NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

 

Toxoplasma abortion in Sheep

Toxoplasmosis ranks alongside enzootic abortion, as one of the two most important causes of abortion in British flocks.  Precise losses due to toxoplasmosis are difficult to assess, because in addition to causing identifiable late abortions, it can result in embryonic/foetal loss at any stage of pregnancy.  A toxoplasmosis problem may therefore only manifest itself in barren ewes, and there are difficulties associated with arriving at a retrospective diagnosis.

 

Toxoplasma gondii is a protozoan parasite, which requires more than one host species to complete its lifecycle.  The organism is primarily an intestinal parasite of cats, and has a wide range of intermediate hosts including sheep and mice.  One cat shedding oocysts can provide sufficient environmental contamination to infect a large number of sheep. 

 

Infection of ewes during early pregnancy may result in embryonic death and resorption.  Infection during mid-pregnancy may result in either foetal death followed by mummification, or foetal retardation due to compromised placental nutrition and foetal infection.  Late pregnancy infection may result in abortion of freshly-dead lambs or birth of weakly lambs with high neonatal mortality rates.  Litter mates may be affected to different degrees. 

 

1

TOXOPLASMOSIS - IN PRACTICE, LATE ABORTED LAMBS

ARE BORN IN VARIOUS STAGES OF DECOMPOSITION

 

Aborted ewes remain clinically normal, and may not be diagnosed until lambing time when they appear as barren ewes.  The classic picture of toxoplasmosis is of late aborted lamb alongside a brown-coloured mummified lamb and placenta.  In endemically infected flocks in the absence of control measures, the abortion/barren rate is highest in ewe-lambs and two-tooths, and low in older ewes, however when an environment is newly contaminated, abortions or high barren rates may occur in all ages of non-immune ewes.

 

Control

 

  • In the face of a late abortion storm, injectable or oral sulphonomide antibiotics can be used to reduce losses.

 

  • Depopulating cats is not successful as a control method for toxoplasmosis, as generally only young cats hunting for the first time shed oocysts, and removing an established cat population allows new young cats to move onto the farm.  Measures should be taken to limit cat breeding, and maintain a healthy adult cat population.

 

·         The coccidiostat drug decoquinate can be fed to pregnant ewes as an aid to prevention of abortions due to toxoplasmosis.  Unfortunately toxoplasmosis can cause abortion or foetal loss at any stage, so to be fully effective decoquinate should be fed throughout pregnancy.  This is cost prohibitive and impractical, partly due to the cost of concentrate feed required as a carrier.  In practice, decoquinate is included only during the final 8 weeks of pregnancy, during which period concentrates are regularly fed. 

 

  • Following exposure, ewes develop a life-long immunity, therefore if exposed to a contaminated environment prior to first pregnancy they don’t abort.  This situation is difficult to reproduce in practice, so vaccination is the most obvious method of control.  Toxovax is very effective in the control of ovine toxoplasmosis.  It has a very short shelf life and must be handled carefully.  A single injection effectively gives life-long protection. 

 

  • Your vet can advise you about the control of toxoplasmosis in your flock.

 

 

Neil Sargison BA VetMB DSHP FRCVS

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