What Do Lung Worms Look Like? Adults, Larvae & Eggs

Lungworms are slender, thread-like roundworms that live inside the airways or lung tissue of their hosts, and their appearance varies considerably depending on the species, the life stage, and whether you are looking with the naked eye or through a microscope. Adults range from barely visible wisps a few millimeters long to pale, coiled worms several centimeters in length. Larvae are microscopic and typically identified by tail shape and internal structures under magnification, while eggs can look strikingly different from one species to the next. Because dozens of lungworm species infect animals from cattle and sheep to dogs, cats, wild boar, and even snakes, no single description covers them all, but there are recognizable patterns at each life stage.

Adult Lungworms at a Glance

Most adult lungworms belong to the nematode (roundworm) group, and they share the basic nematode body plan: an elongated, cylindrical, unsegmented body that tapers at both ends. They are typically white to pale gray or slightly translucent when freshly removed from lung tissue, though some species take on a pinkish hue from the blood-rich environment they inhabit. Their diameter is surprisingly small. In histopathologic cross-sections of Oslerus osleri, a species that forms nodules in the trachea and bronchi of dogs, adult worms measured up to about 0.4 mm across, with a thin outer cuticle only around 4 micrometers thick. That cuticle, a tough but flexible outer layer, is what gives many lungworms their slightly glossy or translucent look when you see them with the unaided eye.

Length varies dramatically by species. Some lungworms of dogs and cats are only a centimeter or two long, while livestock lungworms like Dictyocaulus viviparus in cattle can reach 8 to 10 centimeters. In wild boar, Metastrongylus species cluster in the main bronchi and smaller airways of the caudal (rear) lung lobes, often forming tangled aggregations of worms visible during necropsy.

Despite living in the lungs, these worms do not look dramatically different from intestinal roundworms to the casual observer. The giveaway is location: finding a tangle of pale, hair-thin worms in bronchial passages or embedded in lung tissue rather than in the gut.

Surface Structures Under the Microscope

When researchers examine lungworms under scanning electron microscopy, the surface details become far more interesting than the plain white thread you see with the naked eye. Cystocaulus ocreatus, a protostrongylid lungworm of sheep, illustrates the complexity well. Its mouth sits at the very tip of the head end, encircled by a collar of tissue. The mouth opening itself is shaped like a capital letter Y, formed by three conjoining slits that create three small lips. Just behind that collar, six tiny bumps called papillae ring the head. These papillae serve as sensory organs, helping the worm orient itself in its environment. The entire front portion of the body, in both males and females, is covered in numerous fine wrinkles.

Males and females of many lungworm species look noticeably different at the tail end. Male Cystocaulus ocreatus possess a copulatory bursa, a fan-like structure at the rear used during mating that gives the tail a distinctive flared shape under the microscope. The female, by contrast, has a sharply pointed tail and a bell-shaped genital opening called the provagina. These sex-specific features at the posterior end are often what parasitologists use to tell males from females during identification.

Other species have their own distinguishing traits. Some lungworms of snakes, for instance, possess tiny tooth-like structures called onchia at their head end. A recently described species from the nauyaca viper in Mexico, Serpentirhabdias mexicanus, was distinguished from close relatives partly by having a single excretory gland instead of two, a detail invisible without careful dissection and microscopy.

⁴

What Lungworms Look Like in Cross-Section

Veterinarians and pathologists frequently encounter lungworms not as intact worms but as slices in tissue samples. When a piece of infected lung is preserved, thinly sectioned, stained, and viewed under a light microscope, each cross-section of the worm reveals a characteristic set of layers. In Oslerus osleri infections in dogs, for example, the outermost layer is the thin cuticle. Beneath it sits muscle tissue with a distinctive arrangement. Inside the body cavity, you can see a digestive tract lined by a single layer of epithelial cells that often have a brownish pigment. In females, much of the internal space is occupied by the reproductive tract, which may be packed with developing larvae at various stages of maturity.

Surrounding the worm sections, the lung tissue typically shows an inflammatory response: clusters of immune cells, thickened airway walls, and sometimes areas of tissue damage. In ruminant lungs infected with protostrongylid worms, alveoli (the tiny air sacs where gas exchange happens) may be filled with eggs, first-stage larvae, and a soup of immune cells including macrophages and neutrophils. In some cases, the delicate walls between alveoli remain surprisingly normal despite the presence of parasites; in others, they become noticeably thickened.

⁵

This tissue-section view matters because it is often the first confirmation a clinician gets that a respiratory problem is caused by lungworms rather than bacteria or viruses. The layered, circular worm profiles surrounded by inflammation are distinctive enough to prompt a diagnosis even when no intact worm is recovered.

What Lungworm Eggs Look Like

Not all lungworm species produce eggs that are easily found, because many hatch inside the lungs and only larvae are passed in feces. But when eggs are seen, their appearance depends heavily on the species.

Capillaria aerophila (also called Eucoleus aerophilus), a lungworm that infects both dogs and cats, produces eggs with a very recognizable look. Like other members of the whipworm family, its eggs are barrel-shaped with a plug at each end. What sets them apart from typical whipworm eggs is a combination of features: the two polar plugs are slightly asymmetrical rather than identical, and the outer shell has a net-like surface pattern made up of ridges that branch and reconnect (anastomosing ridges). Under scanning electron microscopy, this surface appears densely striated and textured, almost like woven fabric. By comparison, eggs of the intestinal whipworm Trichuris vulpis are larger and have a thick, smooth outer wall, making the distinction straightforward for anyone with a decent microscope.

⁶

Development inside the egg is slow. After being shed, Capillaria aerophila eggs take roughly 35 days before any internal development is visible, and it is not until about two months later that mobile larvae can be seen wriggling inside the eggshell. This long maturation period explains why environmental contamination persists for extended periods in kennels, catteries, or wildlife habitats.

⁶

For the metastrongyloid lungworms (the group that includes most cat and dog lungworms besides Capillaria), eggs hatch internally, meaning the female deposits eggs into the lung tissue, and first-stage larvae emerge there. The larvae are then coughed up, swallowed, and passed in feces. This is why fecal examination for these species involves looking for larvae rather than eggs.

What Lungworm Larvae Look Like

First-stage larvae, the form most often encountered in diagnostic samples, are tiny. They are typically 200 to 400 micrometers long depending on the species, far too small to see without magnification. Under a microscope, they appear as slender, translucent worm-like forms, much shorter and thinner than adult worms, with a tapered tail. The shape of that tail tip is one of the key identification features: some species have a kinked or S-shaped tail, while others have a straight, pointed tail with a small notch or spine.

Inside the lung tissue itself, larvae are seen alongside eggs in the alveoli of heavily infected animals. In ruminants with protostrongylid infections, first-stage larvae mingle with eggs and inflammatory debris within the air sacs, appearing as small, coiled or curved bodies amid the cellular reaction.

⁵

In dogs, the larvae of Oslerus osleri have been described within the reproductive tracts of adult females cut in tissue sections, appearing as “innumerable mixed immature forms” packed inside the worm’s body cavity. These developing larvae are even smaller than the free-living first-stage forms and are visible only in histologic preparations.

How Lungworms Are Actually Found

Because larvae are microscopic and eggs may never appear in feces for many species, finding lungworms requires specific diagnostic methods. The Baermann technique is the gold standard for recovering first-stage larvae from feces. It works by placing a fecal sample on gauze suspended in warm water; the larvae, which are attracted to warmth and moisture, migrate downward and collect at the bottom where they can be drawn off and examined under a microscope. For cat lungworms like Aelurostrongylus abstrusus and Troglostrongylus brevior, the Baermann method is the only reliable way to detect larvae in fecal samples; standard flotation techniques used for intestinal parasite eggs routinely miss them.

⁷

Dog lungworms similarly require the Baermann approach. In a study of Crenosoma vulpis infection in dogs, researchers collected rectal feces and examined it specifically for first-stage larvae using both the Baermann method and zinc sulfate flotation.

⁸

For Capillaria aerophila, the diagnostic picture is different because this species does pass recognizable eggs in feces. Those barrel-shaped, bipolar-plugged eggs can be found with standard flotation, though they are sometimes confused with whipworm eggs by inexperienced examiners. The textured shell surface and asymmetric plugs, as described earlier, are the distinguishing features.

Adult worms themselves are sometimes discovered during bronchoscopy (using a tiny camera threaded into the airways) or at necropsy. Oslerus osleri adults, for instance, form visible nodules on the tracheal wall that can be seen with an endoscope, and biopsy of those nodules reveals the characteristic worm cross-sections surrounded by inflammation. In livestock, lungworms are often found during post-mortem examination when tangled masses of worms are visible in opened bronchi.

Variation Across Host Species

The diversity of lungworm species is worth appreciating because the worms that infect your dog look and behave quite differently from those in your cat, in farm animals, or in wildlife. In dogs in North America, the respiratory helminths include two capillarids (Eucoleus aerophilus and Eucoleus boehmi), five metastrongyloid species (Angiostrongylus vasorum, Crenosoma vulpis, Filaroides hirthi, Filaroides milksi, and Oslerus osleri), and a lung fluke (Paragonimus kellicotti). Cats share Eucoleus aerophilus and Paragonimus kellicotti with dogs but have their own metastrongyloid, Aelurostrongylus abstrusus. Lung flukes like Paragonimus are flatworms rather than roundworms and look completely different: thick, oval, reddish-brown, and roughly the size and shape of a coffee bean.

In wild boar, Metastrongylus species are the dominant lungworms, accumulating primarily in the caudal lung lobes. These worms are sturdy enough that researchers assign them to species based on morphological features of the tail end, though in one large study about 14% of specimens could not be identified because their caudal end was damaged or lost during collection.

Sheep and goat lungworms, mostly protostrongylids like Muellerius capillaris, Protostrongylus rufescens, and Cystocaulus ocreatus, tend to form distinct nodules or brood nests in the lung tissue rather than living freely in the airways. These nodules can be felt as small, firm lumps on the lung surface during meat inspection and contain a mix of adult worms, eggs, and larvae within a capsule of immune-cell-rich tissue.

Lungworms in Reptiles and Amphibians

Lungworms are not exclusive to mammals. Snakes, lizards, and frogs all have their own lungworm fauna, and these species can look quite different from the mammalian forms. Rhabdiasid lungworms of snakes, for example, are often smaller and more delicate than the robust metastrongyloids of livestock. The recently described Serpentirhabdias mexicanus from the nauyaca viper in Mexico was distinguished from closely related species by fine anatomical details: the number of excretory glands (one rather than two) and the presence of small tooth-like onchia at the head end.

⁴

Reptile lungworms tend to live in the relatively simple, sac-like lungs of their hosts, which are structurally very different from the branching bronchial tree of a mammal. Because reptile lungs have fewer compartments, worms are sometimes visible through the thin lung wall without even opening the organ. In amphibians, rhabdiasid lungworms can be surprisingly large relative to the host, sometimes visibly distending the lungs of small frogs.

For reptile and amphibian keepers, the practical implication is that respiratory symptoms such as open-mouth breathing, excess mucus, or audible wheezing in a pet snake or frog could be caused by lungworms that a veterinarian can physically see during endoscopic examination or identify from a lung wash. The worms themselves, once removed, look like pale, limp threads, broadly similar to their mammalian counterparts but generally finer and shorter.

Why Identification Matters Beyond Curiosity

Knowing what lungworms look like at each stage is not just an academic exercise. In veterinary practice, the specific appearance of a larva’s tail, an egg’s shell texture, or an adult’s bursal structure determines which species is involved, and different species require different treatment strategies. Angiostrongylus vasorum in dogs, for example, is a far more dangerous infection than Crenosoma vulpis, even though both present with coughing and both shed first-stage larvae in feces. Telling the larvae apart under the microscope changes the urgency of treatment.

For livestock producers, the distinction between Dictyocaulus species (which live freely in airways and respond well to anthelmintics) and protostrongylids like Muellerius (which are encapsulated in nodules and harder to reach with drugs) guides both treatment choice and expectations for recovery. The nodular protostrongylids, with their characteristic brood nests visible on lung surfaces, signal a chronic infection pattern that differs fundamentally from the acute bronchitis caused by Dictyocaulus.

Even pet owners benefit from understanding the basics. If your veterinarian mentions finding barrel-shaped eggs with plugs on a fecal float, that points toward Capillaria aerophila and a different treatment approach than if larvae were recovered by Baermann testing, which would suggest a metastrongyloid like Aelurostrongylus in a cat or Crenosoma in a dog. The appearance of the parasite stage found during diagnosis is what drives the clinical decision.

1
PubMed Central. Diagnostic Investigation of Two Cases of Oslerus osleri Infection in Dogs: Microscopic Challenges and Molecular Characterisation
2
International Journal for Parasitology: Parasites and Wildlife. Lungworms (Metastrongylus spp.) and intestinal parasitic stages of two separated Swiss wild boar populations north and south of the Alps: Similar parasite spectrum with regional idiosyncrasies
3
Helminthologia. The fine structures of Cystocaulus ocreatus (Nematoda: Protostrongylidae) and the related lung pathology
4
SpringerLink (Systematic Parasitology). Serpentirhabdias mexicanus n. sp. (Nematoda: Rhabdiasidae), a parasitic lungworm of the nauyaca viper Bothrops asper (Serpentes: Viperidae) in the Mexican Neotropics
5
PubMed Central. Some Pathological Features of Lungs from Domestic and Wild Ruminants with Single and Mixed Protostrongylid Infections
6
PubMed Central. New insights into morphological and biological features of Capillaria aerophila (Trichocephalida, Trichuridae)
7
PubMed Central. Comparison of Different Copromicroscopic Techniques in the Diagnosis of Intestinal and Respiratory Parasites of Naturally Infected Dogs and Cats
8
PubMed Central. Lungworm (Crenosoma vulpis) infection in dogs on Prince Edward Island