Worms on pasture

The survival of the third stage (L3) infective stages of roundworms on pastures determines the level of exposure cattle may receive. Temperature and moisture conditions have the greatest impact on worm survival on pasture (see Table 1).

Table 1. Climate factors contributing to paddock contamination with infective (L3) roundworm larvae (small brown stomach worm; Ostertagia ostertagi, barber’s pole worm; Haemonchus placei, small intestinal worms; Cooperia species and nodule worms; Oesophagostomum radiatum).

 

Life stages

Time or conditions

General effect

Minimum time for worm eggs in the dung pat to develop to infective third stage larvae (L3).

4–10 days

Paddocks can quickly become heavily contaminated during suitable moist conditions.

Temperature and moisture requirements for significant numbers of worm eggs to hatch and become infective larvae third stage larvae (L3).

The dung pat provides sufficient moisture for eggs to hatch to infective (L3) larvae without additional rainfall unless conditions are particularly dry. If a dry crust develops on the dung pat, some moisture may be necessary to break it up.

 

Small brown stomach worm

Temperature: daily maximum >8°C1

Moisture: >10–15 mm rainfall2

 

Barber’s pole worm

Temperature: daily maximum >18°C1

Moisture: up to 50 mm rainfall

 

Small intestinal worms

Temperature: daily maximum >18°C for C. punctata: >12°C for C. oncophora

Moisture: up to 50 mm rainfall

 

Nodule worm

Temperature: daily maximum >18°C1

Moisture: up to 50 mm rainfall

 

1Some hatching of worm eggs of all worm species can occur below these daily maximum levels, but this is usually at a small and insignificant rate.

2Small brown stomach worm eggs can develop at low rates in a relatively dry dung pat.

Unsuitable conditions prevent eggs hatching and developing into infective L3.

Note: The eggs of the small brown stomach worm are more tolerant of cold and dry conditions than other genera.

Eggs and larvae in dung pats during mid-winter and mid-summer may die due to extremes of conditions (temperature and/or moisture).

Adverse weather conditions that prevent egg development reduces the worm-risk of paddocks.

L3 larvae can remain protected within the dung pat for several months (up to 5 months if deposited in spring and 7-8 months or longer if deposited in winter) due to its high moisture content and bulk.

 

When rainfall is continuous, migration of larvae out of the dung pat is continuous. When rainfall is alternated with periods of dryness, migration occurs in waves.

 

Once on pasture, L3 larvae are vulnerable to environmental conditions.

Survival of L3 larvae on pasture:

 

Maximum temperature

Time for 90% of L3 to die

Cold

< 15°C

4 months

Warm

about 22°C

3 months

Hot

about 35°C

1.5 months

Very hot

> 40°C

1–2 weeks

 

L3 larvae do not feed. Once they leave the dung pat their survival time is limited by their energy reserves.

 

While waiting to be ingested by grazing animals, L3 move randomly in drops of moisture. Increased activity in warm weather depletes their energy reserves, hastening death. In extremely hot and dry conditions, larvae on pasture die more rapidly.

Minimum time for infective larvae ingested by cattle to mature and lay eggs (the ‘pre-patent period’).

Small brown stomach worm: 21-24 days

Barber's pole worm: 26-28 days

Small intestinal worms: 11-17 days

Nodule worm: 32-42 days

Pastures are not contaminated during the pre-patent period.

If cattle are drenched with an effective drench, infective larvae can only establish after the protective period of the drench has finished.