Some parasites sidestep cold weather
Parasites take the winter off, right? Not so, says Dr. Gary Zimmerman, veterinary parasitologist, Zimmerman Research, Livingston, MT. “Cattle and pastures can carry a parasite burden throughout the winter— even after temperatures fall below freezing,” Zimmerman says. In fact, inhibited Ostertagia ostertagi, the brown stomach worm, can survive even in the cold northern winters—both in cattle and on pastures. The brown stomach worm undergoes inhibition as a survival mechanism, Zimmerman says. Both cold weather and dry summers can trigger this parasite to lie dormant as early fourth-stage (inhibited) larvae in the abomasums until environmental conditions prevail. A large number of inhibited larvae accumulate in the gastric mucosa, and can persist for weeks or months, sometimes resulting in severe clinical disease when large numbers of worms mature into adults.
“Presence of inhibited larvae causes damage, but even more harm is caused as the larvae come out of arrested development,” Zimmerman says. “They damage the abomasum and adversely affect digestion and feed conversion.”
Protection inside cattle is not the only winter defense practiced by the brown stomach worm. Infective larvae also have been shown to survive the winter on pastures.
“Infective larvae burrow into the ground and can survive a hard winter,” Zimmerman says. “In fall conditions, you can get a lot of these larvae on pasture that survive freezing conditions and continue to infect cattle.” Zimmerman cites a study his research team conducted in Oregon during a twoweek period between Thanksgiving and Christmas when environmental temperatures never got above freezing. Parasitenaive calves turned out on contaminated pastures during this period picked up more than 200,000 nematodes, the majority of which were Ostertagia.
“The best plan for climates with cold winter weather is to treat for parasites in the fall and spring,” Zimmerman advises.
He says this will help get rid of all the parasites the animals have picked up and retained during the grazing season and will help clean out parasites in the inhibited state. Zimmerman recommends waiting, in some cases, until after the first freeze to treat in the fall. He says though the freeze is not likely to kill all the parasites, it may reduce the number of larvae at the ground surface.
“Cows treated in late fall with an endectocide carry more weight through winter, wean heavier calves and, in some cases, breed back faster than nontreated controls within the same herds,” says Dr. James Hawkins, Merial parasitologist and consultant.
“However, it is important to remember that treating for parasites in the fall does not protect cows all year long, which is why a spring treatment also is essential.” Hawkins adds that not all dewormers are effective against inhibited larvae, so producers need to read labels and use a product that is effective against inhibited O. ostertagi.
Zimmerman says treating again in the spring cleans up any residual parasites and kills additional larvae picked up from spring grazing. Pasture management also can help reduce parasite contamination. Zimmerman says that following spring treatment, producers should wait approximately 10 to 14 days for the compound to kill the parasites before moving cattle to a clean pasture. This helps reduce pasture contamination. “The bottom line is that cold winters do not mean that producers can be complacent about parasite control,” Zimmerman says. “Left untreated, O. ostertagi and other parasites will overwinter in cattle and can cause harm to the animal and a producer’s bottom line.” —