Page 730 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Respir atory system: 3.4 Medical conditions of the lower respir atory tr act 705

VetBooks.ir 3.164a 3.164b

Fig. 3.164 Adult Dictyocaulus sp. worms in the (a) bronchus (10× magnification) and (b) bronchiole
(20× magnification). (Photos courtesy Kaori Sakamoto)

smaller (up to 25 cm). Adults live in the small intes- Differential diagnosis
tine and lay eggs containing L1 larvae, which are A variety of causes of pneumonia (bacterial, viral) in
then shed onto pasture in faeces and quickly mature foals and yearlings may have a similar presentation.
(<2 weeks) into the infective L2 larvae inside the egg.
The females produce large quantities of eggs and a Diagnosis
single infected foal can shed millions of eggs each An outbreak of pulmonary disease in foals at pasture is
day. Foals become infected by ingestion of L2 lar- highly suggestive of P. equorum infection. Secondary
vae within eggs from the pasture. After ingestion, bacterial pneumonia can occur. Confirmation is by
L2 larvae migrate through the small intestinal wall faecal egg counts and demonstration of thick-walled
and reach the liver as L3 larvae, via the hepatic por- ascarid eggs.
tal vein. Larvae migrate through the liver and reach
the lungs via the heart and pulmonary artery within Management
2 weeks of infection. L3 larvae migrate through P. equorum can be treated using any of the available
alveolar capillary walls into the airway, where they anthelmintics including benzimidazoles, ivermectin
are coughed up into the pharynx and swallowed. In and moxidectin. Faecal egg count reduction testing
the small intestine, larvae develop into L4 larvae and of foals should be done regularly to detect resistance
then adult worms, thus completing the life cycle. to macrocyclic lactones, in which case early treat-
The pre-patent period is between 3 and 4 months. ment with pyrantel or benzimidazoles may limit
development of high burdens in the intestinal tract
Clinical presentation that predispose to ascarid impactions. Recent reports
Foals between 6 and 12 months old are most com- in multiple countries of resistance to macrocyclic
monly affected, typically towards the end of their lactone anthelmintics (ivermectin, moxidectin) have
first summer at pasture. The disease is sometimes been reported, prompting recommendations to
referred to as ‘summer colds’. Affected foals may perform faecal egg count reduction testing.
have poor body composition. Clinical signs referable
to the respiratory tract include modest pyrexia, nasal Prognosis
discharge, weight loss, coughing, tachypnoea, dys- The prognosis is good for most cases, although
pnoea and crackling and wheezing on auscultation death from small-intestinal rupture in very heavily
of the lung field. Heavy infections of adult parasites infected foals can occur. Solid immunity develops on
can cause death due to small-intestinal obstruction recovery from infection and infections in foals older
and rupture. than 12 months are unusual.