Drugs that fight parasites – Pet tip 256

The thought of parasites makes most pet owners shudder. Just in case we’ve forgotten what they are, a parasite is an organism that lives off another animal without benefit to that animal and without killing that animal. Parasites can be classified into external (ectoparasites) and internal (endoparasites). There are many different types of these critters, from roundworms to hookworms, tapeworms to fleas and ticks. Here is an overview of some of the drugs used to kill them, and a couple of pointers that you as a conscientious pet owner might want to be aware of.

Broad Spectrum Antiparasitics
There are 4 classes of these, and each of them kills parasites in different ways.
The first class is the “Macrocyclic Lactones”, called this because of their chemical structure. These kill both some external and internal parasites. They do this by binding to a structure (specifically, a receptor) on cells that only parasites have, which causes the parasites to become paralyzed and die. These are very safe drugs since the receptor they bind to is specifically found only in the parasite, and not on mammalian cells. Also, these drugs do not cross into the brain at high levels due to a molecule called “P glycoprotein”, which pumps out any drug if it does enter the brain. However an interesting breed-related note comes out of this. Collies, as well as collie crosses and Shetland Sheepdogs, Old English Sheepdogs, German Shepherds, Long-haired whippets and Silken windhounds, are all especially susceptible to a drug in this class known as ivermectin. This is due to the fact that they have a mutation in a certain gene, and thus do not have this P glycoprotein. Thus, the ivermectin can enter the brain and cause adverse effects. Still, it is important to note that this drug is still perfectly safe to give to these breeds as long as it is used only for heartworm prevention, since the heartworm preventive dose is significantly lower than the toxic dose. However, it should not be used for other parasites, since significantly higher doses are needed to kill other parasites.

The second major drug class are the Benzimidazoles. These are truly broad spectrum and kill roundworms, flukes and even some protozoa like Giardia. However, they do not work against most endoparasites. These are safe drugs as well. However, there have been rare cases of bone marrow suppression in dogs using a drug in this class, and acting as a teratogen in cows and sheep. (A teratogen is a substance that causes birth defects.) These drugs act by binding to a certain molecule in parasites, again causing parasite paralysis, but in a different way from the drug class previously mentioned. The end result is the same. One of the drugs in this class (Febantel) should be used in dogs only; it is toxic in the cat.

A third class is the Pyrimidines, again named this due to their chemical structure. These bind to a structure called a ‘nicotinic receptor’, and increase the amount of a molecule called Acetylcholine, which is responsible for nerve firing. The cells of the parasites are thus overloaded with acetylcholine, which causes the cell to fire continually instead of only at certain times, eventually exhausting the cell and causing the parasite to go into a spastic paralysis and die. This is not the right drug to use if the animal has a very high number of parasites. This is because it can cause impactions (blockages) in the intestines, where large numbers of worms are suddenly becoming paralyzed. This can block the gut and lead to great problems. However, it is a safe drug to use in animals that are suspected to have a low parasite burden.

Finally, the last drug class is derived from compounds from a fungus. A synthetic version is called ‘emodepside’. This class of drug kills parasites by inhibiting their mouthparts, thus, they cannot hold onto the gut wall and feed, and die in this fashion. These drugs have a wide spectrum of activity as well, killing both tapeworms and roundworms.