Cattle parasites' growing resistance to dewormers is concerning veterinarians and cattle industry experts, according to Louisiana State University AgCenter veterinarian Christine Navarre.
The issue – known as anthelmintic resistance – has been identified in all commercially available products to control parasites, Navarre says.
"Anthelmintic resistance is an inevitable consequence of the use of anthelmintics over time," Navarre said. "The parasites may be resistant to one or multiple products at the same time."
The parasites can develop genes that allow resistance, or ranches can acquire anthelmintic-resistant parasites when they add new animals to their herds, Navarre notes.
Navarre says the issue is worldwide, and varies from ranch to ranch. One way to determine the existence of parasite problems is diagnostic testing. If resistance issues are identified, producers can slow resistance by maintaining "refugia," Navarre says.
"Parasites in refugia do not have genes for anthelmintic resistance – they are still susceptible to anthelmintics,” she said. “The more refugia in a population, the more the resistance genes in a population are diluted and the more effective anthelmintics will be."
However, refugia is limited when an entire herd is dewormed. Eventually, all parasites in refugia are killed, allowing resistant populations to take over.
"Eventually, there is failure of the dewormer to work as expected," Navarre says.
To control the spread of anthelmintic resistance, cattle producers need a basic understanding of parasite biology and should work with their veterinarian to develop parasite control programs, Navarre said. Additionally, producers should balance the short-term economic benefits of deworming with the long-term effects of resistance.
She recommends that producers avoid eliminating parasites in refugia by not trying to eliminate all parasites on a ranch, and developing a plan to keep overall parasite levels low enough to prevent economic losses while retaining refugia to slow progression of resistance.