What to do when worm reinfection limits lamb growth rates
© Tim Scrivener In the latest in our series, Ben Strugnell of Farm Post Mortems, County Durham, discusses a tough late summer season of parasite challenge in youngstock at grass.
Good-quality grass is your cheapest and best feed, but not if it is full of worms.
This year, many lambs will have been treated, killing non-resistant worms, but then returned to pasture and become reinfected. This is because most wormers only kill worms in the lambs at the time of treatment.
It is likely that the worms completed several lifecycles during the wet summer, meaning the number of eggs on pasture able to infect lambs now is higher than average.
This is reinfection and is different to resistance, where the wormer does not work. The worm burden that lambs develop after this reinfection depends on two things:
- The number of eggs they eat
- How much immunity they have acquired to the species of worm they are eating.
Reinfection always occurs to some extent, especially in autumn, and is worse on some farms this year because of the higher worm burdens in pastures.
Why does weather matter?
Because livestock typically avoid grazing close to droppings, worms depend on moisture to travel from the droppings and up the grass leaf to increase the likelihood of being grazed.
Ideally, we need a hot and dry midsummer period to desiccate dung piles, slow worm larvae development and kill eggs with lethal ultraviolet sunlight.
Average rainfall in England for the second half of summer was as follows:
- July: 120mm = 207% of the average
- August: 72mm = 102% of the average
- September: 81.7mm = 116% of the average.
What parasites will be affecting my lambs?Â
There are 14 gastrointestinal parasites that commonly affect UK sheep farmers.
Of these, Teladorsagia circumcincta and Haemonchus contortus (which live in the abomasum), and Nematodirus battus and Trichostrongylus sp. (which live in the small intestine), are highly pathogenic.
Nematodirus is the only one not referred to as a strongyle.
These parasites latch onto the lining of the abomasum (Haemonchus and Teladorsagia) or small intestine (Nematodirus and Trichostrongylus).
They feed on the mucosa and damage the villi – the folded, protruding surface of the stomach, which has evolved to absorb nutrients.
This compromises nutrient absorption, resulting in runny stools and death through malnutrition.
Nematodirus tends to be a spring issue, although hatches can be delayed. Teladorsagia is notable as it is the species with the most widespread resistance.
Haemonchus is particularly damaging and leads to the most sudden deaths. It is the largest of these parasites, at 1.5-3cm in length, and drinks about 0.5ml of blood a day from the abomasum wall.
Neither ewes nor lambs ever become resistant to it.
The immunity to worms that lambs develop only applies to the species they encounter, and the species tend to become active one after the other, so lambs can often face constant new challenges by worms they have not previously encountered.
How to deal with the problem
The options people have tried are:
Dose and move to clean grazing
Administer a wormer and move lambs to ground where the challenge will be lower, such as a herbal ley, a new ley that has not had years of parasite accumulation, or a field that has had cattle or dry ewes on hoovering up worm larvae.
You must wait several days (ideally four or five) after dosing lambs to avoid populating new pastures with exclusively resistant worms.
House and feed hard
This can be a higher-cost option if ad-lib concentrates are used, but it removes lambs from infective pasture. Very often, 90% of the infectivity comes from 10% of the worms.
It may be best to house the worst affected to help limit burdens for the rest.
They will need supplementary protein to enable their guts to recover – this can easily be provided indoors.
Lambs must be vaccinated against pulpy kidney (and probably pasteurella) before housing, or the change may trigger deaths. Carefully adjust diets from pasture to cereal to avoid losses through acidosis.
Incorporate forage into your system
Before reaching for chemical answers, the solutions may be home-grown, whether that be summer forage (a herbal ley or new grass ley) or a winter forage (brassicas).
Remove affected lambs
The best place for the wormy, non-growing lambs is off your farm. If you sell them, they cannot continue to contaminate your pastures.
Long-acting wormer
Reaching for moxidectin may seem appealing as it continues to kill the worms that lambs eat for a period after dosing.
This persistent activity varies with the preparation (oral, 1% and 2%), and is different for the different species of worms. The long-acting effect is best against Haemonchus and Teladorsagia.
However, note the following:
- Only use it for worms if you are not using it as a scab treatment
- Teladorsagia is the most resistant of sheep parasites
- Moxidectin has shorter persistence against Trichostrongylus worms, which are typically an issue in late summer and autumn
- There is no persistence against Nematodirus
- It has long withdrawal periods (70 and 104 days) for injectables and a 14-day withdrawal for the oral dose
- A faecal egg count reduction test (FECRT) should be done two weeks after treatment with moxidectin to prove that it has killed the worms
New wormer classes
A dose of the orange monepantel (Zolvix) later in the growing season is being championed as a knockout dose.
Combine it with a move to less-infected ground so lambs can hopefully thrive thereafter.
The move must be four or five days after treatment to avoid the risk of selecting for the small number of worms that may be resistant and populating the clean pasture.
Test your wormer
Whatever the product, an FECRT will show whether the worming has killed all species of worms in the lamb at the time, and establish which products work on your farm.
More information is available on the Sustainable Control of Parasites in Sheep (Scops) website.
High faecal eggs counts (FECs) in lambs with a high challenge are ideal candidates for an FECRT, as the level of eggs is high and any reduction will be obvious.
Speciate your worms
Identifying which species of strongyle is affecting your lambs is only possible with veterinary laboratory investigation.
This may be especially useful on the residual eggs after an FECRT, as it is helpful to know which species of worms (if any) are resistant to which wormer.
Weigh, count, and record
Read the Scops website for guidance on FECs and using weight gain data to inform treatment plans.
Weigh lambs every three weeks if you can. Lowland lambs’ daily liveweight gains are typically 300g pre-weaning, 150-200g post weaning and potentially less by autumn, depending on weather.
Lambs that have been reinfected with larval worms may not grow well and this may be your first indication that reinfection is constraining growth rates and you need a different strategy.
Does rotational grazing help?
Rotational grazing has many benefits but parasite control in ewes and lambs is not necessarily one of them.
Sometimes lambs can return to a mass infection on a rotational system, depending on intervening conditions.