Dictyocaulus viviparus

The lung nematode Dictyocaulus viviparus occurs in cattle in many parts of the world, including Canada.  Adult parasites live in the trachea and bronchi.  The life cycle is direct, involving the development of first-stage larvae passed in the faces into infective third-stage larvae.  Infection of cattle is by ingestion of these third-stage larvae.  The pre-patent period is 3 to 4 weeks.  Many cattle infected with small numbers of D. viviparus show few or no clinical signs.  Significant pathology and clinical signs are most common in animals that have not previously been infected and that ingest large numbers of infective larvae over a short time period (days or weeks).  Disease is most commonly associated with this situation - in western Canada in the late summer and fall in calves and other young stock. 

Dictyocaulus viviparus can cause disease at several stages of its development in cattle: pre-patent - when larvae are newly arrived in the lungs cause tissue damage; patent - when adults present in the airways obstruct air flow, and when eggs, first-stage larvae, and exudates aspirated into the bronchioles and alveoli cause tissue damage, including metaplasia of the alveolar epithelium; and post-patent - when many of the adult parasites have been shed as a result of immune-mediated expulsion (a common occurrence with this parasite) and there is a hypersensitivity-like reaction in the alveoli.  Typical clinical signs for all stages of infection include dyspnoea, coughing (often with the head down), and drooling.  Morbidity can be high and there may be some mortality, particularly in animals affected by the post-adult hypersensitivity reaction.

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

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

Adult D. viviparus measure up to approximately 5.5 cm (males) and 8.0 cm (females) in length. The males have a distinct copulatory bursa and short, thick spicules. Adult parasites are usually identified on the basis of their location on the host – they are the only adult nematodes found in the trachea and bronchi of cattle.

The  first-stage larvae of D. viviparus (the stage passed in feces) measure approximately 400 to 450 µ in length, and have a granular intestine and a bluntly pointed tail.

Dictyocaulus viviparus is primarily a parasite of cattle and occurs in many areas of the world, including Canada, where it is seen as an occasional cause of respiratory disease. The parasite has not been recovered from sheep or goats, but it can infect other, free-ranging, ruminants. The exact taxonomy of the parasite from these hosts has not been clearly defined.

Adult D. viviparus live in the in trachea and bronchi. Eggs laid by the females hatch in the airways and first-stage larvae pass in the feces. In the environment, these develop to the infective third-stage. Infection of cattle is by ingestion of third-stage larvae from pasture. These larvae migrate in the vasculature to the lungs, leave the branches of the pulmonary artery and migrate through the lung parenchyma to the airways. The pre-patent period is 3 to 4 weeks. In some animals, larvae entering the lungs undergo inhibited (delayed) development for up to several months. In many animals, especially those with heavier burdens, the majority of the adult D. viviparus are expelled within two to three months of infection. This expulsion is probably a result of an immune reaction which is protective for many cattle after exposure to the parasite during their first grazing season.

Life Cycle: Dictyocaulus viviparus

Dictyocaulus viviparus seems to require access to pasture for successful transmission. The basic annual epidemiological cycle for D. viviparus is similar to that for the GI trichostrongyles of cattle. At the start of the grazing season in most areas of Canada the pastures are probably free of larvae, but inhibited pre-adult larvae and adult parasites may be present in yearlings and adult cattle. These serve as the source of infective larvae for the calves, which develop patent infections, and sometimes clinical disease. Infected wildlife (for example, deer and elk) may (or may not) be reservoirs of D. viviparus for cattle.

The epidemiology of D. viviparus has two special features. First, a dry summer may result in crusting of the surfaces of fecal masses on the pasture and persistence of viable infective larvae in the centres on the masses. Rain in the fall may soften the crusts and release the larvae on to the herbage. Second, infective larvae in the fecal masses may migrate on to the sporangia of the fungus Pilobolus, and when the sporangium bursts, these larvae are propelled up to three metres into the herbage.

Many cattle with low-level D. viviparus infections show no adverse effects. The presence and severity of clinical disease seems to depend on the rate of ingestion of infective larvae. Heavier infections, especially those acquired over a short time period, can cause severe lung pathology and clinical signs, These result from the damage caused by pre-adult larvae migrating in the lungs, the irritation caused by the adults in the airways, and by the aspiration deep into the lungs of mucus, and of eggs and larvae.

Among the significant pathological changes produced by the parasites, beginning towards the end of the pre-patent period and persisting in some animals into the post-patent period, are changes in the structure and function of the alveolar epithelium and the deposition of a hyaline membrane over the alveolar surfaces. Both these lesions can significantly interfere with gaseous exchange.

Clinical signs of lungworm infection can appear before the infection becomes patent and, as a result of parasite-induced damage to the respiratory epithelium, can persist in some animals after the adult parasites have died and been expelled. Clinical signs include difficulty breathing, coughing, noisy lung sounds, emphysema and rapid loss of condition. In Canada, clinical lungworm disease is most often seen in young animals towards the end of their first grazing season.

For clinical lungworm disease, the history and clinical signs are helpful. The diagnosis can be confirmed by detection of first-stage larvae in feces using the Baermann technique. Dictyocaulus viviparus are the only nematode larvae found in fresh cattle feces in Canada. Fecal samples collected from the ground may be contaminated with larvae of free-living nematodes, and these must be differentiated from those of D. viviparus. Similarly, in samples not properly stored, eggs of GI trichostrongyles may hatch, and these first-stage larvae must also be differentiated from those of D. viviparus. Small numbers of D. viviparus are occasionally found in cattle at post mortem. Care is required in assessing the significance of these small parasite burdens as a cause of disease.

There are several products approved in Canada for Dictyocaulus viviparus.  All are effective against both the stages in the airways and the fourth-stage larvae.

Drug(s)	Product(s)
Albendazole	VALBAZEN
Doramectin	DECTOMAX
Eprinomectin	EPRINEX
Fenbendazole	PANACUR, SAFE-GUARD
Ivermectin	VARIOUS
Moxidectin	CYDECTIN

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

Successful control of lungworm in cattle depends on an awareness of the clinical signs, and prompt treatment of all the animals at risk, generally considered as those sharing grazing with the sick animals.  Moving treated cattle to a "clean" pasture (one though to be free of infective larvae) after treatment is also very important.

In Europe, but not in North America, an oral vaccine is commercially available containing irradiated third stage larvae, which undergo development in cattle sufficient to generate a protective immunity, but which do not cause significant pathology and do not mature to adults.  The vaccine is usually given in the spring, before turn-out to pasture.

Dictyocaulus viviparus is not known to be zoonotic.

Panuska C (2006) Lungworms of ruminants. Veterinary Clinics of North America Food Animal Practice 22: 583-593.