PHA 520 - L3 Anti-Parasitics Therapy

What are antiparasitics?

A class of medications used to treat parasitic diseases caused by internal parasites (nematodes, cestodes, trematodes, protozoa) and external parasites (fleas, ticks, mites, lice, biting insects).

What is a parasiticide?

A drug that is more toxic to the parasite than to the host; its use always carries some potential hazard to the host.

What is an ectoparasite?

A parasite that lives on the outside/external surface of the host.

What are examples of ectoparasites?

Fleas, ticks, mites, and lice.

What is an endoparasite?

A parasite that lives inside the host.

What are the major classes/examples of endoparasites?

Nematodes/ascarids (roundworms, hookworms, whipworms, heartworms, lungworms), cestodes (tapeworms), trematodes (flukes), and protozoa (Giardia, Coccidia, Toxoplasma).

What are the properties of an ideal antiparasitic agent?

Effective against ALL parasitic stages of a species; broad spectrum across different genera; non-toxic to host/wide safety margin (at least 4–5×); rapidly cleared/excreted; does NOT induce resistance; economical/reasonable cost.

Does one antiparasitic product possess all the properties of an ideal antiparasitic agent?

No; no single product currently fits all ideal characteristics.

What are the common ectoparasiticides emphasized in this lecture?

Pyrethrins/synthetic pyrethroids, formamidines, avermectins/milbemycins, insect growth regulators (IGRs), and newer flea/tick agents.

What is the general toxicity theme of ectoparasiticides?

Many interfere with neurotransmission, so severe adverse effects are commonly neurological; cats are especially sensitive.

Which ectoparasiticides should NEVER be used in cats?

Permethrin and amitraz.

What is an important toxicity of permethrin?

Permethrin is highly toxic to cats; avoid permethrin-containing dog products such as K9 Advantix in cats.

What is amitraz (Mitaban)?

A formamidine ectoparasiticide/neurotoxin used for generalized demodectic mange in dogs; toxic to cats and rabbits.

What are common adverse effects of amitraz?

Transient sedation for 24–72 hours and other CNS effects; toy/small breeds and aged/debilitated pets may be more susceptible.

What are insect growth regulators (IGRs)?

Agents that kill/prevent development of immature insects and break the parasite life cycle before the parasite reaches adulthood.

How do juvenile hormone-mimicking IGRs work?

They bind juvenile hormone receptors in immature insects and prevent survival/development to the next life stage.

What is lufenuron's mechanism of action?

IGR that inhibits chitin synthesis and prevents development of immature fleas; it does NOT kill adult fleas.

What are the "-laner" ectoparasiticides?

Fluralaner (Bravecto), afoxolaner (NexGard), sarolaner (Simparica), and lotilaner (Credelio); newer-generation flea/tick drugs.

What major precaution applies to the "-laner" drugs?

Use cautiously/avoid in animals with a history of seizures or epilepsy because they may cause neurologic effects such as tremors and ataxia.

What parasites does pyrantel (Strongid, Nemex) treat?

Primarily GI nematodes: roundworms/ascarids and hookworms; also used for pinworms in horses.

What is pyrantel's mechanism of action?

Causes neuromuscular junction depolarization, resulting in paralysis of the parasite musculature.

What are the formulations/brands of pyrantel?

Pyrantel pamoate = Nemex; pyrantel tartrate = Strongid-T.

What parasites does fenbendazole (Panacur) treat?

Broad spectrum: GI nematodes including roundworms, hookworms, and whipworms; lungworms; some tapeworms/flukes; also Giardia.

What is fenbendazole's mechanism of action?

Binds nematode tubulin, preventing glucose uptake and starving the parasite.

What is an important safety characteristic of fenbendazole?

It has very low toxicity and may be safe at doses far above the recommended dose.

What is fenbendazole (Panacur) especially useful for regarding Giardia?

It is very effective and safe and is the drug of choice for Giardia in pregnant animals and cats.

What parasites does praziquantel (Droncit) treat?

Tapeworms/cestodes and flukes; it is essentially 100% effective against adult tapeworms, including Echinococcus.

What is praziquantel's mechanism of action?

Attacks the parasite neuromuscular junction and tegument, causing contraction/paralysis, destruction of the tegument, and parasite death.

What is important for breaking the tapeworm life cycle in addition to praziquantel treatment?

Block transmission through effective flea control and/or preventing predation because fleas or prey may serve as vectors/intermediate hosts.

What is an endectocide?

A drug effective against BOTH ectoparasites and endoparasites.

What type of drug is ivermectin?

A macrocyclic lactone/avermectin endectocide effective against both certain external parasites and internal nematodes.

What parasites does ivermectin treat/prevent in dogs and cats?

Prevents heartworm development; extra-label treatment of demodectic mange, sarcoptic mange, ear mites, and ear ticks; may also kill microfilariae after adult heartworm treatment.

What parasites do macrocyclic lactones such as ivermectin generally treat?

Ectoparasites such as lice, mites, fly larvae, bots, grubs, and some ticks plus endoparasitic nematodes; they have NO activity against tapeworms or flukes.

Why don't ivermectin and related macrocyclic lactones treat tapeworms or flukes?

Tapeworms and flukes lack the glutamate-gated chloride receptors targeted by these drugs.

What is ivermectin's mechanism of action?

Binds glutamate-gated chloride channels in invertebrate nerves/muscles → hyperpolarization → flaccid paralysis → parasite loses attachment and is eliminated.

What is the major breed-related concern with ivermectin?

Many Collies and other herding breeds may carry an MDR1 mutation that increases susceptibility to ivermectin and related drug toxicities, especially at high treatment doses.

What causes heartworm disease?

The nematode Dirofilaria immitis.

How is heartworm disease transmitted?

Through the bite of an infected mosquito carrying infective heartworm larvae.

What is the intermediate host for heartworm disease?

The mosquito; heartworm larvae develop into their infective form inside the mosquito.

What is the definitive host for heartworm disease?

The infected pet (dog, cat, or ferret), where heartworms mature into adults, mate, and produce offspring/microfilariae.

Is heartworm disease directly contagious between pets?

No. Transmission requires a mosquito intermediate host.

Where do adult heartworms live?

Primarily in the heart, lungs, and associated blood vessels.

What are common signs of heartworm disease in dogs?

Cough, fatigue/exercise intolerance, and difficulty breathing; severe disease can cause heart/lung damage.

What is the key principle of heartworm disease management?

Prevention is the best treatment; pets should receive year-round prevention and be tested every 12 months.

How do heartworm preventatives work?

They kill larvae acquired during the previous ~30 days, preventing larvae from developing into adult heartworms; they do NOT kill adult worms.

What should be done before starting heartworm prevention in a dog older than 6 months that has not been receiving prevention?

Test the dog for heartworm disease first.

What drug kills adult heartworms in dogs?

Melarsomine (Immiticide), an organic arsenical adulticide given by deep IM injection.

Are cats treated with heartworm adulticides such as melarsomine?

No; killing adult worms can cause severe inflammatory lung disease and death in cats.

What are examples of heartworm preventatives from the macrolide/endectocide class?

Ivermectin, moxidectin, selamectin, and milbemycin oxime.

What are protozoa?

Single-celled organisms; important GI protozoan parasites include Giardia and Coccidia.

How is Giardia transmitted?

Fecal-oral transmission through ingestion of cysts, often from contaminated water/material; self-reinfection can also occur through grooming.

What are examples of anti-protozoal drugs?

Sulfonamides (e.g., Albon), trimethoprim/potentiated sulfonamides (e.g., Tribrissen), metronidazole (Flagyl), fenbendazole (Panacur), and clindamycin for Toxoplasma.

What is the drug of choice for Giardia according to the lecture?

Metronidazole (Flagyl).

What is the drug of choice for Giardia in pregnant animals and cats?

Fenbendazole (Panacur) because it is effective and very safe.

What protozoan infection is treated with sulfonamides such as Albon?

Coccidia/coccidiosis.

How do sulfonamides work against Coccidia?

They inhibit an enzyme required for folate synthesis, interfering with DNA synthesis and survival.

What protozoan infection is treated with clindamycin?

Toxoplasmosis (Toxoplasma).

What are the FOUR endoparasiticides you especially need to match with their parasites?

Pyrantel → roundworms + hookworms; Fenbendazole → roundworms + hookworms + whipworms + lungworms + some tapeworms/flukes + Giardia; Praziquantel → tapeworms + flukes; Ivermectin → heartworm prevention + certain nematodes/ectoparasites, but NOT tapeworms or flukes.

What are the biggest Lecture 5 cat toxicity takeaways?

NEVER use permethrin or amitraz in cats; cats are especially sensitive to ectoparasiticides and neurologic toxicity.