Worms present a significant production threat to grazing cattle with infection costing the industry millions of pounds a year.  The main worms affecting cattle are stomach, gut worms and lungworms commonly termed roundworms (nematodes)and  tapeworms (cestodes).  Liver fluke (trematodes) are covered in a separate section



Economically, worms are extremely important, causing clinical effects in animals. Even when present in small numbers, with no visible clinical signs, they are responsible for significant production losses.
Common cattle worms and their effects 

  • Stomach and intestinal worms cause diarrhoea, appetite loss and weight loss
  • Lung worms cause coughing and unthriftiness
  • Barbers pole worm causes anaemia and bottle jaw due to blood sucking



Roundworms are cylindrical and vary in length from a few millimetres to several centimetres. They parasitise the small intestine, large intestine, stomach and lungs. 

Generally they are host specific, i.e. sheep nematodes do not infect cattle.  Exceptions are the Barbers Pole Worm and the gut worm Nematodirus battus which can affect either.

Stomach worm

In cattle, brown stomach worm (Ostertagia ostertagi) is by far the most important worm. Disease only occurs in first and sometimes second season grazing stock as immunity is developed over this period. Two types of disease occur; Type I during the summer, as larvae are picked up from the pasture and Type II in early spring when arrested larvae re-emerge from their dormant state in the abomasal wall. Symptoms of the two types of disease are very similar. 



Lungworm is more serious in cattle than in sheep. In endemic areas, first season cattle can be vaccinated prior to turnout.


(Moniezia spp) are the  only tapeworms found in UK cattle as adult worms and live in the small intestine with the head attached to the intestine wall. Segments are shed in the faeces thirty days after infection in quite long chains. Immunity develops rapidly and, although diarrhoea and unthriftiness can be associated with tapeworm, generally clinical and economic losses are minimal. 

To know how to best control parasites, it is vital to understand their life cycles:

Lifecycle of roundworms

  1. Larvae enter the gut, as the host eats infected grass
  2. Larvae mature into egg producing adults and eggs are excreted by the host
  3. Eggs are then spread by hooves and rain
  4. Eggs develop into larvae after 24 hours
  5. At 3-4 days old larvae become infectious
  6. Larvae move throughout pasture and attach to grass


Arrested development (hypobiosis)

Larvae picked up in the autumn, instead of moulting to the next stage, can become dormant. This can be caused by environmental cues such as decreasing day length and temperature. Larvae picked up in spring or summer complete their lifecycle and do not become arrested in this way.

The arrested development of Ostertagia ostertagi in cattle is the most notorious. Arrest occurs at the fourth larval stage when, instead of moulting to the fifth larval/young adult stage, larvae become dormant and hibernate in the gastric glands of the abomasum. 

No further development occurs until it is re-triggered by some kind of stimulus (possibly hormonal or linked to lengthening daylight hours) causing all arrested larvae to resume growth and development. 

In Ostertagia ostertagi this stretches the gastric glands and disrupts the stomach lining as they re-emerge, causing the disease Type II ostertagiasis ‘winter scours’ which is often quite sudden and dangerous.

The mid season rise   

The majority of eggs deposited in April, May and June develop slowly, then as temperatures increase, all reach the infective stage from mid July onwards (the ‘mid season rise’). Development from egg to L3 slows again during the autumn and is it is doubtful that eggs deposited after September ever develop to L3.

Factors affecting infection levels on pasture

  • Worms present.  Some roundworms produce thousands of eggs daily, others only a few hundred. 
  • Stock management. Higher stocking rates produce greater contamination, especially in the right conditions for egg or larval development, such as the spring and summer. 
  • Immune status of animals.  The influence of stocking density will be greatest if all the stock are fully susceptible, or if the ratio of susceptible to immune stock is high.


The aim of any worm control strategy is to prevent the exposure of young susceptible animals to high levels of infestation on pasture in the second half of the grazing season. This can be done using:

  1. Use of low risk pasture/clean grazing
  2. Wormer treatments
  3. Integrated systems incorporating strategies 1 and 2

However control of roundworms by clean grazing alone is impractical on many farms, due to lack of available clean grazing. Wormer [anthelmintic] treatment is often necessary.  


Wormers are vital to control worm infections and protect farmers against production losses that can cost the industry millions of pounds.  

There is a vast array on the market and wormer choice depends on a number of factors: time of year, activity against different parasites, chemical class, mode of action, available handling facilities. 

Due to the potential growth of resistance, the most important differentiating factor between wormers is chemical class: 

Wormer chemical classes

Wormer differentiating factors:

Wormers belonging to these groups will work against most, if not all, species of nematodes and there is little to choose between them in terms of activity. Major differentiating points are:

  • Only wormers in the 1-BZ group have efficacy against tapeworms.
  • Only wormers in the 3-ML group have any persistent activity against nematodes, allowing the dosing intervals to be increased.

Cattle anthelmintics are now available in a number of different forms:

• Drenches • Pour-ons • Boluses • Injectables

Choosing a wormer

Choosing the cheapest anthelmintic can be a false economy. 

The anthelmintic most suited to one farm may be inappropriate for another. Whichever wormer is selected it is important to read label recommendations and dose accordingly.

When to dose cattle

Mature cattle may be relatively immune to worm infections, although there has been work to show that worming adult cattle at certain times of the year can be beneficial for production. Discuss with your vet or prescriber how this may affect your farm.

Calves, after turnout 

Where calves are grazed independently of dams, infection levels are liable to multiply to levels producing a reduction in weight gain and/or clinical symptoms. 

If clean grazing is not available, calves should be wormed regularly after turnout, to suppress egg output in the early part of the season and limit the dangerous build up of infective larvae on the pasture in the second half of the grazing season.

Products with no persistent activity should be used every 3 weeks until the over-wintering infection has declined. Those with persistent activity can be used e.g. 3, 8 and 13 weeks after turnout or longer depending on the product used

To succeed and be cost-effective, control strategies must be applied to set stocked systems (i.e. where the animals stay on the same field/s) otherwise the benefits are lost. 

Treat all animals, as untreated animals would contaminate the pasture. With these systems, a considerable reduction in the numbers of infective larvae on the herbage in the second half of the grazing season will be obtained.

Calves, at housing 

Some larvae will always be present on pasture, so treat calves with an anthelmintic effective against arrested worms at housing, especially if taking place late in the season.

‘Dose and move’ strategy 

If clean grazing is available, a ‘dose and move’ strategy can be adopted. Calves should be dosed immediately before they are moved so that contamination of the clean pasture is avoided. 

Young cattle should not be returned in autumn to pastures they grazed earlier in the year, where high herbage infections may put them at risk from clinical disease or give them large burdens of arrested larvae.

If egg output is not suppressed during the grazing season, there will be a build up of infective larvae on the pasture by late summer/early autumn. This is particularly dangerous, since larvae exposed to declining daylight hours and temperatures are conditioned to arrest their development with the possibility of producing Type II ostertagiasis the following spring. 

If worm burdens are controlled throughout the grazing season the incidence of Type II ostertagiasis will be much reduced.

Anthelmintic resistance in cattle does not appear to be as big a problem as it is in sheep but the responsible use of anthelmintics is always advised.