CVC highlight: Keep respiratory helminths on your radar

Getty Images/Istock VectorsAn adage sometimes used by instructors at medical and veterinary colleges is, "You can't diagnose what you don't look for." That concept applies to infectious diseases, particularly some relatively uncommon helminth parasites that are not readily diagnosed by using the common fecal flotation technique. Here we review a few that infect the lung tissues.

TYPES OF LUNGWORMS

At least seven helminth parasites have been documented in the respiratory tract of dogs in North America. Similarly, three helminths are found in the respiratory tract of cats. The diagnosis of these parasites is infrequent in most regions, and necropsy data are generally lacking. However, the prevalence of these parasites might be underestimated because, in most circumstances, the fecal flotation technique widely and routinely used in veterinary clinics and diagnostic laboratories is not always the technique of choice to identify these parasites.

Eucoleus species

Eucoleus aerophilus (also known as Capillaria aerophila) is a nematode that occurs in the trachea, bronchi, and bronchioles of dogs, cats, and some wild carnivores. In most cases, infections are well-tolerated, but a wheezing or chronic cough is sometimes noted in heavily infected animals. The closely related Eucoleus boehmi is found in the nasal passages and sinuses of dogs and wild canids. Infected animals may have recurrent sneezing issues with or without nasal discharge.

These two species produce bipolar plugged eggs that can be found in respiratory secretions or in feces. Because these eggs resemble Trichuris species eggs, they may be incorrectly identified when casually viewed microscopically in a fecal flotation examination. However, the two genera of eggs can be distinguished by subtle differences in size and egg morphology (the surface of the Eucoleus species eggs is finely textured, while the slightly larger Trichuris species eggs have more prominent bipolar plugs and a smooth surface).

Crenosoma vulpis

Crenosoma vulpis, the nematode lungworm of foxes, occurs in the trachea, bronchi, and bronchioles of wild canids and occasionally in dogs. This parasite is seen most frequently in the northeastern United States and eastern Canada. Infected dogs may show mild to moderate respiratory disease with chronic coughing. The diagnostically important larval stage can be detected by microscopic examination of respiratory secretions or lavage fluids or in feces by using a Baermann technique.

Oslerus osleri and Filaroides hirthi

Oslerus osleri (also known as Filaroides osleri) is another nematode found in dogs and wild canids. The adult parasites typically form nodules at the bronchial bifurcation of the trachea. The nodules protrude into the air space and sometimes cause turbulence that results in chronic coughing. The closely related Filaroides hirthi adult parasites localize in bronchioles and deeper in the lung parenchyma.

These parasites are not typically serious pathogens. They can be diagnosed

> by finding larvae (not eggs) in direct fecal examinations

> by using the fecal concentration technique for larvae (Baermann technique)

> occasionally by finding larvae on fecal flotations

> by seeing microscopic larvae in respiratory lavage fluids.

Additionally, the tracheal nodules of O. osleri may be visualized by endoscopy or bronchoscopy, or possibly as radiodense bulges into the lumen of the trachea on thoracic radiographs.

Aelurostrongylus abstrusus

Aelurostrongylus abstrusus, a nematode found in the lung parenchyma of domestic cats, closely parallels F. hirthi infections in canids. Infections are usually subclinical, and kinked-tail larvae are the diagnostically important stage found in feces or respiratory lavage fluids of cats.

Angiostrongylus vasorum

Angiostrongylus vasorum, the "French heartworm," is a nematode that occurs in the pulmonary arteries and the right side of the heart in dogs and wild canids in Europe and Africa. The fox is the most important definitive host.

The parasite was accidentally introduced into a limited geographic area in eastern Canada and has become endemic in the fox population, occasionally infecting domestic dogs, too. Individual canine cases have been identified sporadically outside of the newly established range for this parasite.

This parasite is typically found by detecting larvae in feces, respiratory mucus, or lavage fluids. Veterinarians in the northeastern part of the United States should be particularly aware of these parasites to prevent further spread into North America.

TREATMENT

Once you've diagnosed an infection with one of these nematode lungworms in one of your patients, treatment should be initiated. Historically, fenbendazole has been used, but a macrocyclic lactone drug, such as milbemycin oxime, moxidectin (at the heartworm preventive dose), or ivermectin (at a higher dosage than heartworm preventive), are generally good choices for treating these infections in dogs or cats.

TAKE-HOME MESSAGE

The next time you deal with a dog or cat with asthma-like clinical signs-chronic coughing, sneezing, or nasal discharge-consider whether that respiratory disease is caused by a helminth parasite. Even though you may never have actually diagnosed a lungworm case in your practice, an infected dog or cat has probably been in your hospital.

SUGGESTED READING

1. Conboy G. Helminth parasites of the canine and feline respiratory tract. Vet Clin North Am Small Anim Pract 2009;39:1109-1126.

Karen Snowden, DVM, PhD, DACVM (parasitology) Department of Veterinary Pathobiology College of Veterinary Medicine & Biomedical Sciences Texas A&M University College Station, TX 77843