Bad Reactions Of Dogs To Flea And Tick Medications

Bad Reactions Of Dogs To Flea And Tick Medications

Sep 21, 2021
25 min

Fleas and ticks are the most common external parasites of dogs. These parasites feed on the blood of their hosts and heavy infestations can cause serious anemia particularly in puppies, senior dogs, debilitated, and those with compromised immune system function. Singapore, with its year-round humid environment, is a perfect place for ticks and fleas to thrive. In countries where there are 4 distinct seasons, flea and tick season usually occur during the warmer months of the year. But in humid Singapore, fleas, and ticks flourish throughout the year.


Fleas are tiny wingless insects with powerful jointed legs that enable them to jump vertically up to 6 inches from one host to the next. They can hitch a ride with passing dogs or cats, and even the shoes or legs of humans. They feed on the blood of their hosts and cause intense itching, irritation, and even hair loss. In fact, adult fleas consume up to 15 times their body weight in blood per day. For some dogs, a flea or two can trigger intense itching because they are allergic to a protein that is present in the saliva of fleas.

Fleas are hardy and ubiquitous creatures. They can survive unfavorable conditions. The flea population is highest during summer when warm weather favors the hatching of flea eggs that are hidden in every nook and cranny of homes. Hatched eggs develop to become larvae then pupae until they become full-grown adults that are capable of laying eggs, the vicious lifecycle of fleas begins all over again.

Fleas are extremely prolific. Under ideal conditions, they can reproduce and develop within a short period of time. Even if you do not see fleas on your pet dog, thousands of flea eggs, larvae, and pupae may be present anywhere in your home and yard. Early signs of fleas should never be ignored because infestations can quickly get out of control. And remember fleas can bite humans too and cause intense itching and discomfort.



Ticks look like little spiders. These 8-legged parasites feed on the blood of their hosts by burying their mouthparts into the skin of dogs. After a blood meal, they can grow. An adult female tick lays eggs in cracks and crevices within the dog’s immediate environment. The humid tropical climate of Singapore creates a favorable environment for the growth and multiplication of ticks all throughout the year. The most common tick that can affect dogs in Singapore is Rhipicephalus sanguineus or the brown dog tick.

Effects of fleas and ticks

In addition to causing anemia and severe itching and irritation, fleas and ticks are also important carriers of a range of diseases.

Fleas can transmit tapeworm, murine typhus, Mycoplasma haemofelis, and cat-scratch disease. They can also trigger hypersensitivity reactions in dogs with flea allergies.

Ticks, on the other hand, are important carriers of canine babesiosis, canine ehrlichiosis, Lyme disease, and tick paralysis.



How to get rid of fleas and ticks

The eradication of fleas and ticks is not a one-time undertaking. Parasite prevention should be a year-round program to ensure adequate protection of dogs not only during flea and tick season but throughout the year. Being hardy creatures, they can survive and continue to feed on their hosts throughout the year.

Ridding your pets and your home of a thriving flea and tick population is easier said than done. All regimens for eradicating flea and tick populations are multi-pronged. They involve undertaking measures to treat the animal and the environment.


Flea and tick treatment in dogs


The traditional approach for getting rid of flea and tick populations typically involves the simultaneous treatment of both the pet and the premises. Traditional flea and tick products are now being replaced by revolutionary new medications. If your pet is infested, it is recommended that you work closely with your veterinarian in creating a preventive health program that involves year-round protection of your pet from external parasites. Even when you choose to buy over-the-counter (OTC) tick and flea products, it is prudent to consult a veterinarian to ensure that the product is effective and safe for your pet.

Flea and tick products for dogs are available as:

  • Oral medications –Usually available as chewable tablets and target adult fleas and ticks, killing them within 24 hours.
  • Topical (spot-on) solutions – Applied along the dog’s back or between the shoulder blades
  • Sprays
  • Collars
  • Shampoos
  • Injectables
  • Flea traps

The top tick and flea medications in terms of efficacy are generally available as oral/chewable tablets and topically-applied solutions called “spot-on”. The dosage of these preparations will depend on the weight of the animal.

While these products are effective in combating the tick and flea population, pet owners should always remember that their active ingredients are chemicals that can be potentially toxic and can cause an adverse reaction in humans and pets alike. It is for this reason that their use should always be done under the supervision of a veterinarian, even the OTC products.

Bad reactions of dogs to oral or spot-on tick and flea medications

Even if these products have been approved and continue to be safe and effective for most dogs, prudent use should always be practiced. Many incidents of adverse reactions to these medications in dogs occurred because of misuse, thus authorities push for the need for clearer and proper labeling of products.

Oral medications

Oral tick and flea medications may be among the most effective products in the market but they are not without potential side effects. Some pills or chews have been observed to have a negative effect on the neurological system leading to the manifestation of nervous symptoms such as seizures or stumbling. The most common active ingredients of oral tick and flea medications for dogs include the following:


Isoxazoline (brand names: Bravecto, Simparica, Credelio, and Nexgard)

Isoxazoline is a class of tick and flea products that are commonly available as pills and chews. But its use has been associated with potential adverse reactions particularly neurologic problems, such as ataxia, muscle tremors, and seizures. Dogs with ataxia lack muscle control causing them to stumble or twitch. Seizures can occur in dogs even without a history of suffering from any seizure episode before. Isoxazoline works by targeting the nervous system of ticks, fleas and other insects.

Many dogs have been given tick and flea products with isoxazoline as the active ingredient without any adverse reaction, however, since the possibility exists, it is recommended to check with your veterinarian so your pet’s medical history can be reviewed to ensure that these products are safe for your dog.


Pyrethrins and pyrethroids

There are active ingredients of insecticides that are used for the treatment of flea and tick infestations. Pyrethrins are extracted from pyrethrum-related plant species, particularly from Chrysanthemum cinerariaefolium. On the other hand, pyrethroids are synthetic-based and have a longer-lasting effect compared to pyrethrins. Some of the common pyrethrins include cypermethrin, permethrin, allethrin, and etofenprox.

Pyrethrins and pyrethroids can exert an adverse effect on the dog’s nervous system. Adverse reactions are more frequent in small dogs, puppies, old, ill, or debilitated dogs. Dogs with body temperatures that are lower than normal limits (such as brought about by sedation, anesthesia, or bathing) are also predisposed to toxicity. Symptoms that are exhibited by affected dogs include:

  • Allergic reactions — itching, hives, congestion, respiratory distress, shock, and in rare cases, death can occur
  • Mild reactions — vomiting, diarrhea, drooling or excessive salivation, paw flicking, mild depression, ear twitching
  • Moderate to serious adverse reactions — persistent vomiting and diarrhea, incoordination, depression, muscle tremors



Many tick and flea medications that contain organophosphates are safe and effective with proper use. Improper use carries with it risks of toxicity and other adverse reactions that can harm a dog’s health. Signs of toxicity associated with these organophosphates-containing products include vomiting, diarrhea, respiratory distress, constricted pupils, muscle tremor, drooling, and weakness. Depending on the ingredients of a product and how much a dog has been exposed to, the onset of toxicity symptoms can be rapid and usually fatal.

Do’s and Don’ts when it comes to using tick and flea products in dogs


  • Consider giving your pet a product that is prescribed by a veterinarian to be sure that is safe and effective. Always seek your vet’s approval before applying any tick and flea medication on your dog.
  • If you are applying topical products to several of your pets in the household, you should separate them until the product is dry, so they won’t lick it off each other.
  • Make a habit of reading product label directions.
  • Pet owners must confirm if a product is appropriate for dogs, and the dog’s weight and/or age.
  • Proper administration and dosing must be followed to help avoid adverse reactions.


  • Topical tick and flea products should never be applied on skin that is irritated, scratched, or broken.
  • Avoid using more than one product at a time unless you are instructed by your veterinarian to do so.
  • When using dips, never submerge a dog into the liquid solution. Instead, the solution should be poured over the dog’s body. Use a sponge to apply the solution to parts of the body that are still dry.
  • When spraying tick and flea products, be careful not to accidentally spray into the dog’s mouth or eyes.


What to do if your pet reacts adversely to a product

If, after giving or applying a tick and flea control product to your pet, negative reactions are displayed, immediately bath your pet with mild soap and rinse him well. Be sure to dry your pet thoroughly after bathing and keep him warm as you bring your pet to your veterinarian. Cases of toxicity can quickly turn for the worse, thus you should seek immediate veterinary intervention for your pet.


Will my pet recover from tick and flea medication toxicity?

Early detection and intervention can go a long way in improving the prognosis and making a full recovery. The treatment regimen to reverse toxicity usually includes intravenous fluids, symptomatic treatment, supportive care, and hospitalization for monitoring.

Non-toxic and natural tick and flea control measures

  • Run a fine-toothed flea comb through your pet’s hair coat every day to catch fleas. Prepare a bowl that contains soapy water so you can dip the comb into it after each sweep.
  • Vacuum carpets, furniture, as well as every nook and cranny of your home to get rid of flea and tick eggs, larvae, and pupae. Be sure to seal the bag and dispose of properly as the eggs inside can hatch and become a source of reinfection.
  • Sprinkle diatomaceous earth (DE) on carpets to eliminate ticks and fleas. You can also spread it around your yard during heavy flea infestations. Be sure to remove any pet from the area and wear a protective mask during the application. Let the powder sit for several hours and vacuum it afterward. You can buy DE from perromart. Do not use diatomaceous earth that is used in swimming pools because these are chemically treated.
  • Some products contain microorganisms that eat fleas (also called “beneficial nematodes”). These products can be sprayed on lawns. Formulas are safe for pets, birds, friendly garden dwellers (like earthworms and ladybugs), as well as humans.
  • Use herbal shampoos that are specially formulated to combat ticks and fleas.
  • There are also flea treats that are formulated to repel fleas.
  • Make a natural flea repellent solution by combining essential oils — tea tree oil, rosemary oil, eucalyptus oil, citronella oil, and peppermint oil. Use 5 drops of each essential oil to one cup of water. Shake it thoroughly and put it in a spray bottle.
  • Fleas are repelled by orange or lemon. Lightly rub freshly-squeezed orange or lemon juice on your pet’s hair coat.
  • A natural repellent for ticks is rose geranium oil which can be applied to the collar of dogs.


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