Roundworm Review .....

4. Periparturient (Spring) Rise - PPR

This phenomenon relates to the increased output of eggs in ewe faeces around lambing time, particularly in spring lambing flocks. It is thought to result from hormonal changes associated with milk production which cause a suppression of the ewe's immune system.

The source of the additional egg output involves:

• Increased egg production by female roundworms as they are less inhibited by the
  ewe's immune system than at any other time of the year
• Maturation of arrested larvae due to decreased host immunity (see below)
• An iIncreased establishment of infections acquired from pastures

  PPR occurs at a time when increased numbers of new, susceptible hosts are available and climatic conditions are favourable for larval development. This ensures the survival and propagation of the worm species. However, ewe immunity to roundworm infestation is rapidly restored at weaning / end of lactation.

  The most significant period for periparturient rise is during the 2 weeks prior to lambing plus the 6 weeks post-lambing, with Ostertagia eggs deposited at this time becoming potentially dangerous for lambs from July to October.

Arrested Larval Development - ALD (Hypobiosis)

Often with the onset of autumn / winter in the Northern Hemisphere, larvae fail to develop into mature adults within the host. The proportion of larvae which become arrested is thought to be a heritable trait and is affected by grazing systems and degree of environmental adversity. This seasonal effect is known as "hypobiosis".

For example in Canada where winters are severe almost all trichostrongyloid larvae become arrested. In the UK where winters are moderate, about 50% - 60% become arrested whereas in the Tropics, very few become arrested. It is thought that a small percentage of ALD could also be a result of acquired immunity in the host.

As mentioned above, the maturation of arrested larvae seems to be linked with the breeding cycle of ewes and occurs at or around parturition. ALD is an important factor in sustaining roundworm populations because
a) it ensures survival during periods when environmental conditions outside the host are not suitable, and
b) the subsequent maturation of arrested larvae increases the contamination of the environment, coinciding with presence of susceptible hosts.

Return to:

1. Background

2. Life Cycles and Stimuli for Hatching and Infection • A typical trichostrongyloid e.g. Ostertagia circumcincta • An Exception: The Nematodirus Life Cycle

3. PGE and Nematodirosis: Symptoms and Diagnosis

4. Periparturient Rise and Arrested Larval Development

5. Host Immune Response

6. Breaking the Cycle - Clean Grazing

7. Anthelmintics Treatments and Conventional Controls

8. Implications for Organic Farmers

9. References

 

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