Oslerus (Filaroides) osleri

Adults of the nematode  Oslerus (Filaroides) osleri live in nodules beneath the tracheal mucosa of dogs in many parts of the world, including Canada, and in coyotes in North America.  Nodules are most common around the tracheal bifurcation.  The life cycle is direct and first-stage larvae are infective.  Probably many infections in dogs are asymptomatic.  When large numbers of nodules are present, however, especially in dogs with small tracheas, the nodules, together with aspirated eggs and larvae and mucus, can cause coughing and sometimes severe respiratory disease and death.  Oslerus osleri can be a particular problem in breeding kennels where the first-stage larvae can be transmitted through the saliva and feces from a dam to the young pups, in which disease can be severe.  Where possible, diagnosis is best done using a bronchoscope to visualise the nodules.  The examination of feces (by Baermann larval sedimentation) or trans-tracheal washes for first-stage larvae is also possible, but there are problems with the sensitivity of these tests.  When treating O. osleri with an antiparasitic drug (fenbendazole or macrocyclic lactones), clinical cure is not synonymous with elimination of infection .

Phylum: Nematoda
Class: Secernentea
Order: Strongylida
Superfamily: Metastrongyloidea
Family: Filaroididae

The Superfamily Metastrongloidea contains many of the lung nematodes of domestic animals, including Metastrongylus species in pigs, Muellerius capillaris of sheep and the other protostrongylids of domestic and free-ranging ungulates, Aelurostrongylus abstrusus of cats, but not species within the genus Dictyocaulus.  

Oslerus osleri is an unusual metastrongylid in that it has a direct life cycle.

Note: Our understanding of the taxonomy of parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on Deplazes et al. eds. Parasitology in Veterinary Medicine, Wageningen Academic Publishers, 2016.

Adult O. osleri are very small, to approximately 5 mm (males) and 15 mm (females) in length, and they are tightly coiled in the tracheal nodules in which they live.

Nodule Section

First-stage larvae of O. osleri are approximately 230 to 265 µm in length and have a characteristic tail. They can easily be distinguished microscopically from the larvae of Strongyloides stercoralis, which have a rhabditiform pharynx.

Oslerus osleri occurs in dogs in many parts of the world, including Canada, and is common in coyotes and other canids in some areas, including western Canada.

Adult O. osleri are located in sub-epithelial nodules in the trachea and large bronchi, especially around the coryna. Many nodules are approximately spherical and may be clustered. The nodules containing mature adult parasites are usually visible grossly, as are the white parasites coiled within them. Occasionally it is possible to see the tail of a female parasite protruding from the nodule.  Females are ovoviviparous; the eggs hatch inside the female.  Both larvated eggs and larvae are passed by the female nematode. The first-stage larvae – which are the infective stage - are released into the tracheal lumen and move up to the pharynx, are swallowed and then pass out in the feces. Infection of the definitive hosts is by ingestion of first-stage larvae. First stage larvae cross the intestinal wall and travel via lymphatics or blood to the lungs. Auto-infection may occur with O. osleri, in which the infective first-stage larvae penetrate into the intestinal wall without leaving the host, migrate to the trachea, and develop into adult parasites. 

In western Canada, most cases of clinical O. osleri in dogs are in individual pet animals, and it is difficult to identify the source of infection, other than where the infection is transmitted to a pup from the dam. In one instance in British Columbia, O. osleri was a major problem in a breeding kennel. In Saskatchewan, the parasite was first identified in dogs used for hunting coyotes. Given the presence of O. osleri in this wildlife host,  it is possible that hunting dogs may have become infected by ingestion of the infective first-stage larvae in the trachea and lungs of the coyotes.  Coprophagia of coyote feces might be another route of exposure.

Many dogs infected with O. osleri, especially those with few nodules, show few adverse effects. The nodules, the larvae and the resulting mucus may interfere with air flow and may cause coughing – typically including retching - especially after exercise. Aspiration of mucus and larvae into lungs may cause bronchopneumonia, especially in heavily infected dogs. Oslerus osleri can lead to very severe clinical signs – including cyanosis and collapse - in some animals, especially pups which acquire heavy infections from their dam.

Oslerosis should be suspected in a dog with a chronic cough and a history compatible with exposure to the parasite i.e. from a kennel or dam with a known case or contact with the carcasses or feces of wild canids (fox, coyote).  The very distinctive tracheal nodules mean that O. osleri is best diagnosed using bronchoscopy. History, clinical signs and radiology can also be helpful. Larval recovery from feces using the Baermann technique has a poor sensitivity, even with repeat samples. A trans-tracheal wash for larvae might be more sensitive. The first-stage larvae of O. osleri are characteristic and can be distinguished morphologically (by an experienced technician) or by molecular means from those of Strongyloides stercoralis, Crenosoma vulpis, and Angiostrongylus vasorum, the only other nematode larvae likely to be found in fresh feces from dogs in Canada, for which dogs serve as definitive hosts.  Other larvae that can be detected in feces of dogs include hatched hookworm larvae (in non-fresh samples), larvae from hatched strongyle or trichostrongyle eggs in livestock feces (from coprophagia), or free living nematodes (from environmental contamination). 

No drugs are approved in Canada for O. osleri in dogs, although several products have been used extralabel, with variable results. Fenbendazole has a label in the UK, and macrocyclic lactones (ivermectin, doramectin) also appear to be effective. Multiple rounds of treatment may be necessary.  Bronchoscopic removal of the nodules may also be attempted, especially if the nodules are seriously obstructing the airway.  With O. osleri, there is an important difference between a clinical cure and eliminating infection. It seems possible to achieve the former in the absence of the latter, and in kennel situations this may be important in relation to control of the infection. 

Effective control of O. osleri in breeding kennels can be very difficult because it is usually necessary to interrupt the very close contact between a dam and her suckling pups. Recognition of the infection in the dams and appropriate treatment, coupled with efforts to maintain a clean environment, is also helpful. The significance of autoinfection has not been explored for O. osleri, but is known to be a problem with the closely related F. hirthi. 

There is no evidence that Oslerus osleri is zoonotic.

Conboy G (2009) Helminth parasites of the canine and feline respiratory tract.  Veterinary Clinics of North America Small Animal Practice 39: 1109-1126.