Dirofilaria immitis

Question 1

What is the vector for D. immitis?

Answer 1

Females of various Mosquito species:
Aedes, Ochlerotatus, Anopheles, Culex, Psorophora

Question 2

What superfamily does D. immitis belong to?

Answer 2

Filarioidea

Question 3

Explain the life-cycle of the worm inside the mosquito and the dog

Answer 3

Mosquito bites infected animal with circulating micro- filariae (L1) –> ingests microfilariae –> microfilariae develop within the mouth of the mosquito –> larvae molt in the mosquito twice (L1-L2, L2-L3) over 2-weeks –> infective –> feeding mosquito infects host by penetrating skin/MM + transmitting L3 larval stage –> molt + migrate to L4 in host tissues within 12d –> final molt to L5 (immature adult) occurs 2-3 months after host infection –> enter vascular system –> right heart –> PA –> mature –> Microfilariae (L1) are released from the mature female worms into the host’s circulatory system 6-9 months after tinfection –> provides source of infection for biting mosquitoes to continue the D. immitis life cycle.

The D. immitis life cycle in cats is shorter, which is attributed to differences in host-species immune response

Question 4

What avarage daily temperature is required for Larva development?

Answer 4

14°C

Question 5

How long can mature worms live within the host?

Answer 5

5-7y

Question 6

When does damage within the host begin after infection?

Answer 6

2-3m after infection + before antigent test would detect infection

Question 7

Where do the primary lesions of D. immitis occur and what is the pathologic change observed?

Answer 7

PA + lungs

–> local damage to the intimal surface with intimal thickening and proliferation

Question 8

How do pulmonary vascular lesions develop in dogs infected with D. immitis?

Answer 8

1. worm-induced damage to the vascular endothelium 2. activation of the immune system –> inflammation, platelet activation, thrombus formation
2. release of vasoactive mediators: endothelin-1, platelet-derived growth factor –> vasoconstriction+
3. increased vascular permeability –> pulmonary edema

Question 9

What causes PAH in dogs infected with D. immitis? What is a sequelae of PAH?

Answer 9

• damage to the pulmonary arteries
• reduced pulmonary arterial blood flow
• pulmonary thromboembolism

–> right ventricular hypertrophy

Question 10

What are damages of the lung tissue observed in dogs infected with D. immitis?

Answer 10

• interstitial inflammation
• hemorrhage
• hemosiderosis
• fibrosis

Question 11

What distal organ may be damaged secondary to immune complex formation?

Answer 11

Kidney - glomerulonephritis

Question 12

Why is heartworm often mistaken for asthma in cats?

Answer 12

immature and mature worms (L3-L5) induce pulmonary and vascular inflammation early in the disease process (3-6 months), resulting in respiratory disease

Question 13

What is the prevalence of heartworm in the US?

Answer 13

Question 14

What other presentations apart from the heart are reported?

Answer 14

brain
spinal cord
eyes

Question 15

What are clinical signs reported with heartworm infection in dogs?

Answer 15

• respiratory: cough, increased RR + effort, dyspnea, hemoptysis, excercise intolerance
• hemoglobinuria (RBC lysis from intracardiac worms)

Question 16

What is caval syndrome?

Answer 16

worms migrate backward into the right heart and vena cava

–> Mechanical obstruction of blood flow through the right heart
–> Severe tricuspid valve dysfunction
–> Hemolysis (destruction of RBCs due to turbulence)
–> Reduced CO + venous congestion

Question 17

How can D. immitis be diagnosed?

Answer 17

Microfilariae: modified Knott test
Adult heartworms: detection of circulating antigens that arise from the uterus of the adult female worm –> ELISA or immunochromatographic techniques
–> highly specific and sensitive
–> echocardiography
–> thoracic radiographs

Question 18

What are reasons for a false negative test for adult heartworms?

Answer 18

all male infection
juvenile worms only
Antigen-antibody complex formation blocking antigent detection

Question 19

What are thoracic radiographic changes indicative of heartworm disease?

Answer 19

14% had normal thoracic radiographs

most common abnormalities:
* enlargement of the pulmonary trunk (DV/VD), RV, and right branch PA

Others: caudal vena cava enlargement, ascites

pulmonary patterns: bronchial, to interstitial secondary to inflammation and fibrosis
alveolar: PTE, hemorrhage, edema

Question 20

What is the recommended treatment for dogs with adult heartworm infection (adulticide treatment)?

Question 21

How can you treat Wolbachia?

Answer 21

Doxycycline 10mg/kg PO BID for 1month (can be reduced to 5mg/kg PO BID if adverse GI signs)

Alternative: Minocycline

Question 22

What is an inevitable consequence of adulticide therapy in heartworm disease?

Answer 22

Thromboembolism of worm fragements

Question 23

What are factors that are associated with a low risk of thromboembolic complications following adulticide therapy?

Answer 23

• no clinical signs+
• normal thoracic radiographs
• no heartworms seen on echo
• no concurrent diseases
• owner able to restrict exercise

Question 24

What are factors that are associated with a high risk of thromboembolic complications following adulticide therapy?

Answer 24

• clinical signs (cough, syncope
• ascites
• thoracic radiographic abnormalities consistent with heartworm disease
• heartworms observed with echo
• concurrent disease
• owner who is unable to restrict exercise

Question 25

Why are multiple melarsomine injections neccessary to treat heartworm disease?

Answer 25

A single injection of melarsomine can kill up to 90% of male worms but only 10% of female worms and it is not effective against microfilariae

Question 26

What are reported complications of melarsomine injection?

Answer 26

* local irritation, swelling or pain at the injection site * reluctance to move * prolonged presence of a firm nodule at the injection site * fever * hypersalivation * lethargy * cough or other heartworm infection-related respiratory signs * paresis and paralysis (if injection is inadvertently made in or near the spinal cord - rare) * embolization to the PA --> severe respiratory distress + hemoptysis

Question 27

What is the main essential treatment for prevention of embolization of PA after adulticide therapy?

Answer 27

Exercise restriction

Question 28

What other treatment options can be used apart from melarsomine to treat adult heartworms?

Answer 28

Drugs: ivermectin or moxidectin Surgical removal --> pretreatment with antihistamine (diphenhydramine 2 mg/kg IM), bronchodilator (albuterol sulfate inhaler) for airway signs related to pneumonitis and heparin 75-150 IU/kg SC q8h if concern for thromboembolic complications

Question 29

What pretreatment is recommended when using macrocyclic lactone preventatitves?

Answer 29

* corticosteroid (dexamethasone 0.2mg/kg IM) * antihistamine (diphenhydramine 2mg/kg IM)

Question 30

What is the prognosis for dogs with caval syndrom without heartworm removal?

Answer 30

guarded to poor patients already showing clinical signs can progress to developing hemolytic anemia, DIC, hepatorenal dysfunction + heart failure --> death within a few days

Question 31

What is the overall prognosis of heartworm disease?

Answer 31

Variable, depends on disease severity + clinical signs

Question 32

What can be used a a preventative for heartworm disease?

Answer 32

macrocyclic lactones: ivermectin, milbemycin, selamectin, moxidectin