Nematodirus in Lambs

While the majority of sheep nematodes make their home in the abomasum stomach, Nematodirus battus is a worm which settles primarily in the small intestine. It follows a slightly different life cycle to other ‘gutworms’, traditionally appearing as a pathogenic parasite towards the beginning of the grazing season, often causing severe, acute disease. This article highlights the key characteristics of Nematodirus, as well as including some advice on control.

2015-02-28; 001Nematodirus is very much a ‘lamb to lamb’ disease, and typically there is just one generation of the parasite per year. Lambs pass out Nematodirus eggs in the faeces in the spring and early summer, which then lay dormant on the pasture for the remainder of the year. In truth, the worm larvae are silently developing within the egg during this time- a process which requires a sustained period of cold weather to complete. As such, it is not until the eggs have laid on the pasture through the colder temperatures of the winter that they are ready to hatch. The result of this unusual life cycle is that eggs deposited by lambs in the early summer lie on the pasture for almost an entire year, before suddenly hatching en mass when the cold winter temperatures recede and the warmer weather of the subsequent spring sets in (see diagram).

This synchronised mass hatching of eggs on the pasture can lead to a sudden outbreak of gastroenteritis among lambs, with symptoms including profuse scouring, ill thrift, weight loss and death, which can follow in as little as a few days. To complicate matters further, it is the larval stage of the Nematodirus worm which causes damage to the intestine, and since standard faecal worm egg counting is only able to detect the presence of adult worms, the parasite can often remain ‘invisible’ to routine monitoring until considerable damage has already been done.

Thankfully, this species of worm has seemingly developed very little resistance to anthelmintics, including benzimadazoles (white wormers). As a result, infection with this parasite is easily remedied. Rather, the key to stemming an outbreak is in early diagnosis and prompt treatment when clinical signs arise. Of course, early treatment of disease caused by Nematodirus also acts to reduce the number of eggs being shed by your lambs, which in turn reduces the infective load waiting for next year’s susceptible animals.

You can also reduce the risk of suffering losses to this parasite by being aware of its unique life cycle and taking management steps to help avoid high risk situations. For example, the highest risk pastures for Nematodirus during any given spring are those which were grazed by 6-12 week old lambs the year before. Avoid grazing these pastures until later in the season (when many of the Nematodirus eggs will have long since hatched, become unviable or ‘burned off’), and you will reduce the chances of seeing problems associated with the parasite. Similarly, putting adult ewes or cattle on such pastures during the spring will help to ‘clean up’ the grazing and make it safer for subsequent occupation by younger lambs. If you operate on non-permanent pasture, choosing to graze 6-12 week old lambs on lays that are to be replaced that year will break the life cycle of the worm and dramatically reduce the risk of problems associated with it.

In summary, the Nematodirus worm can be a rapid killer, and prompt treatment based on clinical signs is required in the case of an outbreak. Faecal egg counts cannot be relied upon to predict imminent disease from this parasite, and so good stockmanship is invaluable in spotting it. In situations where it is feasible, good pasture management and co-grazing can be used as tools to substantially reduce the risk of losing out to this often fatal disease.

For further information regarding how Nematodirus might be affecting your flock, please contact one of our vets or SQPs.

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