Lungworm in Deer: Respiratory Parasites, Coughing, and Herd Impact

Lungworm is a parasitic infection of the lower respiratory tract. In deer, the main lungworms are Dictyocaulus eckerti and Dictyocaulus cervi, close relatives of the cattle lungworm. These worms live in the bronchi and lungs, where they irritate the airways and can trigger bronchitis, coughing, and sometimes pneumonia.

The life cycle is important for herd management. Adult worms in the lungs produce larvae that are coughed up, swallowed, and passed in manure. On pasture, those larvae develop into infective stages, especially in warm, moist conditions. Deer become infected when they graze contaminated forage.

Some deer carry low parasite burdens with few obvious signs. Others, especially fawns, yearlings, stressed animals, or deer on heavily contaminated pasture, can develop more noticeable respiratory disease. In farmed cervids, that can mean slower growth, poorer body condition, and more labor and treatment costs across the herd.

Lungworm in deer is caused by parasitic roundworms in the genus Dictyocaulus. Merck Veterinary Manual lists D. eckerti and D. cervi as lungworms of deer. These parasites have a direct life cycle, which means deer do not need an intermediate host to become infected. Instead, larvae passed in manure mature on pasture and are picked up during grazing.

Risk rises when deer are kept on the same pasture for long periods, stocking density is high, drainage is poor, or weather stays warm and wet. Those conditions help larvae survive and spread farther from manure. Carrier animals can also seed pasture contamination from one season to the next.

Not every coughing deer has lungworm. Your vet may also consider bacterial pneumonia, aspiration, dusty housing, viral respiratory disease, tuberculosis in some regulated settings, or other parasites. That is why testing and herd history matter before building a treatment plan.

Diagnosis usually starts with the basics: herd history, age group affected, pasture conditions, recent deworming history, and a hands-on exam of the deer or representative animals. Your vet will listen for abnormal lung sounds, assess breathing effort, and look for signs of poor thrift or concurrent illness.

Fecal testing is often the next step. A routine fecal flotation may miss lungworm because many lungworms pass larvae, not eggs. A Baermann fecal test is commonly used when lungworm is suspected because it is designed to recover larvae from fresh manure. In herd situations, your vet may test multiple animals to understand how widespread the problem is.

If disease is more severe, your vet may recommend additional work such as bloodwork, thoracic ultrasound or radiographs where practical, and sometimes necropsy of a deceased deer to confirm the diagnosis and rule out pneumonia or other herd threats. In farmed cervids, a diagnosis often combines test results with response to treatment and management findings.

Prevention focuses on lowering exposure, not only giving dewormers. Because lungworm larvae develop on pasture, herd management is a big part of control. Rotating grazing areas, avoiding overcrowding, improving drainage in wet areas, and reducing prolonged grazing on heavily contaminated paddocks can all help lower larval intake.

Work with your vet on a parasite-control plan that fits your herd, region, and season. Blanket deworming without a plan can waste money and may not solve reinfection pressure. In many operations, it is more useful to combine strategic treatment with fecal monitoring, body condition checks, and close observation of young deer.

Biosecurity also matters. New arrivals should be evaluated before joining the herd, and any deer with coughing or poor thrift should be separated when practical until your vet helps clarify the cause. If deaths occur, necropsy can be one of the most useful prevention tools because it helps confirm what is affecting the herd and guides the next steps.