Vet View- Coccidiosis in lambs

Ben Strugnell is a vet based in northeast England, with a background in farm animal disease investigation. In this column he tells us about a project he is currently involved with which ultimately aims to develop vaccines effective in preventing coccidiosis. It is the time of year when lambs are beginning to appear in fields, and the grass is growing. Lambs outside will have survived their greatest challenge - the first 48 hours of life - but cannot quite relax yet as warmer weather and more lambs per acre will soon lead to their next potential threat - coccidiosis. This protozoal parasite is passed out in the faeces of ewes in very small numbers, all the time. It is likely that appreciable numbers also survive on pastures from previous years. In spring, these small numbers of eggs (oocysts) are eaten by the first few lambs on the ground and multiply in their gut lining into very large numbers. The next lamb coming along might get a mouthful of grass contaminated by these very large numbers of eggs. The result is invasion of the gut by millions of eggs all at the same time. These oocysts multiply faster than the lamb’s immune system can stop them, and the result is very severe gut damage, leading to bloody scour, often prolonged reduction in growth rates, and even death. As a rough guide, lambs are generally affected at three to eight weeks of age, which for a flock with lambs in late March/early April, will be about now (mid-April), and after challenged lambs become immune and resistant to infection. The biggest problem is that not all lambs become infected at the same time, so it can be very difficult to know when the right time is to treat them. Treatment/prevention is with coccidiostats, toltrazuril and diclazuril, which state that ‘to obtain maximum benefit, animals should be treated before the expected onset of clinical signs”, which is in the prepatent period (the time after infection but before oocysts are produced in faeces). These compounds may continue to kill coccidia and can therefore reduce oocyst shedding for two to three weeks (toltrazuril is more persistent than diclazuril). Even so, if all lambs are treated, for some it will be too early, and for others too late - and the treatments are not cheap. Keeping lambs tightly batched in ages, and reducing faeces accumulation (for example, around creep feeders) will help to minimise the dose ingested, tipping the balance from severe disease to mild disease, which will be followed by solid immunity. Given the timing challenges, this is a perfect vaccine target. If lambs’ immunity could be raised before exposure, the consequences of ingesting high numbers of oocysts would be less severe, preventing disease, deaths and lost productivity. This is the aim of a project currently underway that is funded by AHDB and BBSRC, and run by The Royal Veterinary College and Farm Post Mortems (in Co. Durham). This project aims to identify parts of lamb coccidial species which would be suitable targets for vaccines (like the famous spike protein of Covid 19). Progress will be reported, and it is to be hoped that in future years, lambs could be vaccinated (possibly at tailing/ringing) rather than playing the Russian Roulette of timing the treatments post-turnout.