Bug Season Isn't Cute Anymore

4 hours ago
9 min read

The parasites coming for your dog

A beautiful dog lays in the grass as the sun sets in the background.

Something is back that we thought we'd seen the last of sixty years ago. And it does not mess around.

The New World screwworm — Cochliomyia hominivorax, if you want to be precise about the nightmare — was eradicated from the United States in the 1970s through one of the most ambitious pest-control campaigns in American history. Federal agencies mass-released sterile male flies until the species essentially ran out of viable mates to reproduce with. It was a genuine triumph.

Then, humans got lax, and it started creeping back north through Central America. Then southern Mexico in 2024. And as of early June 2026, confirmed cases have been detected in southwest Texas and New Mexico — the first cases of New World screwworm in the United States in sixty years. Among them: a confirmed case in a dog.

So. We're doing this.

Meanwhile

If you are not in Texas or New Mexico right now, this is probably not something you need to cancel your summer plans over. The risk for most dogs in most parts of the country remains low. But "low risk" is doing a lot of work in a sentence that also contains the phrase "flesh-eating fly larvae."

And the reality is that the screwworm's return is a useful — if deeply unpleasant — occasion to talk about something we don't spend nearly enough time on: the full roster of bugs that are actively interested in making your dog's life worse.

Because the screwworm is dramatic. But it is not alone.

The New World Screwworm: What It Actually Is

Despite the name, this is not a worm. It's the larval stage of a fly. What makes it so dangerous is that these larvae feed on live tissue — not just dead tissue like most maggots.

Female screwworm flies are drawn to open wounds, even minor wounds and scratches. The flies lay anywhere from 200 to 300 eggs right at the wound's edge. Within 12 hours, those eggs hatch into maggots that start actively feeding. The screwworm has sharp mouth hooks that allow it to burrow — screw — into the wound and feed on healthy tissue. As larvae continue to hatch in waves, the wound gets larger and deeper. It develops a foul smell. It does not heal. Secondary infections set in.

Clinical signs include irritated behavior and head shaking. Infested animals often become depressed, stop eating, and may separate themselves from other animals.

For dogs, depending on the location and severity of the infestation, it can be fatal. If you see larvae in a wound, this is an emergency — not a "let's see how it looks tomorrow" situation.

New World screwworm is a reportable disease in the U.S. If you observe suspicious wounds or maggots, contact your veterinarian, state animal health official, or USDA veterinarian immediately.

Treatment: Vets will remove as many larvae as possible — potentially under sedation in severe cases — clean and treat the wound, and get the dog on an isoxazoline parasite preventative if they aren't already on one. The FDA has authorized emergency use of nitenpyram tablets, an over-the-counter drug that can kill most screwworm larvae within hours of the first dose. A second dose may follow six hours later; wound care continues after.

Prevention: Keep companion animals on year-round ectoparasite control. The isoxazoline products many pets already take for heartworm and fleas also guard against screwworm. Monitor wounds — including old tick bite sites — closely and often. Assess your pet's environment for things that could cause injury: metal collars or chains, wire fencing, sharp vegetation. Small wounds can fly under the radar. Right now, they need to be on yours.

The Rest of the Lineup

The screwworm is the newcomer getting all the press. But the bugs that have been here the whole time are still very much clocked in.

Fleas

Fleas are easy to dismiss as a nuisance problem. They are, in fact, a disease vector with a sideline in causing misery.

Fleas can carry tapeworms, which pets may ingest while grooming, leading to intestinal infections. Young, small, or compromised pets can develop anemia from a severe flea infestation. Some dogs are allergic to flea saliva — flea allergy dermatitis — which turns every single bite into an inflammatory event. One flea. Weeks of itching, hot spots, and hair loss.

The evidence of fleas is usually flea dirt before it's the fleas themselves: tiny black specks that look like coffee grounds in the coat. Comb them onto a damp white paper towel. If they turn red, that's digested blood. That's fleas.

Treatment involves treating the dog, treating the environment, and treating the yard — because the flea you see on your dog represents a fraction of the infestation living in your carpet and furniture. Prevention is monthly protection, applied consistently.

Ticks

Ticks are the overachievers of the parasite world. They can transmit a slew of diseases, including Lyme disease, tick-borne relapsing fever, Rocky Mountain spotted fever, babesiosis, anaplasmosis, ehrlichiosis, and hepatozoonosis, among others. They can also, while feeding, release a toxin that causes tick paralysis — a condition that can mimic neurological disease and resolves when the tick is removed, which would be satisfying if the tick weren't usually attached somewhere hidden.

Tick checks after time outdoors are non-negotiable. Use fine-tipped tweezers or a tick removal tool, grip as close to the skin as possible, and pull straight out with steady pressure. No twisting, no burning, no petroleum jelly. Those methods fail and create additional risk.

Prevention is year-round. Ticks are not a warm-weather-only problem.

Heartworm

Let's talk about the one that will break your heart before it kills your dog.

Heartworms are transmitted to pets by mosquitoes. Once inside the body, heartworm larvae mature and settle in the heart, lungs, and blood vessels, causing progressive damage. They migrate, mature, and take up residence. They can grow to be a foot long or longer. And your dog can live with them — sometimes for years — before showing obvious signs of illness.

By the time a dog is coughing, struggling to exercise, losing weight, or breathing hard, the damage is well underway. In dogs, heartworm can be treated — but surgery may be required, making it a costly and painful disease to manage.

And the treatment is almost as brutal as the disease.

The American Heartworm Society recommends a three-injection protocol using melarsomine dihydrochloride, combined with a 28-day course of doxycycline twice daily and a monthly preventative. The injections go deep into the large muscle mass along the spine, usually requiring sedation. Many dogs experience significant pain following melarsomine injections, so preemptive pain control is part of the protocol.

Then comes the part people don't fully reckon with until they're living it.

When heartworms die, pieces of the decomposing worm bodies can block blood vessels in the lungs, causing a potentially fatal pulmonary embolism. If the dog's heart rate is increased by exercise or excitement, worm pieces can be forced into the tiny blood vessels of the lungs. So for the duration of treatment, strict cage rest is required beginning with the first injection and extending until six to eight weeks after the final treatment. Leash walks are acceptable, but dogs should never be allowed to run.

Let that sit for a second. Your dog, who has no idea what is happening to them, cannot run. Cannot play. Cannot have the burst of excitement that dogs have when they see their favorite person walk through the door — because that spike in heart rate could be fatal. One quick burst of speed chasing a squirrel could bring on an embolic complication. There are documented cases of heartworm-positive dogs dying mid-stride.

The three-injection melarsomine protocol is 98% effective at killing adult worms. But the road there is not gentle, and some dogs sustain permanent cardiovascular damage even after successful treatment.

And the cost. Treatment runs anywhere from $1,000 to $3,000 or more depending on disease severity, geographic location, and what complications develop. For families already stretched thin, that number makes every decision harder.

I'm watching this play out right now in shelters across the country. Dogs are coming in heartworm-positive — not uncommon, and not surprising given the numbers — but the positive result complicates everything. It extends their length of stay. It adds to the shelter's medical costs. It adds a layer to the adoption conversation that some families aren't prepared for or able to take on. These are not permanently broken dogs. Many of them are young. Many of them are sweet as hell. They need treatment, they need time, and they need someone willing to see past a line on a test result. If you are that person, shelters need you badly right now.

Intestinal Parasites: Roundworms, Hookworms, Whipworms

These don't get the dramatic press of the external parasites, but they're extremely common and some carry zoonotic implications — meaning your kids and your dog are sharing more than the couch.

Roundworm eggs can survive in soil for years. Dogs pick them up through contaminated environments, and puppies are frequently born with them or acquire them through nursing. Roundworms can cause serious illness in children, particularly when eggs are ingested from contaminated soil or surfaces.

Annual fecal testing is the baseline standard of care. It's not glamorous. It's also one of the most useful things a routine vet visit actually does.

The "Cold Kills the Bugs" Myth Needs to Retire

Northern winters used to pull more weight. The thinking went: cold kills mosquitoes. No mosquitoes? No heartworm. Save a few bucks and take a break from the pills. It was never completely sound, but it was at least directionally reasonable for a while.

It's not anymore.

Winter seasons are becoming shorter and warmer, which means mosquitoes remain active longer. More than 80% of the U.S. population lives in urban areas, where temperatures can run 10–20 degrees warmer than surrounding rural areas. Heat islands — the trapped warmth in buildings, pavement, parking lots — create pockets where mosquitoes continue to breed and survive well past the first frost. Some mosquito species can overwinter by hibernation in warmer areas like damp basements and sewers, emerging as potentially infective adults. In central Ohio, a mosquito coming out of a basement on a mild February afternoon is not a hypothetical.

Here's the thing that makes the "seasonal" logic especially risky: heartworm preventatives don't work forward — they work backward. Each dose eliminates larvae acquired in the previous month. Missing even one dose allows larvae to mature past the stage that preventatives can target. So if you stop the preventative in October and your dog gets bitten by an infected mosquito on a warm November afternoon — a perfectly plausible scenario — January's dose doesn't catch it. You've got a window. And windows are how worms get in.

Heartworm disease has been diagnosed in pets across all 50 states. Both the American Heartworm Society and the FDA recommend that all dogs in the U.S. be on heartworm preventatives year-round. Not most of the year. Not most states. All of them, year-round. The "it's fine to stop in winter" era is over. We just haven't fully updated the cultural assumption to match.

On the Subject of Cheap Ivermectin from the Farm Store

Here's the appeal: ivermectin is the active ingredient in Heartgard, one of the most widely prescribed heartworm preventatives in the country. It works. It's been working for decades. And if you go to a rural farm supply store, you can buy ivermectin paste or pour-on solution off the shelf at a fraction of the cost of a branded vet product. People do this. There are whole corners of the internet that tell you how.

Here's the problem, stated plainly: equine paste contains 18,700 mcg of ivermectin per gram — a concentration so high that a safe dose for a dog requires measuring a tiny fraction of a milliliter with laboratory-level precision. Dosing errors are common and can be fatal. The paste is calibrated for horses weighing hundreds of pounds. A single gram contains more ivermectin than a medium-sized dog's entire safe annual heartworm dose.

Dosage isn't even the whole problem. Up to 75% of some herding breeds carry the MDR1 gene mutation, which makes them far more sensitive to ivermectin. Dogs with the MDR1 mutation can be affected at doses as low as 0.1 milligrams per kilogram, compared to normal dogs at 2.5 milligrams per kilogram. That's a 25-fold difference in sensitivity. The neurological effects — ataxia, tremors, blindness, coma — can develop within hours. FDA-approved heartworm prevention products have been tested in dogs with the MDR1 mutation and found to be safe at label doses. A blob of horse paste eyeballed out of a tube has not been.

There's also a specific danger for dogs who already have heartworms. OTC ivermectin at prevention doses will not kill adult worms — that requires melarsomine. But giving a heartworm-positive dog ivermectin without proper staging and veterinary oversight can trigger a rapid die-off of microfilariae that causes a severe systemic reaction, and the increased embolism risk can be life-threatening.

The cost frustration behind the farm-store impulse is real and worth naming. Veterinary care is expensive, preventatives are expensive, and people are trying to protect their dogs without going broke. That's not irrational. But the answer to expensive preventative care is not a product designed for a 1,200-pound animal. It's advocating for better access to low-cost veterinary services — which is a whole other post.

The Throughline

Here's what all of these have in common: most of them are either preventable, treatable when caught early, or both. And most of the prevention strategies overlap. Year-round parasite protection — the kind that covers fleas, ticks, heartworm, and increasingly, screwworm — is not overkill. It's the minimum.

The screwworm's return is unnerving precisely because we thought we'd solved it. We did solve it, for sixty years, through sustained, coordinated effort. That's worth noting. It's also worth noting that it came back.

The bugs are always working. Your dog's protection probably shouldn't be seasonal.

If you think your dog may have been exposed to New World screwworm, contact your veterinarian immediately. This is a reportable disease. Suspected cases can also be reported to the USDA APHIS at 1-866-536-7593.