Selected helminthoses in domestic ruminants: Infections of the lungs and trachea
Selected helminthoses in domestic ruminants:
Infections of the lungs and trachea
Author: Prof Joop Boomker
Licensed under a Creative Commons Attribution license.
Dictyocaulus filaria is the lungworm that occurs in the trachea and bronchi of sheep and goats
world-wide. The life cycle is direct and the developmental period is 28-30 days. Most eggs hatch in the
bronchi and are coughed up and swallowed. Those not hatched do so in the intestine. L1 is present in the
faeces and will only migrate from the dung pad when sufficient moisture is present. Larvae do not feed
but live off the granular material present in the intestinal cells. They can easily overwinter for periods up
to 100 days, provided there is enough moisture. Infection takes place per os and larvae move to the
mesenteric lymph nodes. The worms migrate through the lymphatic ducts to the thoracic duct, from there
to the heart and the lungs and will eventually burst through the lung alveoli to reach the bronchioli and
bronchi, where they mature. Males are present from day 6 and females from day 8 after infection.
The pathogenesis can be divided into 4 phases:
a)
The penetration phase, during which the larvae migrate via the lymphatics and lymph nodes to the
lungs and lesions are not yet apparent.
b)
The pre-patent phase, when the larvae arrive in the lungs. They cause alveolitis, followed by
bronchiolitis and finally bronchitis as they become mature and move to the bronchi. Cellular
infiltrates (neutrophils, eosinophils and macrophages) temporarily plug the bronchioli, causing the
collapse of groups of alveoli. This is responsible for the first clinical signs.
c)
The patent phase is associated with 2 main lesions, namely a parasitic bronchitis, characterized by
the presence of many adult worms embedded in white frothy mucus. Secondly, a parasitic
pneumonia occurs, characterized by collapsed areas around infected bronchi. The pneumonia is
the result of aspirated eggs and L1 which act as foreign bodies and provoke pronounced
polymorph, macrophage and multinucleated giant cell infiltrations. Varying degrees of oedema and
emphysema may also be seen.
d)
The post patent phase is normally the recovery phase after the adult lungworms have been
expelled. The lung tissue organizes and clinical signs abate. In a small percentage of animals,
however, a flare-up of clinical signs may occur during this time. This is due to one of 2 causes,
epithelialization of the lungs that hinders gaseous exchange and may result in interstitial
emphysema and lung oedema, and secondary bacterial infection, which results in an acute
interstitial pneumonia.
1|Page
Selected helminthoses in domestic ruminants: Infections of the lungs and trachea
Within 2 - 3 weeks after infection the animals’ breathing becomes shallow and the head raised; a violent
cough, and a profuse discharge of mucus from the nostrils is also present. Affected animals lag behind
the rest of the flock. Emaciation and prostration may set in before death.
There are small foci of pneumonia, resulting from the 5th stage worms breaking through the alveoli.
Frothy fluid is present in the alveoli and terminal bronchioles and there may be oedema and emphysema
of the interlobular septa. Most of the bronchioles contain plugs of exudate. As the worms mature and
move to the bronchi these plugs of exudate resolve. The mature worms in the bronchi are easy to see.
They are surrounded by a frothy bronchial exudate and they cause atelectasis and emphysema
secondary to the bronchitis, and which is provoked by the eggs and larvae.
The lung lesions appear as fairly large wedge-shaped areas, dark red or grey in colour, usually situated
at the posterior border of the diaphragmatic lobes and are slightly sunken below the surface of the
surrounding tissue. In cases where bacterial infections took place, the lesions show all the changes
associated with a purulent pneumonia. There is no pleuritis.
Sheep become resistant after the initial infection and larvae of subsequent infections become arrested
and are destroyed in the lymph nodes. They do not reach the lungs. Animals are equally resistant if they
are treated at different stages of the development of the worms. This is one of the few nematodes against
which a good vaccine has been developed.
The larvae are extremely sensitive to heat and desiccation but are resistant to cold. In southern Africa
these worms are therefore usually seen in isolated areas with generally low temperatures and adequate
moisture. Irrigated pastures may provide a potential focus of infection in areas free of D. filaria.
Dictyocaulus viviparus is the lungworm of cattle. It is present world-wide and is a problem in especially
Europe and North America, where cattle are housed in barns during winter. The life cycle is direct and the
developmental period is 24 days or more. Most eggs hatch in the bronchi and are coughed up and
swallowed. Those not hatched do so in the intestine. L1 is present in the faeces. The infective larvae are
relatively inactive and there is little movement / migration from faecal pads onto herbage. During heavy
rainfall or irrigation, larvae will move onto herbage but only for a limited distance.
The pathogenesis of the disease in cattle is the same as that for sheep and goats. Few other changes
are noted, but in severe cases calves may die during the 3rd week after infestation. This is accompanied
by moist rales, and rapid respiration (up to 200/min). Depending on the size of the infection, coughing
may persist for weeks or months.
The epidemiology is largely unknown. The parasite is present in isolated foci in the mist belt of the
Drakensberg in Natal and Transvaal, and is rife on irrigated pastures. Optimum conditions for larval
survival are cool moist conditions and development can take place at temperatures as low as 5°C.
Muellerius capillaris is found in the lungs of sheep and goats, and snails and slugs are the intermediate
hosts. They are common in the winter rainfall areas of the Cape and cause greyish nodules, 2-3 mm to 2
cm in diameter, to form in the parenchyma of the lung.
2|Page
Selected helminthoses in domestic ruminants: Infections of the lungs and trachea
Dictyocaulus in the trachea of a bovine (top left), and at the bifurcation of the trachea in a sheep. Note the
large amount of foam (top right). Fairly large wedge-shaped areas, dark red in colour develop as a result of
the aspiration of eggs and larvae (bottom left) and oedema and emphysema often occurs (bottom right)
Nodules caused by Muellerius capillaris in the lungs of a sheep
3|Page