Lesson 6 Nematodes Part 2


Strongyles and hookworms- stomach and intestinal worms


The strongyles and hookworms infect the gastrointestinal tract of dogs, cats, horse, ruminants and swine. In this group are the hookworm of the dog and cat, and the strongyles and trichostrongyles of the horse, ruminants, and pig. They all produce the typical strongyle egg, which may vary slightly in size and shape depending on the parasite species, but all the eggs exhibit some degree of morula formation and have a thin shell.


The hookworms of the dog and cat are found in the genus Ancylostoma and Uncinaria.  Hookworms attach to the lining of the small intestine and suck blood. When they move to a new site, the old site will ooze blood into the lumen and be passed down into the feces. As the blood passes through the tract, it is partially digested and makes the feces appear dark and tarry. The animal will also be anemic. This is a common sign of what is known as high intestinal bleeding from any cause, but when we see it, we always want to rule out hookworm infection. During its life cycle the hookworm also migrates through the host and causes a cough as the migration occurs in the lungs and the larva are coughed up and swallowed to get to their final destination. Like the roundworm of the dog, the hookworm can also infect the fetus transplacentally before birth and through the milk after birth. In addition, when the eggs hatch out in the environment, the larva are capable of penetrating skin and then migrating to the intestinal tract. What you don’t want to hear is that these same larva can also penetrate our skin. However, the larva seem to only migrate aimlessly in human skin and rarely reach our intestinal tract. The migration tract becomes inflamed and can be seen as squiggly red itchy lines in the skin, known as cutaneous larval migrans. This is a good reason not to go barefoot in moist warm soil that is frequented by dogs. Standard fecal flotation detects the eggs, and they can be identified at 10X.


Ancylostoma spp.


Hookworm of the dog courtesy of Jean Holtzen


Strongyles of the horse infect the large intestinal tract and can be a major cause of colic. They are classified into two groups: the large strongyles and the small strongyles. Strongylus vulgaris is an example of a large strongyle, well known for its migration in the mesenteric artery during its life cycle. As a result of the migration, thrombi form and may cause colic during these episodes.  The small strongyles are also known a cyathostomes. The prepatent period for large strongyles on average is about 9 months, while that of the small strongyles is about 6 to 12 weeks. Many members of these groups also exhibit hypobiosis. Transmission is by the fecal oral route. Because of the heavy fecal contamination of their environment, and taking into account the prepatent periods of the strongyles, deworming programs for the horse have been developed. Horses are now routinely dewormed several times a year, sometimes as frequently as every 6-12 weeks or so. In some cases, daily administration of an anthelminitic in the feed is recommended. With these methods, the pasture contamination is decreased and the re-infection rate decreases also.


The eggs of the horse strongyles are easily found on standard fecal flotation and they all look alike no matter what species or size of strongyle. The eggs typically are elongated ovals and have a morula of several cells, sometimes up to 16 or more. Horse feces may also be tested for egg counts, to give an estimate of ongoing pasture contamination and also to judge the effectiveness of the deworming program. If the egg count starts to rise before the next scheduled deworming, the deworming schedule will be readjusted. With the increased use of anthelmintics, there has been concern that resistant strains of parasites are developing from overuse of these drugs. Therefore, some advise to check egg counts on a regular basis, deworm, and then recheck the egg count in 2 months. In this scenario, deworming is recommended only when the egg counts start to rise. There will be a predictable rise in the spring when hypobiosis tends to end.


  Strongyle egg of the horse


For a review of a fecal egg count technique, click here


The trichostrongyles and strongyles of ruminants infect the gastrointestinal tract.  Trichostrongyles are so called because they are very tiny and hairlike, but don’t let their small size fool you, they can be busy little bloodsuckers, causing a severe anemia. There are numerous species and they produce typical strongyle eggs containing a morula. Some infect the compartments of the stomach and some infect the intestines. Nematodirus spp. is a trichostrongyle that has a fairly distinctive egg in that it is very huge and is tapered on each end, whereas the other eggs in this group are more oval and blunted on each end. Often there is more than one strongyle or trichostrongyle species infecting a single animal which can worsen the clinical signs. If Haemonchus, Ostertagia, and Trichostrongylus are present at the same time, it is known as the HOT complex because clinical signs and prognosis are usually so much worse with this overwhelming combination of parasites.


  Bovine strongyle/trichostrongyle, courtesy of Jean Holtzen


 Nematodirus (large egg) with other strongyles/trichostrongyles in background, courtesy of Amy Baeza


The strongyles and trichostrongyles of large animals tend be limited to one type of host, that is, either the equines, ruminants, or swine. However, the one exception is Trichostrongylus axei which is capable of infecting all three types of animals: horses, ruminants, and swine. Sometimes you may find large animal strongyle eggs on fecal flotations from dogs, but dogs do not get infected with large animal strongyles. This means that if you find a large strongyle egg on a dog fecal, it is likely he was eating horse feces (as dogs like to do). The strongyle egg just passes on through, but does not infect the dog. In this scenario, the finding of a large animal strongyle egg in dog feces is termed a "spurious parasite", because although it is a parasite, its finding in dog feces is not indicative of a true parasitic infection of the dog.  If you have trouble telling which parasite the egg came from, measure the ova. The difference in size will help you differentiate whether the egg is from the dog parasite or a large animal parasite. Ancylostoma eggs are usually in a size range of 56-65  by  37-43 um, whereas the strongyles of large animal parasites are usually a bit larger.


Deworming schedules of ruminants emphasizes the phenomenon of hypobiosis which is somewhat predictable for many of the parasites, such as Ostertagia spp. Hypobiosis tends to end at the end of winter and at the time of parturition. Pasture contamination accumulates as the summer progresses. As a result, there is a spring rise and a later summer rise of worm burdens in livestock with strongyles. Therefore, many deworming schedules will include a deworming in the spring, and mid to late summer, fall, and at periparturient times. Of course rotation of pastures is important as well in controlling exposure of the parasite to the host. During dry seasons, larva are less likely to survive on pastures.


Threadworms- the Strongyloides spp.


Strongyloides  spp. are known as threadworms because they are tiny and threadlike. They inhabit the intestinal tract. Their claim to fame is that only the female is parasitic and even more special, the parastitic females are parthenogenic, producing fertile eggs without the aid of a male worm! The eggs are reminiscent of the typical strongyle type eggs, but usually contain a larva when passed. In some cases, the eggs embryonate and hatch out, so that larva rather than eggs are found in the feces. Larva can then infect another host or reside in the environment. Larva that remain in the environment survive as free living forms in the soil, and have a more traditional reproduction that includes male and female worms. They can live for several generations in this manner before infecting another host. The routes of infection are fecal-oral, through contact with contaminated soil, or through the milk. Many of the larva infecting the host will migrate to the mammary tissues. Another reason to drink only pasteurized milk! Interesting factoid: Orphaned bottle fed animals are much less likely to get Strongyloides infections.


The strongyloides tend to be host specific, but some are shared between a few hosts. For example, Strongyloides stercoralis infects dogs, cats, and humans. Strongyloides westeri infects horses,  Strongyloides papillosus infects ruminants and so on. The larva or eggs can be seen on fecal flotation.


  Strongyloides spp. larva and ova


Strongyloides spp.ova (embryonated)




Worms that infect the lungs or airways include Aleurostrongylus abstrusus, Filaroides spp. and Dictyocaulus sp. Obviously these organisms will cause illnesses with respiratory signs. The eggs are thin shelled like a strongyle type egg, but larvated when released. Often by the time the eggs are coughed up, swallowed and passed in the feces, they have hatched out and you will see larva and not eggs. When the larva are ingested they migrate from the intestinal tract to the respiratory tract to continue the life cycle. The larva can be detected on fecal flotation. Specific identification as to the species of worm the larva originiates from is sometimes determined by fecal culture techniques using the Baermann apparatus. It is also possible to detect the eggs or larva from a tracheal wash or some exudate that the animals snorts or spits out.


For a review of fecal culture techniques using the Baermann apparatus, click here.


Aleurostrongylus abstrusus is the lungworm of cats and is usually only found in cats that hunt. Current information indicates that a slug or snail is the intermediate host, but why a cat would eat such a thing calls that into question. Another theory is that there is a paratenic host, such as a rodent or bird who does eat snails and slugs and could be a source of infection to cats. The larva is interesting to find as it typically is coiled in an S shape or to some it looks like a treble clef. For photos of Aleurostrongylus abstrusus, see page 20 figure 3-14, page 38 figures 4-31 and 4-32 in your textbook.


Filaroides spp. infects dogs and typically is found in the airways such as the bronchi and bronchioles. But some species infect the lungs. The life cycle is direct. And usually only larva are found on fecal flotation. The larva is often coiled. For a photo of Filaroides spp, see page 38 figure 4-33 in your textbook.


Dictyocaulus spp. are the lungworms of large animals such as cattle and horses. There are several species of Dictyocaulus spp., each somewhat specific for its host, and are usually found in the airways. Larvae are the forms usually detected in specimens. See page 46 Figures 4-50 in your text book.


Meningeal Worm


Infections with Paralaphostrongylus tenuis, also known as the meningeal worm or brain worm, deserves special mention because of the increasing popularity of nontraditional species, such as llamas. The adult worms reside with the meninges of white-tailed deer and amazingly are nonpathogenic. Larvae undergo migration and are eventually passed out into the feces. The larvae have a distinctive kinked tail with a dorsal spine. Larvae passed in the feces are acquired by the snail intermediate host. The snail containing the infective larvae are then eaten by the host while grazing or browsing. In hosts other than white-tailed deer, the migrating larva cause severe tissue damage in the central nervous system and ultimately death.  Species that are susceptible to CNS disease from this worm include llamas, sheep, goats, moose, caribou, elk, and mule deer. Keeping any of these animals in an area where there is a large population of potentially infected white-tailed deer puts them at risk for infection with the meningeal worm. Strategically timed monthly treatments will control the larval stages, but not the adult parasite. Measures to control snails are also helpful. 


For more information on the meningeal worm in llamas go to:





The Trichuris spp. are also known as the whipworms.  There are several species. Trichuris vulpis infects the dog, Trichuris ovis infects ruminants, and Trichuris suis infects the pig. The adults have a fine hairlike head and a rather fat and curved tail end.  It reminded someone of a whip with a handle, hence the name whipworm. The thin head is embedded in the wall of the large intestine or cecum and that fat tail end part hangs out into the lumen of the intestine. Typically this parasite induces a chronic recurring colitis, in which mucus and diarrhea spotted with fresh blood is produced intermittently. The route of infection is fecal-oral.


Trichuris eggs are very resistant in the environment and can be present for many seasons, so soil contamination with the eggs becomes well established and a source for reinfection.  The parasite may produce eggs sporadically, so on some days there may be large numbers of eggs and on other days there may be very low numbers. On the low egg number days, the fecal tests may be “negative” for ova. Therefore, if whipworm infection is suspected but the fecal test shows no evidence, some veterinarians might deworm based on the occult parasitism concept. We might also recommend retesting the feces at random intervals over the next few days to see if the parasite can be documented. Since the prepatent period is about 3 months, monthly deworming for 3 months may be done.


The eggs of Trichuris spp are brown football shaped eggs with knobs on each end. They can be seen on routine fecal flotation.


  Trichuris spp. egg




The worms in the capillarid group infect the intestinal tract, respiratory tract, or urinary tract. Many of the worms have had the genus changed from Capillaria to some other term, so you may see both terms used for some species. The egg that is produced is the capillarid egg, which is similar to the trichurid egg. The capillarid egg is oval, barrel, or lemon shaped depending on the species, but it has a knob on each end like the trichurid egg. The knobs are sometimes off set from the center making some of the eggs look a little lopsided, or assymetrical. The outer shell may have a rough or netted appearance. The color of the egg is usually a much lighter yellow color or even colorless. Since the eggs are often very similar to Trichuris spp. eggs, you may mistake them for a whipworm egg. Sometimes, so called intractable whipworm infections are actually Capillaria infections, which may require different medications.


Eucoleus (Capillaria) spp. inhabit the respiratory tract. Depending on the species, worms  can be found in the sinuses, bronchial tree, or lungs. Eucoleus aerophilia inhabits the bronchial tree of the dog. The capillarid eggs are released, coughed up, and swallowed, then are passed out into the feces where they can be detected with fecal flotation, or tracheal washes and swabs. The life cycle may be direct or use the earthworm as an intermediate host. For a photo, see page 39 figure 4-35 in your textbook.


Pearsonema (Capillaria) plica inhabits the urinary bladder. The capillarid egg is passed out in the urine, where it can be found easily in the sediment during urinalysis. The earthworm may be an intermediate host, or a paratenic host. For a photo, see page 40 figure 4-39 in your textbook.


    Photos of Capillaria plica found during urinalysis at low and high power. The urine sediment has been stained. Photo courtesy of Anne Widell and David Morris, DVM.


Oxyurids- Pinworms


Pinworms are scavengers that generally inhabit the colon and rectum of omnivores and herbivores, including the horse, rabbit, rodents, and humans. Carnivores do not get pinworms, so the pet cat or dog are not to blame for any pinworm infection of people in their household. Pinworms are very host specific, meaning each pinworm species will only infect one type of host. Humans have their own pinworm species, as do the horse, rabbit, and rodents. The significance of this is that humans do not get pinworms from animals. Animals do not get pinworms from humans. Horses get pinworms from other horses. Humans get pinworms from other humans and that is that!!


Transmission of pinworms is via the fecal oral route. During the life cycle of many pinworm species, the female worm will come out at night and lay her eggs around the anus, then returns to the rectum. The pinworm of the horse is Oxyuris equi. Horses often exhibit signs of an itchy anus by rubbing their tail against the fence, doors, or trees. The hair on the tail will become roughed up and disheveled as a result so even if you don’t see the horse doing this, you can tell by looking at the tail that the behavior has been occurring.


The eggs from the pinworm of the horse are large and easily seen on the 10x objective. They are an elongated oval, slightly flattened on one side with an operculum or little flat lid on one end. The eggs could be detected in fecal flotation, but a reliable technique is the scotch tape test. The sticky side of scotch tape is applied against the perianal and anal area to hopefully collect some eggs deposited there. The tape is examined for eggs by placing it sticky side down on a slide. The eggs may be larvated.


  Oxyuris equi ova, courtesy of Jean Holtzen


The pinworm eggs of other species from the rabbit or rodents typically look like bananas, instead of like that of the horse. Some of the pinworm species of these hosts do not lay eggs on the anus, so you may only find the eggs on fecal flotation. For photos to compare how the various pinworm species appear, see figure 4-64  on page 53, figure 4-76 on page 59, figure 4-77 on page 60, figure 4-82 on page 63, and figure 4-87 on page 66 in your textbook.


Filarial worms


Worms that reside in tissue or vascular spaces and produce microfilaria are known as filarial worms. Microfiliaria are only produced if the female is fertilized by the male. The most famous species in this group are Dirofilaria immitus, Acanthocheilonema (Dipetalonema) reconditum, Elaephora scheideri, and Onchocerca cervicalis. The microfilaria either circulate in the blood or the skin, and are transmitted by a biting insect that serves as the intermediate host.


Dirofiliaria immitus is the heartworm of the dog, but it also infects the cat, ferret, and wildlife such as wild canids and marine mammals (pinnipeds). Wild canids such as the wolf and coyote serve as natural reservoirs. The adult worms normally reside in the right ventricle and pulmonary artery, but sometimes will end up at aberrant sites. Over time, heartworm infections cause exercise intolerance which progress to heart failure. In the cat, clinical signs often resemble asthma. There must be at least one pair of female and male worms to produce microfilaria which then circulate in the peripheral blood. Mosquitoes are the intermediate host and acquire the microfilaria during feeding. After development in the mosquito, the L3 is transferred to a new host when the mosquito feeds again. The larva reside in the tissues and after development to more mature stages, migrates to the heart. The prepatent period is about 6 months. Prevention of the parasite infection is aimed through routine periodic treatment of the dog with medications that affect the larva present in the tissues but have not traveled to the heart yet. The dog can be bitten by an infected mosquito, but because of preventative medication, the larva do not complete the life cycle.


If a dog becomes infected with heartworms, then the treatment for it requires a careful approach to address both the elimination of the adult worms and the microfilaria. Once the adult worms start to die, they will be passed out into the pulmonary circulation where they potentially form emboli. The dog may become very ill with fever and dyspnea a few days after treatment is initiated, and some die as a result. This adverse reaction depends to some extent on the medication that is chosen as well as the worm burden the dog has. The microfilaria will continue to circulate for some time, and can cause immunologic damage to organs such as the kidneys. Therefore, at some point during the treatment process, medications are given to clear the microfilaria.  


The identification of heartworm infections in the dog is through ELISA testing which tests for the presence of heartworm antigens, whether or not microfilaria are present. The ELISA test is the favored test, but examination for microfilaria are still necessary to assess response to treatment of patent infections. Microfilaria can be found at the buffy coat in spun PVC tubes, concentrated using the Knott’s test or Difil filtration technique, or in a peripheral blood smear. Microfiliaria are often not present in heartworm infections of the cat or ferret.


--Microfilaria in peripheral blood at low power. Red blood cells tend to clump around a microfilaria in a small open space so that you can spot them more easily.


Microfilaria in peripheral blood at high power


Microfilaria filter test, courtesy of Jean Holtzen. The microfilaria tend to straighten with this technique.


For a review of the Difil filtration test for circulating microfilaria , click here.


For a review of the buffy coat examination technique, click here.


For a review of the modified Knott's technique, click here.


For more details and up to date information about heartworms go to




Acanthocheilonema (Dipetalonema) reconditum is a filarial worm of the dog that resides in the subcutaneous tissues and usually causes no problems. The intermediate host is the flea. Acanthocheilonema (Dipetalonema) produces a circulating microfilaria that is easily confused with that of the heartworm. The tail of Acanthocheilonema (Dipetalonema) displays a button hook and the head is blunt. The microfilaria of the heartworm has a tapered head and a straight tail. Sounds simple, but the truth is, these differences are hard to appreciate, and the ELISA test for heartworm antigen is useful in telling you which one is the likely one you have in that patient.


Elaephora spp. is the arterial worm of sheep. The microfilaria are present in the capillaries of the skin on the face. Hair loss and dermatitis of the face is a sign of infection with this parasite, a condition called “sorehead”. It is transmitted by horseflies. Demonstrating the microfilaria through biopsy of the skin is the recommended method of diagnosis.


Onchocerca cervicalis adults live in the ligamentum nuchae of the horse. The microfiliaria are present in the skin and are ingested by the intermediate host, the biting midge (looks like a fat gnat), during feeding. After some development in the little fly, another feeding on another horse releases infective larva into a new host. Horses infected with Onchocerca typically have a dermatitis on the face, neck, and ventral midline which is more common in the summer months, and often called “summer mange”. Microfiliaria can be demonstrated with skin biopsy techniques. 


The Trichina worm


We haven’t really emphasized the parasites of swine, since most of the diagnostic stages of their parasites are similar to those of other hosts. But here is a special one for you to know: Trichinella spiralis. Trichinella worms inhabit the intestinal tract and muscle tissue of pigs, and some wildlife. The worms are so tiny that they lie along side the villi lining the intestinal tract. The female deposits small prelarval stages into the tissue. The larval forms migrate through the circulation to other tissues where they become encysted. The most common regions for larval cysts to form are in the muscle tissue. Other animals become infected with Trichinella by eating infected tissue through predation, cannibalism, and carrion eating. Humans are infected with Trichinella by eating undercooked infected pork or game. Once ingested, the cycle repeats and new larva then migrate to the tissue of the new host, causing damage, and eventually become encysted. There may be no clinical signs, or if there was a heavy exposure, clinical signs can be severe and lead to death, especially if the heart muscle is parasitized. Disease cause by Trichinella spiralis is termed trichinosis or trichinellosis.


Trichinella larva are detected in tissue by performing a squash prep of muscle tissue, usually the diaphragm. The larva can be seen coiled up in a little circle. Trichinella is considered endemic in the United States.


For a review of the tissue squash preparation, click here.


Controlling infections has been aimed at forbidding the feeding of uncooked offal to pigs and cooking pork until it is well done before human consumption. Deep freezing will also kill the larva found in pigs, but not those strains found in wildlife. Microwaving pork has been found to be unreliable in killing all the larva, because the center of the food is not completely heated sometimes. See page 58 figure 4-74 and 4-75 for a look at the larva neatly coiled in muscle tissue.


   Trichinella spiralis larva in muscle tissue courtesy of CDC


Lesson 6 Writing assignment


Please email your answers to maller@nvcc.edu



  1. Let’s pretend! I am a new puppy owner, your clinic has just given me deworming medication, and has dispensed a second dose to be given in 2 weeks. My reaction is this: “why do I have to deworm him again- shouldn’t the medication work the first time?”  or how about this one—“The medication should work the first time-you all are just trying to make extra money off me by selling me medications I don’t really need”  So your job as vet tech is to explain things to me, what would you say? (in a nice professional way)



2. Why would you perform vomit flotation in addition to fecal flotation?


3. What causes summer sores in horses?


4. When a worm is vomited up, how would you determine what kind it is?


5. Where do puppies and kittens acquire roundworm infection?


6. Why test for intestinal parasites if an animal has a cough?


7. What is a morula?


8. How do puppies and kittens acquire hookworm infection?


9. Why are deworming programs for horses recommending deworming occur at least every 2 months and sometimes sooner?


10. What is a cyathostome?


11. T or F  Horses can get pinworms from humans.


12. What is the pinworm of the dog and cat?


13. How in the world do respiratory parasites get their eggs and larva into the feces if they don’t infect the intestinal tract?


14. How in the world do respiratory parasites infect the respiratory tract if their eggs or larva are ingested and not inhaled?


15. How could a dog be positive for the ELISA test for heartworm, but not have any circulating microfilara?


16. How could a dog have circulating microfilaria but be negative for the ELISA test for heartworms?


17. What swine parasite will infect humans if infected undercooked pork is eaten?


18. Which nematode is transmitted by mosquitoes?


19. Which nematodes produce larva in the feces?


20. List the organisms from this lesson that you can detect in

a.       feces

b.      urine

c.       blood

d.      skin biopsy

e.   scotch tape collection (perianal area)


You can use the nicknames and not the latin names if you want.

21. For which organism is the Difil performed?

22. For which organisms is the Baermann apparatus used to study?

23. Fecal flotation is used to detect which organisms from this lesson?

24. Why are fecal egg counts done?

25. Whipworms are not found in which domestic species in the United States?

26. Why do we care about the meningeal worm of white-tailed deer?

Additional references


Samples OM: “Understanding parasite problems in Large Animals” Veterinary Technician January 2003, pp. 28-35   


Bowman DW, Lynn RC, Eberhard ML: “Georgi’s Parasitology for Veterinarians 8th ed” WB Saunders Phildelphia, 2003, pp. 155-230



Photos courtesy of CDC Public Health Image Library http://phil.cdc.gov/Phil/default

Microfilaria photo courtesy of Kerri McCombs, future LVT

Photos also courtesy of the following (through grant from Hill's Pet Nutrition, Inc.):

Jean Holtzen, Omaha College of Health Careers

Sarah McLaughlin, St. Lawrence College

Stuart Porter, Blue Ridge Community College

Pam Baker, Seneca College

Amy Baeza, SUNY College Delhi New York

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