By
Parasites aren't just about itching — they carry heartworm and blood-borne parasites that can be fatal. A vet lays out a parasite-prevention and deworming schedule by age, plus a tip on choosing drugs safely with MDR1 gene testing in at-risk breeds.
The questions I hear most often are "When should I deworm?" and "How often should I apply flea/tick treatment?" As a veterinarian, let me lay out a parasite-prevention schedule for both internal and external parasites by age, plus a safety tip many owners have never heard.
| Age | Internal (GI worms / heartworm) | External (ticks / fleas / mites) | Frequency |
|---|---|---|---|
| 2, 4, 6, 8 weeks | Liquid dewormer for round/tapeworms | No chemicals yet (manual removal + clean bedding) | Every 2 weeks early on |
| 2–6 months | Start heartworm prevention + deworming | Start spot-on / chewable (by weight) | Every month |
| 6 months & adult | Continue heartworm prevention + deworming | Chewable (1–3 months) or spot-on | Per product cycle (1 or 3 months) |
This is very important and many owners have never heard of it. Some breeds carry a genetic risk called the ABCB1 / MDR1 mutation, a gene that produces a protein protecting the brain from chemicals. If mutated, the dog becomes highly sensitive to the toxicity of certain parasite drugs (e.g. macrocyclic lactones such as ivermectin).
Global statistics on the MDR1 mutation by breed (PMC)
| Breed | % with mutation | Risk level |
|---|---|---|
| Collie | 55–57% | Very high |
| Australian Shepherd | 17–46% | Very high |
| Longhaired Whippet | 42% | Very high |
| Shetland Sheepdog | 7–35% | High–moderate |
| Miniature Australian Shepherd | 20–26% | Moderate |
| White Swiss Shepherd | 14% | Moderate |
| German Shepherd | 6–7% | Low |
| Border Collie | 1–2% | Low |
| Mixed (herding mix) | 2–7% | At risk if this ancestry is present |
Many owners mistakenly think, "If my dog has been fine on a low monthly preventive dose, I don't need gene testing." In reality, testing matters most when a dog needs drugs at a high therapeutic dose — for example, generalized demodicosis (wet mange), which requires macrocyclic lactones at many times the usual preventive dose (WSU).
Parasite prevention and treatment isn't guesswork — it's a science that demands precision. Following the schedule consistently, combined with knowing your dog's genetics, is the best shield you can give them.
1. If my dog has the MDR1 mutation, which parasite drugs should I avoid?
For at-risk breeds (e.g. Collies, Australian Shepherds) with the MDR1 mutation, the body is highly sensitive to certain chemicals. The drugs to watch are macrocyclic lactones such as ivermectin at high therapeutic doses, plus any non-standard injectables. Standard monthly preventives dosed by weight are usually safe, but should be used under veterinary guidance.
2. Do senior dogs still need parasite prevention?
Very much so. Even senior dogs remain at risk from heartworm, ticks, fleas, and GI worms throughout life, so consistent prevention per the product cycle is still necessary. The vet may adjust the dose or schedule to suit a senior dog's health.
3. Can newborn puppies use flea/tick spot-on right away?
No. In the newborn stage (2–8 weeks) a puppy's skin is very thin and the liver and kidneys aren't fully developed, so you shouldn't use over-the-counter spot-on or oral products yourself. For fleas or ticks, focus on manual removal or consult a vet for a gentle puppy-specific spray.
