States with year-round warm climates, such as Texas, provide excellent conditions for mosquitoes, and heartworms become a greater threat wherever mosquitoes thrive. These parasites live in your pet’s heart and lungs, but the larvae travel throughout the body and sometimes wind up in the eyes. April is Heartworm Awareness Month and the Envision More Veterinary Ophthalmology team is sharing information about heartworm disease, potential ocular effects, and how to protect your pet.

Heartworm risk for dogs and cats

Heartworms are transmitted by mosquitoes, which can easily enter homes through a tear in a screen or an open door. This means that all pets are at risk for heartworm infection, including those who live exclusively indoors. In fact, studies show that one in four infected cats never goes outside. Heartworm has been found in all 50 states, but is most common in Southern, coastal states, where mosquitoes are plentiful. Incidence in each county or state is unpredictable and can change from year to year based on weather patterns, mosquito activity, and infected pet transport.

Heartworm transmission in pets

Heartworm larvae circulate in infected pets or wildlife who are reservoirs, and mosquitoes can ingest the larvae when they bite. The larvae stay inside the mosquito for 10 to 14 days until they reach an infective stage, are deposited on the new host’s skin when the mosquito bites, and enter the host through the bite wound. The larvae then travel through tissues to reach the new host’s heart and lungs, where they mature into adults after six to seven months and begin reproducing.

Common heartworm signs in dogs and cats

Heartworm affects dogs and cats differently, although they share some similar signs. Dogs are a natural heartworm host and can hold 30 to 100 or more worms, but infected cats usually host only one to three. Despite the lower worm burden, cats can mount a significant immune response that causes asthma-like respiratory disease, and occasionally sudden death. 

Heartworm infections are often silent for several months or years, until the worms cause enough inflammation and damage for clinical signs to appear. Signs may include:

  • Coughing
  • Fatigue
  • Weight loss
  • Vomiting
  • Abnormal fluid accumulations
  • Heart failure
  • Collapse or death

Ocular heartworm involvement in pets

Eyes have an excellent blood supply, which means larvae from the bloodstream or those migrating through tissue can end up inside a pet’s eye. The larvae can penetrate into the conjunctiva, cornea, or the eye interior, causing inflammation and damage along the way. Ocular heartworm larvae can be seen on an eye examination, and must be removed surgically prior to treating the pet for systemic heartworm infection. These pets may also require medications to reduce inflammation and prevent secondary infections inside their eyes. 

Heartworm diagnosis and treatment for dogs and cats

Heartworm infections can be difficult to detect in their early stages, because heartworm blood tests can detect only adult female worm proteins (i.e., antigens). This delay is the reason why we recommend annual screening tests for all dogs, and repeating testing after six months for rescued or stray pets with unknown histories. Cats usually need combined antigen and antibody tests to detect heartworms, and may also need X-rays or other imaging tests.

Infected dogs can be treated with a specific drug protocol that requires hospitalization, close monitoring, and several months of strict cage rest. Treating large worm numbers can be risky, although most dogs recover from their infection. Cats cannot be safely treated for heartworms and can only be closely monitored and provided with supportive medical therapy until the worms die naturally in two to three years. Unfortunately, the first heartworm disease sign in some cats is collapse or sudden death. 

Heartworm prevention for dogs and cats

Heartworm infections are dangerous for pets and treatment is costly and risky, making heartworm prevention clearly the best option. Your primary veterinarian can prescribe a monthly heartworm prevention medication that can safely and effectively kill heartworm larvae before they mature into adults, and before they can make their way into your pet’s eyes. Multiple oral and topical options are available, so finding an easily administered formulation is no problem.

The risk of eye problems with heartworm disease is low, but the systemic effects can be deadly. Protect your pet with year-round, monthly, cost-effective heartworm preventives recommended by your primary veterinary team. If your heartworm-positive pet has ocular signs, contact the Envision More Veterinary Ophthalmology team to schedule a visit and consultation.