Parasites: The problem (Proceedings)

Although sheep and goats get numerous types of parasites, Haemonchus contortus is the most important nematode and most significant parasite with respect to anthelmintic resistance, and will be the focus of this discussion.

Small ruminant producers usually want their veterinarians to give them a deworming schedule including how often to deworm, and with what product. They would also like this schedule to work every year in all situations. Unfortunately, with the anthelmintic resistance problems of Haemonchus, this is impossible. Parasitism is not an anthelmintic deficiency. It is a pasture problem. The 4 P's of parasitism are: permanent pasture perpetuates parasitism. Even if stocking rates are suitable for proper forage usage, if small ruminants are allowed to graze an entire pasture without rotation, deadly infestations of Haemonchus can result.

Veterinarians need to urge their producers to get away from anthelmintic use, and move towards management of pastures and better culling practices. Veterinarians should not give advice to producers about parasite control without knowledge of the farm environment and management. This requires physically visiting the farm or seeing detailed photographs. This will be met with resistance by the many producers, but is essential. The situation is so critical, that those who take their veterinarians advice will likely stay in business, and those who don't, will not. There are farms in which no dewormer is considered effective at treating Haemonchus. Without pasture management and proper culling practices, most of the animals will not survive. A salvage type deworming program called FAMACHA is now being recommended. For more information, visit the website: www.scsrpc.org

Anthelmintic Resistance

Anthelmintic resistance is a natural biologic response to drug administration. It is caused by frequent anthelmintic administration and treating all animals at once. Even very effective dewormers are not 100% effective. When used this way, the few parasites that survive are "super worms". They are anthelmintic resistant and produce more eggs per worm, so the offspring of these parasites are all resistant and buildup of a resistant population is rapid.

Anthelmintic Resistance Facts:

• natural biologic response to drug administration

• genotypic resistance shows up long before phenotypic

• resistant parasites produce more eggs per worm

• causes

o frequent anthelmintic administration

o treat and move (what we recommended for years)

o treating when few larvae on pasture

o treating everything at once

• resistance shared by products in a class (ivermectins, levamisole, benzimidazoles)

• benzimidazole resistance is for life

• moxidectin is effective against ivermectin resistance only because more potent

• ivermectin resistant worms will quickly become moxidectin resistant

Diagnostics

To make recommendations in the FAMACHA program, it is essential to know what dewormers work at what level. This requires a fecal exam. (Remember to keep fecal samples cold. If kept warm, like in a truck for a few hours, the eggs will hatch and not be seen on fecal exam for eggs) Eggs per gram should be calculated on fecal samples taken before deworming then repeated in 7 days. Dewormers do not decrease the EPG by at least 90% are considered ineffective. However, the actual percent effectiveness should be recorded for all dewormers tested, as even those partially effective may be used at times.

Another test is the Drenchrite® test offered at times through The University of Georgia's Parasitology Lab. Although the cost is ~$350, resistance patterns to all dewormer classes can be determined with one test, which is actually less expensive than running multiple fecals for several dewormers.