Difference between revisions of "Toxoplasmosis - Sheep"

Revision as of 14:37, 13 August 2010

Description

Toxoplasmosis is the disease caused by Toxoplasma gondii, an intracelluler protozoan parasite. Although the definitive host is the cat, T. gondii can infect all mammals including man and is a significant cause of abortion in sheep and goats. Toxoplasmosis does not seem to cause disease in cattle.

Life cycle of Toxoplasma gondii. Source: Wikimedia Commons; Author: LadyofHats (2010)

Life Cycle

There are three infectious stages of Toxoplasma gondii: 1) sporozoites; 2) actively reproducing tachyzoites; and 3) slowly multiplying bradyzoites. Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are containted within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue.

In naive cats, Toxoplasma gondii undergoes an enteroepithelial life cycle. Cats ingests intermediate hosts containing tissue cysts, which release bradyzoites in the gastrointestinal tract. The bradyzoites penetrate the small intestinal epithelium and sexual reproductio ensues, eventually resulting the production of oocysts. Oocysts are passed in the cat's faeces and sporulate to become infectious once in the environment. These can then be ingested by other mammals, including sheep.

When oocysts are ingested, T.gondii intiates extraintestinal replication. This process is the same for all hosts, and also occurs when carnivores ingest tissue cysts other animals. Sporozoites (or bradyzoites, if cysts are consumed) are released in the intestine to infect the intestinal epithelium where they replicate. This produces tachyzoites, which reproduce asexually within the infected cell. When the infected cell ruptures, tachyzoites are released and disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly and, if the cell does not burst, they eventually encyst and persist for the life of the host.

Signalment

Diagnosis

Clinical Signs

Clinical outbreaks of toxoplasmosis are sporadic
- Immunity is acquired before tupping
- Significant ill-effects are unlikely if immune ewes are infected during pregnancy
- Not shed from sheep to sheep so predicting outbreaks is difficult

Laboratory Tests

Pathology

Aborted ewes show focal necrotic placentitis with white lesions in the cotyledons and foetal tissue

Treatment

Toxovax vaccine
- - Live, avirulent strain of Toxoplasma
  - Does not form bradyzoites or tissue cysts
  - Killed by host immune system
  - Single dose given 6 weeks before tupping
  - Protects for 2 years
  - Immunity boosted by natural challenge
- Medicated feed can be given daily during the main risk period
  - 14 weeks before lambing
- The best method of protection is to prevent cats from contaminating the pasture, lambing sheds and feed stores

Prognosis

Links

References

Buxton, D (1990) Ovine toxoplasmosis: a review. Journal of the Royal Society of Medicine, 83, 509-511.
Innes, E A et al (2009) Ovine toxoplasmosis. Parastiology, 136, 1887–1894.
Buxton, D et all (2007) Toxoplasma gondii and ovine toxoplasmosis: New aspects of an old story. Veterinary Parasitology, 147, 25-28.
Dubey, J P (2009) Toxoplasmosis in sheep — The last 20 years. Veterinary Parasitology, 163, 1-14.

@@ Line 3: / Line 3: @@
 ==Description==
-Toxoplasmosis is the disease caused by ''Toxoplasma gondii'',  a protozoan parasite. Although the definitive host is the cat, ''T. gondii'' can infect all mammals including man and is a significant cause of abortion in sheep and goats. Toxoplasmosis does not seem to cause disease in cattle.
+Toxoplasmosis is the disease caused by ''Toxoplasma gondii'',  an intracelluler protozoan parasite. Although the definitive host is the cat, ''T. gondii'' can infect all mammals including man and is a significant cause of abortion in sheep and goats. Toxoplasmosis does not seem to cause disease in cattle.
+[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]
+===Life Cycle===
-===Life Cycle===
+There are three infectious stages of ''Toxoplasma gondii'': 1) sporozoites; 2) actively reproducing tachyzoites; and 3) slowly multiplying bradyzoites. Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are containted within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue.
-[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]
-'Toxoplasma gondii'' has three infectious stages: 1) sporozoites; 2) an actively reproducing stage called tachyzoites; and 3) slowly multiplying bradyzoites. Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are containted within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue. Transplacental infection is also possible.
+In naive cats, ''Toxoplasma gondii'' undergoes an enteroepithelial life cycle. Cats ingests intermediate hosts containing tissue cysts, which release bradyzoites in the gastrointestinal tract. The bradyzoites penetrate the small intestinal epithelium and sexual reproductio ensues, eventually resulting the production of oocysts. Oocysts are passed in the cat's faeces and sporulate to become infectious once in the environment. These can then be ingested by other mammals, including sheep.
-In naive cats, ''Toxoplasma gondii'' undergoes an enteroepithelial life cycle. Cats become infected by ingesting intermediate hosts containing tissue cysts, which release their bradyzoites in the gastrointestinal tract when the wall is digested. Bradyzoites then penetrate the small intestinal epithelium and produce five types of schizonts, which then give rise to merozoites. Male and female gamonts are formed from merozoites, which fertilise to form a macrogamont. A wall forms around the macrogamont to produce an oocyst, which is passed in the faeces approximately three days after ingestion of the tissue cyst. Initially, these oocysts are unsporulated and are therefore not infectious, but after 1 to 5 days sporulation occurs to produce two sporocysts, each with four infectious sporozoites. This sporulation is dependent on temperature and aeration, and sporocyts can remain viable in the environment for several months. As cats generally develop immunity to ''T. gondii'' after the initial infection, they will only shed oocysts once in their lifetime.
-When other, non-feline, carnivores consume tissue cysts or oocysts from cat faeces, ''Toxoplasma gondii'' initiates extraintestinal replication. This process is the same for all hosts, and does not vary with the form of the parasite ingested. Bradyzoites and sporozoites, from cysts and oocysts respectively, are released in the intestine and infect the intestinal epithelium where they replicate. This produces tachyzoites, which are lunate in shape, about 6 microns in diameter and possess the ability to multiply in almost any cell type. The infected cell ruptures to release tachyzoites which then disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly and, if the cell does not burst, they eventually encyst and persist for the life of the host.
+When oocysts are ingested, ''T.gondii'' intiates extraintestinal replication. This process is the same for all hosts, and also occurs when carnivores ingest tissue cysts other animals. Sporozoites (or bradyzoites, if cysts are consumed) are released in the intestine to infect the intestinal epithelium where they replicate. This produces tachyzoites, which reproduce asexually within the infected cell. When the infected cell ruptures, tachyzoites are released and disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly and, if the cell does not burst, they eventually encyst and persist for the life of the host.
 ==Signalment==