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The cysts are the larval form of the tapeworm Taenia serialis. Lagomorphs such
as jack rabbits and rabbits are intermediate hosts for the cestode Taenia
serialis. The definitive hosts are dogs and foxes, which ingest the larvae by
eating infected lagomorphs. Adult tapeworms are found in the small intestine of
dogs and foxes, which pass gravid proglottides or tapeworm segments containing
eggs in the feces. Jack rabbits ingest the eggs by eating vegetation on which
the infected carnivore h as defecated.
Once the eggs reach the intestinal tract of the lagomorph, the embryos hatch
and migrate to intermuscular connective tissue. In this tissue, the larva
transforms into a bladder-shaped structure with one or more inverted scolices
(a specialized anterio r segment of the tapeworm or "head") in an invaginated
portion of the wall. If the bladder wall has a germinal layer capable of
producing multiple scolices, it is referred to as a coenurus or multiceps.
Another example of this type of cestode is Coenur us cerebralis, the larval
stage of the dog tapeworm Multiceps multiceps. It is the causative agent of an
uncommon disease of the central nervous system of sheep known as "gid," in
which a larval bladder is localized in the brain or spinal cord.
If the larva has a solid caudal portion and a bladder-like proximal portion, it
is called a cysticercus. An example is Cysticercus pisiformis which occurs on
the peritoneum and liver capsule of rabbits, squirrels, and other small
rodents. The a dult stage of this parasite is Taenia pisiformis and is found in
the small intestine of the dog, cat, fox, wolf, and other carnivores.
Generally, because the coenurus is a single bladder with multiple scolices, it
grows to 4-5 cm in diameter, and occurs s ingly or occasionally as two or three
bladder-like cysts. In contrast, because cysticercus contains only one scolex,
it grows into a fluid-filled sphere 15-18 mm in diameter, and often occurs as
multiple cysts resembling a bunch of grapes. Other modificat ions of the
bladder include the strobilicercus, with an invaginated scolex attached to a
small bladder by a segmented portion; and the echinococcus or hydatid cyst,
with a germinal layer that produces brood capsules within which scol
The scolices in all the larval stages possess suckers and hooklets identical to
the adult stage. When ingested by the definitive host, the scolex evaginates
and attaches itself to the intestinal mucosa. Growth of the tapeworm then
proceeds by prolifer ation of segments at the posterior extremity.
Lagomorphs of many species are infected with Taenia serialis1. The distribution
of T. serialis is more restricted, and the rate of infection is less than for
T. pisiformis 2. The coenurus generally occurs on the side of lagomorphs, in
the intermuscular connective tissue between the ribs and hips, and over the
ribs. In these locations, the coenuri are generally non-pathogenic and finding
them is incident al. Those coenuri that develop in areas other than
intermuscular connective tissue, for example in the abdomen or brain, can
compromise the host. Coenurus serialis has occurred in laboratory rabbits3, but
is more likely to be seen now in pet rabbits, in which T. serialis is enzootic4.
The Plum Island researchers first suspected that the cysts were
sialoceles--cysts that form in an obstructed salivary duct due to the dilating
effect of the contained salivary gland secretion--because of their location
along the path of the parotid sa livary duct. However, this did not seem likely
when they saw a cyst on the chest wall of another jack rabbit.
An alternate consideration was that the cysts might be the subcutaneous larvae
of botflies (Cutebra spp.). Several species of Cutebra frequently infect wild
lagomorphs. However, the subcutaneous swellings containing the larvae are
characterized by a fistula or air-hole for the larva to breathe. The
researchers did not find holes in any of the cysts. When the researchers
dissected the cysts, they found numerous scolices, which identified the cyst as
The researchers? VHD susceptibility studies indicated that wild North American
leporids were clinically unaffected by experimental exposure to the VHD virus5
Dr. Gregg is a veterinary pathologist at the Plum Island Foreign Animal Disease
Diagnostic Laboratory, Greenport, NY. The opinions and assertions contained in
this article are the author?s own, and are not to be construed as official or
as reflecting t he views of the USDA. The research described was conducted
according to the regulations of the Animal Welfare Act, and the principles set
forth in the NIH Guide for the Care and Use of Laboratory Animals.
Hofing, G.L., and Kraus, A.L. Arthropod and Helminth Parasites. In: Manning,
P.J., Ringler, D.H., and Newcomer, C.E., eds. The Biology of the Laboratory
Rabbit (2nd ed). Academic Press Inc., San Diego, CA, pp. 231-257, 1994.
Pfaffenberger, G.S. and Valencia, V.B. Helminths of sympatric black-tailed
jackrabbits (Lepus californicus) and desert cottontails (Sylvilagus audubonii)
from the high plains of eastern New Mexico. J. Wildl. Dis.; 2 4: 375-377, 1988.
Hamilton, A.G. The occurrence and morphology of Coenurus serialis in
rabbits. Parasitology; 40: 46-49, 1950.
Goudswaard, M.F. and Thomas, J.A. Coenurus serialis infection of a white
rabbit. Vet. Rec.; 129: 295, 1991.
Gregg, D.A., House, C., Meyer, R. et al. Viral hemorrhagic disease of
rabbits in Mexico: epidemiology and viral characterization. Revue Scientifique
et Technique, Office International des Epizooties; 10:435-451, 1991.
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