The parasitic roundworms of the genus Dictyocaulus are a significant threat to livestock health, particularly in cattle, sheep, and goats. These nematodes cause dictyocaulosis, a respiratory disease often called “husk” or parasitic bronchitis due to the characteristic, harsh cough it causes in infected animals.
The Dictyocaulus Parasite and Resulting Disease
The genus Dictyocaulus consists of several species of lungworms, which are slender, whitish-gray nematodes up to eight centimeters long. The primary species affecting livestock are Dictyocaulus viviparus in cattle and Dictyocaulus filaria in sheep and goats. These parasites establish themselves in the trachea, bronchi, and bronchioles, leading to inflammation and obstruction of the airways.
The presence of adult worms and their eggs, combined with the host’s inflammatory response, causes a buildup of mucus that blocks air passages. This physical obstruction severely impairs the animal’s ability to breathe, often resulting in respiratory distress. Young animals in their first grazing season are the most susceptible to severe clinical disease.
The Life Cycle and Transmission Routes
Dictyocaulus species follow a direct life cycle, requiring no intermediate host. Adult females in the host’s lungs lay eggs, which rapidly hatch into first-stage larvae (L1) within the airways. These L1 larvae are coughed up, swallowed, and passed out into the pasture through the feces.
Once in the feces, the larvae molt twice to become the infective third-stage larvae (L3) in as little as five days under warm, moist conditions. The L3 larvae are sluggish and do not migrate far from the fecal pat on their own. Instead, they move onto the sporangia of the fungus Pilobolus, which grows on herbivore dung.
When the fungal sporangium bursts due to high internal pressure, the L3 larvae are forcibly launched up to three meters away, scattering them onto the surrounding grass. The host animal then ingests the infective L3 larvae while grazing. After ingestion, the L3 larvae penetrate the intestinal wall and travel through the lymphatic system to the mesenteric lymph nodes, where they molt into the fourth-stage larvae (L4).
The L4 larvae use the blood and lymph systems to reach the lungs. They break into the alveoli and migrate to the bronchi to mature into adult worms, completing the life cycle within about three to four weeks.
Recognizing Clinical Signs and Confirming Infection
The most characteristic sign of dictyocaulosis is a persistent, deep, and harsh cough, often called “husk” or “hoose.” Infected animals also display an increased respiratory rate, difficulty breathing (dyspnea), and often stand with their necks extended to ease air passage. In severe cases, the damage caused by the larvae and adult worms can lead to secondary bacterial pneumonia, weight loss, and death.
Diagnosis relies on clinical signs, herd history, and laboratory confirmation. The gold standard for identifying a patent infection is the Baermann technique, which isolates and concentrates L1 larvae from a fresh fecal sample. This technique is effective because Dictyocaulus larvae are the only nematode larvae typically found in the fresh feces of cattle.
Serological tests, such as Enzyme-Linked Immunosorbent Assays (ELISA), are also available to detect specific antibodies against D. viviparus in milk or serum. While these tests provide evidence of exposure, interpretation can be complicated by persistently high antibody titers. Post-mortem examination can confirm the diagnosis by revealing adult worms tangled within the trachea and bronchi.
Strategies for Management, Treatment, and Prevention
Management involves therapeutic treatment of infected animals and proactive measures to reduce pasture contamination. Several classes of anthelmintic drugs are highly effective against all stages of the parasite. Common choices include macrocyclic lactones (ivermectin, moxidectin) and benzimidazoles (fenbendazole, albendazole).
When treating severely affected animals, supportive care is necessary, especially for those with advanced respiratory distress. This includes non-steroidal anti-inflammatory drugs and antibiotics for secondary bacterial infections. Levamisole may be recommended because it paralyzes the worms, allowing them to be coughed up and expelled, which is often preferable to the worms dying in situ and causing an allergic reaction.
Prevention strategies focus on interrupting the life cycle and promoting protective immunity. For cattle, an effective oral vaccine consisting of irradiated third-stage larvae is available in some regions. The vaccine is administered in two doses, stimulating a strong immune response that prevents clinical disease, though it does not completely stop natural infection.
Strategic grazing management is a non-chemical control method. This involves rotating young, susceptible stock away from pastures grazed by older animals that may be shedding larvae. Using anthelmintics strategically, such as pulse treatments, can suppress pasture contamination while still allowing low-level exposure necessary for the natural development of long-term immunity in young stock.