The Case of the Hidden Heartworms
Bane is a 5 year old Doberman Pinscher that came to see Dr. Garcia last winter to discuss having a mass on his chest removed as it had started bleeding and was causing him discomfort. During Bane’s pre-surgical screenings, he tested positive for heartworm disease. Heartworm (Dirofilaria immitis) is, as its name implies, a parasitic worm that lives in the heart and surrounding blood vessels. If left untreated, heartworm disease can lead to serious medical problems including: heart failure, respiratory disease, and a life-threatening condition known as caval syndrome. Fortunately for Bane, his disease was identified before he was showing any symptoms which increases the chance of a complete recovery. Let’s discuss heartworm disease in a bit more detail, and then we’ll get back to Bane’s case.
Heartworm is transmitted from other infected canids (e.g. dogs, coyotes, foxes, etc…) via an intermediate host (mosquitoes). When a mosquito feeds on an infected animal, it ingests the immature larval form of the worm called microfilaria. The microfilaria goes through a maturation process inside the mosquito over about two weeks. Once this has occurred, the microfilaria is then passed to another host when the mosquito feeds again. Over the next two months in the new host, the larvae then move from the tissue to the blood stream, eventually developing into an adult worm that takes up residence in the heart or large vessels near the heart. The mature worms live there happily, breeding, and creating new microfilaria (babies) thus, the cycle continues.
Most dogs do not show signs of being infected with heartworms until very late in the disease. Adult heartworms can live for several years inside their preferred host (dogs) during which time, they cause increasing inflammation and damage to the blood vessels and chambers of the heart. In addition to this damage, large numbers of worms can cause mechanical blockage preventing blood flow which, when severe, leads to caval syndrome and ultimately, death. It is precisely because these infections go unnoticed for so long that it vital to keep our dogs on monthly preventives and perform annual heartworm testing. The sooner the infection is caught, the better the outcome!
Now, let’s get back to Bane’s case. During his standard pre-surgical screening, Dr. Garcia ordered a Heartworm Antigen 4DX test. This test looks for little pieces of the adult worms (antigen) which are given off by pregnant female worms and was positive in Bane’s case. It also screens for the most common tick borne diseases which, fortunately, Bane tested negative for. When a dog tests positive for heartworm on the 4DX test, a confirmatory test is typically ordered to ensure that infection is truly present. This test can be looking for antigen a different way (sending to the laboratory for a special test called an ELISA) and/or looking for the immature worms (microfilaria). In this case, Dr. Garcia ordered some additional testing including chest x-rays to make sure that Bane’s heart was safe to undergo anesthesia. The rest of Bane’s pre-surgical screenings came back excellent so Dr. Garcia decided it was safe to proceed with his mass removal. The surgery went well and a plan to treat his heartworm disease once he had healed from surgery was developed.
Treatment for heartworm disease involves several steps which we followed in Bane’s case:
First, it is important to start the dog on a monthly heartworm preventive that is Ivermectin based. This helps kill the immature worms before they can be transmitted and prevents the dog from being infected with more worms while treatment is undertaken. At the time of diagnosis, the patient is also started on an antibiotic (Doxycycline) to kill a species of bacteria called Wolbachia which helps the heartworm survive and reproduce. In some cases, the patient is also started on a course of steroids to help reduce inflammation caused by the worms which can worsen as the worms start to die.
Two months later, the patient is ready to start a potent anti-parasitic drug (Melarsomine) administered to start killing the adult heartworms. This medication is administered by a veterinarian deep into the muscle of the dog’s lower back. The treatment consists of a total of three injections. The first injection is given and the patient monitored closely with strict exercise restriction for one month. It is very important that patients undergoing heartworm treatment be prevented from engaging in vigorous activity or becoming overly excited. If a pet is too active, dying heartworms can dislodge and cause a blockage in the heart or arteries which is life threatening! If all goes well after the first injection, the patient then receives two injections 24 hours apart a month after the first injection was given. Bane completed this series, staying with us several times during the process and he quickly became a staff favorite!
Heartworm disease can cause irreversible changes to the heart and lungs of the patient even after treatment which is why year round heartworm prevention is so important. Fortunately for Bane, his disease was caught early and we were able to treat it before it became too serious. We’re happy to report that Bane was back in for a follow-up with Dr. Garcia and recently tested negative for heartworms!