Dogs are natural carriers of heartworms, which means that their bodies offer the perfect conditions for heartworms to live comfortably and complete their life cycle successfully. The heartworm’s affinity for the canine host environment has led to incredible strides in heartworm prevention for pet owners in the form of monthly heartworm medication tablets. However, problems may arise and, in the event of the unexpected, pet owners should be as informed as possible about what heartworms are, how they are contracted, and their life cycle.
What are Heartworms?
Heartworms are small, thread-like parasitic roundworms that live in the heart and pulmonary arterial system of dogs and other animals. They reproduce rapidly and infect multiple parts of the heart and major arteries, causing serious damage that often results in the death of the host. In rare cases, heartworms migrate to unusual parts of the body such as the eyes or the brain, causing seizures, blindness, and cognitive dysfunction.
How Heartworms are Contracted
The main carrier of heartworms is mosquitos. Mosquitos are essential to the heartworm life cycle-- they incubate inside a mosquito host during their larval stages, where the temperature for this particular stage of growth is prime for rapid development. When external temperatures become warm in the springtime, heartworm development is prolific. Many mosquitos carry host-ready worms to new carriers in spring.
The action of transmission is cyclical. Mosquitos extract heartworms from infected hosts when feeding normally, then incubate them without being affected. Once the larvae mature, the mosquito passes the mature worm onto a new host when feeding.
Maturation Inside the Host Dog
Though heartworms enter their canine hosts in a near-adult stage, they require the nutrition provided by the dog’s muscle tissue and blood vessels to reach the final stages of growth.
• Maturation to the fourth level of growth occurs at the site of the mosquito bite. Growth typically takes 7-14 days.
• Budding heartworms migrate to the chest and stomach areas, where they wait 45 to 60 days before molting. They enter the fifth stage of growth and wait an additional 30-60 days before entering the bloodstream and moving to the pulmonary artery.
• After another 3-4 months, heartworms reach nearly (or exceeding) a foot long, and shortly thereafter begin mating. Young larvae, called microfilariae, are deposited in the gut to await maturation inside a mosquito host.
Image Source: www.heartwormsociety.org
Diagnosis of Heartworms in Dogs
There are currently 3 methods of testing whether your dog has contracted heartworms or not:
1. Microfilarial Detection - A direct blood smear test that attempts to identify microfilariae presence, a sign that heartworms present in the dog have successfully matured and mated. However, since the volume of larva does not correspond to the amount of heartworms present, the test cannot identify the severity of the infection.
2. Antigen Tests - A test that looks for the antigens released from the adult female heartworm’s reproductive tract. While relative accuracy is high, a number of problems still prevent 100% accuracy. Results will show negative for the first 5-8 months of infection, and will always produce a negative result in all-male infections.
3. X-rays - Only effective for a diagnosis of severity of infection. The x-rays reveal which parts of the body are affected by the infection so a prognosis can be developed.
Outward signs and symptoms of heartworm infection are largely nonexistent. Your dog’s behavior may change due to chronic pain in the affected areas of the body, but inflammation and blood blockage is undetectable from the outside. Shortly before the most severe symptoms, including death, your dog may faint and be unable to awaken. See a veterinarian immediately if this occurs.
Treatment and Prevention of Heartworms
A qualified veterinarian should always carry out heartworm treatment. Treatment comes with serious risks that may affect the heart, liver, and kidney, so your dog must be physically evaluated beforehand.
Late-stage infections can be treated with arsenic-based pet medicines that target and poison living heartworms inside the pet’s body. However, your dog must be able to process and filter the arsenic safely or the medication may lead to even more serious health issues. Following the death of the worms, dogs must rest for an extended period of time so the body can absorb what is left of the worms.
In advanced heartworm cases, worms can be removed surgically, but the risks of heart surgery are extremely high. Surgery is only indicated in the most dire cases of heartworms.
Preventive Heartworm Medication
Prevention of heartworm infection is typically administered by one of several heartworm medications commonly available at stores all over the United States. Most are anti-parasitic drugs that have indications for multiple parasites, so they can protect animals (and humans) from many different kinds of infections. These medications work by causing a temporary rise in the permeability of cell membranes which cause parasites to become paralyzed and die.
Whether you’re disciplined with your preventive medication or not, all pet owners should have their pets tested once a year for heartworms to prevent rare cases of infection that could lead to permanent damage or death.