Heartworm in Dogs | Comprehensive Guide

Cause of Heartworm Disease:

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Heartworm disease is caused by the parasitic worm Dirofilaria immitis. This parasitic worm is transmitted through mosquito bites. These worms primarily reside in the heart and pulmonary arteries of dogs, leading to severe cardiovascular issues.

Note: More than 70 species of mosquitoes are reported to transmit Dirofilaria immitis. Among these Aedes, Anopheles, and Culex are the topmost genera.

Host Susceptibility:

Heartworm or Dirofilariasis is mainly a problem of dogs especially those that are 3-8 years of age and mostly live outdoors in endemic areas. Cats are less commonly affected.

Some reports suggested that patent infections are also reported in wild animals and they seem to be reservoirs of the pathogen responsible for canine heartworm. Among wild animals, wolves, coyotes, foxes, California gray seals, sea lions, and raccoons are on the top list.

Heartworm Lifecycle in Canines:

Canine Heartworm or Dirofilariasis lifecycle is complex and involves several stages in its pathogenesis. This disease primarily affects the cardiovascular system of dogs, leading to potentially severe health issues. Let’s have a look at the lifecycle of dirofilariasis.

Stage 1: Infestation and Maturation of Microfilariae

Adult heartworms residing in an infected dog’s heart release microscopic offspring called microfilariae into the bloodstream.
Mosquitoes feeding on an infected dog become carriers, ingesting these microfilariae along with the blood meal.
Inside the mosquito, microfilariae undergo a maturation process over the course of about 10 to 14 days, becoming infective larvae.

2: Transmission to a Host Dog

When an infected mosquito bites a susceptible dog, it injects these infective larvae into the dog’s bloodstream.
The larvae migrate through the dog’s tissues, with a preference for blood vessels and muscles, while undergoing further development.

3: Larval Migration and Molt

The larvae continue to migrate through the host dog’s tissues for approximately two to three months, creating inflammation and initiating an immune response.
During this migration phase, they undergo two molts and change into another form called L3, also known as the infective larval stage.

4: Reach the Heart and Pulmonary Arteries

Once the L3 larvae mature, they find their way to the heart and pulmonary arteries, where they grow into adult heartworms.
Adult heartworms can measure up to a foot in length and can live for several years within these critical cardiovascular structures.

5: Reproduction and Microfilariae Production

Adult male and female heartworms reproduce, with females releasing microfilariae into the bloodstream, completing the life cycle.
The microfilariae, being microscopic, circulate within the bloodstream, allowing mosquitoes to ingest them when they feed on the infected dog.

What is Wolbachia and its Role in Dirofilariasis?

Wolbachia is a type of bacteria that plays a significant role in the life cycle and pathogenesis of heartworm (Dirofilaria immitis) and other filarial parasites. Here’s a closer look at the role of Wolbachia in the context of heartworm disease:

1. Symbiotic Relationship:

Wolbachia bacteria have a symbiotic relationship with many filarial parasites, including heartworms. This means that the bacteria live inside the cells of these parasites and influence their development and survival. The interaction between Wolbachia and heartworms is considered obligatory, as the bacteria provide essential nutrients and metabolites to the worms, without which the worms might not complete their life cycle successfully.

2. Impact on Reproduction:

Wolbachia bacteria play a critical role in the reproduction of heartworms. Female heartworms infected with Wolbachia produce microfilariae (offspring) that carry the bacteria. The presence of Wolbachia within these microfilariae is believed to influence their ability to develop and be transmitted by mosquitoes. Wolbachia may also play a role in manipulating host immune responses to facilitate the survival of heartworms in their hosts.

3. Inflammatory Response:

Wolbachia can trigger an immune response in the host, leading to inflammation. This immune response is partially responsible for the symptoms associated with heartworm disease, as well as for some of the damage caused to the heart and blood vessels. The inflammation caused by Wolbachia can contribute to the severity of the disease.

4. Target for Treatment:

The role of Wolbachia in the life cycle of heartworms has been leveraged for treatment. Certain heartworm medications, known as macrocyclic lactones (e.g., ivermectin), not only target the heartworm larvae but also have an impact on Wolbachia. When these medications are administered, the Wolbachia bacteria within the heartworms are affected, leading to reduced viability and disruption of the heartworm life cycle. This makes the treatment more effective in eliminating the parasites.

Heartworm in Dogs Signs:

The clinical signs of heartworm infection in dogs are listed below:

Persistent Cough: Dogs with heartworm infection often develop a persistent cough, which can be dry or may produce mucus. This cough is a result of the worms’ presence in the lungs and airways. (See Kennel Cough)
Lethargy: Infected dogs may display increased fatigue and reduced energy levels due to the strain on their cardiovascular system caused by heartworms.
Exercise Intolerance: Heartworm disease can lead to reduced exercise tolerance in dogs. They may tire more quickly during physical activities and may show reluctance to engage in play or walks.
Rapid or Labored Breathing: Heartworm-infected dogs might experience rapid or labored breathing, especially after exertion or during rest. This is due to the compromised lung and heart function.
Weight Loss: Unexplained weight loss can be a sign of heartworm infection. The metabolic demands of the infection and the strain on the dog’s body can lead to a loss of body condition.
Bloated Belly: In advanced cases, heartworm infection can cause an enlarged abdomen or a bloated appearance. This can result from fluid accumulation or enlarged organs due to the presence of the worms.
Collapse: Dogs with severe heartworm disease might experience episodes of collapse, particularly during exercise. This can be a result of reduced oxygen supply to the body.
Cyanosis (Blueish Gums or Tongue): Cyanosis, characterized by blueish gums, tongue, or other mucous membranes, can occur due to reduced oxygen levels in the blood, a consequence of heart and lung compromise.
Coughing Up Blood: Severe cases of heartworm infection can lead to coughing up blood, known as hemoptysis. This is a serious sign that requires immediate veterinary attention.
Heart Murmurs: Veterinarians may detect abnormal heart sounds or murmurs during physical examinations of heartworm-infected dogs. These murmurs are a result of the strain on the heart.

dog is coughing beacuse of heartworm infection

Mosquito, the carrier of heartworm disease

It’s important to note that the severity and combination of clinical signs can vary depending on the stage of the infection, the number of worms present, and the individual dog’s immune response.

Note: Heartworms primarily affect the heart, but they can occasionally be detected in the stool. There are several other worms that are detected in the feces of dogs. If you are curious about what are these worms or if there is any zoonotic concern, then this masterpiece is for you.

Diagnosis of Heartworm in Dogs: From Detection to Confirmation

Detecting heartworm disease in dogs involves several methods to accurately identify the presence of the parasites. Let’s explore some of these methods:

1. Serology: Antigen Test

Think of this test like a detective’s clue. It looks for tiny pieces released by heartworms in a dog’s bloodstream. If these pieces are found, it means heartworms are present. It’s like finding a fingerprint that belongs to the heartworm.

2. Detection of Microfilariae: Direct Smear

Imagine taking a super close-up picture of the blood. In this test, we look for baby heartworms called microfilariae. They’re like the babies left behind by the big heartworms. If we see these babies, we know their parents are somewhere in the body.

3. Modified Knott Test

This is like using a special tool to gather clues. We mix the dog’s blood with a special solution, and then we filter out the baby heartworms. After that, we can look at them under a microscope and see if they’re there.

4. Millipore Filter Technique

Think of this like a sieve that catches heartworm evidence. We pass the dog’s blood through a super tiny filter, and any tiny heartworm pieces get caught. Then, we can see if the evidence is there.

For detail on Microfilaria, you can visit this

These methods help veterinarians figure out if a dog has heartworms. Remember, it’s important to catch heartworms early to keep our furry friends healthy and happy. If you suspect your dog might have heartworms, talk to a vet for the right tests and treatment.

Veterinarian performing a heartworm test on a dog

Dog being administered heartworm preventive medication

Treatment Options of Heartworm (Dirofilariasis) in Dogs:

When a dog is diagnosed with heartworm disease, the right treatment can help them recover. Veterinarians use specific medicines to get rid of the heartworms. Let’s explore the process:

1. Immiticide (Melarsomine) Treatment:

Think of Immiticide as a warrior against heartworms. It’s given as an injection, and the treatment usually has two stages:

Dose: The dose of Immiticide depends on the dog’s weight and the severity of the infection.
Frequency: The injections are usually given 24 hours apart for the first two injections. Then, the third injection is given after a month.

2. Doxycycline Treatment:

Doxycycline is like the helper that prepares the body for battle. It’s an antibiotic that’s given before the Immiticide treatment.

Dose: The dose of Doxycycline depends on the dog’s weight.
Frequency: It’s usually given once or twice a day for about 4 weeks before the Immiticide injections.

3. Heartworm Preventive Medication:

After the treatment, it’s crucial to prevent new heartworm infections. Heartworm preventive medicines are like shields to keep the heartworms away.

Dose: The dose depends on the specific preventive medication prescribed.
Frequency: These medicines are usually given monthly, as advised by the veterinarian, to keep the dog safe from new infections.

Remember, every dog is unique, so the treatment plan might vary based on factors like the dog’s health and the severity of the infection. It’s important to follow the veterinarian’s advice closely to ensure the best outcome for your furry friend’s heart health.

Note:

Thiacetarsamide sodium, given IV b.i.d. for 2 days; restricted activity (no running) for 3-6 wks

Diethylcarbamazine, given p.o. daily during the mosquito season and for 2 months after, recheck yearly – effective against molting larvae

Melarsomine dichloride 2 injections I.M. 24 hours apart

Ivermectin, given once a month, recheck at least yearly – effective against L4

Heartworm Prevention for Dogs:

Heartworm prevention for dogs is a crucial aspect of ensuring your furry friend’s well-being. Canine heartworm disease, caused by parasitic worms transmitted through mosquito bites, can lead to severe health issues if left untreated. To shield your beloved pet from this potential threat, it’s essential to employ effective preventative measures. Here are some key points on how to prevent heartworm in dogs:

1. Regular Use of Heartworm Medication:

Administering veterinarian-prescribed heartworm preventive medication is a primary defense against this parasitic menace. These medications come in various forms, such as chewable tablets, topical treatments, and injections.

2. Annual Heartworm Testing:

Regular heartworm testing, usually conducted during your dog’s annual veterinary check-up, helps catch any potential infections early on. This proactive approach enables timely intervention and prevents the disease from progressing to more advanced stages.

3. Minimizing Mosquito Exposure:

Reducing your dog’s exposure to mosquitoes plays a pivotal role in heartworm prevention. This can be achieved by keeping them indoors during peak mosquito activity times, using mosquito nets or screens, and eliminating standing water sources where mosquitoes breed.

Exploring Natural Heartworm Prevention: A Holistic Approach.

Natural Heartworm Prevention:

For those who prefer natural approaches, there are alternative methods that can complement traditional prevention. While natural prevention methods may not replace medication, they can offer additional protection:

Garlic: Small amounts of garlic in your dog’s diet have been suggested to repel mosquitoes due to their sulfur compounds.
Neem Oil: Neem oil can be diluted and applied to your dog’s fur to create a natural barrier against mosquitoes.
Herbal Repellents: Certain herbs like rosemary, lavender, and lemongrass have mosquito-repelling properties. They can be used in essential oil form (properly diluted) or as part of homemade sprays.
Healthy Diet and Exercise: Maintaining your dog’s overall health can also contribute to their immune system’s ability to fight off infections, including heartworm. Regular exercise and a balanced diet boost their resilience.

Incorporating these measures into your pet care routine demonstrates your commitment to safeguarding your dog’s health and happiness. Remember, consulting with your veterinarian is essential before making any changes to your dog’s preventive regimen to ensure its safety and well-being.

Conclusion:

In conclusion, understanding and addressing the threat of heartworm in dogs is not just a responsibility, but a demonstration of our unwavering commitment to our loyal companions. The potential consequences of heartworm disease are far too serious to overlook, from the discomfort it causes to the severe health risks it poses. Through consistent use of veterinarian-recommended preventive medications, regular heartworm testing, and smart mosquito control practices, we can substantially reduce the risk of our furry friends falling victim to this insidious parasite.

Whether opting for traditional preventive measures or exploring natural alternatives, the safety and well-being of our canine companions remain at the forefront. By arming ourselves with knowledge, engaging in proactive prevention, and consulting with veterinary experts, we can provide our beloved dogs with the protection they need to live long, happy, and healthy lives, free from the grip of heartworm disease.

References:

Nelson, R. W., & Couto, C. G. (Eds.). (2020). Small Animal Internal Medicine. Publisher.

Merck Veterinary Manual. (2020). Canine Heartworm Disease. Retrieved from there