Lungworm infection, or hoose, is one of the most significant respiratory diseases of cattle in northern Europe.
It is caused by the lungworm Dictyocaulus viviparus, which is a roundworm (nematode) parasite similar to gutworms.
As the name suggests, it completes its life cycle in the lungs rather than in the gastrointestinal tract.
According to Animal Health Ireland (AHI), the clinical signs of infection in cattle include coughing and difficulty breathing, especially when animals are being moved.
The disease is commonly described as ‘parasitic bronchitis’ and is also commonly known as hoose, as mentioned above, or husk. It can result in death where serious infections occurs.
As deaths from hoose can occur with very little warning and at various times of the year, it is essential that farmers consult their veterinary practitioner when drawing up their parasite control programmes.
Lungworm life cycle
The picture above shows the life cycle of lungworm. According to Animal Health Ireland (AHI), when the L3 stage larvae on pasture are ingested, they pierce the intestinal wall and move through the lymphatic system and bloodstream to the lungs.
In the lungs they leave the blood and penetrate lung tissue, where they grow rapidly and mature into adult worms.
Within 24-28 days of ingestion, eggs are laid by adult female worms in the large airways and are then coughed up and swallowed by the host animal. During the passage through the intestine, the eggs hatch and the immature larvae (L1) are excreted in the dung.
The rate of development of the free-living stages depends on environmental conditions. If the weather is warm (20°) and humid, infective larvae may be on pasture within seven days or less of being passed through faeces.
Larvae may be dispersed from the dung by a fungus (Pilobolus) or by the splashing effects of rain. This means that pastures can become contaminated with infective larvae very quickly.
Older animals (yearlings and adults) may serve as carriers over winter as some adult worms will survive in the lungs (either as fully mature or hibernating immature adults).
Infective larvae are relatively short lived. They survive for a few weeks in hot, dry conditions.
However, they may survive in pasture regrowth after silage is made and also over the winter on pasture, in enough numbers to cause disease in susceptible animals turned out to pasture in early spring.
The numbers of larvae present, their survival and their rate of development on pasture are therefore very variable and unpredictable.
Signs and symptoms
Some of the signs of lungworm are as follows:
- Mildly affected animals will have an intermittent cough, especially after exercise;
- Moderately affected animals will cough frequently while resting and have an increased respiratory rate;
- Severely affected animals will have difficulty breathing and may adopt a mouth-breathing stance with the head and neck out-stretched, mouth open and the tongue protruding;
- Lung damage can be severe and some of the pathology is irreversible, so deterioration of clinical cases and mortality can occur despite successful removal of the worms with an anthelmintic;
- Adult cows may also have a severe drop in milk yield. This can be observed before coughing is seen;
- Affected cattle have an increased susceptibility to other respiratory pathogens such as viruses and bacteria.
Close monitoring for early clinical signs of respiratory disease, particularly coughing, is the best approach for detection of lungworm infection.
Hoose is frequently suspected on clinical grounds alone, but other respiratory diseases must also be ruled out.
It may be confirmed by submitting dung samples for identification of lungworm larvae, but deaths from acute hoose can occur even before larvae begin to appear in dung samples.