Heartworms

Question 1

Dirofilaria Immitus (hosts)

Answer 1

Heartworm
Final Hosts: Dogs, wild canids (rarely cats, horses, ferrets, humans, etc.)
Intermediate Hosts: Mosquito; rarely flea
Site of Infection: Right ventricle, pulmonary artery, occasionally right atrium and caudal vena cava

Question 2

Dirofilaria Immitus (Morphology)

Answer 2

Morphology: Long, slender, cylindrical
Males have spiral tail
Females up to 30 cm; males up to 20 cm

Question 3

Microfilariae

Answer 3

Blood-borne L1 larvae

Question 4

Acanthocheilonema (Dipetalonema) reconditum

Answer 4

microfilariae that can be confused with Dirofilaria Immitus, need to learn to differentiate

Question 5

Difference between D. Immitus and A. Reconditum

Answer 5

D. immitus________A. reconditum
——————— ——————–

307-322 mcm\_\_\_\_\_\_269-283 mcm
Undulating\_\_\_\_\_\_\_\_Rapid, progressive
Straight tail\_\_\_\_\_\_\_\_Button-hook tail
Tapered anterior\_\_\_\_Blunt anterior
No cephalic hook\_\_\_\_Cephalic hook (40X)

Question 6

Heartworm Life Cycle

Answer 6

Mature worms in right ventricle 
microfilariae in bloodstream  ingested by IH during blood meal 
develops to L3 in 8-18 days 
mosquito feeds on another dog  L3 migrates through piercing wound 
to resting site in SQ, muscle, or fat where it remains for about 4 months, and molts 2X  venous circulation  right heart (85-120 days post-infection).

Prepatent period about 6 ½ months – therefore disease of adult dogs

Question 7

Diagnosis

Answer 7

Direct Smear
Capillary Tube Method
Modified Knotts Technique
Filtration Techniques
Di-Fil®
Lyse-Alive®
Serologic testing for occult heartworm disease

Question 8

Clinical Signs/Pathology

Answer 8

General, early signs
Anorexia; dyspnea; cough, especially with exercise
End-Stage HWD
Ascites, emaciation, hydrothorax, SQ edema, pulmonary edema, syncope, epistaxis, oliguria, hemoglobinuria, passive congestion of the intestines may lead to bloody diarrhea, collapse, death
Pathology
Right heart failure
Lung, liver, kidney involvement

Question 9

Vena Caval Syndrome

Answer 9

When large numbers of young adults move into venous circulation at one time
Death due to liver failure within a few days of clinical signs
More common in dogs less than 3 years old
Only viable treatment is surgical removal of worms obstructing the venae cavae

Question 10

Mosquito Control

Answer 10

Preventatives
Diethylcarbamazine citrate (DEC) – first one used – given once/day – effective only during larval molts – not currently in use
Ivermectin (Heartgard Plus™/generics now available) – given once/month – effective against larval stages.  Newer studies indicate that this medication will prevent most new infections in heartworm positive dogs, and that up to 95% of worms will be gone after dosing consistently for two years.  Observe for side effects for six hours after first dose.

Question 11

Heartworm Preventatives

Answer 11

Milbemycin oxime (Interceptor™, Sentinel™, Trifexis ™)
Given once/month
Moxidectin (ProHeart™)
Given by injection every six months
Selimectin (Revolution™)
Applied topically once/month
Others, used less often

Question 12

Heartworm Treatment

Answer 12

Immiticide™ (Melarsamine dihydrochloride)
Administered as IM injections in epaxial muscles
As two injections 24 hours apart, or
Modified treatment – 1 injection; then two injections 24 hours apart approximately 1 month after initial injection – currently recommended
Also recommended to precede tx with Doxycycline for one month, hw preventative for 3 doses, and post-tx with prednisone for one month
Causes slow kill of adult worms over 1 month period. Severe exercise restriction important precaution.
Historically –
Older adulticide was Caparsolate Na administered in 4 IV injections 12 hours apart
Significant toxicity to liver and kidneys
Tissue sloughs if injected perivascularly

Question 13

Microfilaricide

Answer 13

Ivermectin (Ivomec™)
Mixed 1 cc concentrate to 9 cc propylene glycol
Administered p.o. at rate of 1 cc/44 lb BW
Monitor for 6 hours after administration for SE
DO NOT administer to Collies or Collie Xs (herding dogs) – crosses blood-brain barrier in some lines

Milbemycin oxime (Interceptor™)
Administer preventative dose, and monitor carefully for side effects.  Dogs with heavy microfilariae burdens have significant chance of developing shock post-treatment.

Follow-up recheck scheduled 1-3 weeks after microfilaricide. Dog should test negative for microfilariae, and be placed on preventative, if not already using.
Ancillary medications may be used during treatment – Prednisone is most common

Question 14

Pulmonary Thromboembolism – Most common complication of heartworm treatment

Answer 14

Fever
Anorexia
Dyspnea
Cough
Treatment involves cage resting, antibiotics and anti-inflammatories