A question on rotating anti parasitics

Asked September 17, 2016, 2:11 PM EDT

The current best practice for antihelminthics in worms is to not rotate wormers, but to use one as long as it is effective. It used to be reccommended you change wormers regularly. However, apparently the current advice for beekeeping is to rotate miticides to prevent resistance. Is there any research to support a difference in the types of parasites which would support rotation for one and not the other, or is one industry following improper procedures? Thanks.

Clackamas County Oregon beekeeping parasites wormers miticides

2 Responses

Dr. Susan Kerr at WSU confirmed that the latest recommendation for using dewormers on livestock is to not rotate treatments. However, parasitic Varroa mites breed VERY quickly and have already become resistant so some of the over-used miticides (fluvalinate, amitraz and coumafos) in several areas. If you 'google' information on those pesticides in regard to Varroa you will find an abundance of research.

It is standard practice now for all ARTHROPOD pests to routinely rotate chemicals that use different modes of action whenever possible to avoid resistance to our limited "toolbox" of products. Parasitic "worms" are completely different critters and I don't know of research as to why antihelminthics are not rotated.

There is some discussion as to whether Varroa can ever develop resistance to the organic acid miticides often used now but why chance it?

Additional information from Dr. Kerr 9/21/2-16
"The current movement is toward de-emphasizing the use of dewormers in parasite control. We are trying to get people to manage livestock internal parasites via selection of animals that survive/thrive despite the presence of parasites; environmental sanitation; pasture management (not grazing below 3"; rotating pastures; prolonged pasture rest; etc.); attention to nutritional quality and quantity; and routine monitoring of individual animals to note the need for treatment. Individual animals in need of deworming would then be treated with a chemical dewormer known to be effective on that farm, as evidenced by >95% reduction in fecal egg count after treatment. Using a dewormer so rarely will delay the development of resistance to this product. If resistance develops, there will be 2 other dewomer families to go to next. That still doesn’t leave many options, so we have to do all we can to make deworming of individual animals a rare event. Managers work on leaving “refugia” in the environment—parasite genetics that are susceptible to the effects of dewormers instead of genes that are resistant to chemical dewormers. Every deworming episode decreases refugia and increases resistance.

Rotating dewormers will hasten the development of resistance to all 3 current dewormer families. Think of it like taking ONE penicillin tablet this month, then ONE cephalexin tablet next month, then ONE erythromycin tablet next month… with each treatment, the “wimpy” susceptible bacteria are killed, but the remaining bacteria are more hardy and resistant. If one (stupidly) took ONE penicillin tablet this month and next and next, one would be selecting for penicillin-resistance bacteria, but at least there would be plenty of other antibiotics to turn to if needed.

A good site to read more about these concepts is http://www.wormx.info/anthelmintic-resistance if you are interested in more details."


Thank you both for your response. I guess I don't understand the difference between the phyla, though. Veroa mature in 8 days, and produce 5 or 6 young per breeding. Haemonchus mature in 21 days, but produce 5,000 or more eggs per day. There is obviously a significant number of eggs that don't survive to be eaten by an herbivore and infect the host, but it's not unreasonable to assume that equivalent parasite loads would be passed on. I'll have to do a bit more research. :-)