Cattle grubs have pretty much been forgotten by Canadian cattlemen over the past 20-plus years. Most of our young producers may not even know what I am talking about but those in their 50s will remember the vast amount of hide and internal damage caused by warble larvae migrating close to the spine or the esophagus, depending on the species, and then cutting breathing holes in the hide when they pupate.
The cattle then try to wall them off with what is called the “warble.” I personally have not seen one of these breathing holes for over 20 years and basically had thought the warble fly had been all but exterminated from Canada.
Dr. Joyce Van Donkersgoed and her team did a couple of slaughter audits in the 1990s and came up with a very low incidence of damage from cattle grubs after treatment, something in the order of 0.1 per cent. And it decreased too less than hundredths of one per cent about five years later. So from a very low level we experienced a tenfold decrease in just five years. This would support the fact we just don’t see these pests because the products were very effective at killing them and almost all producers treated their cattle.
Pour-on cattle grub treatments started in the 1950s with products similar to organochlorines but these were not 100 per cent efficacious. Then the organophosphates came along with products such as fenthion (Spotton) that were quite efficacious that also controlled lice and then progressed up to the macrocyclic lactone family which includes all the ivermectin products. The warble is very sensitive to these macrocyclic lactones and as a result clinical cases have all but been eliminated in Canada. The real issue becomes, would they return any time soon if we were to stop treating?
Dr. Doug Colwell, a parasite specialist at the Lethbridge research station did a lot of blood sampling of cattle from 2008 to 2010 looking for cattle exposed to warble flies during the summer. The antibody evidence would suggest warbles are still out there and between 25 and 50 per cent of the cattle population show seroprevalence (exposure) to the warble fly.
There must be some resident cattle that are not treated which harbour the larval forms of this parasite and allow the life cycle to be completed. The larvae will survive in both horses and bison but the evidence suggests they don’t survive to become a viable fly. Very recently a few bison hides were seen with “warble” holes so we know the larvae reach the back for sure. Horses were once highly affected but the decreased incidence in cattle has allowed their problems to disappear, as well.
A few ranches were also sampled and one showed an increased incidence of exposure during two wet years. Another had not used any macrocyclic lactone treatments the previous year and its exposure rate was definitely higher the following year.
Dr. Colwell and others have found that a certain amount of grubs in the larval stages are killed during their migration through the body and don’t make it through the skin along the back. If even one larva survives we will see it inside the “warble.”
Several countries including Great Britain launched aggressive campaigns to eradicate this pest and were successful, although it took many years. Other European countries have tried and failed because neighbouring countries did not implement a similar program.
In the past I found the majority of warble problems was on farms that did not treat.
Essentially prevention involved treating the warble larvae in the early fall before it migrated too close to the spine. If one recalls there was a time restriction on the earlier treatments to not treat between November 1 and January 31 to avoid the possibility of swelling around the spine and paralysis from killing the larvae as they migrated by the spine on their way to lie along the back.
In my entire career I only witnessed this once and it was with a product called Lysoff (fenthion) that was indicated for lice only but the warning against this possible outcome was always on the label. With the advent of the macrocyclic lactones (ivermectin-type products) they were supposed to kill the larvae slower so there would not be as much reaction and I personally never saw a problem with them regardless of when the cattle were treated.
The ivermectin products effectively kill cattle grubs at a very low dosage. Europeans killed the cattle grub larvae effectively using a microdose of 1:100 of the label strength.
Going forward questions will be asked in light of developing internal parasite resistance with the macrocyclic lactones. I do not have the answers to these questions but in light of the warble being extremely sensitive to macrocyclic lactones perhaps a greatly reduced concentration product could be developed for warbles. Pyrethroids used for flies and lice do not work on internal worms or warbles.
Do we need lice- and warble-specific products redeveloped? The evidence suggests warbles will come back if they are not treated but how long will that take? Should we treat for warbles using the macrocyclic lactones every two or three years? Eradication is a possibility but of course the U.S. would need to be on the same program.
There should be a system for reporting warble damage at packing plants, feedlots or veterinary clinics so cases could be traced.
Another complication is the organic, natural or branded beef programs that steer clear of macrocyclic lactones and thus become a possible source of future contamination.
Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.