Parasite management - Resistance is futile....

Parasite management is a concern for anyone raising livestock.  Coming from a horse and cattle background, frequent treatment for internal parasites was the norm for me.  We commonly dewormed our cows several times a year.  The horses were dewormed fall and spring.  I don’t remember anyone ever talking about resistance to these drugs being an issue, although in cattle it is now becoming one.  The first year we had our sheep, when I consulted my veterinarian about what dewormer to administer, he cautioned me about the potential for drug resistance to form without careful parasite management.  It became a bit of an obsession with me – to learn everything I could about the topic.  I’ve been reading every article or website I can find and have attended several webinars and lectures on the subject.  There are several things a shepherd can do to reduce the chances of developing drug resistant parasite populations on their farm.

The best way to avoid problems is proper pasture management.  Worm eggs shed in manure can mature into microfilaria in as little as six days.  The microfilaria (immature worms) crawl up the grass where they are ingested by a passing host animal to continue their lifecycle. Pasture rotation helps prevent parasite infestation in two ways – first, because most microfilaria live in the lower 2” of grass,  if sheep are not allowed to graze the grass below 4” in height, it decreases the chances they will ingest the microfilaria.  Second, if the host organism (sheep) isn’t present for a long enough period of time, the parasites die and break the lifecycle.  The best way to avoid drug resistant parasites is not having to use drugs to kill them in the first place.  Obviously, a lot of small farms do not have the ability to rotate animals every 4-5 days to stay ahead of the parasite cycle.  It is estimated that parasites can survive in pastures up to 120 days in cool moist environments.  In hot dry areas the sunlight and dry conditions kill the eggs and microfilaria more quickly.  Sunlight, dry soil, and frost kill the eggs and the immature worms.  Depending on your climate, pastures need to rest from 4-12 weeks in between grazing cycles.  This requires a large enough land base as well as infrastructure (fencing, water systems, shelters).  One good strategy includes permanent perimeter fence with internal portable fencing to make temporary grazing areas.  We have found this to work well on our farm as long as we can keep the net fences hot!

If pasture rotation is not an option on your farm, the next best thing is to make sure you are treating how and when necessary.  First, find out if your flock has a heavy worm load and what kind of internal parasites are present.  We use a FEC or FOC (fecal egg count or fecal oocyte count).  Although there are other tests available like FAMACHA, they are useful for determining if you have a heavy parasite load but not necessarily the parasite type.  This is critical information because different drugs are effective for different parasites.  Available anti-parasitic drugs come in three classes, and each drug can have different effectiveness against an internal parasite species or groups of species.  If worms on your farm develop a resistance against a drug, they will be resistant to any drug in that class.  Unfortunately, our choices for treatment are quite limited with no new drugs on the horizon.  Flock management strategies to reduce the chances of infection are key.

There are a few treatment protocols to help slow the development of drug resistance.  If it is possible to test each animal individually via FEC, only those animals with internal parasites can be treated with the appropriate drug for the species of parasite.  Another option where testing individuals isn’t possible is to treat 75% of the animals in the flock each time – the idea here is that some portion of the remaining worms will be susceptible to the drug, decreasing the population of resistant parasites.  Some veterinarians suggest using drugs from two different classes at the same time – preferably one injectable and one either topical or in drench form.  Then switching up the drugs at the next treatment cycle.

If parasites are a problem on your farm, it is very important to carefully track individual animals and treatments.  Some animals are naturally more resistant to internal parasites as are some breeds.  Most heritage breeds of sheep have a higher genetic resistance to internal parasites.  One of the many reasons they are so awesome!  It may be necessary to cull individual animals from your flock that are infected more often than the rest of your sheep to prevent development of drug resistance.  Some individuals are just more susceptible to internal parasites than others.  Nutrition plays a big role in internal parasite management.  Animals that receive proper nutrition and are in good body condition are more able to handle some presence of internal parasites.  Adequate food, proper mineral supplements, fresh water, and a non-stressful environment help decrease the potential for animals to succumb to internal parasite infections.

The bottom line – reducing the chances of developing drug resistant parasites on your farm is critical.  There are many different actions you can take to increase your chances of success.  There is a lot of information available regarding management of internal parasites in small ruminants.  Working closely with your veterinarian, develop a plan for prevention and treatment of internal parasites in your flock.  Then track the progress so you can make adjustments as necessary.



University of Idaho webinar -