Parasites are the quietest of the routine health threats to dogs, in part because the ones that matter most are invisible until they are expensive, and in part because prevention works so well that a protected dog never shows the problem. For Dogs (Canis lupus familiaris), the parasite program runs on two fronts, external pests that live on the skin and internal organisms that live in the gut, blood, and heart, and the modern standard is year-round prevention prescribed by a veterinarian rather than a seasonal campaign.
External parasites
Fleas are the most familiar, and their biology explains why household infestations feel sudden: the adult fleas on the dog are a small fraction of the population, with eggs and larvae developing in carpets, bedding, and yard soil for weeks before the adults that bite ever appear. Signs range from scratching and flea dirt, the black specks that smear red on a wet paper towel, to the flea-allergy dermatitis that turns a few bites into weeks of skin disease, and the tapeworm connection matters as much, since fleas carry the tapeworm larvae that dogs swallow while grooming. Control is systemic rather than spot-treatment: the dog's preventive, the household's bedding and floors, and the yard's shaded dirt all enter the plan at once, because treating only the animal leaves the environment to refill it.
Ticks matter less for their bite than for what they carry, and the disease list is regional: Lyme disease from the deer tick across the eastern and upper-midwestern United States, ehrlichiosis and Rocky Mountain spotted fever in their own geographies, and anaplasmosis where the vectors overlap. Transmission takes hours of attachment, which is why the daily tick check after grass and woods, ears, armpits, and groin in particular, works alongside prevention, and the tick is removed with tweezers or a hook at the skin line rather than burned or painted. Mites complete the external set: sarcoptic mange mites burrow and itch fiercely and are contagious between dogs, demodex mites live harmlessly on most dogs and overpopulate in young or immunocompromised individuals, and ear mites favor puppies and cats. The itchy, hairless, scabby dog has earned a veterinary skin exam rather than a guess, because the treatments differ by mite.
Intestinal worms
The roundworms, hookworms, and whipworms of the canine gut are ubiquitous, and puppies are born into the problem: roundworm larvae cross the placenta and the milk, which is why every puppy is dewormed on a schedule from two weeks of age regardless of what its feces show. Adult dogs pick the worms up from contaminated soil, from hunting and scavenging, and from fleas in the tapeworm's case, and the signs range from nothing at all to the pot-bellied, dull-coated, sometimes vomiting puppy of the classic description. Fecal testing is the diagnostic engine, run at the puppy series, then annually for most adult dogs, because the worms shed eggs intermittently and the microscope sees what the eye cannot.
The zoonotic footnote makes the routine a family-health issue as well. Roundworm and hookworm larvae can infect people, children especially, through contaminated soil and sand, and the larvae migrate through human tissue in the conditions whose names, visceral and cutaneous larva migrans, most households never hear until they do. The prevention is the same program the dog needs anyway: routine deworming on the veterinarian's schedule, feces picked up everywhere, sandboxes covered, and hands washed after yard and garden work.
The puppy deworming schedule
Puppies run the tightest parasite calendar in the field, because roundworm and hookworm larvae cross the placenta and the milk and mature inside the puppy faster than any symptom appears. The standard schedule deworms at two, four, six, and eight weeks of age, then follows with fecal checks through the vaccination series, because the first treatments miss larvae that were still migrating when they ran. The schedule is why breeders' records matter at purchase, and why the first veterinary visit asks for them; a puppy from an unknown-history source simply restarts the series under the veterinarian's direction. Adult dogs keep the lighter version, an annual or twice-annual fecal and a deworming when the test or the lifestyle calls for it, since the hunting dog, the scavenger, and the dog park regular all qualify for tighter loops. The household that follows the puppy schedule once understands the whole system, because the schedule is simply the life cycle of the worms, interrupted at every stage.
Heartworm, the quiet threat
Heartworm is the parasite that redefined the prevention calendar, because its vector is the mosquito, its geography is wherever mosquitoes survive, and its disease is silent until it is severe. The life cycle runs through the mosquito's bite, which delivers larvae that migrate and mature in the heart and pulmonary arteries over roughly six months, and the adult worms produce the coughing, exercise intolerance, and heart failure that present late. Treatment exists but is arduous and costly, months of restricted activity and injections that carry their own risks, which is why veterinary medicine's position is categorical: prevention is cheaper, safer, and easier than cure by every measure.
The prevention standard is year-round dosing with the products a veterinarian prescribes, monthly chewables and topicals among them, with an annual antigen test to confirm the program is working, because a missed dose or a resistant strain is the scenario the test exists to catch. Puppies start prevention young, the product choices weigh the dog's size and the household's compliance style, and the regions where mosquitoes never truly freeze are the reason the seasonal myth, no mosquitoes, no heartworm, no longer survives its own weather data. Cats share the risk in their own way, which matters in the multi-species household, but the dog is the classic host and the routine's anchor.
What treatment involves
When prevention fails, heartworm treatment is the reason prevention exists. The adulticide protocol runs over months rather than days: staged injections of melarsomine deep into the lumbar muscle, preceded by weeks of stabilization with preventives, doxycycline against the worm's symbiotic bacteria, and anti-inflammatory support, all wrapped around the strictest exercise restriction most owners ever impose on a dog, because a dead worm lodging in a pulmonary artery of an exercising dog is the treatment's central risk. The cost runs to four figures in many practices, the dog's activity shrinks for the duration, and success still depends on catching the disease before the heart and arteries remodel permanently. The slower-kill protocols that some households request trade duration and uncertainty for lower cost, and the veterinary consensus treats them as a compromise rather than an equivalent. Every element of the experience, the expense, the confinement, the risk, prices the monthly chewable against the disease it replaces, which is the arithmetic the prevention chapter keeps making.
What treatment involves
When prevention fails, heartworm treatment is the reason prevention exists. The adulticide protocol runs over months rather than days: staged injections of melarsomine deep into the lumbar muscle, preceded by weeks of stabilization with preventives, doxycycline against the worm's symbiotic bacteria, and anti-inflammatory support, all wrapped around the strictest exercise restriction most owners ever impose on a dog, because a dead worm lodging in a pulmonary artery of an exercising dog is the treatment's central risk. The cost runs to four figures in many practices, the dog's activity shrinks for the duration, and success still depends on catching the disease before the heart and arteries remodel permanently. The slower-kill protocols that some households request trade duration and uncertainty for lower cost, and the veterinary consensus treats them as a compromise rather than an equivalent. Every element of the experience, the expense, the confinement, the risk, prices the monthly chewable against the disease it replaces, which is the arithmetic the prevention chapter keeps making.
Protozoa and the waterborne cases
Giardia and coccidia are the single-celled intestinal parasites that fill the diarrhea file, and both travel through contaminated water and soil. Giardia is the stream-and-puddle parasite of the hiking dog and the kennel, causing soft, greasy stools that wax and wane, and its cysts survive outdoors for months, which makes the contaminated yard and water bowl part of the treatment plan. Coccidia favors puppies and crowded facilities, and both organisms are diagnosed on fecal testing rather than assumption, because the treatments differ and the signs overlap with the bacterial and dietary causes of diarrhea. The prevention mirrors the water chapter: clean standing water out of reach, feces removed daily, kennel and crate surfaces disinfected, and the hiking dog drinking from carried or filtered water rather than the stream.
Building the prevention routine
The routine that handles all of these is short: year-round broad-spectrum prevention covering fleas, ticks, and heartworm as the veterinarian prescribes for the region, deworming and fecal testing on the schedule the dog's life stage demands, the daily tick check in season, and the yard and household hygiene that starves the environmental half of every life cycle. The calendar matters more than the brand, because the missed dose is the opening every parasite life cycle waits for, and the records, product, date, dose, join the medical file where the next veterinarian reads them. Parasites have co-evolved with dogs for millennia, and the modern household does not defeat them; it simply stands between them and its dog, on schedule, all year.