The stress of lambing often means a higher parasite burden, but mature sheep that are healthy are often immune to the effects of parasites.
Photos by Sally Colby.
Those who raise sheep have probably heard of the four S's, the mantra of experienced shepherds: sick sheep seldom survive.
Part of that mantra stems from the fact that sheep (and goats) do a pretty good job disguising the fact that they're ailing; their survival depends on it. If sheep show any sign of illness, they become an easy target for predators. Good flock or herd managers can usually tell when an animal is sick, but illness due to parasitism can be more difficult to determine.
The concept of deworming sheep has changed drastically since the days of offering a free-choice phenothiazine salt block. With such blocks, some sheep were dewormed, others were not. Throughout the 1960s and 1970s, shepherds often included monthly deworming, usually with thiabendazole, as part of a flock management plan. Thiabendazole was relatively effective, especially against roundworms. Some shepherds checked ewes' mucous membranes, and if the color was bright red, she probably received less dewormer. If her color was poor, she received more. Immediately after deworming, sheep were housed on a lot where worms would be expelled, and then sheep were moved to a "clean" pasture.
However, like all animals, the parasites of sheep and goats follow the rule of survival of the fittest. The strongest worms survive treatment, are expelled in feces, consumed through grazing and have the perfect opportunity to multiply.
Through overuse, under-dosing and off-label use of anthelmintic products, parasites of small ruminants have become resistant to the products available for deworming. Today, instead of talking about the four S's of raising sheep, shepherds are concentrating on the three R's: resistance, resilience and refugia.
The concept of refugia was key when chemicals such as DDT were being used to kill mosquitoes in Africa. Susceptible insects died when treated, but insects that were resistant to chemicals would reproduce and thrive. One way to slow this process was to provide a "refuge," where susceptible insects would mate with resistant insects, producing offspring that were less resistant to chemical treatment.
The concept of refugia in relation to small ruminant parasites was first discussed when flocks in Australia and New Zealand started showing signs of resistance. In the parasite life cycle, worms mate in the gut and produce eggs, which hatch and continue as larvae. Larvae crawl up blades of grass in pastures and are consumed by the host animal (sheep) and the cycle continues.
Dewormers will kill susceptible worms (and sometimes eggs), but resistant worms survive. Those resistant worms mate and eventually create a "super population" that continues to thrive. In order to dilute this effect, worms that are still susceptible to deworming products must mate with resistant worms.
Sparse, overgrazed pastures often harbor excessive worm larvae, but can be used immediately after deworming so sheep deposit larvae-laden feces that will eventually help to dilute the resistant population.
To create refugia, some animals are left untreated. Worms that are "in refugia" are those that have not been exposed to treatment. While in refugia, resistant worms mate with susceptible worms, and the population is diluted. Refugia can also be achieved by placing treated animals on a lot or pasture that is known to be "wormy." If treated animals are moved to a clean pasture, surviving worms (the strongest, most resistant worms) will mate with other resistant worms that survived treatment.
Since creating refugia means that some animals must be untreated, how does the sheep or goat owner know which animals to skip when it's time for deworming? Rather than basing treatment on a calendar schedule, shepherds can deworm animals less frequently, with emphasis on the poorest-performing and most highly stressed animals in the flock. Those animals are typically ewes with multiple lambs and older animals that can't eat properly due to lost teeth. Mature animals should, theoretically, have developed a degree of immunity to a parasite load and should not be routinely dewormed unless total flock deworming is based on a combination of FAMACHA scores and fecal egg counts.
Although it seems counterintuitive, treated young animals should be placed on a contaminated, or "dirty," pasture so that resistant larvae can mate with less-resistant larvae. The resulting parasites from these matings end up back in the animal, and the next time that animal is treated, the parasites it harbors are more likely to die from treatment.
Refugia will not prevent resistance, but it can be used to help delay resistance in a flock or herd. A combination of FAMACHA, fecal egg counts and fecal egg reduction tests can help sheep and goat owners get the most effective use from available anthelmintic products.
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania.