Before using adjustable-dose injector guns for animal medicines thisspring, farmers should be advised to make sure they perform the primingroutine correctly.
Pfizer VPS livestock vet Dave Gilbert says that incorrect priming may cause some guns to under-dose.
He said: “The best advice is to follow the instructions on the gun’s packaging. If that isn’t available, then ask whoever supplied it. The most common routine is to set the gun to maximum dose, then prime it with medication before adjusting for the actual dose required.
“Clearly, an important consequence of under-dosing is that treatment may only be partially successful. However, on top of failing to get value for money from the treatment, there is also the potentially much more serious risk of helping develop medicine-resistant target organisms.”
Mr Gilbert explained that, among the population of any disease organism, some individuals are more susceptible than others to the medications used to kill them. When animals are under-dosed for a pathogen or parasite, the treatment may still kill the more susceptible part of the population while some of the less susceptible ones survive, he warned.
He said: “This causes a ‘survival of the fittest’ selection process for future generations consisting predominantly of organisms that are less susceptible to the medication that previous generations. Simplistically, this is how anthelmintic-resistant worms or antibiotic-resistant bacterial develop.”
When administering liveweight-dependent medicine doses, Pfizer also advises weighing a few of the biggest animals being treated and setting the dose according to the heaviest. Other precautions that all farmers should maintain are:
 to avoid non-essential use of medications, and
 to use every medication strictly according to
(a) the manufacturer’s recommendations and
(b) advice from the veterinary practice or animal health merchant who supplied it.