Parasitology Lecture 13 (Heartworms)

Question 1

Discuss Dirofilaria immitis

Answer 1

• Adults live in the pulmonary artery:
  - dogs
  - cats
  - gferrets

    - biologically vectored by mosquitoes
    - Filarid nematode - does not produce eggs
             - produces live **microfilariae**
             -

Question 2

Discuss the dirofilaria immitis life cycle

Answer 2

• Adults in pulmonary artery mate and produce microfilaria (do not produce eggs)
• Microfilaria in blood are picked up by the mosquito when it feeds on dog
• Microfilaria develop in mosquito to the L3
• Mosquito bites dog and deposits the L3 on the skin,, which wiggle in via the bite wound. Molts to L4 in the skin i the first few days after the bite
• L4s migrate through tissues for ~2motnths
• L5s enter circulation and are carried up to pulmonary arteries
• Finish maturation to adult and mate in pulmonary arteries (takes about 4 mopnths to mature)
• Microfilaria appear in blood 6 months after infection of the dog

Question 3

Discuss the important points o fhte Dirofilaria Imitis life cycle

Answer 3

• **Microfilariae ** are infectious to the mosqiito
• L3s are infectious to the dog
• ## It takes about 6 months ffrom the tume of the L3 infection by mosquito until microfilaria appear in the blood

Question 4

Discuss the opathologic effects of Dirofilaria Immitis

Answer 4

_Severity depends on:
- number of worms
- health and immune status of the host
- duration of infection
- host activity level

Mechanical trauma nd damage from worm antigens and excretions:
- inflammation of the tunica interna
- arterial fibrosis
- thrombus fomration
- pulmonary embolism –> dead worms
- Pulmonary hypertension

Caval syndrome: occus when worms fill the right atrium and ventricle > tricupsid valve and insufficiecny + pulmonary hypertension
- characterized by: 1) acute severe lethargy + 2) hemoglobinemia + 3) hemoglobinuria –> fatal if not treated surgicallu wotjom 1-2 days

Clinical signs: exercise intolerance and lethargy
syncope –> fainitn from the lack of blood flow to the brain
soft cough
acute respiratory distress
sudden unexpected death

Question 5

Discuss the diagnosis of Dirofilaria Immitis

Answer 5

1) blood antigen tests –> SNAP tests –> detects a uterine protein –> adult female worms must be present
2) Microscopic exam for microfilaria –> direct blood smear or Knott’s test
3) imaging –> rads, EKG

Question 6

Besides dirfilaria, what is another nematode that may be present in the blood of dogs in the US?

Answer 6

Acanthocheilonema is a non pathogenic nematode that lives in the subcutis of dogs.

When diagnosing this nematode, Acanthocheiloema has 1) very few microfilaria and 2) their wiggles takes them places (progressive movement) which is different because Dirofilaria has 1) high numbers 2) Wiggly but dont go anywhere (nonporgressive)

Question 7

When considering Dirofilaria immitis, discuss Wolbachia

Answer 7

Heartworms harbor a symbiotic bacterium called wolbachia:
- Dirofilaria rely on WOlbachia to remain healthy (if you kill the bacteria, you incapacitate the worm)
- Release of wolbachia from dead worms causes inflammatiry repsonses, especially in the lungs and kidneys (missin note)
- Diropfilaria require wolbachia to develop into effective stage in mosquito

Question 8

Discuss the treatment of Dilofilaria immitis

Answer 8

• In all cases of heartworm, strict exercise restriction must be enforced
• Most severe: Caval syndrome worms in the right heart require prompt surgical remocal
• Mild to severe clinical signs: stabilization first as needed: corticocsteroids, fluids, diurietics, vasodilators, positive ionotropic agents

Macrocyclic lactones –> moxidectin and others
- Kills microfilaria and immature L3 and L4 worms

Doxycycline:
- Kills wolbachia, potentiates the effects of the other drugs, prevents secondary inflammation

Melarsomine –> arsenical
- kills adults and late L5 stages

Question 9

Discuss specific treatment and life cycle fo Dirfilaria immitis

Answer 9

Question 10

Discuss Dirofilaria Immitis in terms of prevention

Answer 10

Macrocyclic lactones - ivermectin, selamectin, moxidectin, milbemycin, eprinomectin
- oral and topical are monthly
- Parenteral mexodectin can be given eevery 6 months or 12 months

Ectoaparasiticides –> mosquito reppelant
- pyrethrins
- neonicitinoids –> imadocloprid, dinotefuran
- insect growth regulators –> lufenuron, pyriproxyphen

Question 11

Disccus Dirofilaria immitis in relatioin to cats

Answer 11

Heartworm disease in cats is refrred to as: Haertworm associated respiratory disease (HARD)
- pathologic effects are in the lungs: cough, dyspnea, wheezing:
- due to:
- 1) aberrant migration of immature worms in lung
- 2) death of immature worms with resultant dead worm emboli

Diagnosis is difficult due to very low worm burdens and the propensity for clinical signs to be due to immature worms

Treatment: Only recommended if the cat is clinically affected (has respiratory disease and heartworm infection)
1) Melarsomine is contraindicated in cats
2) Glucocirticoids only remmeber that disease in cats is due to the immune response to migrating larva and dead worms