Lungworm Infections in Dogs

What causes lungworm infection?

A lungworm infection is caused by one of several parasitic roundworms. In North America, the parasitic lungworms that infect dogs are Eucoleus aerophilus (formally known as Capillaria aerophilus), Oslerus osleri (formally called Filaroides osleri), Crenostoma vulpis (fox lungworm), Eucoleus boehmi, and Filaroides hirthi. Angiostrongylus vasorum (French heartworm) may also be considered a lungworm due to the lung damage it causes. It is an emerging parasite in North America, currently only seen in the Atlantic provinces of Canada and the eastern U.S.

Eucoleus aerophilus is primarily known as a parasite of foxes but it has also been found in dogs, cats, and other carnivores. The other parasites infect dogs and other related members of the canid family.

The adult worms of Eucoleus boehmi live in the nasal cavity of dogs and related canids. Adult worms of Oslerus osleri and Crenostoma vulpis live in the trachea and bronchi. Filaroides hirthi adults live in the lungs. Angiostrongylus vasorum live in the arteries of the lungs.

How do these parasites cause infection?

Dogs can get a lungworm infection by swallowing infective stages of parasitic lungworm. The exact way that a dog is infected varies according to the life cycle of the parasite. 

Adult, female worms lay their eggs in an infected dog’s alveolar sacs in the lungs. The eggs hatch, then the larvae move to the upper airways (bronchi) and are coughed up and swallowed. The larvae are excreted with feces or coughed up in sputum. Some lungworm species require an intermediate host to ingest this stage of larvae, such as a slug, snail, or frog. After the larvae enter the slug, snail, or frog, they develop into their infective stage, capable of infecting other dogs. Dogs can become infected by eating slugs, snails, or frogs, or by eating another animal that has eaten a slug or snail, such as a snake, lizard, mouse, or bird. 

Other species of lungworm are transmitted to a susceptible animal through saliva or nasal secretions or feces ingestion. Dogs may also become infected by food or water that has been contaminated by infectious forms of the parasite. Oslerus osleri is transmitted mainly through the saliva and transmission most commonly occurs when a mother dog grooms her puppies.  

For most other lungworms, infectious forms of the parasite are transmitted through the environment. Once swallowed, infective stages of the parasite complete their life cycle by migrating through the body from the intestinal tract and to their destination: the nose, the bronchi, or the lungs. 

Are there areas where these infections are more common?

• Eucoleus aerophilus has been found worldwide.
• The prevalence of Eucoleus boehmi is not well known, but it seems to be worldwide in distribution.
• Filaroides hirthi occurs sporadically and periodically appears in some kennels or in individual dogs.
• Oslerus osleri tends to occur sporadically and in certain regions. It is relatively common in coyotes in Nova Scotia, Canada.
• Crenostoma vulpis is widely distributed around the world in the fox population and consequently, in dogs that share the same environment. In Canada, it has spread from Atlantic Canada into other provinces, including Quebec and Ontario.

What are the clinical signs of lungworm infection?

The clinical signs of a lungworm infection depend on the number of parasites present in the lungs, the immune status of the infected dog, and the site of the infection. Puppies with lungworm infection tend to be more affected than adult dogs.

Signs can include a slight increase in breathing rate, moderate coughing or sneezing when stressed, severe coughing, wheezing, respiratory distress, and exercise intolerance. Signs are generally caused by the body’s reaction to the parasites. In some cases, there are no visible symptoms of the disease, and the infection may be discovered incidentally when your veterinarian performs tests for another reason.

How is lungworm diagnosed?

Your veterinarian may suspect your dog has a lungworm infection based on clinical signs and potential exposure to the parasite. Radiographs (X-rays) of the infected cat’s chest or upper airways may show abnormal spots or nodules in the affected airways or lung lobes.

If the dog shows signs of breathing difficulty, the X-rays may also show signs of pneumonia, called "granulomatous pneumonia," particularly if Filaroides hirthi or Eucoleus aerophilus are present. Depending on the case, your veterinarian may recommend an examination of the airways with an endoscope (called bronchoscopy) to obtain samples of cells from these nodules. Samples may also be obtained through a transtracheal wash or a bronchoalveolar lavage. For these procedures, sterile saline is flushed into the trachea or deeper in the lungs, and then fluid is retrieved for analysis to identify the organism or characteristic cells (eosinophils) that may suggest the organism is present. 

There are no blood tests to diagnose one of these parasitic infections. A definitive diagnosis of a lungworm infection is usually made by identifying parasite stages in the feces of an infected animal. The eggs of Eucoleus Aerophilus and Eucoleus boehmi may be detected in a routine fecal flotation test or in an examination of saliva or nasal discharges. The diagnosis of Filaroides hirthi, Oslerus osleri, and Crenosoma vulpis is made by detecting larvae in the fecal sample. Your veterinarian may recommend a Baermann test as well as a standard fecal test to aid in the diagnosis of these parasites. 

Could the symptoms be caused by something else?

Wheezing, coughing, and exercise intolerance are non-specific symptoms, so other diseases can cause them, including asthma, bacterial or viral infections, heart disease, cancer, or chest trauma. Your dog’s age will help your veterinarian decide the most likely cause of the symptoms. In young outdoor dogs, parasitic lungworm infection is a more likely diagnosis, while in older dogs, another disease may be more likely.

How is lungworm treated?

The treatment goals are to eliminate the parasite and control symptoms caused by inflammation. The parasites can be killed with specific antiparasitic drugs, depending on the type of lungworm. Different medications include ivermectin, fenbendazole (Panacur™), moxidectin + imidacloprid (Advantage Multi®), milbemycin + afoxolaner (Nexgard Spectra®), and milbemycin oxime + praziquantel (Interceptor Plus™).

If your dog’s symptoms are moderate, your veterinarian may prescribe medication to alleviate inflammation. In rare cases where the dog has severe respiratory distress, hospitalization for oxygen therapy or other supportive treatments may be necessary.

What is the success rate with treatment?

Generally, the prognosis for recovery is good, although the success rate for treatment depends on the severity of the symptoms and the number of parasites present.

Can my dog pass the infection to other pets?

It depends on the species of lungworm that is causing the infection.

• If the infection is caused by Crenostoma vulpis, the infection cannot be passed directly from one dog to another because the parasite needs an intermediate host (snail or slug) to complete its life cycle.
• If the infection is caused by Eucoleus aerophilus or Oslerus osleri, dogs can become infected from your dog’s feces or saliva.

Can my dog be re-infected?

Yes, if your dog is re-exposed to a contaminated environment, or continues to prey on the intermediate host species, the dog can be infected again.