I will be the first to say I have a few Vets. One of them is a ‘Farm’ Vet and the other is a specialist in surgery. Both have their amazing talents, and I value them both. I have a great relationship with them. I have had some Vets in the past that I felt were more interested in writing prescriptions than they were in the health of my pets. In this article I am not implying that giving your dog nothing to prevent heartworms is ok. This short article is an attempt to showcase some research that is not well known by pet owners in an attempt top educate them on the heart worm process. Now my disclaimer…..I am not a Vet, nor do I have any plans to become a Vet. 😉
Most of us have had the experience of going to our Vet and leaving with ‘Heartguard’ after getting a lecture from our Vet on the dangers of Heartworms. We forgot about the cost, since we are determined to be the best doggie owners and will pay the price no matter what. Years ago, I would ask the same question to myself “Should I Give My Cane Corso Heartguard?”. Well the answer for me after extensive research was clear. Yes, and No. Yes to Heartguard and no to year round treatment.
The Fact is ..Veterinarians and pharmaceutical companies are working together in a marketing campaign to scare pet owners into giving year-round heartworm preventatives to dogs. They tell us to do give this medication to improve protection for dogs and cats, however, the facts point to a different story.
Unless you live in the warmest parts of the U.S., Heartworms are a seasonal issue. In my opinion, there seems to be little evidence to give heartworm medicine to pets year-round.
Let’s take a look at the heartworm infection process. You will see that getting infected is not as easy as we are led to believe:
heartworm is called Dirofilaria immitis. There must be Several steps completed in order for your to give dog to acquire a dangerous heartworm infestation:
Step 1: To infect your dog, you need mosquitoes. More specifically, you need a hungry female mosquito of an appropriate species. Female mosquitoes act as incubators for premature baby heartworms (called microfilariae). Without the proper mosquito, dogs can’t get heartworms.
Step 2: Our hungry mosquito needs access to a dog already infected with sexually mature male and female heartworms that have produced babies.
Step 3: The heartworm babies must be at the 1st stage of development when the mosquito bites the dog.
Step 4: 10 – 14 days later — if the temperature is right –the microfilariae mature inside the mosquito to the infective 3rd stage then migrate to the mosquito’s mouth.
Step 5: Mosquito transmits the 3rd stage larvae to your dog’s skin with a bite. Then, if all conditions are right, the 3rd stage larvae develop in the skin for three to four months (to 5th stage larvae) before making their way into your dog’s blood.
Step 6: Only if the dog’s immune system doesn’t kill these worms do the heartworms develop to adulthood.
Conclusion- It takes almost six months for the surviving larvae to achieve maturity. At this point, the adult heartworms may produce babies if there are both males and females, but the kiddies will die unless a mosquito carrying L3′s intervenes. Otherwise, the adults will live several years then die.
In summation, a particular species of mosquito must bite a dog infected with circulating L1 heartworm babies, must carry the babies to stage L3 and then must bite your dog . The adult worms and babies will eventually die off in the dog unless your dog is bitten again! Oh, and one more thing.
Heartworms are transmitted by mosquitoes. Heartworm larvae, called microfilaria, live in the blood and are sucked up by the bug. Once inside the mosquito, they must further develop before they can infect another dog. For that to occur, outside temperatures must remain above 57 degrees, day and night, for a certain period of time. The warmer the temperature, the faster the larvae will mature. If the temperature drops below critical level, larval development will stop; but the larvae don’t die—development will re-start at the same point when the weather warms back up. Larvae reach their infective stage in 8 to 30 days (the latter being the entire lifespan of the average mosquito).
In many areas of the country (northern and mountain states, for instance), such warm temperatures simply don’t exist for most of the year, and sustained warm temperatures don’t occur until at least June. Only in Florida and south Texas is year-round heartworm transmission possible. Within 150 miles of the Gulf Coast, heartworm risk exists 9 months out of the year. In the rest of the country, heartworm transmission is possible between 3 and 7 months out of the year. Hawaii and Alaska have each had a few cases of canine heartworm, but the incidence in those states is very low.
It should be obvious that during seasons where there are no mosquitoes, there is no risk of heartworm. Evidently that does not stop people prescriptions from Vets for heartguard, even for puppies living high in the Colorado mountains. At that altitude, temperatures are never warm enough for heartworms!
There are some Vets who agree with treating for heartworm only when necessary, and example is Dr. Roman.
Dr Margo Roman is Founder of Integrative Health Pet Expo in Massachusetts, she begins medication six weeks after mosquitoe season begins. This allows 2 weeks for the microfilariae (baby heartworms) to mature inside a mosquito to the infective stage and be transferred to a dog, plus 30 days additional days covered by the medication working backwards to kill those babies.
When Should You Stop Heartworm “Preventatives”?
Dr. Roman recommends stopping meds after the first frost for people living in an area with cold winters. In other areas, vets recommend stopping 30-45 days after weather is consistently below 57 F degrees and you see no mosquitoes. I feel Dr. Romans approach makes the most sense to me!
When an infected mosquito bites a dog or cat, the microfilaria are deposited on the skin, where they crawl into the bite wound and enter the bloodstream. Inside the body, they grow and progress through other larval forms. In dogs, the heartworm’s natural host, larvae migrate to the heart and eventually develop into adult worms, reproduce, fill the blood with microfilaria, and pass it on to the next mosquito.
Heartworm preventative drugs do not kill adult heartworms, but they do kill microfilaria up to a certain stage of development. Currently it is believed that larvae under 6 weeks old are affected. This means that in order to prevent heartworms from reaching adulthood, the preventative can be given up to 6 weeks after the mosquito bite and still work. The recommendation is to give the drugs every 30 days, purportedly because once-a-month dosing is easier for most people to remember (and, coincidentally, it also sells more drugs). Preventatives should be given starting 4-6 weeks after the earliest possible infection date and continue 4-6 weeks after the last possible infection date. In most states, protection should be continued through November or December. In southern Texas and Florida, year-round preventatives may be needed. Local conditions may vary from year to year.
We are often led to believe that Heartworm infection is rapid and will kill your dog overnight! This is far from the truth! It takes about three months for microfilaria (baby worms) to grow inside your dog to a larval stage, and even longer for these larva to mature into adult heartworms. If your dog is dosed with a simple Ivermectin treatment at any time during this period before adult worms are present (a period that lasts about three months long), the larvae will never develop into adult worms, and will die. Fact is that a single dose of Ivermectin will stop heartworm dead up to 3 months after your dog is first infected.
The most common preventative drugs for heartworm are ivermectin (Heargard®) and selamectin (Revolution®). While these drugs are generally safe and effective, there are always exceptions. Toxicity associated with ivermectin include depression, ataxia (balance problems), and blindness, but these are uncommon at the doses used in heartworm preventatives. Selamectin is also used to treat ear mites and some worms; adverse reactions include hair loss at the site of application, diarrhea, vomiting, muscle tremors, anorexia, lethargy, salivation, rapid breathing, and contact allergy.
Notes – The Companion Animal Parasite Council (CAPC) reported that mounting evidence suggests that preventatives may be susceptible to a very serious problem: resistance. This is similar to the situation with antibiotics, where massive and unnecessary over-use has caused many bacteria to develop resistance to one or more drugs, creating super-infections, and making many antibiotics useless. The CAPC report states: “There is a growing body of anecdotal reports and experimental evidence that currently available heartworm preventives (macrocyclic lactones) may not be completely efficacious in preventing heartworm infection in dogs. Reports of resistance for dogs in the region [south-central U.S.] have resulted in confusion about how best to prevent infection in veterinary patients.” If ivermectin and related drugs lose their effectiveness, that will be trouble indeed, since these drugs are also used in the treatment of heartworm infections.