diseases sheep-health reproduction parasites

Toxoplasmosis in Sheep: Understanding and Preventing Abortion

A comprehensive guide to Toxoplasma gondii infection in sheep, including transmission, prevention, and management strategies

Dr. Veterinary Team •

What is Toxoplasma gondii?

Toxoplasma is a single-celled parasite that must penetrate animal cells for its development. The parasite has two separate life cycles:

  • Asexual cycle: Can occur in the cells of many mammals
  • Sexual cycle: Can only occur in cat cells

The parasite is found in brain and muscle tissue of many rodents that serve as its reservoir, but it is only spread through cat feces.

The Life Cycle in the Farm

  1. Cat Infection:

    • A cat (young, not yet infected and immune) hunts an infected rodent
    • Within 3-4 days, the cat excretes millions of oocysts (parasite eggs) in its feces

  2. Sheep Infection:

    • Sheep become infected by eating feed (straw, hay) contaminated with cat feces
    • Oocysts are resistant to environmental conditions for up to two years

  3. Important Notes:

    • An infected sheep cannot infect other sheep
    • An infected sheep develops lifelong immunity
    • Cats also become immune after the first infection

Sheep Infection During Pregnancy

The most severe infection in sheep occurs when they are infected for the first time during pregnancy. The parasite infects the placenta and fetus, and the outcome depends on the stage of pregnancy when infection occurs.

Infection in Early Pregnancy (Months 1-2)

  • Causes fetal death
  • Results in abortion or resorption
  • Ewes will return to heat and may conceive again

Infection in Mid-Pregnancy (Months 3-4)

May cause:

  • Fetal death and mummification (fetus shrinks and is covered by placenta)
  • This can lead to:
    • Abortion
    • Delivery of mummified fetuses at term
    • Sometimes delivery of a dead lamb one week before or at term
    • Possible delivery of live but small and weak lambs
    • Many will die from starvation, hypothermia, or other diseases

Infection in Late Pregnancy (Month 5)

  • Mothers typically deliver live, normal lambs
  • Both lambs and their mothers will be immune to Toxoplasma for the rest of their lives

Prevention and Treatment

Natural Immunity Development

After natural infection with the parasite, the sheep acquires lifelong immunity. Therefore, it is important to infect sheep when they are empty and to infect ewe lambs before first breeding. This ensures they develop immunity and prevents:

  • Abortions
  • Delivery of dead lambs
  • Delivery of small and weak lambs

Exposure Methods

Exposure to the parasite is achieved by:

  • Feeding with feed exposed to cat feces
  • Grazing in pastures containing cat feces with oocysts

If exposure did not occur in time, it is very important to try to prevent cats from reaching the feed and barn during pregnancy.

Medical Treatment: Monensin

Usage:

  • When given to mothers in the second half of pregnancy
  • May reduce the number of abortions
  • Reduces delivery of dead lambs
  • Reduces delivery of weak live lambs

Important Considerations:

  • The decision to use monensin is made by the treating veterinarian
  • Monensin at high levels above permitted is very toxic
  • Prohibited for use in European Common Market countries
  • Must be added to the feed at a licensed feed mill
  • The substance is not palatable, so attention must be paid to ensure it is eaten by the sheep

Alternative Treatment: Decoquinate

There is a claim that giving ‘Decox’ can help. However, the effectiveness of the substance against the parasite has not yet been proven.

Clinical Signs of Infection

In Non-Pregnant Sheep

  • Usually no clinical signs
  • Develops lifelong immunity

In Pregnant Sheep (First Infection)

Depending on stage of pregnancy:

  • Early pregnancy: Abortion, return to heat
  • Mid pregnancy:
    • Mummified fetuses
    • Weak or dead lambs at birth
    • Early abortion (1 week before term)
  • Late pregnancy: Normal live lambs (both immune)

Signs of Abortion Storm

  • Multiple abortions in the flock
  • Weak lambs at birth
  • Small lamb size
  • High lamb mortality
  • Mummified fetuses

Diagnosis

Diagnosis is typically made by:

  • Clinical signs: Pattern of abortions, weak lambs
  • History: Presence of cats, first pregnancy
  • Laboratory tests:
    • Serological testing of ewes
    • Placental tissue examination
    • Fetal tissue analysis
    • Detection of Toxoplasma in tissues

Management Strategies

Cat Management

  1. Prevent cat access to:

    • Feed storage areas
    • Barns (especially during pregnancy)
    • Hay and straw storage

  2. Control rodent populations:

    • Reduces cat attraction to farm
    • Breaks the life cycle

Feed Management

  1. Protect stored feed from cat contamination
  2. Cover hay and straw properly
  3. Avoid feeding on ground where cats may defecate

Flock Management

  1. Expose replacement ewe lambs before first breeding:

    • Allow controlled exposure when empty
    • Ensure immunity development

  2. Quarantine new additions:

    • Test for immunity status
    • Allow exposure period if non-immune

  3. Timing considerations:

    • If possible, plan exposure during non-breeding season
    • Allow time for immunity to develop before breeding

Key Takeaways

  • Toxoplasma is spread only through cat feces
  • Infected sheep develop lifelong immunity
  • Greatest risk is first infection during pregnancy
  • Timing of infection during pregnancy determines outcome
  • Prevention focuses on controlled exposure before breeding
  • Cat and rodent control are essential
  • Monensin may help in pregnant ewes (veterinarian decision)
  • Feed protection is crucial during pregnancy

When to Consult Your Veterinarian

Contact your veterinarian if you experience:

  • Abortion storms in the flock
  • Multiple weak or small lambs at birth
  • Mummified fetuses
  • High neonatal lamb mortality
  • Unexplained return to heat in ewes
  • Known cat access to feed during pregnancy

Your veterinarian can:

  • Confirm diagnosis through testing
  • Recommend prevention strategies
  • Advise on monensin use if appropriate
  • Help develop flock management plans

Prognosis

With Proper Management:

  • Excellent prevention through controlled exposure
  • Lifelong immunity after first exposure
  • Minimal production losses

Without Management:

  • Potential for significant abortion storms
  • High lamb mortality
  • Reduced productivity
  • Economic losses

The key is establishing immunity in all breeding females before they become pregnant.