Rainy Season Flea Check: 4 Spots and a 48-Hour Gap

Rain does not bring fleas. It keeps the humidity above 70 percent, and that is what hatches the eggs already in your carpet. In a heated room at 24 to 28 C with 80 to 95 percent relative humidity, cat flea eggs hatch in 2 to 4 days instead of 10, so a cat that was clear in September can be covered in 3 weeks. The check takes 3 minutes; the treatment takes 12 weeks.

Temperature, humidity, and the 2-to-10-day egg

Cat fleas follow a fixed script. An adult female lays 20 to 50 eggs a day, and those eggs fall off the cat wherever it sleeps. Hatching takes 2 to 10 days, fastest at 24 to 29 C and 70 to 85 percent humidity. Below 50 percent humidity the eggs dry out and die; below 13 C development stops; above 35 C most larvae die. Larvae then take 6 to 12 days to pass 3 stages, and the pupa sits for 5 to 14 days, or up to 6 months if it is waiting for a host to walk past. The whole cycle is 21 to 28 days at room temperature.

That arithmetic explains the seasonal trap. Only about 5 percent of a flea infestation is on the cat as adults; roughly 50 percent is eggs, 35 percent larvae and 10 percent pupae, all in the environment. Killing the 5 percent on the cat while leaving the 95 percent in the carpet is why the fleas come back on day 21.

The 4 places to check, in 3 minutes

Use a fine-toothed flea comb with teeth 0.3 to 0.5 mm apart, and work in daylight. Part the coat against the grain and look at the skin, not the fur. Four sites carry almost all the evidence: the tail base, the groin and lower belly, under the chin and jaw, and between the toe pads of the hind feet. Flea dirt is darker than dandruff, grit-shaped, and turns rust red when you press it onto a damp paper towel, because it is digested blood.

Site	Seconds to check	Comb strokes	What you find
Tail base	45	10	Densest flea dirt; the first place to look
Groin and lower belly	45	10	Live fleas, red papules, licked-bare skin
Under the chin	30	8	Adults on a cat that cannot reach to groom
Between the toes	30	8	Itching, brown staining, chewed nails
Sleeping spots	30	0	Eggs and dirt in the bed at 1 to 2 per cm

Counting fleas tells you which problem you have

After the first dose, count what you comb out each morning for 7 days. Fewer than 2 fleas a day means the product is working and the environment is catching up. Five to 10 new fleas a day, 2 weeks in, means the room is still producing them: vacuuming is too rare, or the pupae are still emerging. More than 20 on a kitten, or a cat with pale gums, is a veterinary visit rather than a shopping problem, because 100 fleas can take enough blood from a 1 kg kitten in a week to cause fatal anemia.

Itching that continues after the fleas are gone is usually flea allergy dermatitis, where 1 or 2 bites set off a reaction on the whole back half of the body; the pattern and the treatment window are described in the itching and sores guide.

The dose, the expiry, and the 48-hour bath rule

Spot-on products differ in duration, and the label is the only reliable number. Fipronil and imidacloprid formulations are labelled for 30 days; selamectin for 30 days; fluralaner for 12 weeks. Anything past its expiry date, or a tube opened 2 seasons ago, may be at half strength. Water is the other variable: shampoo strips the lipid layer that holds the drug, so do not bathe the cat for 48 hours before or 48 hours after application, and do not use a medicated shampoo in the same week. A cat that swims or gets a bath on day 3 of a 30-day product has effectively lost 3 days of cover.

Never use a dog spot-on on a cat. Products containing 45 to 65 percent permethrin are common for dogs and cause tremors, hypersalivation and seizures in cats within 1 to 3 hours; check every ingredient against the toxic substance list before anything goes on the skin. Kittens under 8 weeks and cats under 1 kg usually need a vet-weighted dose rather than a tube size chosen by guess.

Environment: the 3-part routine

1. Vacuum every 2 days. Sofa seams, carpet edges, under the bed, the cat bed and the car seat. Ten strokes per square metre, then seal the bag or empty the canister outside within 5 minutes. Vacuuming removes 30 to 50 percent of eggs and also triggers pupae to emerge, which is why it belongs on the same day as the dose.
2. Wash bedding at 60 C or above, weekly. A 30-minute hot cycle plus 30 minutes in a dryer kills every stage. A cold wash moves the eggs around.
3. Use an insect growth regulator on the room. Methoprene or pyriproxyfen sprays stop larvae from becoming adults and cover the 6-month pupal reservoir; treat 2 rounds, 3 weeks apart, and keep cats out until the surface is dry.

Recheck schedule

Comb on day 7, day 14 and day 30, at the same time of day, and write the count down. Day 7 checks the product, day 14 catches a missed dose, day 30 is the point where a new generation would appear if the environment was not treated. Rainy weeks are the ones to watch: 3 days of 90 percent humidity is a 2-day hatch, and the second wave that follows a warm autumn runs on the same clock.

When to stop treating at home

• Vomiting, drooling or wobbling within 24 hours of a dose: bathe off the product with dish soap and call the vet.
• Bald patches, broken skin or a cat that cannot stop scratching: this needs a diagnosis, not a stronger product.
• Pale gums, lethargy or a kitten that stops feeding: possible anemia, same-day visit.
• No improvement after 8 weeks of correct dosing and weekly washing: the source is probably outside the house.

Frequently asked