Canine Heartworm Disease

What is known as heartworm?

Dirofilaria immitis

What are the natural definitive hosts of Dirofilaria immits?

Dogs, coyotes, wolves

What animals can serve as a definitive host but are not the natural definitive hosts for Dirofilaria immitis?

Cats, ferrets, foxes, seals, sea lions, etc.

What is the intermediate host for Dirofilaria immitis?

Mosquitos

What is the geographic distribution of Dirofilaria immitis?

Europe, Asia, Africa, North and South America

What is the one state that Dirofilaria immitis is not endemic in?

Alaska

Where is the highest incidence of Dirofilaria immitis in the U.S.?

South east

Mississippi and Ohio river valleys

California north central coast

Sierra Nevada Foothills

Where is the lowest incidence of Dirofilaria immitis in the the U.S.?

Rocky mountains

Pacific northwest

Upper plains states

Why is Dirofilaria immitis spreading?

Increased transportation of dogs from southern enzootic areas

Where are adult Dirofilaria immitis primarily found?

Pulmonary arteries

What is the secondary location of Dirofilaria immitis adults ?

Right heart

What kind of worm is Dirofilaria immitis?

Pulmonary-arterial worm

What is the lifecycle of Dirofilaria immitis?

Adults in pulmonary artery reproduce and release microfilariae

Microfilariae are deposited into circulation where a mosquito ingests them

Microfilariae develop into L1 in the mosquito, then into up to L3

When the mosquito feeds on an animal, the L3 are released onto the skin and migrate into the subcutaneous tissue

L3 develop into L4 in the subcutaneous tissue

L4 migrate to the thorax and molt into immature adults

Immature adults enter circulation and go to the pulmonary arteries to mature

What is the prepatent period for Dirofilaria immitis?

6-7 months

How long post-infection does it take until a heartworm test comes back positive?

6-7 months

How long can microfilariae survive in vascular system?

1-2 years

What is the infective stage of Dirofilaria immitis?

L3

When can Dirofilaria immitis be present in the right heart?

During heavy infections or in small breed dogs

How do L3 larvae infect a dog?

By entering into the wound made by a mosquito after it leaves

Where do L3 go after being deposited onto the skin?

Into the subcutaneous tissue

Where are L3 and L4 Dirofilaria immitis found?

In the subcutaneous tissue or muscle

What stages of Dirofilaria immitis are found in circulation?

Microfilariae

Immature adults

Mature adults

What is the diagnostic stage of Dirofilaria immitis?

Antigen produced by female adults

Microfilariae found in circulation

When do preventives work on Dirogilaria immitis?

Between the L3 stage to L4 stage

When do treatments work for Dirofilaria immitis?

During the adult stage to the microfilariae stage

What determines how rapidly clinical signs appear in a Dirofilaria immitis infection?

Number of worms

Duration of infection

Size of the animal

Exercise level of the animal

When is pathology caused by Dirofilaria immitis?

When adults are mature in the pulmonary artery

What kind of disease are adult worms?

Vascular disease

What is the pathology of adult Dirofilaria immitis?

Inflammation of vascular endothelium leads to roughening and thickening of vessels

Plasma leaks into lungs causing pulmonary edema

Pulmonary hypertension results in right ventricular hypertrophy and ultimately right sided heart failure

What is the main pathological disease caused by Dirofilaria immitis?

Progressive pulmonary hypertension

What happens when adult Dirofilaria immitis worms die?

More inflammation occurs

Thrombus formation around the worm

Vascular fibrosis around the worm

When radiographic changes are seen with a Dirofilaria immitis infection, what does that tell you?

It's late in the disease process

What radiographic changes can be seen in the late stages of heartworm disease?

Reverse D shaped heart due to right atrial enlargement

Thickening of the pulmonary arteries

What happens to the endothelium of the pulmonary arteries during a Dirofilaria immitis infection?

It becomes roughened and thickened

What can adult Dirofilaria immitis worms cause?

Obstruction of blood vessels, heart chambers, and valves

Type III hypersensitivity resulting in glomerulonephritis

What type of hypersensitivity is seen in a Dirofilaria immitis infection?

Type III

What can the type III hypersensitivity reaction result in?

Glomerulonephritis

What is the life threatening condition seen in heartworm infections?

Caval syndrome

What is Caval syndrome?

Life-threatening condition when a large mass of worms becomes situated in the right ventricle, right atrium, and often the vena cava causing sudden death due to cardiac failure and hepatic insufficiency

What causes Caval syndrome?

Exposure to a large number of infected mosquitos over a short period of time depositing many L3

What can Caval syndrome result in?

Sudden death due to cardiac failure and hepatic insufficiency

What pathology is caused by microfilaria?

Obstruction of fine capillaries

Immune responses to microfilaria can result in immune complexes in the glomeruli and impairment of kidney function

What bacteria is associated with Dirofilaria immitis?

Wolbachia pipientis

What is Wolbachia pipientis?

Symbiotic, gram negative intracellular bactieria associated with heartworms

In what stages of Dirofilaria immitis is Wolbachia pipientis present?

In all life stages

When can Wolbachia pipientis be released?

During worm death

During microfilariae production

What happens when Wolbachia pipientis is released?

Bacteria itself and its endotoxins can exacerbate lung injury

How do Dirofilaria immitis get infected with Wolbachia spp.?

Maternal transfer

What is the disease cause by heartworm?

Progressive pulmonary hypertension

When does pathology from heartworms start?

Immediately once matured