Watch out for worms in cattle

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Internal parasites or worms, have been a bigger problem than usual this year. The milder summer last year meant worm larvae survived throught the summer and numbers have built up and are now affecting stock. Young animals are especially at risk, but we are seeing problems in older cattle as well.




What worms do cattle 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 such as Cooperia.


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

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

The animal eats the larvae and it then burrows into the wall of the fourth stomach, the abomasum. After moulting to become early fourth stage larvae (L4), development may continue without delay or be interrupyed 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 the adult worms appear 3-4 weeks after infection with L3 larvae.


The Disease:

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 generally 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 (the abomasum). 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 benzimidazole 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 Ostertagia is a problem, the use of "mectins" is generally recommended.

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.

There are also doubts about how well "Pour-On" drenches actually get to 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 

Usually adult cattle are relatively resistant to worms because of their age and previous infection (as younger stock). Thus, routine treatment of adult cows is not generally recommended and only the few individual animals that show signs of disease generally need to be drenched.


Young Stock:

Young stock do not have immunity against worms and so are at high risk of disease. Young calves generally do not need drenching before 12 weeks of age, unless they are highly stocked.

Heifers and steers may need drenching at intervals of 6 -12 weeks 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 pasture 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



 Young Animals:

Worm egg counts done on dung samples are used to indicate the worm burden in young cattle. Egg counts are less reliable in cattle older than 12 months. Use egg counts to monitor the level of worms and determine if drenching is required. Egg counts can also be used to measure the effectiveness of drenching by conducting a second egg count two weeks after drenching.

Adult Cattle 

Blood tests can be done to indicate worm damage in the abomasum. This can be of great value in older cattle.