Mosquito repellents in the prevention of heartworm disease
5-year-old dog living in the Ebro delta, a hyperendemic area for heartworm. It receives a monthly macrocyclic lactone and the owner asks whether adding a repellent spot-on makes sense.
Which statement is correct?
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A. A permethrin spot-on is enough and the macrocyclic lactone can be stopped
No repellent prevents every bite; a single infective bite is enough to transmit larvae. The macrocyclic lactone remains essential.
B. Macrocyclic lactones already repel mosquitoes, so the repellent adds nothing
Macrocyclic lactones do not repel: they kill the larvae that have already entered the dog after the bite.
C. Repellents are contraindicated together with macrocyclic lactones
There is no general contraindication; indeed, some products combine both. Always check the product information of each product.
D. The repellent complements the macrocyclic lactone: it reduces bites and transmission, and is useful in high-prevalence areas
Correct. This is the "double defence": the repellent reduces bites (and the infection of mosquitoes feeding on microfilaraemic dogs), and the macrocyclic lactone kills any larvae that do get in.
lightbulb Key takeaway
In Spain, Culex bites mainly at dusk and at night, whereas the spreading tiger mosquito (Aedes albopictus) also bites during the day. Repellents also help against Dirofilaria repens and, if they contain pyrethroids, against sandflies. Never use them in cats if they contain permethrin.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.