Dictyocaulus arnfieldi

The lung nematode Dictyocaulus arnfieldi occurs in horses and other equids in many parts of the world, including rarely in Canada..  The life cycle is direct and similar to that of D. viviparus in cattle.  Larvated eggs or rarely first-stage larvae are passed in the faeces and develop into infective third-stage larvae in the environment.  Infection of horses is by ingestion of these larvae.  In horses the parasite rarely completes its development to the adult stage in the bronchi and trachea but occasionally larvae migrating through the lung parenchyma can result in significant pathology and clinical signs, notably dyspnoea and coughing.  Full parasitic development and patency are more common in other equids, particularly donkeys, and these can serve as a source of infection for horses.

Dictyocaulus arnfieldi is not known to be zoonotic.

Phylum: Nematoda
Class Rhabditea
Subclass: Rhabditia
Order: Strongylida
Superfamily: Strongyloidea
Family: Dictyocaulidae

The closest relatives of D. arnfieldi are the lungworms D. viviparus of cattle and D. filiaria of sheep, all of which have similar life cycles.

Adult D. arnfieldi measure up to approximately 3.5 cm (males) and 6.0 cm (females) in length. The males have a distinct copulatory bursa and short, thick spicules.

The larvated eggs of D. arnfieldi (the stage passed in feces) measure up to approximately 100 μm by 60 µm, and contain a first-stage larva when freshly passed.

Dictyocaulus arnfieldi occurs in donkeys, and also in horses and other equids, in many parts of the world. In horses, the pre-adult larvae rarely complete their development and so infections in this host are rarely patent. Patent, but not necessarily clinically significant, infections are, however, common in donkeys, and such infections occur occasionally in Canada.

Adult D. arnfieldi live in the bronchi and sometimes trachea of donkeys and its other hosts. Larvated eggs, and very rarely first-stage larvae, pass in the feces. In the environment, the eggs hatch and the larvae released undergo two moults to the infective third stage. Infection of the donkey is by ingestion of the infective larvae. Larval migration in donkeys follows the same route a D. viviparus in cattle. The pre-patent period of D. arnfieldi in donkeys is approximately 12 to 14 weeks. In horses, the infection very rarely becomes patent.

 Life Cycle: Dictyocaulus arnfieldi

As donkeys appear to be the "normal" host for D. arnfieldi, they are probably the major reservoir of the parasite for horses. For significant transmission among or between any of the hosts, access to pasture is required. Co-mingling horses and donkeys on pastures may lead to transmission of D. arnfieldi to the horses, and perhaps to clinical disease.

In donkeys, D. arnfieldi is sometimes associated with clinical signs, although heavy infections may lead to coughing and respiratory distress. In horses, the pre-adult larvae, particularly if present in large numbers, can result in a range of respiratory symptoms, including coughing.

No products are approved in Canada for treatment of Dictyocaulus arnfieldi in horses or donkeys, but extralabel ivermectin and possibly fenbendazole may be effective.

Additional information on the product mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of D. arnfieldi in horses depends most importantly on preventing contact at pasture between horses and donkeys and other hosts in which the parasite may be patent.

Dictyocaulus arnfieldi is not known to be zoonotic.