Introduction
Parasite preventives are some of the most common products a veterinary technician dispenses. You weigh the patient, choose the right size of chew or pipette from the veterinarian's order, show the client how to apply it, and answer the phone when a cat starts twitching after the owner used the dog's flea product. To do that well you need to know the main drug classes, what each one kills, which species it is labeled for, and the one warning that goes with it.
This guide covers the macrocyclic lactones and the MDR1 gene, the isoxazoline flea and tick drugs, permethrin toxicity in cats, the common dewormers, heartworm prevention and adulticide basics, and the year-round control that CAPC (the Companion Animal Parasite Council) recommends.
Key facts
- Ivermectin, milbemycin, moxidectin and selamectin are macrocyclic lactones. They act on glutamate-gated chloride channels in the parasite.14
- Dogs with the MDR1 (ABCB1) mutation lack a working P-glycoprotein pump, but all macrocyclic lactone heartworm preventives are considered safe in them at approved doses.4
- The isoxazoline class (afoxolaner, fluralaner, lotilaner, sarolaner) carries a label warning for tremors, ataxia and seizures.78
- Cats are more sensitive to pyrethroids than dogs. Severe cases usually follow a dog-only spot-on applied to a cat.910
- CAPC recommends annual heartworm antigen and microfilaria testing for all dogs and year-round preventives.5
- Melarsomine is given by deep intramuscular injection in the epaxial (lumbar) muscles, L3 to L5 only, and is not recommended for cats.64
- Puppies and kittens should get dewormer starting at 2 weeks of age and every 2 weeks until regular broad-spectrum control begins.11
How the VTNE tests this
- Match a drug or class to its target parasite or mechanism (for example, praziquantel for tapeworms). Domain 1 Task F; K8.
- Recognize a species or breed warning: permethrin on cats, ivermectin at livestock doses in herding breeds. Domain 1 Task F; K11.
- Choose the right client instruction for a scenario, such as what to do after a dog spot-on is applied to a cat.
- Calculate a dose or injection volume from mg/kg and a concentration in mg/mL.
- Know heartworm testing and prevention schedules and the technician's role during adulticide treatment.
Core concepts
How dewormers kill worms
Most anthelmintics (drugs that kill worms) either paralyze the parasite or damage its cells. Spastic or flaccid paralysis lets the host's normal gut movement push the worm out.1
- Benzimidazoles (fenbendazole, febantel, which the body converts to an active benzimidazole) block tubulin polymerization, so the worm's cells cannot work normally.1
- Tetrahydropyrimidines (pyrantel) are cholinergic agonists at nicotinic receptors. They cause paralysis.1
- Macrocyclic lactones (avermectins such as ivermectin and selamectin; milbemycins such as milbemycin oxime and moxidectin) act on glutamate-gated chloride channels.1
- Emodepside binds a latrophilin-like receptor and the slo-1 ion channel. It stops pharyngeal pumping, then the worm is paralyzed and dies. It works against nematodes resistant to other classes.1
- Praziquantel is the drug of choice for tapeworms (cestodes). It disrupts calcium channels and paralyzes the tapeworm.1
Macrocyclic lactones and the MDR1 gene
The MDR1 gene (also called ABCB1) codes for P-glycoprotein, a pump that moves certain drugs out of cells, including out of the brain. Dogs with the mutation have a deficient pump and are at greater risk of toxicosis from certain drugs, including macrocyclic lactones.4 WSU reports that up to 75% of some breeds carry the mutation. Herding breeds, long-haired whippets and silken windhounds have the highest rates, and it also occurs in mixed breeds.3
The risk depends on the dose. All macrocyclic lactone heartworm preventives are considered safe in MDR1 dogs at approved doses.4 The danger comes with the much higher ivermectin doses once used for mange: Merck states ivermectin is contraindicated in Collies and Collie crosses for that use, that toxicity may be seen in any breed, and that MDR1 testing may be needed first.2 Loperamide (Imodium) at diarrhea doses causes neurologic toxicity in MDR1 dogs and should be avoided in all of them.3
Isoxazolines
The isoxazolines are oral or topical flea and tick drugs given monthly or, for fluralaner, every 12 weeks.7 Every product label carries the class warning: the class has been associated with neurologic adverse reactions including tremors, ataxia and seizures, even in animals with no history of them. Labels advise caution in dogs with a history of seizures and in cats with a history of neurologic disorders.78 The manufacturers of NexGard, Bravecto and Credelio tested their products in MDR1 dogs and saw no adverse effects.3
Pyrethrins and permethrin
Pyrethrins come from chrysanthemum flowers. Pyrethroids such as permethrin are synthetic, more stable and more potent, and are common in dog spot-ons.10 They keep nerve sodium channels open too long, so nerves fire repeatedly.10 Cats are more sensitive than dogs because their liver is poor at glucuronide conjugation, so they clear the drug slowly.910
Heartworm prevention and treatment
Macrocyclic lactone preventives kill L3 and early L4 heartworm larvae. They come as monthly oral or topical products and as 6-month or 12-month injections.4 Year-round use is the only way to reliably prevent infection, because mosquito seasons are hard to predict.4 Preventives do not kill adult worms, and the adulticide melarsomine does not kill newly introduced larvae.5 Microfilariae appear in the blood 6 to 7 months after infection, which is why a test too soon after exposure can be negative.5
Doxycycline is used before adulticide to kill Wolbachia, bacteria that live inside heartworms. This improves adulticide efficacy and reduces lung damage from dying worms.5
| Class (examples) | Parasites covered | Species | Key warning |
|---|---|---|---|
| Macrocyclic lactones (ivermectin, milbemycin oxime, moxidectin, selamectin) | Heartworm larvae; many also intestinal worms; selamectin also fleas, ear mites, sarcoptic mange | Dogs, cats, ferrets (by label) | MDR1 dogs at high (non-preventive) doses |
| Isoxazolines (afoxolaner, fluralaner, lotilaner, sarolaner) | Fleas and ticks | Dogs; some products cats | Tremors, ataxia, seizures |
| Pyrethrins and pyrethroids (permethrin) | Fleas, ticks; permethrin also repels mosquitoes on dogs | Dog-only permethrin spot-ons | Toxic to cats |
| Benzimidazoles (fenbendazole, febantel) | Roundworms, hookworms, whipworms; Giardia | Dogs, cats, livestock | Albendazole not advised in dogs or cats |
| Tetrahydropyrimidines (pyrantel) | Roundworms, hookworms | Dogs, cats, horses | Does not kill tapeworms |
| Praziquantel, epsiprantel | Tapeworms, including Dipylidium | Dogs, cats | Reinfection unless fleas are controlled |
| Emodepside (with praziquantel) | Roundworms, hookworms, tapeworms | Cats | Read label for age and weight limits |
| Melarsomine (organic arsenical) | Adult heartworms | Dogs only | Deep IM, L3 to L5 only; exercise restriction |
Sources: Merck Veterinary Manual (anthelmintic action, heartworm, giardiasis); CAPC heartworm, ascarid and Dipylidium guidelines; DailyMed labels for Revolution, Bravecto and Diroban; WSU MDR1 list.1456121314
Procedure or technique
Dispensing a parasite preventive
- Weigh the patient on the day you dispense. Products are sold in weight bands, and the label sets a minimum age and weight. Bravecto for dogs, for example, is labeled for dogs 6 months and older weighing at least 4.4 lb.7
- Check the species on the box. A product for dogs is not a product for cats, even if the active ingredient sounds similar.
- For heartworm products, confirm the dog has a current heartworm test on record. CAPC recommends annual antigen and microfilaria testing.5
- Note any seizure history in the record and flag it for the veterinarian before an isoxazoline is dispensed.7
- Show the client how to give or apply the product and write the next due date. Explain that the dose is year-round.4
Assisting with melarsomine
- Weigh the dog accurately. The dose is 2.5 mg/kg, which equals 0.1 mL/kg of the reconstituted product (25 mg/mL).6
- Wear gloves or wash hands well. The label warns to avoid human exposure.6
- Restrain the dog so the veterinarian can give a deep intramuscular injection into the epaxial (lumbar) muscles at L3 to L5. No other muscle group, never IV, and avoid superficial injection or leakage.6
- Record which side was injected. The usual schedule is one injection, then two injections 24 hours apart about 1 month later.5
- Teach strict exercise restriction. CAPC advises no excitement or exercise beyond slow walking for at least 1 month after each set of injections.5
- Book the recheck. Treated dogs may be antigen tested 6 to 7 months after therapy, or after 1 year.5
| Patient | What CAPC or Merck recommends |
|---|---|
| Puppies and kittens | Dewormer from 2 weeks of age, every 2 weeks until regular broad-spectrum control starts; fecal testing at least 4 times in the first year |
| Adult dogs and cats | Year-round broad-spectrum control; fecal testing at least 2 times a year depending on lifestyle |
| All dogs (heartworm) | Annual antigen and microfilaria test; preventive year-round |
| Dogs in highly endemic areas | Twice-a-year antigen testing may be advised |
| Cats (heartworm) | Prevention recommended for all cats; preventives labeled for kittens over 6 or 7 weeks old |
Sources: CAPC ascarid and heartworm guidelines; Merck Veterinary Manual, heartworm disease.1154
Flowchart
Species differences
- Dogs: herding breeds may carry MDR1. Preventive doses are safe, but livestock ivermectin products or high mange doses are not.42 Melarsomine is labeled for dogs only.6
- Cats: never use dog permethrin products. Melarsomine is not recommended in cats because of the risk of severe lung inflammation and death; affected cats are started on a macrocyclic lactone preventive instead.4 Antigen tests alone are too insensitive to screen cats for heartworm.4 Emodepside is paired with praziquantel in a cat product.12
- Ferrets: only topical moxidectin with imidacloprid is FDA approved for heartworm prevention. Adulticide treatment has a mortality of about 50%.4
- Horses and livestock: large-animal ivermectin products are not the same as canine preventives. WSU lists specific heartworm preventives as tested safe in MDR1 dogs and advises asking for dosing guidance for other ivermectin formulations.3 A dog that eats a horse dewormer paste needs a call to the veterinarian.
Calculation
Worked example: melarsomine volume
A 22 kg dog is ordered melarsomine at 2.5 mg/kg. The reconstituted vial contains 25 mg/mL.6
Step 1, dose in mg: 22 kg x 2.5 mg/kg = 55 mg (kg cancels).
Step 2, volume: 55 mg / 25 mg/mL = 2.2 mL (mg cancels).
Check: the label shortcut is 0.1 mL/kg, and 22 kg x 0.1 mL/kg = 2.2 mL. The two answers match.
Safety, legal and client education
- Melarsomine is an arsenical. Wear gloves or wash hands; rinse eyes with plenty of water after a splash and see a physician after any accidental exposure.6
- Tell clients that flea control is part of tapeworm control. Dipylidium is treated with praziquantel or epsiprantel, but pets reinfect from fleas.12
- Explain that isoxazoline labels carry a seizure warning, and ask owners to report tremors or wobbliness after a dose.7
- Tell owners never to use a dog spot-on on a cat. The most severe pyrethrin poisonings in cats come from dog-only products, which hold a higher pyrethroid concentration.10
- Repellents and isoxazolines can reduce heartworm transmission but are not a stand-alone heartworm program.4
- Ask clients to pick up feces right away, as CAPC advises for roundworm control.11
Common mistakes and exam traps
- Saying heartworm preventives are unsafe in MDR1 dogs. At label doses they are considered safe; the danger is high or livestock doses.4
- Thinking preventives kill adult heartworms. They kill larvae. Melarsomine kills adults.5
- Choosing pyrantel for tapeworms. Pyrantel targets nematodes; praziquantel targets cestodes.1
- Bathing a permethrin cat in hot water. Use lukewarm water: hot increases absorption and cold worsens tremors.10
- Injecting melarsomine anywhere convenient. It goes only into the lumbar epaxial muscles, L3 to L5.6
Quick review
- Macrocyclic lactones act on glutamate-gated chloride channels.
- Benzimidazoles block tubulin; pyrantel is a nicotinic agonist.
- Praziquantel is the drug of choice for tapeworms.
- Emodepside acts on latrophilin and slo-1 and works on resistant worms.
- MDR1 dogs: preventive doses are safe; avoid high ivermectin doses and loperamide.
- Isoxazolines carry a tremor, ataxia and seizure class warning.
- Cats are sensitive to permethrin; treat with lukewarm bath and methocarbamol as ordered.
- Annual heartworm antigen and microfilaria test; year-round preventive.
- Melarsomine: 2.5 mg/kg deep IM at L3 to L5, dogs only.
- Strict exercise restriction for at least 1 month after each set of injections.
- Deworm puppies and kittens from 2 weeks of age, every 2 weeks.