With lambing safely over for most flocks, the lengthening and often balmy days of early summer are often seen as a pleasant interlude before haymaking picks up the pace. These halcyon days are not an excuse to leave the flock to its own devices though! Early summer often produces lots of warm, wet weather which is just what nematodirus need to thrive.

This parasite is a very common round worm which can have devastating consequences for lambs up to 6 months of age. Fortunately, lambs begin to build up resistance at around 3 months of age and by 6 months most are well able to resist infection. Until then, you need to keep a sharp eye out for any signs of infestation.

It isn't the adult worms that do the damage, but large numbers of infective larvae burrowing into the gut simultaneously. When this happens the signs are quite graphic. You'll probably notice any of the following, watery scour, (diarrhoea), heads down, drooping ears and generally looking "tucked up." They also get very thirsty due to dehydration caused by the scouring so another give away is grouping around the water trough. Left untreated, lambs experience a lot of pain and decline rapidly.

Fortunately, nematodirus can be effectively controlled. Being aware of potential worm problems and having a good management system can do a lot to ensure your flock doesn't get devastated by an outbreak. Treating symptoms early will prevent lambs losing too much condition and ensure they recover quickly.

As nematodirus is picked up during grazing, access to clean pasture plays a big part in worm control. Splitting a large field into smaller paddocks that can be rotationally grazed will help to break the worm cycle. This will be further aided if it is also grazed by other animal species. Cattle always follow sheep and if you can graze one field by cattle one year and swap with the sheep next year, this will have a good effect on breaking and reducing the worm cycle. Grazing cattle and sheep together will also help, but no where near as much as alternating.

By the way, it isn't realistic or even desirable, to expect your pasture to be completely uncontaminated by worms. Even if you plough and reseed, fields can become infected with nematodirus larvae which have laid dormant and subsequently burrowed back to the surface.

No matter how good your grazing system is, you will probably still have to use some form of worm drench most of which fall into one of three categories. It's worth knowing what these are as there is a real risk of worms building up resistance if you stick with one type of drench and some have effects on meat and milk consumption.

Benzimidazoles. These are effective against adult and larval worms but do not kill arrested larvae. They deprive parasites of their food supply causing worms to starve to death. The disadvantage is a long withdrawal period due to slow cleansing from blood so you would have to take care if using on lambs almost ready for slaughter.

Imidazolthiazoles. These clear from the body rapidly which means a short withdrawal period. Especially effective against nematodirus infection. They are also good against the arrested larvae of Ostertagia and lungworm but not liver fluke. They work by paralysing worms which are then digested and destroyed by the host.

Avermaectins. These are highly effective against all stages of roundworm development except for the eggs. They are not good for treating liver fluke or tapeworm and have a long withdrawal period for meat. They should not to be used in milking flocks either as the same applies to milk. They work by causing paralysis of parasites which are then dislodged and excreted.

As you can see, you will need to know that you have made the correct diagnosis to ensure you treat with the best drench and then you'll have to remember to select an alternative drench from one of the other groups the following year.

Administering the correct dosage and at the right times is also important. Over dosing can lead to toxicity and under dosing simply won't work and may increase the risk of resistance. Instructions are always included on the packaging so follow these carefully and if you need to dose according to weight, don't guess! The time spent weighing can pay dividends in the long run.

Wormers can be administered by injection, but the most usual way is by drenching, i.e. through the mouth so you'll need good handling facilities! So far as I'm aware, the pour-on wormers that are used on cattle are not effective in sheep due to the thickness of the wool.

After worming, move lambs onto clean grazing, if you don't, they will quickly re-infect themselves. Don't forget, lambs will build up resistance to nematodirus infection so the older they get, the less susceptible they will be. To encourage this resistance, increase the distance between worming as the summer wears on. As a guide, you may need to worm for nematodirus 2-3 times during May/June and less frequently there after.

Also, don't forget to worm the ewes! Although they are not susceptible to nematodirus, they do carry other worms so drench before turning out.