Dictyocaulus viviparus, commonly known as the bovine lungworm, is a parasitic nematode that significantly impacts cattle health worldwide. This parasite primarily resides in the trachea and larger bronchi of infected animals, leading to a respiratory disease often referred to as “husk” or “hoose.” The infection can affect cattle of all ages, though first-season grazing calves are frequently the most susceptible. This parasitic bronchitis can result in reduced weight gain in calves and yearlings, and decreased milk yield in dairy cows.
Understanding the Lungworm and its Life Cycle
Dictyocaulus viviparus is a roundworm belonging to the Trichostrongyloidea superfamily. These adult worms, which can reach up to 8 cm in length for females and 5.5 cm for males, live within the airways of their bovine hosts. The life cycle of this lungworm is direct, meaning it does not involve an intermediate host.
Female worms lay eggs in the cattle’s airways, which then hatch into first-stage larvae (L1) within the host’s lungs. These L1 larvae are coughed up, swallowed, and subsequently passed in the animal’s feces. Once in the environment, these L1 larvae develop into infective third-stage larvae (L3) in as little as one week under ideal conditions. These L3 larvae are then ingested by grazing cattle from contaminated pastures.
After ingestion, the L3 larvae penetrate the intestinal wall, migrate through the lymphatic system and bloodstream to the lungs, where they mature into adult worms, completing the cycle. The period from ingestion of infective larvae to the shedding of new larvae in feces, known as the pre-patent period, is three to four weeks.
Clinical Signs and Impact on Livestock
Cattle infected with Dictyocaulus viviparus exhibit a range of clinical signs, with severity depending on the number of ingested larvae and the animal’s immune status. A common symptom is a harsh, spasmodic cough, which can worsen after exercise. Infected animals may also display an increased respiratory rate, difficulty breathing, and in severe cases, open-mouth breathing.
Beyond respiratory distress, affected cattle can experience significant weight loss and reduced feed efficiency. In dairy cows, a notable decrease in milk production is a common consequence. While clinical disease is most frequently observed in young, first-season grazing calves, older cattle that have not developed or maintained immunity can also succumb to the disease. Severe infections can lead to complications such as emphysema and secondary bacterial pneumonia, potentially resulting in mortality.
Diagnosis and Treatment Approaches
Diagnosis of Dictyocaulus viviparus infection combines an assessment of clinical signs with specific laboratory tests. A history of exposure to contaminated pastures and the presence of characteristic respiratory symptoms like coughing and increased respiratory rate are strong indicators. To confirm the diagnosis, fecal examination for the presence of first-stage larvae is a standard method.
The Baermann technique is widely used for this purpose, where larvae migrate out of a fecal sample suspended in water, allowing for their collection and microscopic identification. These L1 larvae are distinctive, being the only nematode larvae found in fresh cattle feces. Early diagnosis is important to prevent the progression of severe disease. Treatment involves the use of anthelmintics, commonly known as dewormers, which are effective against various stages of the lungworm within the animal.
Several types of anthelmintics, including macrocyclic lactones and benzimidazoles, are approved for treating Dictyocaulus viviparus. Severely affected animals may also require supportive therapies, including antibiotics for secondary bacterial pneumonia and non-steroidal anti-inflammatory agents.
Prevention and Control Strategies
Preventing Dictyocaulus viviparus infection and controlling its spread involves a combination of management practices and targeted interventions. Strategic deworming programs are a common approach, where anthelmintics are administered at specific times to reduce pasture contamination and prevent clinical disease. The timing and frequency of these treatments depend on factors such as the grazing season and the persistence of the chosen dewormer.
Pasture management plays a significant role in reducing the larval challenge. Avoiding overgrazing helps minimize the ingestion of infective larvae, as a higher density of animals can lead to increased pasture contamination. Rotational grazing, which involves moving cattle to fresh pastures periodically, can also help disrupt the parasite’s life cycle by allowing time for larvae on the vacated pasture to die off. Fencing off damp or wet areas of pastures is also beneficial, as these conditions favor larval development and survival.
Vaccination offers another layer of protection against lungworm. A live attenuated vaccine is available. This vaccine helps calves develop immunity before they are exposed to contaminated pastures. While vaccination provides strong protection against severe disease, it does not always prevent all worms from completing their life cycle, which can contribute to the development of natural immunity from ongoing low-level exposure. An integrated approach combining vaccination, strategic deworming, and sound pasture management is the most effective way to manage bovine lungworm.