Cysticercosis and hydatid echinococcosis, dangerous parasitic larval forms in rabbits

Abstract

Flat segmented worms or tapeworms can infest wild and domestic rabbits, as well as other leporid species such as the American cottontail or the hare.

Full text

Cysticercosis and hydatid echinococcosis, dangerous
parasitic larval forms in rabbits
Esther van Praag
Flat segmented worms or tapeworms can infest wild and domestic rabbits, as well as
other leporid species such as the American cottontail or the hare.
Infestation by tapeworms is rare in rabbits
living in a well maintained and protected
area such as a pen, a hutch or an
apartment, but not impossible ... Different
species of parasitic tapeworm parasite the
digestive system of wild and domestic
rabbits. Some develop into adulthood and
are, therefore, able to reproduce and
produce eggs. They include the leporid
tapeworm Leporidotaenia sp., the rabbit
Figure 1: The proximity of canids infested by a tapeworm or Echinococcus and the presence of their
feces in the grass permits the transmission of parasite eggs to the rabbit.
Picture: MediRabbit.com

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tapeworm Cittotaenia variabilis, the
tapeworm Ctenotaenia ctenoid, the
porcupine tapeworm Monoecocestus
americanus and the Mosgovoyia pectinata
americana tapeworm. A healthy rabbit is
rather resistant to tapeworms. Indeed, the
presence of a few worms only has been
observed; there is rarely an overpopulation
as may be the case with roundworms, e.g.
the rabbit pinworm Passalurus ambiguus.
Intermediary host: the rabbit
Other tapeworms remain blocked at an
intermediate larval development stage in
the abdomen or in the subcutaneous tissue
of rabbits. The rabbit tapeworm (Ta enia
pisiformis) is a parasitic worm that infests
the intestines of carnivores: dogs, foxes and
sometimes cats. The development of the
parasite occurs in two steps. The adult stage
takes place in the definitive host, e.g. the
dog. Th e pa r a si te colo n i zes the sm a l l
intestine (duodenum, jejunum and ileum)
and may become as long as 2 meters
(6.6 feet). Adults segments containing the
reproductive organs and mature eggs
 proglottids, are shed with the feces.
Before it can infest another dog, the
parasite must necessarily go through the
larval stage, called cysticercus. The latter
occurs in rabbits. These get infested after
eating fresh grass contaminated by eggs of
the parasite. Cysts develop in the abdomen,
the peritoneal cavity, and in the liver
(Figure 2). Their size ranges between 2 and
3 cm (0.7” to 1.18”), but they can reach up
to 8 cm (3.15”) in diameter. The vesicles
contain a clear fluid and the invaginated
scolex of the larval tapeworm (protoscolex).
The development of the larvae is blocked at
this stage. The growth into adult parasites
able to reproduce will only take place after
ingestion of viscera of an infested rabbit by
a fox or dog.
Once the cysticercus has reached the
canid intestine, it rolls up like a sock and
Figure 2: Whitish cysts or cysticerci of Taenia sp. with larvae attached to the intestine wall.
Picture: Prof. R. Hoop

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allows the larva to attach itself to the wall.
Another canid tapeworm uses this mode
of transmission: Taenia serialis, with an
intermediate developmental stage in hares
and rabbits and an adult stage in dogs and
foxes. Its incidence in the pet rabbit is rare.
Cysts of this worm develop in the
subcutaneous and muscle tissues.
Clinical effects of cysticercosis
Larvae emerging from the ingested eggs
migrate from the intestine to the liver via
the portal vein, which transports blood from
the digestive organs to the liver. Their
presence causes granulomatous hepatitis:
inflammation, local necrosis of liver cells,
infiltration of white blood cells and scaring.
The structure of the liver tissue changes
and will be replaced in time by connective
tissue. After 15-30 days, the larvae migrate
to the liver parenchyma and cysts develop.
A severe cysticerci infestation causes
digestive problems such as enteritis and
promotes the development of ulcers of the
intestine. The rabbit becomes emaciated
and is extremely weak (Figure 3). It can die
suddenly.
Aberrant migration of the larvae is
possible. Some tapeworm larvae have
completed their migration into the peritoneal
serous liquid, in the eye or lungs of a rabbit.
In the latter, they may occupy the whole
pulmonary cavity, causing acute respiratory
distress. The presence of cysticerci and
calcified cysts in the brain may cause
seizures, hydrocephalus and meningitis.
Figure 3: Extreme thinness in a rabbit may indicate the presence of cysticerqui in the abdomen. Picture:
Anonym

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Hydatid echinococcosis
Echinococcus granulosus is a parasitic
cestode that infests canids and other
carnivorous animals (Figure 4). It is
dangerous to man too and may cause
important damages to tissues. Echinococcus
granulosus is a cosmopolite parasite but
different strains have different intermediary
hosts depending on continents.
The North American strain is predominant
in regions with boreal forests and tundra in
Eurasia and North America. It infests
wolves, deer and bison. The South European
strain is found in pastoral areas and infests
domestic animals (dogs and ungulates) and
wild animals (canids, wild ungulates and
sometimes wild rabbits).
Life cycle
The development of Echinococcus
granulosus requires an intermediate
herbivorous host and a final host that
belong to the canids. The adult parasite
measures between 2 and 11 mm (0.08” and
0.4”) and is composed of 2 to 7 proglottid
segments.
Worms produce up to 1000 eggs every
two weeks. The segments containing eggs
are released in the intestine of the host and
are eliminated in the external environment
with the feces. Eggs are dispersed into the
outside environment by the wind, water or
flies and contaminate the vegetation eaten
by herbivores.
The rabbit is an accidental intermediate
host after ingestion of contaminated food or
dirty water. It is affected by the larval phase
called hydatid echinococcosis. Once
ingested, eggs release tiny larvae with
hooks in the small intestine of the host.
After crossing the intestinal wall, the larvae
migrate via the blood or lymphatic
circulation to organs such as the lungs and
liver, but also to the nervous system, brain,
spleen, kidney or bone marrow. Each larva
will form a cyst, a hydatid unilocular vesicle
filled with a colorless fluid. The cysts
continue to grow slowly and can reach a size
of 30 cm (11.8”) in some herbivores.
Echinococcus larvae multiply and wait for
the next stage of infestation inside
structures adhering to the inner wall of the
cysts. A rupture of the vesicle allows a
secondary infestation of organs or ingestion
by their definitive canid hosts.
Pathogenesis
The severity of this parasitosis depends
on the size of cysts and their number. In
Figure 4: Adult parasitic tapeworm
Echinococcus granulosus.

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general, echinococcosis presents no clinical
signs as cysts are small and do not damage
surrounding tissues unless they develop in
the tissues of the nervous system and brain,
or compress organs such as lungs.
Clinical manifestations
An affected rabbit may show a decreased
appetite (anorexia) and appear lethargic.
Clinical signs vary according to which organs
are affected.
Lungs: Respiratory distress, dyspnea;
Deep irritating cough;
Pain.
Liver: Compression of liver tissue;
Blockage of blood vessels;
Inflammation of the cystic canal
of the gall bladder;
Pain.
Kidneys: Increased size of the kidney;
Proteinuria and hematuria;
Urinations anomalies;
Pain.
When a vesicle ruptures, the larvae can
spread throughout the body of its host and
infect further organs. The presence of free
larvae in the blood can trigger a fatal
Figure 5 : A : Intrathoracic, extrapulmonary hydatid cyst (arrowheads) in the mediastinal region of the
lungs of a rabbit. Diffuse consolidation of the right lung and emphysematous changes in the
apical lobes of both lungs are visible. B : The roughly conical hydatid cyst with clear content
and whitish « sand »(arrowheads). C : Protoscolices D : showing the calciferous corpuscles
(arrowhead) and the rostellum (arrow). E : Rostellar hooks (F) consisting of the small and lare
hooks with the characteristic handle, guard and blade (Bars in C, D, E and F = 20 micron).
Reprinted from: C. Sreekumar, A. Kirubakaran, R. Venkataramanan, P. Selvan, R. Anilkumar,
M. Iyue (2010) Spontaneous primary intrathoracic, extrapulmonary hydatid cyst in a broiler
rabbit. Helminthologia, 47 (3):193-195
http://link.springer.com/article/10.2478/s11687-010-0029-4

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allergic or anaphylactic reaction.
Rarely a cyst becomes sterile as a result
of bacterial invasion, and calcifies.
Without treatment, the size of cysts
continues to increase slowly. Compression of
surrounding tissues may cause irreversible
chronic or acute lesions with fibrosis of the
surrounding tissue.
Diagnosis
Diagnosis is difficult. The observation of
cysts and the identification of the parasite
are often accidental during an autopsy. In
case of suspicion of hydatid echinococcosis
in a rabbit, it is important to confirm the
presence of a cyst in the lung or in the
abdomen by ultrasound or x-ray.
A biopsy of the cyst content can be
attempted using an ultrasound-guided fine
needle. It helps differentiate a hydatid cyst
from an abscess or a tumor. The presence of
a protoscolex or larvae in the sample helps
confirm the diagnosis.
If the rabbit presents a deep irritating
(dry) cough reminding a bark, it is
important to differentiate hydatid echinococ-
cosis from pneumonia.
If the cyst is calcified, detection of
antibodies or PCR testing is required to
confirm the diagnosis. These methods are
expensive.
Treatment
The treatment of cystic echinococcosis is
difficult, risky, and results are not
guaranteed.
The administration of antiparasitic drugs
such as mebendazole, albendazole,
niclosamide and prazinquantel kills 60-85%
of viable cysts. They also help to reduce
their size, to eliminate parasitic larvae, to
avoid a relapse with a new growth of cysts
and the spread of parasitic larvae in the
body of the host. This approach enables,
furthermore, to reduce tissue damage and
relieves pain.
The rabbit must be closely monitored
after the administration of medications.
Indeed, the dead larvae of the parasite can
trigger an inflammation of the surrounding
tissue and a reaction of the immune system.
It is also possible to aspirate the content
of one or more cysts with a needle thanks to
the ultrasound-guided technique and
replaced it with an antiparasitic solution.
Surgical resection of the vesicles is also
possible, but results are not always
satisfactory. There are also risks. Indeed, if
the cyst accidentally ruptures during the
procedure, larvae can invade the blood
circulation. A secondary invasion of organs
becomes possible, which may cause the
death of the animal.
Acknowledgement
Big thanks to Richard Hoop, professor of
poultry diseases at the institute of veterinary
bacteriology, Vetsuisse Zurich, Switzerland, for
his permission to use his picture.
References
Allan JC, Craig PS, Sherington J, Rogan MT,
Storey DM, Heath S, Iball K. Helminth
parasites of the wild rabbit Oryctolagus
cuniculus near Malham Tarn, Yorkshire, UK. J
Helminthol. 1999;73(4):289-94.
Barker IK. The penetration of oncospheres of
Taenia pisiformis into the intestine of the
rabbit. Can J Zool. 1970 Nov;48(6):1329-32.
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Bundesen PG, Janssens PA. Biochemical tracing
of parasitic infections. II. Taenia pisiformis in
rabbits--a quantitative study. Int J Parasitol.
1971;1(1):15-20.
Craig PS. Circulating antigens, antibodies and
immune complexes in experimental Taenia
pisiformis infections of rabbits. Parasitology.
1984;89 ( Pt 1):121-31.
Flatt RE, Moses RW. Lesions of experimental
cysticercosis in domestic rabbits. Lab Anim
Sci. 1975;25(2):162-7.
Foronda P, Del Castillo A, Abreu N, Figueruelo E,
Piñero J, Casanova JC. Parasitic helminths of
the wild rabbit, Oryctolagus cuniculus, in
different bioclimatic zones in Tenerife, Canary
Islands. J Helminthol. 2003 Dec;77(4):305-9.
Foronda P, Valladares B, Lorenzo-Morales J,
Ribas A, Feliu C, Casanova JC. Helminths of
the wild rabbit (Oryctolagus cuniculus) in
Macaronesia. J Parasitol. 2003;89(5):952-7.
Heath DD. The migration of oncospheres of
Taenia pisiformis, T. serialis and Echinococcus
granulosus within the intermediate host. Int J
Parasitol. 1971 Sep;1(2):145-52.
Heath DD, Chevis RA. Duration of immunity to
Taenia pisiformis larvae in rabbits. J Parasitol.
1978;64(2):252.
Heath DD, Christie MJ, Chevis RA. The lethal
effect of mebendazole on secondary
Echinococcus granulosus, cysticerci of Taenia
pisiformis and tetrathyridia of Mesocestoides
corti. Parasitology. 1975;70(2):273-85.
Hörchner F, Langnes A. The efficacy of
mebendazole and praziquantel on larval
taeniids from mouse, rabbit and pig.
Tropenmed Parasitol. 1977;28(1):44-50.
Lepitzki DA, Woolf A, Bunn BM. Parasites of
cottontail rabbits of southern Illinois. J
Parasitol. 1992;78(6):1080-3.
Meléndez RD, Yépez MS, Coronado A.
Echinococcus oligarthrus cysts of rabbits in
Venezuela. J Parasitol. 1984;70(6):1004-5.
Owiny JR. Cysticercosis in laboratory rabbits.
Contemp Top Lab Anim Sci. 2001;40(2):45-8.
Rickard MD. Hypothesis for the long term survival
of Taenia pisiformis cysticerci in rabbits. Z
Parasitenkd. 1974;44(3):203-9.
Rickard MD, Coman BJ. Studies on the fate of
Taenia hydatigena and Taenia ovis larvae in
rabbits, and cross immunity with Taenia
pisiformis larvae. Int J Parasitol.
1977;7(4):257-67.
Schantz PM, Lord RD. Echinococcus in the South
American red fox (Dusicyon culpaeus) and the
European hare (Lepus europaeus) in the
Province of Neuquén, Argentina. Ann Trop
Med Parasitol. 1972;66(4):479-85.
Soveri T, Valtonen M. Endoparasites of hares
(Lepus timidus L. and L. europaeus Pallas) in
Finland. J Wildl Dis. 1983;19(4):337-41.
Sreekumar Kirubakaran CA, R. Venkataramanan
Selvan RP, Anilkumar R, Iyue M. (2010)
Spontaneous primary intrathoracic,
extrapulmonary hydatid cyst in a broiler
rabbit. Helminthologia, 2010;47:193-195
Thomas H, Gönnert R. The efficacy of
praziquantel against experimental
cysticercosis and hydatidosis. Z Parasitenkd.
1978;55(2):165-79.
Varela-Diaz VM, Coltorti EA. The presence of host
immunoglobulins in hydatid cyst membranes.
J Parasitol. 1973;59(3):484-8.
Worley DE. Quantitative studies on the migration
and development of Taenia pisiformis larvae in
laboratory rabbits. Lab Anim Sci.
1974;24(3):517-22.