Dictyocaulus viviparus

Introduction

Dictyocaulus viviparus is the bovine lungworm (a member of the Trichostrongyloidea). They are white thread-like worms, less than 8cm long, which are found in the trachea and larger bronchi and are responsible for parasitic bronchitis (syn. Bovine dictyocauliosis, husk, hoose). There has been an increase in the incidence of husk in recent years; first season calves are particularly affected, although yearling and adult cattle may also succumb to the disease. Lungworm is responsible for reduced weight-gain and deaths in calves and yearlings and lowered milk-yield in dairy cows. A closely-related species is also responsible for one of the most important diseases of farmed deer. The parasite is of welfare importance if clinically affected animals are left untreated.

Life-Cycle

1. Adult worms in tracheae and larger bronchi – females lay embryonated eggs
2. Embryonated eggs are coughed up and swallowed
3. Eggs hatch during passage through the intestine
4. First stage larvae passed in fresh faeces (as D. viviparus is not a metastrongyloid, the larvae do not have an S-shaped tail like other lungworms)
5. Development to second and third (infective) stage larvae in dungpat on pasture
6. Ingestion of infective larvae at grazing
7. Infective larvae swallowed, pass through alimentary tract, penetrate wall of intestine and migrate via lymphatics → blood circulation → lungs
8. Ascend respiratory tree and mature to adult lungworms. Prepatent period 3 and a half weeks

• Dictyocaulus viviparus causes parasitic bronchitis

Parasitic Bronchitis - Cattle

Dictyocaulus viviparus (Image sourced from Bristol Biomed Image Archive with permission)

Parasitic bronchitis (Image sourced from Bristol Biomed Image Archive with permission)

• Found in trachea and large bronchi
• Causes parasitic bronchitis, synonyms: bovine dictyocauliosis, husk, hoose
• Primary infection:
  • Penetration phase (week 1)
    • Larvae migrate to lungs, no clinical signs
  • Prepatent phase (weeks 1-3)
    • Development and migration of larvae -> bronchiolitis -> eosinophilic exudate -> air passage blocked -> alveolar collapse (distal to blockage) -> clinical signs (tachypnoea, coughing)
  • Patent phase (weeks 4-8)
    • Egg-producing mature worms
    • Bronchitis - due to mature worms
    • Parasitic pneumonia - due to aspiration of eggs and larvae -> cellular infiltration of neutrophils, macrophages, giant cells
  • Postpatent phase (weeks 8-12)
    • Majority of worms are expelled
    • In 25% of cases clinical signs may reappear as a result of alveolar epithelialisation
    • May be together with interstitial emphysema and pulmonary oedema, or secondary bacterial infection
• Reinfection syndrome:
  • Immune cattle show clinical signs only if exposed to large numbers
  • Pathogenesis - large numbers of larvae reach bronchioles where they are killed by immune response
  • Pathology - parasite granulomata (grey-green, 5mm diameter, macrophages, giant cells, eosinophils) and eosinophilic plugs in bronchioles

Above from RVC Parasitology study guide (2005-2006)

• Preferentially in dorsocaudaland ventrocaudal regions

• Histologically
  • Bronchial epithelium may show hyperplasia due to the chronic irritation
  • Cross-sections of the parasites
  • Exudate contains many eosinophils
  • Foci of necrosis in the rest of the lung tissue due to aspiration of eggs and larvae fromhese adults
  • In mild infestations, the adults are normally expelled in two months - self cure
• Repeated infestation and secondary bacterial infection are common

• Two other types of lesion in lung tissue due to this worm have been reported:
  • Nodules (2-4mm in diameter) with greenish centres in the reinfection of an immune animal - the host mounting a successful defence and preventing larval migration
  • Pulmonary oedema and emphysema - thought to be a hypersensitivity response to a massive invasion of larvae in previously- sensitised animals - the gross and microscopic appearance is similar to that of fog fever