Endoparasite Resistance

What is tolerance?

The drug is not effective against the parasite and has not ever been

What is resistance?

Ability of worms in a population to survive drug treatments that are generally effective against the same species and stage of infection at the same dose

What causes resistance and what is it a result of?

Changes in allele frequency of resistant genes

Result of drug selection

What is an example of tolerance?

Pyrantel is not an effective treatment against whipworms

What is a single nucleotide polymorphism?

A DNA sequence variation occurring when a single nucleotide in the genome is altered

T/F resistance is actually the change in allele frequency, not the induction of new alleles?

True

T/F resistance occurs in an entire drug class?

True

Why are new drug classes not the answer to resistance?

You need to change the approach of how you use them

When do you suspect resistance?

FEC remains high or clinical signs persist following treatment

What should you also consider besides resistance?

Treatment failure

How can treatment failure make you think there is resistance?

Inadequate dose

Drug inactive due to storage, expiration

Parasites not susceptible

FEC performed improperly

What are some practices that encourage resistance?

Frequent treatments

Selecting for multiple generations

Treatments that target multiple life stages

Treating all animals at the same time

Treating when few larvae on pasture

Treating and moving to clean pasture

What are the large animal goals of deworming?

Maintain herd health

Reduce environmental contamination

Preserve anthelmintic efficacy

What are the small animal goals of deworming?

Keep the animal healthy

Reduce environmental contamination

Protect public health

How do you reduce environmental contaminatoin?

Selective treatment

Quarantine new arrivals until FEC is negative

Reduce stocking density

How do we select animals that are treated/untreated?

Don’t treat the best looking of the herd

Average daily gain

Bulk testing

How do you select what sheep/goats to treat?

FAMACHA

Average daily gain/body condition score/ milk production

FEC

What is the FAMACHA program?

Reduce selection pressure on parasite population

Leave others with smaller worm burdens

Selecting for herd immunity

Why doesn’t resistance seem so bad?

Resistance is relative

Killing some relieves symptoms

Rotational deworming is used

Success is rarely monitored

How can you determine if your dewormer is effective?

Fifteen animals per treatment group with UNIFORM age, breed, history etc

How do you dose each animal the same when examining treatment efficacy?

Dose to the heaviest of each same age group

When should fecal samples be collected to examine dewormer efficacy?

Time of treatment

14-21 days after treatment

How can you “diagnose” resistance?

DrenchRite lab test (larval development assay)

PCR identification

What are the large animal key points?

Goals are to keep heard healthy while maintaining efficacy

Refugia is the key to maintaining dewormer efficacy

Quantitative flotations are the best to monitor efficacy

Where is heartworm resistance to macrocyclic lactones most prominent?

Southern Mississippi delta region

What do you need to do before deciding you have a resistant heartworm case?

Confirm product administration

Review prior heartworm tests

Why is diagnosing HW resistance not practical?

Requires animal infection study

Microfilaria suppression test

How did HW resistance likely occur?

Vets tried a slow-kill with macrocyclic lactones given to heartworm infected dogs

What are some ineffective methods to prevent HW resistance?

Only dosing for part of the year (prevention when there is not HW does not make resistance)

What can we do with heartworm positive dogs that cannot undergo melarsomine treatment?

Macrocyclic lactone preventative monthly

28 days of doxycycline

Where did resistant Ancylostoma caninum resistance come from?

Racing greyhound farms (now spread into other breeds)

What are Ancylostoma caninum resistant to sometimes?

Pyrantel

Benzimidazoles

Macrocyclic lactones

Before deciding you have Ancylostoma resistance what do you need to do?

Confirm owner compliance

Remember hookworms can become patent infections between monthly doses (dose every 2 weeks to break lifecycle)

FECRT

What is Dipylidium caninum developing resistance to?

Praziquantel

What can you do to protect your dog from Dipylidium caninum resistance?

Always keep on flea preventative

What are the small animal key points?

Cannot leave infected animals untreated

Proper use of monthly preventatives is unlikely to cause resistance

Don’t just jump to resistance right away