An Ounce of Prevention

Parasite prevention medications, along with vaccines, have extended the lifespan of our companion animals. A hundred years ago, cats and dogs were primarily outside animals- because who would want to sleep in bed with someone who is flea bitten and tick infested that poops wriggling worms?

This millennium, thankfully, we have safe and effective medications that prevent infections with intestinal parasites that can infect humans, kill disease carrying ticks and itch inducing fleas, and prevent fatal heartworm disease.



The first time I saw the consequences of heartworm disease was during my emergency internship. I walked into the hospital ward to find the walls covered in blood and my fellow intern looking pale and shaken.

One of our patients had been admitted for having trouble breathing secondary to heartworm disease, and during the night the inflammation had eroded into large blood vessels and the poor dog had coughed out most of his blood volume before dying. Horrifying- especially since this was preventable. 

Heartworm is transmitted by the bite from mosquitoes that are infected with heartworm larva (Dirofilaria immitis). The larvae mature under the skin of the dog, then migrate through the tissues of the dog into the circulation and end up in the blood vessels of the lungs and heart.

The incidence has been increasing in Massachusetts as in the rest of the country, likely due to warmer weather, a huge number of bats dying from White-nose syndrome, and with the influx of disease from dogs brought up from the southern US.

Yearly blood testing for this disease is recommended. It takes up to six months after infection for the heartworm test to be positive, as the test is for the antigen (protein) found in the adult female heartworm. Infection is prevented by using a year-round, monthly heartworm prevention medication such as Sentinel.

Heartworm infection in dogs is fatal over time if left untreated. Death occurs by heart failure, bleeding into the lungs, worm embolism, or secondary kidney damage. Treatment is expensive and has significant risks; year round prevention is much safer for your dog and less expensive than treatment. Even more exciting- the same heartworm prevention medication also controls hookworm infection and removes infection with roundworms and whipworms. Major prevention gold.



Ticks in the Concord area are epic, as my son would say- last spring I saw a dog with well over 50 ticks embedded under the fur of his hind legs (he is a long dog, and clearly the ticks were unimpressed by the topical preventative applied to the front end). Ticks are spider-like parasitic insects that feed by biting their host and feeding on blood. They are attracted to the smell, warmth and vibration of an animal passing nearby. When they bite and suck blood, and especially when they detach after their blood meal, ticks will regurgitate part of their stomach contents into their host. This is how tick borne diseases- Lyme, anaplasma, ehrlichia, Rocky Mountain Spotted Fever, and babesia- are transmitted.

In our practice in West Concord, a high percentage of dogs test positive for Lyme and anaplasma. While these diseases will not transmit directly from the dog to you, you are in the same environment as your dog and precautions are advised. Monthly year-round tick prevention is recommended- the black legged (deer) tick, which transmits Lyme disease, quests for a blood meal year- round.

A hard frost does not kill ticks, and ticks continue to be active in certain environments, like the woods, and find warmer microenvironments in the leaf litter where they wait for a meal.

Our practice recommends the monthly oral product Nexgard, which works well in our patient population.



Fleas are thankfully not as common as ticks in Concord, but infestations with resistant fleas are becoming very common in the South. Fleas are small dark insects that look like tiny bugs quickly running through the fur. Fleas bite through the skin and drink blood, and cause itching which can be very intense in a flea allergic pet.

Flea dirt (which is the stool from fleas) looks like small black specks that turn red when moistened, since flea dirt is composed of digested blood. The adult fleas that are seen are only the tip of the iceberg- each adult female lays 50 eggs a day, which hatch into maggot-like larvae.

Once fleas are in the house, intensive frequent vacuuming and washing of bedding, furniture and carpeting are necessary to control the infestation. The adult fleas have to be prevented from feeding (which they need to do to lay eggs) by treating every pet in the house with a preventative. The eggs hatch over a period of three months, so even if the adult fleas are killed more adults will appear. Prevention is best in the form of monthly year-round medication. We recommend the monthly oral product Nexgard. I have never seen fleas in a dog on flea prevention.

We are lucky to be living in a time when parasites- hopping, squirming, biting- do not need to be accepted as the cost of living with pets. Heartworm/intestinal parasite prevention and flea/tick prevention should be part of your basic pet routine. The alternative is gross at best, and life threatening at worst.


Dr. Astrid Kruse graduated from Tufts University Cummings School of Veterinary Medicine in 2003. After completing a year long rotating small animal medicine and surgery internship, she joined the staff at a large general practice hospital on the North Shore, where she enjoyed forming long term connections with her patients and clients. She is happy to be a part of the team at Domino Veterinary Hospital in Concord, MA. Her special interests include dog and cat internal medicine, preventative health care, rabbit and guinea pig medicine, soft tissue surgery, and dentistry in all species.