Parasitology Flashcards
Equine
Anthelmintic Resistance in cyathostomins (small strongyles) is less common with what drug class?
Macrocyclic Lactones
Avermectins and Milbemycins
Equine
What happens when cyathostomins are killed by an anthelmintic drug?
Larvae repopulate the large intestine
L3 = infective stage that is ingested. L3 enters LI mucosa and develop and synchronously emerge as L4 larvae. L4 develop into adults in the cecum & colon.
What anthelmintic can kill hypobiotic larvae of small strongyles?
Moxidectin
Fenbendazole shown to be effective, but has INCR RISK for resistance!!
Adult worms = easily removed (as long as the population hasn’t developed resistance to the chosen drug).
Encysted small strongyle larvae are much more difficult to remove. The cysts protect the parasite within the intestinal wall and prohibit the drugs from reaching the larvae.
There are three classes of anthelmintic used in the treatment of small strongyles:
Benzimidazoles – e.g. fenbendazole and oxfendazole
Macrocyclic lactones (ML) – e.g. ivermectin and moxidectin
Tetrahydrophyrimidines – e.g. pyrantel salts
Fenbendazole shown to control adult and developing larval stages of small strongyles and a higher dose has been found effective during the encysted stage. However,increasing resistance to fenbendazole among populations is limiting effectiveness
While ivermectin is extremely effective for removing small strongyles in adult, luminal larval, and developing stages, it has low efficacy against inhibited stages.
Moxidectin may be the most effective option for small strongyle treatment, as it works against all stages including inhibited larvae, provides ongoing protection against re-infection, and requires less frequent treatment.
Effect of warmer temperatures on strongylids
Warmer temps (~April through August) = faster development of eggs into L3 (in pasture)
- switch from colder to warmer temps causes re-emergence of L3 into L4 (in intestinal lumen)
What time of year can you expect to see highest Fecal Egg Counts of strongylids in northern temperate areas?
towards end of summer (increasing number of horses becoming infected by L3 via ingestion in pasture, and by end of summer peak infections have occurred)
Which animals should be put out on pasture that has the fewest worms?
Most vulnerable
- young
- early lactation offspring
- other immunocompromised animals
Put the non-lactating, immune animals on contaminated pasture
In order to aid with killing of L3 in pastures via depletion of metabolic reserves (aka no grazing), how long can you leave pasture ungrazed? How long can animals graze on a fresh pasture before re-infection occurs?
- Leave a pasture ungrazed for ≥ 60 days
- Use fresh pasture for no longer than 5 days to avoid re-infection
Can also harvest regrowth for hay / plow up land & replant to expedite
Which strongylid can infect two different species?
Haemonchus contortus (small ruminants + small calves)
Egg Reappearance Period (ERP)
the length of time it takes cyathostomin eggs (small strongyles) to appear AFTER tx.
- reflects maturation of larvae NOT removed by tx (NOT NEW INFECTION)
- i.e., only Moxidectin can eliminate encysted larvae (late L3 + L4) and adults
What does the AAEP currently reccomend for strongylid parasite control?
ID the high shedders (horses with higher egg burdens) by performing 1-2 fecal exams per year (change from cold-> warm temps; end of summer)
- Horses < 3 years old treated as high shedders REGARDLESS of FEC b/c of their higher sus to all parasites
Form of Targeted Selective Treatment (TST)
Why don’t Fecal Egg Counts reflect clinical parasitism for strongylids?
B/c the strongylosis is dependent on the worms LARVAL stage (synchronous emergence of L4 larvae!)
When is the best time to use dewormer?
Right before grazing season
grazing season = May to October
What integrated parasite control only works for horses?
Manure removal: improves pasture utilization
- reduces roughs (areas of tall grass where they defecate)
- reduces lawns (areas of shorter grass where they prefer to graze)
What is Targeted Selective Treatment
Formof anthelmintic tx that focuses on soley treating groups/individuals that need it most (aka are most susceptible) in order to decrease chances of anthelmintic resistance developing
What is TST and why is it needed?
- Repeated whole-group anthelmintic treatments encourage the development of anthelmintic resistance
- One solution is to treat only those individuals that need it. This will protect them and reduce overall levels of infection in the herd, while also being efficient and leaving some worms unexposed to anthelmintics, so slowing the development of drug resistance.
What are Fecal Egg Counts used for?
To ID high contaminators (high shedders), usually for breeding purposes
Food Animal - Cattle
Resistance to bovine anthelmintics have been described for all groups of what parasites?
Ostertagia ostertagi and Cooperia oncophora
