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An ideal year to watch for lungworm infection

Every year in later summer and early fall we see a few outbreaks of lungworm in cattle on pasture. This is especially true if the year has been wet and the life cycle of the lungworm can be completed. This year could be the perfect storm in parts of Western Canada.

The life cycle of the lungworm (dictyocaulus viviparous) starts with the animals picking up infective larvae off grass. The ingested larvae migrate through the intestine into the bloodstream and end up in the lungs as adults. They cause damage to the air sacs in the lungs and the adults live in the bronchial tubes.

The adult worms produce a tremendous number of eggs, which are coughed up by the beef animal and swallowed again. The eggs pass into the intestinal tract, and change to larvae by the time they are passed out onto pasture in the manure. The entire cycle takes about one month.

Finding these larvae in manure is one of the ways veterinarians can diagnose the condition in an animal. It takes a different test than looking for the intestinal worm eggs.


The Baerman technique is done on a handful of fresh manure and takes a few hours to run. With manure samples that arrive in the morning, we can give producers an answer the same day. Most clinics are set up to run this test in-house. Finding even one larval lungworm is significant and necessitates deworming. It is probably best to test several manure samples.

Other than checking manure samples, cattle can be checked clinically or autopsies can be done if any have died. Clinically we may see cattle doing poorly in spite of good grass conditions. There is often a number with very prominent coughing. Some may have varying degrees of diarrhea. With herd involvement there is always great variation with some doing very poorly, others just a bit rough-haired, and still others looking almost normal. Generally the younger cattle (calves and yearlings) are most susceptible as any previous exposure to the pest yields some immunity.

If we listen to the lungs there is often evidence of emphysema caused by the lungworm larvae damaging the air sacs. We may even get a secondary bacterial or viral pneumonia due to the stress lungs are under. In severely affected cattle, even with successful treatment, the damage is done resulting in a poor-doing animal with reduced lung capacity. Through autopsy an experienced vet will notice changes to the lungs and actual adult lungworms can be found in the bronchial tubes and trachea. It is very easy to see how lung capacity has been diminished.


The lungworm can survive in our Canadian winters but most of the exposure comes from carrier cattle shedding the larvae on the pastures. Generally there is a buildup of the pest by midsummer in affected areas. Treatment with the endectocides such as Dectomax or Ivomec are somewhat effective and can be used as part of the routine treatment in the fall on asymptomatic carrier animals.

If a clinical diagnosis is made in the summer we sometimes have the dilemma of how to treat if cattle cannot be easily rounded up. The drug fenbendazole (Safeguard) is very effective against lungworms. It comes as either a drench, crumble or concentrated feed additive. The product can either be mixed in grain and fed in feeders as a one-time treatment or fed in reduced amounts over three to six days. For example, feeding one-third the required amount in grain for three days in a row. This ensures most of the cattle will get it.

A technique found successful is scripting the concentrate into the trace minerals. This requires a veterinary prescription as it is not an approved method for administration. Most cattle, young or old, consume minerals at their leisure. This product if put out just before maximum exposure to lungworm (that appears to be around July first in northern Alberta) goes a long way to reducing the incidence.

The Safeguard is mixed in with many assumptions. We assume adult cattle will eat 30 to 60 grams of mineral per day and that all cattle will eat minerals at least every five to seven days. Some cattle get over treated but at least there is a very good likelihood most cattle will get treated and reduce contamination for the rest or subsequent animals that may get reintroduced. On average 80 to 90 per cent of the cattle get enough medication to clear the lungworms.

When treating clinical cases, the symptoms such as coughing or respiratory problems will actually increase for a few days. This is because all dead worms need to be coughed up and swallowed. Killing all adult worms may cause a reaction but the animal must be dewormed so treatment is necessary. Any pneumonia complications may need to be treated with antibiotics at this time.


Pastures that have had previous lungworm problems are the ones we really concentrate on. Over a few years treatment possibly can be phased out, but always be on the lookout for the clinical signs reappearing.

Bison are very susceptible to lungworms and the same Safeguard can be used but under a veterinary prescription. The endectocides if given are used at the same rate as cattle. Horses can get cattle lungworms as well, but all you will see is clinical evidence as they are a dead-end host so no larvae will be shed in the manure.

As an indicator, any time cattle are coughing have them checked because summer lungworms on pasture are a real possibility and often get overlooked. Bring several manure samples into your veterinarian if you suspect the pest and follow their recommendations for treatment. †

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



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