Introduction

• Cause: infection with nematode parasite, Dirofilaria immitis Dirofilaria immitis.
• Signs: very variable ranging from vague malaise to chronic respiratory disease and death.
• Diagnosis: frequently requires application of multiple diagnostic tests, some of which may need to be repeated.
• Treatment: in absence of clinical signs it may be prudent to allow time for spontaneous cure; otherwise, supportive treatment and/or surgical extraction is preferable to destroying them.
• Prevention: ivermectin, milbemycin oxime, moxidectin or selamectin. Disease can be easily prevented.
• Prognosis: if adult worms present, risk of sudden death exists.  21% died as a result of heartworm disease within 4 years of diagnosis.

Pathogenesis

Etiology

• Dirofilaria immitis, large nematode parasite located in pulmonary arteries.

Predisposing factors

General

• True prevalence of heartworm infection in cats is probably understated due to diagnostic limitations and tendency of cats to exhibit only transient clinical signs or die without confirmation of infection.
• Infection common where parasite is endemic and if patients are not receiving chemoprophylactics.
• Much lower prevalence of adult worm infections than in the dog population - approximated at 5-15% of the rate in unprotected dogs in any given area. However tissue phase infections and subsequent short lived juvenile worm vascular infection rates are felt to occur at the same rate as dogs.

Pathophysiology

• Clinical importance of heartworm infection amplified in cats as even as small number of heartworms are potentially life threatening.
• If present, clinical signs typically develop during 2 stages of the disease:
  • Arrival of heartworms in the pulmonary vasculature.
  • Death of adult heartworms.

Stage 1- Arrival of heartworms in the pulmonary vasculature

• Coincides with arrival of immature adult worms in the pulmonary arteries and arterioles - 3-4 months post-infection.
• Early signs due to acute vascular and parenchymal inflammatory response to newly arriving worms and subsequent death of most of these worms.
• Often misdiagnosed as asthma or allergic bronchitis Allergic bronchitis.
• Part of syndrome known as Heartworm-associated respiratory disease (HARD).
• Signs diminish as worms mature.
• Most notable microscopic lesion is occlusive medial hypertrophy of small pulmonary arterioles.
• Once infection is established evidence suggests that live heartworms are able to suppress immune function, allowing cats to tolerate infection without apparent illness until worms begin to die.

Stage 2 - Death of adult heartworms

• Initiated by death of adult worms.
• Degenerating parasites result in pulmonary inflammation and thromboembolism.
• Often leads to fatal acute lung injury.
• Reactions can occur even in single-worm infections upon death of that worm.
• Caval syndrome (common dogs) occurs rarely in cats due to light worm burden.
• Arterial intimal proliferation resembles characteristic arteritis found in dogs; located in major lobar and peripheral pulmonary arteries but are localized and ordinarily fail to cause sufficient obstruction to produce clinically significant pulmonary hypertension Pulmonary arterial hypertension (PHT) due to small worm burden.
• Right ventricular hypertrophy and right heart failure less common in cats than in dogs.

Timecourse

• Disease has insidious onset - from months to years after infection.
• Adult worms survive for 2-4 years in cats, compared to longer survival in the definitive host (dog).

Epidemiology

• Transmitted by many spp of mosquitoes but mainly members of the family, Culicidae.
• Require climate to support viable mosquito population and sustain sufficient heat to allow maturation of ingested microfilariae into infective L3.
• Transmission decreases in winter months but presence of microenvironments suggests risk of heartworm transmission never reaches zero.
• Introduction of microfilaremic dogs or wild canids are important factors contributing to further dissemination of the parasite.

Diagnosis

Treatment

Prevention

Outcomes