp.,
developing clinical signs of disease, supporting the Escherichia coli, Klebsiella spp., Enterobacter spp. and