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
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.
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,
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.
courtesy of Jean Holtzen
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.
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
Figures 4-50 in your text book.
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
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.
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.
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.
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
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
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
--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
Microfilaria in peripheral blood at high power
Microfilaria filter test,
courtesy of Jean Holtzen. The microfilaria tend to straighten with
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,
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
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
For a review of the tissue squash preparation,
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
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
- 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
2. Why would you perform vomit flotation in addition to fecal
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
e. scotch tape collection (perianal
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
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.
Photos courtesy of CDC Public Health Image Library
Microfilaria photo courtesy of Kerri McCombs,
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