Worms in dairy cattle (Parasitic gastroenteritis(PGE))
Cattle can be parasitised by many different species of intestinal worms. This infection is otherwise known as parasitic gastroenteritis (PGE).
Lungworm and Liver fluke are also common parasites of cattle but these will be dealt with in more detail in other sections
Cattle pass worm eggs in their faeces. These eggs then develop to the infective larval stage in the pat (which usually takes around 2 weeks under optimal conditions). The cows are then infected when they eat these larval stages which have by now migrated to the pasture (uneaten larvae at the end of a grazing season can potentially survive in the pasture acting as a source of infection to the next seasons grazing animals). There is then further development through several more larval stages in the cows gastrointestinal system before the worm becomes sexually mature and can start producing eggs. The development inside the cow usually takes about 3 weeks to reach the egg producing stage.
Therefore the whole cycle from egg to mature, egg producing worm takes around 5 weeks – however, in some cases the development within the cow can stop and the larvae can become dormant allowing them to re-emerge up to 6 months later.
Worm life cycle diagram - coming soon!
There are some slight differences between species of worms but the main ones that affect cattle more or less follow this life cycle.
In general, how bad a worm problem becomes depends on the pasture contamination levels. It is useful to understand a little about how cattle develop PGE (Parasitic Gasroenteritis) disease.
Grazing animals can pick up infections early in the grazing season from larvae that have survived over winter on the pasture – this can cause disease if the pasture burden is very heavy but usually these overwintered larvae act as a low level source of infection that allows calves to become infected and start shedding worm eggs onto the pasture during the grazing season. Disease picked up from pasture in the same grazing season such as this is known as type I disease.
Overwintered larvae on pasture begin to die off in the spring and there are very few detectable by June. Most pastures not grazed in the spring are therefore reasonably safe to graze in mid- summer – especially if there has been a high rate of grass growth as this further dilutes the larvae.
As the summer progresses and the weather becomes warmer the development of the egg to the infective larvae becomes faster – in fact most eggs deposited in the early spring reach the infective stage at about the same time (usually around mid July). This causes a higher pasture burden and makes disease more likely from July to October.
As autumn approaches the larval development slows down again and also a higher proportion of larvae in the animal become dormant as mentioned earlier. These can begin developing again towards the spring and if numbers are high can cause clinical disease as they emerge simultaneously as adult worms (known as type II disease). Otherwise, these reanimated larvae contribute to the re-infection of the pasture.
There have been 2 risk factors associated with Type II disease that have been particularly identified:
- Grazing calves on a pasture between May and July and then moving them to aftermath grazing before returning them to the original pasture in autumn. This means there will potentially be a high infective larval burden on the pasture that, when eaten, can become dormant in the animal.
- In dry summers the larvae become trapped in the crusts of pats – they are then released when it rains. If this is late in the season it can result in a high number of infective larvae on the pasture in autumn which are eaten and then become dormant in the animal.
Do adult cattle need worming? Calves usually develop an immunity to worms during their first grazing season which can wane over the winter making second season grazers susceptible to re-infection. However immunity usually is quick to be re-established and symptoms are usually mild and transient. Third season grazers and onwards should have a solid immunity to worms and should not need treating.
The main signs of a high worm burden is diffuse diarrhoea and subsequent weight loss/reduced weight gain. Some calves may exhibit fluid build up under their jaw due to protein loss in the diarrhoea.
Potentially, worms can be a problem in any herd. It is always worth sampling and monitoring the situation amongst calves exhibiting signs of disease as well as those that appear healthy. It is important to discuss strategies to reduce potential problems from parasites with your vet.
The cost can come from loss of production, reduced growth rates and conception rates in heifers with an increased age at first calving. Also poorly considered worming can use unnecessary wormer which is expensive both in the cost of the drug and the labour costs in administering it.
Faecal samples can be used to count worm egg levels to give an idea of the burden
There are 3 main types of wormer:-
- Group 1 – Benzimadazoles – these are known as the white drenches. These kill both adult worms and their eggs.
- Group 2 – Levamisoles – These are known as the yellow drenches. These only kill adult worms.
- Group 3 – Macrocyclic Lactones – These are known as the clear drenches. These can have extended durations of action and also kill external parasites such as lice.
Remember never underdose animals with a worming product and use them as recommended by the manufacturer. It is useful to weigh animals to accurately assess what dose to use. Also remember to calibrate the injectors/ dosing guns etc to ensure a full dose is given every time.
Underdosing has the potential to select for worms with an increased resistance to that particular wormer.
Dependent on the damage caused most animals will recover from moderate parasite burden. However parasites can slow the growth and development particularly of calves and it has been suggested that burdens in adults if treated can improve fertility rate sand production.
It is better to try and prevent a worm problem than to treat it after it has clinically affected the animal
It is unrealistic to expect your cattle to be free of worms. However s ensible treatment protocols should reduce the number of worms your cattle encounter. Remember cattle should have a fairly solid immunity to worms by their third grazing season.
Your vet will be able to discuss with you a sensible worming protocol for your farm. Every farm is different and what works for one farm may not work for another.
Continuous blanket treatment with wormers may not be the best option as this can allow for resistant worms to emerge and also will not allow the cattle to build up such an effective natural resistance.
The COWS (Control of Worms Sustainably) program suggests sensible precautions to prevent resistant worms building up on your farm. These are as follows:-
- Work out a control strategy with your vet
- Quarantine incoming animals and develop a sensible treatment protocol before introducing them to your pastures
- Test that your worming protocols are effective.
- Adminster wormers effectively (don’t underdose and follow the manufacturers instructions)
- Use wormers only when necessary.
- Select the appropriate wormer for the job.
- Adopt strategies to preserve a worm population on your farm that is susceptible to common worming products.
- Reduce your dependence on wormers by adopting good management strategies to avoid exposure to high worm burdens.
You can read in more detail about the COWS report by downloading the manual. Just click here to visit the website.