CYSTICERCUS OVIS, sheep measles, parasitic tapeworm of SHEEP and GOATS. Biology, prevention and control
Last Updated on Tuesday, September 10 2013 13:08
Written by P. Junquera
Cysticercus ovis is the larval stage (cysticercoid, metacestode) of Taenia ovis, a tapeworm parasite of dogs and other canids (coyotes, wolves, foxes, very occasionally cats, etc.), that has sheep and goats as well as various wild ruminants as intermediate hosts.
Cysticercus ovis occurs worldwide, mainly in rural areas of countries with large sheep populations. Regional incidence varies a lot. Unexpected outbreaks can happen due to climatic conditions that favor the survival of eggs in pastures or the activity of wild canids that carry the disease.
The disease caused by this and other cysticercoids is called cysticercosis. In this particular case it is also called sheep measles.
Is livestock infected with Cysticercus ovis contagious for humans?
NO. The reason is that Taenia ovis is not a human parasite. See the life cycle below.
You can find additional information in this site on the general biology of parasitic worms and/or tapeworms.
Final location of Cysticercus ovis
Cysticercoids of Taenia ovis are found mainly in the muscles of sheep and goats. The predilection site of adult tapeworms in their final host (dogs and other canids) is the small intestine.
Anatomy of Cysticercus ovis
In sheep and goats Cysticercus ovis cysticercoids appear as small whitish cysts filled with fluid that contain an immature worm. They have the size of a grain rice (~4 to 9 mm). Infected meat can have dozens of such cysts. They are found mainly in the skeletal muscles and those of the heart, the diaphragm, and the jaws. Several months after infection the cysts die and become calcified.
Visit the page on Taenia species for additional information of the anatomy of adult Taenia tapeworms.
Life cycle and biology of Cysticercus ovis
As all tapeworms, Taenia ovis has an indirect life cycle, with dogs and other canids (foxes, wolves, coyotes, very occasionally cats) as final hosts, and sheep and goats as intermediate hosts.
Sheep and goats become infested when ingesting food or water contaminated with eggs or gravid segments of Taenia ovis through herbage or stored feed. A single gravid segment can have uo to 75,000 eggs! Contamination of sheep feed can occur through contamination with dog feces. The eggs can remain infective for up to 12 months. Intrauterine infection of unborn lambs has been also reported.
Once ingested by sheep or goats the young larvae hatch out of the eggs in the gut, go through the intestinal wall, reach the blood stream and migrate to a muscle, where they encyst. The cysts need 10 to 12 weeks to complete development. The cysts may remain infective for dogs for up to one year.
Dogs and other canids become infected when eating insufficiently cooked meat contaminated with cysts. Once in the dog's gut, the cysts release the young tapeworms, which attach to the gut's wall and start producing segments. Within 5 to 8 weeks (prepatent period) the tapeworms mature and start shedding eggs.
Harm caused by Cysticercus ovis, symptoms and diagnosis
Cysticercus ovis is not pathogenic for sheep or goats and usually the infection causes no clinical signs, unless a vital organ (e.g. the heart) is massively infected, which is very unsusual. However, infections have a substantial economic impact because the whole carcass is condemned at slaughter. An epidemic outbreak can be economically devastating.
So far diagnosis on sheep or goats is only possible post-mortem after carcass examination.
Infection of dogs with Taenia ovis are usually benign as well. They can cause diarrhea and loss of appetite, but they often cause no clinical signs.
Prevention and control of Cysticercus ovis
Although sheep measles is not harmful the intermediate hosts and not contagious for humans, the FAO recommends condemnation of heavily infected carcasses, and this is usually followed worldwide.
Best prevention consists in avoiding the contamination of sheep feed (fresh pasture as well as hay, silage, and other stored feed) or water with dog feces that may contain tapeworm eggs. It must be considered that the eggs may remain infective in the feed after silage or other processing of hey or fodder (pelletizing, fermentation, etc.).
At the same time dog scavenging on potentially contamined cadavers or offal has to be avoided. If this cannot be avoided, in endemic regions is is advisable to regularly deworm dogs (particularly working and guard dogs) at least 3 to 4 times a year, especially if sheep measles has been found in the region. There are numerous dog anthelmintic products that control such infections. They contain active ingredients with broad-spectrum anthelmintic efficacy such as benzimidazoles (e.g. fenbendazole, febantel, mebendazole) or specific taenicides such as praziquantel and epsiprantel, the latter often in combination with nematicides (e.g. levamisole, milbemycin oxime, pyrantel, etc.) to cover a broader spectrum of worms. Scavenging on potentially infected
Most of these dog dewormers are available in formulations for oral delivery either as solids (tablets, pills, etc.) or as liquids (drenches, suspensions, etc.). There are also a few injectable taenicides in some countries (mainly with praziquantel).
So far there are no antiparasitic medicines for external use (spot-on - squeeze-on - pipettes, shampoos, soaps, sprays, powders, insecticide-impregnated collars, etc.) that control established tapeworm infections on dogs.
The regular use of anthelmintics is usually not indicated for preventing sheep or goat infections with Cysticercus ovis. There are reports that praziquantel is effective, but results can be unreliable.
Several classic anthelmintics such as macrocyclic lactones (e.g. ivermectin, doramectin, selamectin, etc.), levamisole, tetrahydropyrimidines (e.g. pyrantel, morantel) and piperazine derivatives are not effective at all against Cysticercus ovis or whatever adult tapeworm or cysticercoid, neither on dogs, nor on sheep, goats or other livestock.
There are so far no vaccines that would protect sheep or goats against Cysticercus ovis. A lot of research is being done in the development of a vaccine against sheep measles, but so far there are no commercial vaccines in most countries. To learn more about vaccines against parasites of livestock and pets click here.
Biological control of Taenia tapeworms respectively Cysticercus ovis (i.e. using its natural enemies) is so far not feasible.
You may be interested in an article in this site on medicinal plants against external and internal parasites.
Resistance of Taenia spp to anthelmintics
So far there are no reports on resistance of Taenia spp, respectively Cysticercus ovis to anthelmintics.
This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that either the product was unsuited for the control of Cysticercus ovis, or it was used incorrectly.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Cysticercus ovis control.