Dirofilaria immitis (1)

Question 1

What is the common name?

Answer 1

Heartworm

Question 2

What are the final hosts?

Answer 2

Dogs

Cats

Many other mammals (wild cats, wild carnivores, ferrets)

Question 3

What are the intermediate hosts?

Answer 3

Mosquitoes

Question 4

Describe adults

Answer 4

Long and slender

20-30 cm

In distal parts of pulmonary arteries of caudal lung lobes
- Large number or nematodes - Also found in right heart and caudal
vena cava

Question 5

Describe the microfilaria

Answer 5

300 micro m long

Found in blood

Tapered anterior end, straight tail

Slow, non-progressive movement in fresh blood smears

Question 6

Explain the life cycle

Answer 6

L3 is infective

Adult females release Mff in bloodstream - circulate for up to 3 years

Female mosquitoes consume Mff during blood feeding

Mff develop in mosquito to infective larvae in salivary glands

Inoculated into dogs when mosquito feeds - can inoculate up to 12 at a
time

Larvae mature into adults in heart
- Infective L3 to L4 in days
- Takes 3-4 months to move through subcutaneous and thoracic
tissue (L4) - what makes it hard to diagnose
- Molts to L5 (immature adult) and infiltrates circulation until it ends
up in distal parts of caudal pulmonary arteries
- Stays there for 2-3 months until it becomes an adult that can
reproduce

Adults survive for several years

Question 7

What are the sites of infection?

Answer 7

Adults - distal parts of pulmonary arteries

When large numbers are present, right heart and caudal vena cava

Question 8

Describe the Pathogenesis

Answer 8

Prevalence can range from very high -> very low

 - Enable high prevalence
        - Environmental
        - Mosquito population
        - Dog population

Endothelium destroyed - Larvae knock the endothelium away
destroying fluid control mechanism

Edema/Thrombus formation

 - Sponge cake texture -> brownie texture of lungs
 - Results in cough

Increased resistance

Wolbachia spp.

Right side of heart works harder because damage to arteries causes thickening of walls, narrows lumen

Important to rest dog during treatment

Cough

Adult nematodes destroy endothelium of small pulmonary arteries, particularly caudal lung lobes

Predisposes to thrombus formation and increased vascular permeability results in intestinal and alveolar edema

Pulmonary arteries dilate proximal to narrowing

Release substances which inhibit vasodilation, cause bronchoconstriction

Substances leak into the interstitium of lungs

Cause inflammation, decreases oxygen transfer, lead to irreversible fibrosis in capillary beds

Intracellular bacteria contribute to Pathogenesis

“Postcaval/Vena caval Syndrome” - endemic areas

Question 9

Explain the involvement of Wolbachia spp.

Answer 9

Plays an important role

Doxycycline

Gram-negative bacteria that works well with heartworm

Plays important role in metabolic function and reproduction of heartworm

Necessary to get rid of it first

Question 10

Describe “Postcaval/Vena caval Syndrome”

Answer 10

First season dogs exposed to infected mosquitoes

Heavy burdens

Also in right atrium, even caudal vena cava

Results in incomplete closure of tricuspid valve

Increased resistance to flow in posterior vena cava

Question 11

What are the clinical signs?

Answer 11

6+ months usually asymptomatic

No consistent relationship between number of nematodes and severity of clinical signs

Severity determined by pulmonary blood flow

Small number of nematodes in regularly exercised dogs = more severe
pathology
than large
number of
nematodes
sedentary
Weight loss

Anorexia

Chronic non-productive cough

Labored respiration

Exercise intolerance

Question 12

How do you diagnose?

Answer 12

Must understand life cycle - long PPP, so HW will fool you

Occult infections - have heartworms but no Mff in bloodstream

Radiology - useful in characterizing severity of pulmonary lesions

Antigen test - most common/definitive diagnostic tool

 - Detects antigens released by female adult heartworms 
 - Highly specific
 - False positive rare
 - Sensitivity increased - best when 3 or more adult females
 - False negative can occur when only 1-2 females present

Eosinophilic pneumonitis

Echocardiography (EKG) - help determine severity

Mff tests

 - Used in conjunction with antigen test
 - Direct blood smear
 - Concentration tests

Question 13

How do you prevent?

Answer 13

Prevention is much safer and more economical than treatment

Start preventatives at 6-8 weeks of age

Test dogs before beginning preventative

Placing dogs on prophylaxis (kills L3 and early L4)
- Administered monthly - year round preferred
- Designed to kill any L3 inoculated in the animal and early L4s
- Limited to no efficacy against late L4s and L5s
- All prophylactics = MCLs
- If prophylactic has high efficacy against Mff and dog has many,
can result in shock

Mosquito control important

Preventatives
- DEC - Daily oral, and must make sure dog is Mff free
- Macrocyclic Lactones
- Ivermectin
- Milbemycin
- Selamectin
- Moxidectin
- Safe and easy
- May have additional activity against intestinal and external
parasite
- Larvae acquired the previous month are killed within a day of
dosing
- Problems occur if preventative missed - this accounts for
prevention “failures” in
practice

Question 14

How do you treat?

Answer 14

3 components of program:

 - Place animal on prophylaxis
 - Kill Mff to stop dog being source of infection
 - Adulticide therapy 

Improve clinical condition of the animal and to eliminate all life stages of the heartworms (microfilariae, larval stages, juveniles, and adults) with minimal post treatment complications

Doxycycline

Manage clinical signs

Decrease activity - CAGE REST!

Question 15

What is important about Dirofilaria immitis?

Answer 15

It is ZOONOTIC