Heartworm Disease Flashcards
1
Q
Describe the basic requirements for heartworm transmission
A
- Reservoir of infection
- Domestic dogs, feral dogs and wild canids
- Competent mosquito vector
- Favourable climatic conditions
- Minimum 57 degrees F (13.9 C) for development of L2 to infective L3 stage
2
Q
Briefly document the life cycle of the heartworm Dirofilaria immitis
A
- Lifecycle takes ~ 7-9 months (210-270 d)
- Female mosquito ingest microfilariae by feeding on infected host
- Ingested larvae transform into L1 within mosquito
- L1 molts twice over the next 2-4 weeks depending on the average temperature
- L3 are infective
- L3 deposited onto dogs skin in hemolymph and enter SC space through the mosquito bite wound
- L3 molt to L4 in the SC space
- L4 migrate through SC space and muscle towards the thorax
- By 50-70 days - final molt into juvenile worm
- Juvenile worms penetrate muscle and eventually the blood vessels
- Carried to the heart in the venous circulation
- By 70-120 days all juvenile worms have entered the pulmonary circulation
- Final maturation to adult worms occurs in the pulmonary arteries
- Microfilariae are produced by 180-270 days
- Adult worms can live up to 5-7 years
3
Q
Describe the primary pathogenesis of heartworm infection
A
4
Q
Describe the role of wolbachia in the pathogenesis of canine heartworm infection
A
5
Q
Define and describe caval syndrome
A
- Caval syndrome is a life threatening complication of obstruction to right heart blood flow due to the migration of adult heartworm into the right atrium. The onset is usually sudden / acute
- Retrograde movement of worms into the right atrium impedes the function of the tricuspid valve and causes direct obstruction to right atrial inflow from the large veins
- Haemolysis occurs due to increased obstruction and RBC sheer stress within the right heart
- Signs of forward failure or right sided congestive heart failure may be present
- Anaemia, haemoglobinemia and hemoglobinuria are common findings
6
Q
Describe the common clinical findings in dogs with heartworm infection
A
- Many are subclinical, especially in quiet placcid dogs
- Mild cough initially
- Exercise intolerance / lethargy
- Unthrifty appearance
- Abnormal lung sounds
- With worsening disease
- Pulmonary thromboembolism
- PH with right heart failure
- Abnormal heart sounds
- Caval syndrome
7
Q
List the testing options to investigate a suspected heartworm infection in a dog
A
- Thoracic radiographs
- Blood - CBC, biochemistry
- Urine - especially protein and bilirubin
- Microfilarial test
- Serology - antigen detection
8
Q
Describe the diagnostic tests for and possible findings in canine heartworm infection
A
- Radiographs:
- Changes characteristic of right sided heart disease and pulmonary hypertension
- Caudal lobar arteries may be enlarged, tortuous or truncated
- Varying degrees of pulmonary parenchymal disease
- Clinical Path
- Suggestive but not pathognomonic
- eosinophilia, neutrophila, non-regenerative anaemia, thrombocytopenia
- With caval syndrome - anaemia, hyperglobulinemia, hyperbilirubinemia
- +/- proteinuria
- Microfilarial detection
- direct exam of anti-coagulated blood with concentration technique - modified Knott’s or Micropore filter
- Only present in ~ 20% of infected dogs
- Serological testing - antigen detection
- POC or lab based well tests
- Enzyme immunoassay
- Detect glycoprotein secreted primarily by female worms
- Sensitivity of 95%, Sp 100% when 3 or more female worms are present
- In-clinic tests have varied sensitivity from 50-72% when compared to necropsy results
- POC or lab based well tests
9
Q
Briefly describe the serological tests available of heartworm detection
Note the situations when results may be inaccurate
A
- The serological tests for heartworm detect glycoprotein produced by the adult female worm/s
- Enzyme based immunoassay is the primary methodology
- Quantitative testing is available, but does not have a direct relationship to the number of adult worms
- Juvenile female and male worms will not be detected
- Ag not present until 5-7 months post-infection
- Low worm burdens (< 3 adult females) may produce false negative results
- PCR tests have been considered and developed - uncertain
10
Q
List the therapeutic recommendations for heartworm therapy
A
- As infected dogs may harbour L3, L4, juvenille and adult worms, treatment needs to be directed at each life stage
- Melarsomine
- Only FDA approved adulticide
- Macrocyclic lactones - avermectins and milbemycins
- Ivermectin, moxidectin etc
- Targets L3/L4 tissue stages and microfilariae
- Doxycycline
- Targets the Wlbachia endosymbiont
- Pretreatment with doxycycline reduces the pulmonary inflammatory reaction to dying/dead adult worms
- Prednisolone
- Helps to reduce the periascular inflammation associated with PTE
11
Q
Briefly describe the current recommended heartworm treatment protocol
A
- Treatment must be directed at all life stages
- Juvenile worms are not susceptible to the various treatments, so time is required to allow their development into adults
Day 0 - heartworm positive
- Treat severe clinical signs
- Macrocyclic lactone - pretreat with antihistamine and corticosteroid if microfilariae are present
- Prednisolone for 1 week
- Doxycycline for 4 weeks
Day 30 - Macrocyclic lactone
Day 60 - Macrocyclic lactone
Day 61 - Melarosomine and tapering prednisolone course
Day 90 - Macrocyclic lactone and Melarsomine
Day 91 - Macrocyclic lactone and Melarsomine
- Tapering prednisolone for 1 month
12
Q
Compare Dirofilaria Immitis infection aetiology in cats with that seen in dogs
A
- Mosquitos will feed equally on cats and dogs
- L3-L4 occurs similarly in cats soon after infection
- Most juvenile worms die 3-4 months post infection
- Small numbers of worms may mature to adults
- The adult heart worm life span is reduced in cats at 2-4 years
- Microsilariae are only produced in ~ 20% of mature worm infections in cats
- Microfilariae are removed by the cat’s immune system within 2 months and not produced again
13
Q
Describe the pathogenesis of heartworm disease in cats
A
- There is an acute vascular inflammatory reaction to the arrival of juvenile worms into the pulmonary circulation
- 42-75 days post infection
- Arteriolar muscular hypertrophy and inflammation
- This pulmonary inflammatory reaction does not require adult heartworms
- The primary disease in cats is referred to as HARD - heartworm associated respiratory disease
- Adult worms are able to down-regulate the activity of the pulmonary intravascular macrophage
- May see resolution of signs
- When the adults die, a more severe form of disease occurs
- Acute, severe inflammation to the degenerating worm together with thromboembolism can cause sudden death in 10-20% of cats
- Chronic respiratory disease represents the third stage of infection in cats that survive
- Arterial intimal proliferation is usually localised
- Generalised PH is uncommon - right heart failure is uncommon
14
Q
Describe the common clinical findings in cats with heartworm disease
A
- Coughing or dyspnoea - 64%
- Intermittent vomiting (unrelated to eating) - 38%
- 28% of infected cats remain asymptomatic
- Possibly due to the fact that exercise intolerance and lethargy are not recognised
- Ascites, hydrothorax, chylothorax, pneumothorax, ataxia, seizures and syncope have been reported but remain rare
15
Q
List and describe the diagnostic tests for cats with suspected dirofilaria immitis infection
A
- Thoracic radiographs
- Can be supportive/suggestive only
- Arterial enlargement strongly supportive
- Detects pulmonary parenchymal inflammation
- Echocardiography
- May see adult worms if present - 64% in one study of 22 cats
- Antigen test
- Positve result is highly specific, sensitivity of 50-86% when mature worms are present
- Negative results with male only infection or with low worm burden or when only juveniles are present
- Antibody test
- Detects Abs directed at late L4, juvenile and adult worms
- Abs confirm recent infection but do not confirm disease causation
