Heartworm in Dogs Flashcards
1
Q
Heartworm life cycle
A
- mosquito bites heartworm infected dog and aquires microfilaria or L1 larva
- heartworm larve migrate to mosquito salivary gland undergoing 2 molts to become infective L3 larva (10-20 days)
- mosquito bites dog and infects dog with L3 heartworm larva
- larva develop and migrate through the tissues to the lungs where the L5 larval stage penetrate the blood vessels (45-60 days)
- adult heartworms in the pulmonary artery mate and produce new baby heartworm larva L1
- another dog is bitten by an infected mosquite and cycle starts over
2
Q
pathophysiology of heartworms
A
- pulmonary vascular effects
- direct endothelial contact -> inflammatory mediators released -> Ag-Ab complexes -> dilation and blunting of pulmonary arteries
- pulmonary parenchymal effects
- local inflammatory response -> PMNs and Eos infiltrate parenchyma OR thromboembolism and embolism of dead worms
3
Q
diagnosis of heartworms
A
- blood tests
- microfilarial tests
- antigen tests
- antibody tests
- thoracic radiographs
- +/- echocardiography
4
Q
Heartworm antigen test
A
- screening test of choice in dog
- very sensitive and nearly 100% specific
- detect Ag shed by mature females, can’t detect male only or immature infestations
- usually a 4DX test (rickettsial diseases)
- can be used to montior adulticide tx
5
Q
when should heartworm antigen testing be done?
A
- before starting prophylaxis (<6 months old)
- no/inadequate prophylaxis
- antigen and microfilaria test prior to starting prevention
- adequate prophylaxis
- antigen testing every year
6
Q
heartworm microfilarial tests
A
- blood smear-insensitive
- Knott concentration- filter tests
- 75% sensitive with patent infestations
- not recommended asa sole screening test-only usually performed after a positive antigen test
7
Q
what are the advantages of testing for Microfilaremia?
A
- validates a + antigen test
- identifies the patient as a reservoir of infection
- alerts you to the possibilty of severe reaction when using a microfiliaricide, and may alter your choice of medications
8
Q
heartworm occult infestations
A
- worm infestation without microfilaremia
- prepatent- only L5
- single sex infestations (cats)
- immune destruction of microfilaria (cats)
- macrocyclic lactone administration post infestation
9
Q
What do these diagnostic results mean?
+ Ag, + microfilaria
+Ag, - microfilaria
puppy (<6mo) with microfilaria
A
patent HW infestation present
occult HW infesttion present
maternal transmission of L1, HW infestation not present
10
Q
diagnosis of heartworms
A
- clinical presentation
- thoracic radiographs
- echocardiography (dogs)
11
Q
clinical presentation of heartworms
A
- most dogs with heartworm disease are asymptomatic when diagnosed
- dry, hacking cough
- exercise intolerance
- lethargy
- caval syndrome
12
Q
caval syndrome
A
- “class 4”
- worms become entwined in tricuspid apparatus
- transient or sustained decrease in pulmonary BF allows worms to “fall” from the pulmonary artery to the right atrium
13
Q
complications of caval syndrome
A
- severe, acute tricuspid regurgitation
- volume-overloaded right heart resulting in right-sided CHF
- volume-underloaded left heart, decreased CO, and shock
- shear stress of RBC’s across worms
- intravascular hemolysis, hemoglobinemia, and hemoglobinuria
14
Q
clinical signs of caval syndrome
A
- sudden onset of severe lethargy
- course right sided systolic murmur
- dyspnea
- pale mucous membranes
- hemoglobinemia/hemoglobinuria
- jugular pulsations
- shock
- +/- ascites, liver enlargement
15
Q
treatment of caval syndrome
A
- supportive care
- surgical removal of worms
16
Q
diagnosis of heartworms
A
- thoracic radiographs
- echocardiography
17
Q
thoracic radiographs for diagnosis of HW
A
- can be normal
- pulmonary arteries (enlargement, tortuosity)
- caudal lobar arteries most often affected
- lung parenchyma (interstitial to alveolar patterns)
- right sided cardiomegaly
18
Q
echocardiography for diagnosis of HW
A
- not necessary for work-up
- not efficient method of dx, but can confirm caval syndrome
- RV dilation or hypertrophy
- tricuspid or pulmonic valve regurgitation
- evidence of pulmonary hypertension
- visualization of worms confirms dx
19
Q
classification scheme for HW disease
A
- class 1, 2, 3
- 1 mild, 3 severe
20
Q
pretreatment evaluation for HW
A
- thorough history (clinical signs/energy level)
- confirmatory heartworm test
- CBC, chemistry, UA
- ideal but not always necessary prior to tx
- thoracic radiographs
- severe changes: risk of post-adulticide complications
21
Q
Melarsomine (Immiticide)
A
- only adulticide approved by FDA
- deep IM injection
- “fast kill” vs “slow kill”
22
Q
“fast kill” Immiticide
A
- previously recommended for class 1-2 dogs
- 2 injections 24 hours apart
- no longer recommended (only special cases)
- kills 90% of adult worms
23
Q
“slow kill” Immitide
A
- split dose protocol
- now recommended for all HW positive dogs
- 1 injection
- 1 month later, 2 injections 24 hours apart
- kills 98% of adult worms
24
Q
side effects of Immiticide
A
- pain inflammation at injection site
- NSAIDs (not with Pred!)
- allergic reaction
- all dogs should be pre-treated with Benadryl
- pulmonary inflammation/edema
- mild cough, lethargy, fever
- pulmonary thromboembolism
25
Medications prior to adulticide therapy
* doxycycline
* macrocyclic lactones
* +/- corticosteroids (if clinical signs warrant)
* +/- antihistamines (if giving MLs and MF+)
* aspirin (not indicated)
26
doxycycline for HW
* treatment for Wolbachia, an obligate, intracellular symbiotic bacteria harbored by D. Immitis
* reduces Wolbachia in all stages of heartworms
* supresses microfilaremia
* may improve pulmonary and renal inflammation
* should be give before adulticide
27
macrocyclic lactones
* controversial- potential resistance
* give with doxy to further decrease MF
* microfilariacide
* treatment for 2-3 mos prior to adulticide
* eliminates susceptibility gap created becomes of Mealrsomine's unconfirmed efficacy against HW stages \<4 mos
28
post adulticide complications
* **pulmonary thromboembolism**
* worm fragments from dead worms lodge in distal pulmonary arterioles and capillary beds, blocking BF
* clinical signs can range from asymptomatic to sudden death, but include cough, dyspnea, lethargy, pale mm, and collapse
* risk increases with exercise, presence of radiographic changes or CS prior to therapy, concurrent pulmonary disease
29
30
treatment of PTE/pneumonitis
* cage rest
* steroid therapy
* supportive care +/- hospitalization
* O2 therapy
31
Surgical extraction of HWs
* most often indicated in **life threatening caval syndrome**
* remove adults via jugular vein
* blindly, fluoroscopic, or echo guidance
* pros: improved survival, quick recovery, low complication risk
* cons: anesthetic risk, expensive for clients, still need adulticide
32
alternative HW therapies
* long term macrocyclic lactone administration
* macrocyclic lactone/doxycycline
33
long term macrocyclic lactone administration for HW
* "super slow kill"
* not recommended
* will shorten life span of adult worms, but may take ~2 years
* strict rest still recommended for entire tx
* continued pathology during tx course
* debate about resistance
34
macrocyclic/lactone doxycycline
* reserved for cases when Melarsamine therapy is impossive
* more rapid adulticidal therapy than MLs alone
* decreases reproduction/sterilizes worms, so combo prevents microfilaria and transmission to other dogs
* antigen test every 6 months
* exercise restriction throughout the entire course of tx
35
HW prevention
* start no later than 8 weeks of age
* test first if starting after 6 months or a dog that has missed prevention
* if negative, repeat in 6 months
* current available HW preventatives are **macrocyclic lactones**
36
macrocyclic lactones
* excellent therapeutic/toxic ratios
* affect microfilaria, L3, L4, and young adult HWs (with continuous use)
* nearly 100% effective when given according to label
* **Ivermectin** **(Heartgard), Milbemycin Oxime (Sentinel, Trifexis), Moxidectin (Pro Heart, Advantage Multi), Selamectin (Revolution)**
