Pathogenesis of Dictyocaulus viviparus

Pathogenesis:

This may be divided into four phases:

1.      Penetration phase: Days 1-7

·         During this period the larvae are making their way to the lungs and pulmonary lesions are not yet apparent.

2.      Prepatent phase: Days 8-25, (By larva)

·         This phase starts with the appearance of larvae (L₄) within the aveoli and causes alveolitis followed by brochiolitis and finally bronchitis (As immature larvae move to bronchioles to bronchi to become mature).

·         Break down of the capillary and  alveolar wall (Collapse of alveoli)  followed by hemorrhage and necrosis

·         Cellular infiltrates of neutrophils, eosinophils and macrophages temporarily plug/block the lumina of the bronchioles.

·         Mucous containing immature lungworms are found in air ways (can be seen in low power microscope)

·         Heavily infected animal, whose lungs contain several thousand developing worms, may die from day 15 onward (due to respiratory failure following the development of severe interstinal emphysema and pulmonary oedema)

·         If larvae die. Granulomatous reaction may lead to grossly visible foci of consolidation distributed through out the lungs.

3.      Patent phase: Days 26-60 , (By Adult)

This is associated with two main lesions:

Adult in trachea and bronchi

·         First, a parasitic bronchitis characterized by the presence of hundreds or even thousands of adult worms in the forthy white mucus in the lumina of the bronchi.

·         The bronchial epithelium is hyperplastic (epithelization and hyaline membrane formation) and heavily infiltrated by inflammatory cells, particularly eosinophils.

·         Secondly, the presence of dark red collapsed area around the infected bronchi

·         There is a parasitic pneumonia caused by the aspiration of eggs and L₁ into the alveoli

·         These ‘Foreign bodies’ quickly provoke dense infiltrates of polymorphs, macrophage and multinucleated giant cells around them.

·         Depending on the extent of the infection, there may be varying degree of interstinal emphysema and oedema.

4. Post patent phase: Days 61-90, Recovery stage

·         The clinical signs are abating including cessation of coughing.

·         The bronchi are still inflamed

·         The residual lesions such as bronchial and peri-bronchial fibrosis may persist for several weeks or months.

·         Severe Epithelialization is due to proliferation of type two Pneumocytes on the alveoli giving the appearance of a gland like oragan

·         Post patent parasitic bronchitis is found in convalescing animals due to the dissolution and aspiration of dead or dying worm material into the aveoli along with super imposed bacterial infection leading to interstitial pneumonia.