1. The most important features in the development of Ostertagia infections are:
- Weather conditions cause long delays in the development of larvae from eggs and their migration from dung pats to pasture.
- The weather in spring and autumn is the most favourable for the development of infective larvae.
2. Dung pats provide protection for infective larvae that may be released whenever sufficient rainfall occurs.
3. Weather conditions determine that the greatest number of larvae are present on pastures from mid-autumn till mid-spring:
- Large numbers of larvae acquired over a short time in winter and early spring by young cattle leads to 'Type 1' disease.
- After August, most larvae acquired by cattle become dormant in the stomach wall, and remain so for months. Resumption of their development leads to 'Type 2' disease in autumn.
4. Guidelines for strategic drenching of autumn- and spring-calving herds are shown in Table 1.
In south-eastern Australia, Ostertagia ostertagi (small brown stomach worm) is the major cause of disease and production losses from worm infections in cattle. Other species of worms are also present, such as Trichostrongylus axei (stomach hair worm) and intestinal worms (mainly Cooperia oncophora). 'Barbers pole worm' (Haemonchus placei) is also occasionally found in northern Victoria and can cause severe disease, especially in calves, because it sucks blood.
Monitoring the worm egg counts of young cattle will often show a majority of Cooperia eggs, mainly because the females of this species lay more eggs than Ostertagia. Although Cooperia is rarely important on its own in beef cattle, it does exacerbate the production losses from Ostertagia. This is because the damaged small intestine is less able to compensate for the damage to the 4th stomach (abomasum) caused by Ostertagia.
There are a few important differences between worm infections of sheep and cattle:
- the major species involved are different, so paddocks grazed by cattle are of low-risk for sheep, and vice-versa.
- In cattle, Ostertagia can become 'arrested' with large populations of larvae accumulating in the stomach wall during late spring/summer, then simultaneously resuming development and maturation in autumn.
- not all drenches kill the arrested larvae.
- adult cattle generally develop good immunity ('resistance') to worms and so don't need routine treatment after their second calf unless they are running at higher stocking rates or are in poor body condition, especially in higher rainfall areas (more than 550-600 mm annual rainfall).
- worm egg counts in the dung are not a reliable indicator of worm burdens after about 6 months of age, so monitoring egg counts is less useful than in sheep.
The worm life-cycle
Cattle are usually infected with a mix of different worm species, all of which have a simple, direct life-cycle. Ostertagia is the most important production-limiting species, and so has been studied in most detail, but the life-cycles of the other worms are quite similar:
- Female worms lay eggs that are excreted in dung.
- These hatch and develop to an infective larval stage in the dung-pat.
- The infective larvae migrate onto the pasture and, when eaten by cattle, can develop to adult worms in the 4th stomach within 3 weeks.
However, lengthy time delays are usual for the Ostertagia life-cycle. These are influenced mainly by weather, but also by factors within the cattle. Understanding these relationships, both within cattle and in the dung pat on the pasture, is important for the effective control of Ostertagia infections.
Most cattle are infected with moderate numbers of adult Ostertagia that release eggs into the dung every day. Weaners and yearling cattle usually have the highest egg counts (up to 500 epg), although diseased animals can have counts up to 1000 epg. Most cattle over 12 months-old have low egg counts, but even a cow with counts as low as 2 epg can deposit up to 50,000 eggs in a single day!
Development of Ostertagia eggs and larvae
Warmth and oxygen are needed for eggs to hatch, but cool conditions and a lack of oxygen in the dung pat often delay this stage of the life-cycle. In summer, hatching takes 4-12 days, but this increases to 5-10 weeks in winter when cold and soggy dung pats have low amounts of oxygen.
The dung pats must begin to dry out before the eggs start hatching and larval development gets underway. Thus, eggs in the outer crust of the pat always hatch before those in the centre. Consequently, the proportion of eggs that develop to infective larvae, and the time taken for development, varies enormously at different times of the year.
Long delays also occur before infective larvae move out of the dung pat onto the pasture. Some are eaten by cattle, but only a small proportion will actually complete their life-cycle. Moisture is essential, and so larval migration is dictated by rainfall and evaporation.
For example, summer storms will often not soak the pats sufficiently, and so larvae remain inside, protected from the drying effect of the sun, for 4-5 months. However, more consistent rainfall at the autumn 'break' starts the migration process, although it still takes 6-8 weeks from the break before peak numbers of larvae are available on pasture. This migration process occurs in dung pats deposited at different times of the year - even pats that were deposited in the previous winter and did not disintegrate during spring will release some larvae after the autumn rains.
Thus, few infective larvae are available on pasture from mid-spring till autumn, but large numbers are found from mid-autumn until mid-spring.
Weather also effects the survival of infective larvae. In the dung pat they can survive fluctuating weather conditions for many months, but once they migrate to pasture they are killed by just a few days of heat and drying. However, larvae that migrate onto pasture after the autumn rains will persist until warmer weather in mid-spring finally kills them off.
Types of Disease
Early in the 1960's, it was found that 3 'phases' of Ostertagia infection produced 2 quite distinct diseases, 'Type 1' and 'Type 2', both characterised by diarrhoea, weight loss and ill-thrift. The third phase is not clinically obvious, but is a pre-requisite for Type 2 disease.
Type 1 Disease develops when young cattle (up to 18 months-old) take in large numbers of infective larvae over a short period (4-8 weeks) during winter and spring. These larvae develop to adult Ostertagia in 3 to 4 weeks, severely damaging the structure and function of the lining of the stomach. This causes scouring, reduced growth rate and loss of weight. Most animals in a mob will be affected, but respond well to treatment with a drench.
The Pre-Type 2 phase occurs from August to December. A gradually increasing proportion of larvae eaten over this time become arrested in their development, 4-6 days after infection. These early 4th stage larvae are only 1.5 mm long and penetrate deeply into the stomach glands, where they remain dormant for 3 to 9 months. In this state they cause little or no damage. However, enormous numbers (millions) accumulate in the stomach wall, and starting about March, resume their development to become adult worms within 3 to 4 weeks.
Type 2 Disease occurs when thousands of dormant larvae simultaneously complete their development to adult worms, causing severe damage to the stomach, diarrhoea, weight loss and ill-thrift.
Because of the irregular development of dormant larvae, usually only a few animals in a mob are affected with Type 2 disease at any one time. Typically this occurs in late summer and early autumn, when pastures are dry and worms are not suspected as the cause of ill-thrift.
Two-year-old heifers, from 4-6 weeks before autumn calving are particularly susceptible, but older cows and mature bulls also may be affected anytime during autumn or winter. Response to treatment is often poor, and drugs that remove only the adult worms allow further waves of larvae to mature, causing more damage to the stomach wall. This may take months to repair, and so persistent ill-thrift and weight loss is common.
Controlling worm infections in cattle
Strategic control (keeping parasite numbers below those that will cause production losses) is far superior to a 'curative' approach (treating only when animals are obviously diseased). This is because, by the time disease is recognised, significant production losses will already have occurred. Strategic control can be achieved by:
1. Limiting the contamination of pasture with worm eggs when conditions for development are most favourable, namely autumn and spring. Effectively, this means using drenches at times when they will have the most effect on the life-cycle.
2. Avoiding grazing young cattle on paddocks where high contamination with eggs and larvae is known to have occurred. This requires some system of grazing management.
Both the time of calving and local weather conditions will play a major role in the selection of strategic drenching and grazing management strategies to control Ostertagia infections.
Guidelines for the timing of strategic drenches in autumn- and spring-calving herds in the high rainfall areas of Victoria are summarised in Table 1. It is important to realise that these are broad recommendations, and can vary between production systems, localities and seasons. Individual producers should seek advice from an experienced animal health adviser for a control program specific to their farm.
Table 1. Suggested timing of drench treatments for strategic worm control in beef cattle in south-eastern Australia
A) Autumn calving
Class of cattle
1st & 2nd calvers
B) Spring calving
Class of cattle
1st & 2nd calvers
âˆš treat in all years; (âˆš) treat low condition score (2 or less) or scouring animals only. Treatment will be more effective if combined with a change to ‘low-risk’ pastures, especially for young stock; (âˆš) * these treatments are more likely to be necessary with higher stocking rate systems or in higher rainfall areas; (âˆš) # the need for and exact timing of these treatments will vary from year to year. Indicators for treatment are low feed availability (eg. less than 1000 kg green Dry matter/ ha), low animal condition score or sudden loss of condition, seasonal conditions that limit pasture growth or favour increased larval availability (eg. cold wet winters, or warm dry ones) and the history of worm control on a particular farm.
Integrating grazing management with drug treatments (strategic drenching) produces more effective worm control programs.
- Short term rotational grazing - Because of the long time delays in the Ostertagia life-cycle, rotational grazing at 5-10 week intervals is of no benefit for worm control.
- 'Low risk' paddocks - As for worm control in sheep, there are opportunities to exploit longer periods of pasture spelling, such as grazing the aftermath from hay, silage or cereal cropping.
- Alternate grazing of sheep and cattle - This is the most effective use of grazing management because most sheep worms don't infect cattle, and vice-versa. A January and June sheep-cattle interchange is commonly recommended, but the optimal timing depends upon local seasonal conditions. Unfortunately, this strategy is often overlooked because of fears that cattle will suffer from grazing sheep pastures. This can be overcome by shifting the sheep 2-3 weeks before the cattle, allowing pastures time to recover before cattle grazing.
Types of drenches
Cattle drenches fall into the same categories ('groups') as sheep drenches. Combinations of drench groups are not used because, unlike the situation in sheep, drench resistance is still uncommon in cattle worms (apart from ML resistance by Cooperia).
The macrocyclic lactones ('MLs', eg. Ivomec®, Cydectin®, Dectomax® & other brands) are the most effective compounds, but more expensive.
- The 'white' drenches (Panacur®, Systamex®, Valbazen®) work reasonably well and are cheaper, but they don't come in pour-on formulations and so are less convenient to use. Their effectiveness against arrested Ostertagia larvae is less and more variable than the MLs.
- Clear (levamisole) drenches (Nilverm® and other brands) are even cheaper, but are not effective against arrested Ostertagia and so they shouldn't be used for drenches given between December and May.