WORM CONTROL - PART 2 - an excursion into the lives of worms

The basics of worm control in the 21st century

Worm control programs developed mid last century were primarily to control the deadly large strongyles and bloodworms. Now our major threats in Australia and world-wide are roundworms (a major parasitic pathogen in foals and weanlings), tapeworms (implicated in most ileal impactions and colic) and multi-drug resistance of cyathostomes. Because of very different biology, life-cycles and host-parasite dynamics, the strategies designed for controlling large strongyles are not appropriate or very effective against roundworms, tapeworms and cyathostomes. Decades of frequent worming have selected for high levels of anthelmintic drug resistance and emphasizes that the traditional approaches for parasite control are not sustainable and new strategies are needed.

Over 60 years ago worm treatment of all horses on a property at the same time every 6-8 weeks, rotation of wormers and FEC were recommended. All except the latter are now ‘out-dated, dangerous and old school’ because the unexpected consequence was the development of multi-drug resistance. Drugs that were once effective are now no longer so. Whereas once our goal was to clear a horse of all worms, due to drug resistance (the major equine wormers belong to just three chemical groups) the new goal is to focus on those horses that are shedding the most worm eggs and contaminating pastures and to use the appropriate drug at  the appropriate time – both of which require a FEC.

MANURE FAECAL EGG COUNTS: The FEC is a snapshot in time – numbers of eggs in manure change with season, age and stress. Many factors may raise or lower a horse’s shedding level, so it is important to continue to monitor FECs at least yearly, even on horses who are previously low shedders.

FAECAL EGG COUNT REDUCTION TESTS: The presence and extent of resistant worms in your horses and on your pastures can be estimated using the faecal egg count reduction test (FERT) – a FEC done before and 7 to 14 days after using a wormer – a reduction of less than 90% is suggestive of resistance. Test each anthelmintic class at least once every three years and base you control program on drug efficacy and horses’ shedding status. Also useful is the egg reappearance period (the time from treatment to the reappearance of eggs in the manure as new adult worm populations establish), which can be used to determine the treatment interval for your particular control plan.

Now let’s have a peep into the lifestyles of horse worms – because there-in lies the key to controlling them.

Roundworms The important thing about roundworm eggs is their extreme persistence in the environment. Their thick wall (which protects them from freezing cold, drying-out and disinfectants) allows them to remain viable for up to 10 years and their sticky coating enables them to adhere to a variety of surfaces - including boots, stable walls, fence posts, bedding, soil and the hair coat or udder of a mare - and not be washed away by hosing. It is really important to discuss the timing and choice of wormer with your veterinarian who may recommend a FEC. Risks increase when stocking rates are high, the same paddocks are used season after season or soil and bedding become contaminated.

Tapeworms A FEC can confirm the presence of egg-producing adults in the intestine but repeat samples may be necessary. Because tapeworm segments are also shed in the manure, a piece of sticky tape applied around the anus and tail base may detect them. Tapeworms are usually diagnosed with a serum or saliva antibody test. Treatment in early summer and autumn with a praziquantel compound can half the risk of tape-worm associated colic, but there is no way to control the pasture mite. Tapeworm eggs continue to be shed and remain infective following treatment, so stabling/yarding for 3 days and correct disposal of manure is advisable. Don’t spread uncomposted manure on paddocks and because larvae and tapeworm mites migrate across grass manure heaps should be distant from grazing areas. Praziquantel wormers should be used sparingly due to early evidence of small strongyle resistance.

Cyathostomes eggs hatch within 2 days in summer and 2 weeks in winter – making picking up the manure every second day in summer and autumn a very useful way to control the numbers of infective larvae on the grass (provided heavy rainfall does not disperse the material). The larvae move 15-30cm from the manure to the surrounding grass and wait to be eaten by a horse. They survive for 4 weeks in summer and 20 weeks in autumn and winter -  highlighting the inadequacy of short-term pasture spelling for all but the hottest months. Manure dries out within 6-8 days in summer and 2-10 weeks in winter and the larvae may wait for rain before maturing and migrating out onto the grass – making spring and autumn rains a time of massive pasture contamination. Eradication of small strongyles is not feasible, because infections are ubiquitous and no drug currently available can completely eliminate the larvae hibernating in the intestinal wall, infective larvae on grass can be long-lived and there are usually few opportunities for the long-term spelling or resowing pastures. Although a FEC can confirm infection with egg-producing adults, it cannot detect the presence of hibernating worms. However, knowledge of the life-cycle offers good opportunities to manage and control cyathostomes.

*Photo - Small strongyles (cyathostomes) in the intestine

Foals are at increased risk due to the peri-parturient relaxation of resistance (PPRR) whereby a reduction in the mares immune-suppression of worms allows her resident population to massively increase egg output when foals are born – maximising the worms chances of infecting horses.  ‘Clean’ pastures are best for foals because although adult horses observe areas for grazing as distinct from areas for voiding urine and manure, foals show no such aversion to grazing close to manure piles.

Alternate grazing Grazing cattle and sheep on horse paddocks can reduce the number of infective larvae on the grass. However, cattle tend to eat the upper layers of pasture and most larvae live at the bottom, so they could lead to a higher number of infective larvae per kg of grass. On the other hand, one sheep grazing pasture with 3000 larvae/kg of grass could remove 90,000 infective larvae every day.

There are no blanket recommendations as each property and horse is different and regional and seasonal weather patterns vary so much. Routine FEC are a direct measure of the rate at which pasture contamination is taking place. They also confirm the continuing efficacy of the drugs used and can be used to determine optimum treatment intervals. Work with your vet in conducting a FEC on your horses and developing the best, most economical and sustainable control program. Under-dosing increases resistance so take care to always use the correct dose of wormer – based on the weight of your horse using scales or a weigh tape (available at Jenquine.com). Worming too frequently and/or with the wrong drug wastes money and encourages resistance.

Support your dung beetle population! Dung beetles help expose larvae to dessication and some wormer drugs kill dung beetles. Some fungi can parasitise worms and the recent availability of nematophagous (ie worm-eating) fungi that will destroy larvae in the manure is an exciting development.

Dr Jennifer Stewart
Equine Clinical Nutrition

DISCLAIMER All content provided in this editorial is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your veterinarian.

*Photo - Small strongyles (cyathostomes) in the intestine