By Dr Charlie Blackwood

Internal parasites or WORMS have been a bigger problem than usual the last few years. Some information about Worms in Cattle.

What Worms Do Cows Get

The main gut roundworm of cattle is Ostertagia ostertagi, known commonly as the brown stomach worm. Usually control of Ostertagia will also control other roundworms of lesser importance such as the small intestinal worm (Cooperia sp). Cooperia is not killed by all drenches, however. Lung worm are a separate problem.

Life Cycle

Ostertagia and other roundworms of cattle have a simple direct life cycle. An important feature of this life cycle is that it consists of two stages; the free-living stage on pasture and parasitic stage in cattle.

Worm eggs are passed out in dung and moult into second and third stages (L2 and L3). The third stage move onto pasture and can survive for many months depending upon how hot and/or are the dry conditions.

The animal eats the larvae and it then burrows into the wall of the fourth stomach, called the abomassum. After moulting to become early fourth stage-larvae (L4), development may continue without delay or be interrupted by a period of up to several months. The lining of the abomasum is significantly damaged when the larvae emerge as immature adult worms.

Large numbers of L4 larvae tend to become inhibited in their development if they are ingested during spring and early summer. These can cause a serious type-2 disease when they resume growth and emerge into the abomasum during late summer and early autumn.

If the L4 larvae develop directly, that is if they don’t become inhibited, then adult worms appear 3-4 weeks after infection with L3 larvae.

The Disease

As explained above, there are two types of disease caused by Ostertagia. The signs of each type result from the same damage to the fourth stomach or abomasum:

Type-1 disease usually occurs in calves and young cattle that have high burdens of adult worms in winter and spring. This disease follows rapid infection with large numbers of L3 larvae from heavily contaminated pastures in the autumn and winter after weaning. Dairy calves typically suffer type-1 disease at 5-6 months. Beef cattle are affected at 15-20 months.

Type-2 disease occurs especially in beef cows calving for the first and second time in the autumn and winter. This coincides with the stress of calving and the emergence of thousands of inhibited L4 larvae from the lining of the fourth stomach. Severe scouring, loss of weight and even death may result. Frequent drenching may be needed just to keep these cattle alive.


The main drenches used are the “mectins” ( or ML drenches). Benzimidazole or “white” drenches are still sometimes used. Levamisole drenches are used less commonly these days.

All are effective against adult worms in the abomasum. They differ in their activity against the developing and inhibited L4 stages of larvae. The mectins are the most effective against L4 larvae, the benzimidazole drenches follow the mectins and the levamisole group is the least effective. Some of the benzimidazoles drenches have lesser and a more variable effect on the immature and inhibited stages of Ostertagia than previously thought.

Where stocking rates for cattle are high or ostertagiasis is a problem, the use of “mectins” is generally recommended.

Which Drench?

There is no single answer as to which drench to use. For adult cattle, in many cases the cheapest ML group drench will do the job. In younger cattle, the more advanced ML drenches are often better.

Drench Resistance

To date some cases of drench resistance in cattle roundworms have been identified in Australia. Resistance has been found in beef properties in south west Victoria. More work is being done on this topic.

Resistance has been increasing at an alarming rate in recent years on beef rearing farms in New Zealand and South America. Resistance in now widespread in these countries and mainly involves resistance in Ostertagia to the white drenches and in the small intestinal worm (Cooperia) to the mectin drenches. Resistance appears to be associated with intensive rearing or frequent drenching in these countries.

There are also doubts about how well Pour On drenches actually get to the worms. Studies show cattle take in much of the pour on drenches by licking each other rather than through the skin, as previously thought.

If possible, use injectable or oral drenches.

Control Programs - Dairy Cattle

In beef cattle and sheep, we rotate paddocks and use cropping to help lessen worms. Unfortunately, this is harder to do in dairy stock.

Adult Cattle
Our usual advice is that adult dairy cows are relatively resistant to worms because of their age and previous infection. Thus, routine treatment of adult dairy cows is not generally recommended and only the few individual animals that show signs of disease generally need to be drenched.

It is debatable whether low levels of infection with worms affect production of milk. Many conflicting studies have been published and it has been generally accepted that the cost of treating dairy cows to control low burdens of worms exceeds the gains of any additional milk production. Some trials have recorded responses after treatment with mectins of up to 0.5 L of milk per cow per day. However, better nutrition of the herd will often produce far greater benefits than drenching.

In recent years, however, we have seen significant numbers of cows in some herds which are scoury, not putting on weight as expected and milking below expectations. Blood tests have confirmed worms as being a significant problem. As previously stated, we believe this is due to the mild season, together with our relatively high stocking rates. Drenching has been required for these herds.

Young Stock
Young stock do not have immunity against worms and so are at high risk of disease, although dairy calves only rarely need drenching before 12 weeks of age.The mild season, using paddocks continually to avoid Johnes disease and high stocking rates again combine to cause problems.

Heifers may need drenching at intervals of 6 weeks to three months from the time of weaning until 8 months of age. The exact frequency will depend on the degree of contamination of the pasture. Irrigation assists the movement of infective larvae from the dung pat to the herbage in much the same way as abundant summer rain does. For this reason there can be significant infections in cattle on irrigated pasture in summer as well as in autumn and winter.

Checking for Worms and Drench Efficacy

Young Animals
Worm egg counts conducted on dung samples are used to indicate the worm burden in young cattle. WECs are less reliable in cattle older than 12 months. Use WECs to monitor the level of worms and determine if drenching is needed or not. You can measure the effectiveness of drenching by conducting a second WEC two weeks after drenching.

Adult Cattle
Blood tests can be done to show worm damage in the previous one to two months. This can be of great value in older cattle. A milk test is available and promoted by some drench companies and suppliers. The science behind this test seems sound, however, deciding what levels are significant is difficult to decide.

Worms – one of the many topics covered in our Herd Health Management Course.

2017 Course Dates 21, 23, 28 and 30 March 2017.

More detail on the Herd Health Management Course can be found via this link! 

To enrol call the Farm Desk on 5561 7666.
Places are limited so get in early.