Helminthology

Helminths of ruminants

Author: J BOOMKER

Respiratory system

With the exception of two Dictyocaulus species –Muellerius capillaris and two Mammomonogamus species –no adult helminths occur in the lungs or trachea of ruminants. Several others, however, may pass through the lungs as larvae on their way to their favoured sites elsewhere in the body. Infections with lungworms are not always obvious, clinically speaking. However, in cattle in Europe and sheep in Africa they invariably cause disease in na ïve animals, and, when not treated, often have fatal consequences.

Lungworms

Dictyocaulus species

The Dictyocaulus species belong to the superfamily Trichostrongyloidea, family Dictyocaulidae –and are colloquially known as lungworms. They are large, milky-white worms that are easily seen in the trachea of their hosts. Males are 3 to 8 cm long and females 5 to 10 cm long. The spicules are stout, boot-shaped, and intricately sculptured. Females are ovoviviparous and lay eggs with a thin shell containing a fully developed first stage larva.

Distribution

Dictyocaulus species occur worldwide and are particularly important in temperate climates.

Hosts

Sheep, goats and occasionally some antelope are the hosts of D. filaria , and cattle, deer, reindeer, water buffaloes and camels are the hosts of D. viviparus .

Life cycle

The life cycle of Dictyocaulus species is direct (monoxenous) and the helminths are sometimes referred to as 'geohelminths'. Eggs may hatch in the lungs, but are usually coughed up and swallowed, and the first stage larvae then hatch when they pass through the intestinal tract. The first stage larva of D. filaria has a small cuticular knob at the anterior extremity, which is lacking in D. viviparus . Brown food granules are present in the intestinal cells of the larvae of both species, and the free-living stages live off the food granules. After a few days, the larvae reach the second stage but retain the cuticle of the first stage. Once they reach the third (infective) stage, the cuticle of the first stage is cast off, but that of the second stage is retained. The infective stage is reached after six or seven days.

Infection of the host occurs per os. Larvae penetrate the intestinal wall and pass to the mesenteric lymph nodes –where the third moult takes place. The now fourth stage larvae pass via the lymph and blood vessels to the lungs where they are trapped in the capillaries and break through into the alveoli. They then migrate to the bronchi and trachea to mature into the adult male and female worms. The pre-patent period is about 28 days for D. filaria , and 22 days for D. viviparus .

Transmission

Although D. filaria is cosmopolitan in its distribution, it is only responsible for sporadic outbreaks –even in warmer areas such as parts of Africa and the Mediterranean. Larvae are extremely sensitive to heat and desiccation, but are resistant to cold and can overwinter in colder areas. Carrier animals are important as a source of pasture contamination in Africa, where the climate is often unsuitable for larval survival. Outbreaks of disease are usually seen after prolonged rain around the time that lambs and kids are weaned. Fountains, marshes, streams and rivers provide adequate moisture for the survival of free-living stages, and irrigated pastures can be potential foci of infection.

D. viviparus is similar to D. filaria in many respects, but its epidemiology is largely unknown. Isolated foci of infection occur wherever the climate is suitable for the survival of free-living stages –usually in the cool, moist parts of a region. The parasites are highly prevalent on irrigated pastures. In temperate regions like Europe and North America, these parasites are particularly important (see Urquhart et al . (1991) and http://www.merckmanual.com for a detailed description of the epidemiology in these countries).

Socio-economic importance

Outbreaks of parasitic bronchopneumonia occur sporadically, and then mostly on irrigated pastures, or in the cool, moist areas of a country. Animals lose condition, and unless treated, some deaths may occur.

Pathogenesis

The pathogenesis of dictyocaulosis can be divided into four phases:

  1. The penetration phase (days 1 to 7) during which the larvae migrate from the intestine to the lungs via the mesenteric lymph nodes and lymphatic system. Neither clinical signs nor pulmonary lesions are seen.
  2. The pre-patent phase (days 8 to 25) starts when the larvae arrive in the lungs. They cause alveolitis, followed by bronchiolitis, and finally bronchitis as they reach the bronchi. Cellular infiltrates (neutrophils, eosinophils, and macrophages) temporarily block the bronchioli –causing atelectasis when groups of alveoli collapse. The first clinical signs –tachypnoea and coughing –are now observed. Heavily infected animals may start dying from day 15 onwards, due to respiratory failure after the development of severe interstitial emphysema and lung oedema. Epithelialisation and the formation of hyaline membranes in the alveoli commence at this stage.
  3. The patent phase (days 26 to 60) is associated with two main lesions –a parasitic bronchitis and a parasitic pneumonia respectively. The former is characterised by the presence of many adult worms in the bronchi and distal trachea, which are embedded in a frothy white mucus. There is severe damage to these tissues, which is manifested by a hyperplastic epithelium infiltrated by inflammatory...

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