heartworm

Question 1

What are some potential risks of adulticidal heartworm treatments?

Answer 1

Dead worms can result in thromboemboli.

Dead worms can liberate Wolbachia bacteria, which are highly immunogenic.

Question 2

What can you do to try to prevent thromboemboli from adulticidal heartworm treatments?

Answer 2

Limit activity level of the dog.

Assess patient’s overall cardiopulmonary health.

Assess patient’s overall worm burden with ELISA test.

Question 3

Macrocyclic lactones target which phase of the heartworm life cycle?

Melarsomine targets which phase of the heartworm life cycle?

Answer 3

Macrocyclic lactones target L3-L4 larvae.

Melarsomine targets adult worms.

Question 4

Describe the “treatment gap” in heartworms.

Answer 4

At about day 30 post-infection, L4 larvae become resistant to macrocyclic lactones. They remain resistant to treatment until day 80, when they begin to develop into adults. After this, worms are susceptible to melarsomine.

Question 5

You diagnose a patient with heartworm. How should you start your treatment?

Answer 5

Put patient on macrolide preventative 2 months before melarsomine treatment. This helps to eliminate microfilariae from the blood stream.

Waiting 2 months before melarsomine treatment insures that any larvae currently in the treatment gap range will have time to mature into melarsomine-sensitive adults.

Question 6

You might consider adding a doxycycline regimen (10mg/kg BID) to your heartworm patient before treating with melarsomine. Why is this?

Answer 6

Doxycyline targets the worms’ intracellular Wolbachia.

This weakens the worms and decreases the amount of bacteria that will be released after the worms die.

Question 7

How does eliminating Wolbachia affect a developing Dirofilaria embryo?

Answer 7

Eliminates asymmetrical division of cells, a process critical for normal development.

Question 8

Describe the dosing regimen for melarsomine treatment of HW.

Answer 8

• First dose: 2.5mg/kg IM
• 30 days later: 2.5mg/kg IM
• 24 hours later: 2.5mg/kg IM

INSURE EXERCISE RESTRICTION!

Question 9

What are the potential signs of thromboembolism in a patient receiving melarsomine treatment?

Answer 9

Low fever

Cough

Hemoptysis (coughing up blood)

Signs are more lilkey in heavily-infected or CHF dogs

Question 10

How would you check for the efficacy of your melarsomine treatment (i.e. are the adults eliminated?)

Answer 10

Heartworm antigen test 6 months after beginning treatment

Question 11

You’ve heard that you can also eliminate adult heartworms through administration of heartworm preventatives (instead of pulling out the big-gun melarsomine treatment). Is this a good idea?

Answer 11

NOPE

Ivermectin takes 1-2 years to eliminate a heartworm infection. During this time, the adults continue to cause disease. The prolonged treatment period may also select for macrolide resistance.

Question 12

When should you start puppies on heartworm preventative?

Answer 12

No later than 8 weeks of age

Question 13

An owner brings you his 4 month old puppy and wants you to check him for heartworm. What is your response?

Answer 13

Facepalm

The prepatent period of Dirofilaria is 6-9 months. In most cases, there is no point in heartworm testing a dog that is younger than 7 months.

Question 14

What kinds of patients will develop vena caval syndrome in heartworm infections?

Answer 14

Heavily infected (>60 worms) young male dogs (<3 years of age) in highly endemic areas.

Question 15

What are the symptoms associated with vena caval syndrome?

Answer 15

Anorexia, severe lethargy, weakness, dyspnea

Hemoglobinuria

Hemolytic anemia,

Question 16

What causes vena caval syndrome?

Answer 16

Worm burden is so high that the worms back up into the right heart, caudal vena cava, and hepatic veins.

Worms can block the tricuspid valve and congest the liver, causing a heart murmur and hepatic insufficiency.

Question 17

Which test is pathognomonic for vena caval syndrome?

Answer 17

Echocardiogram

Worms are so numerous that you will be able to visualize them with imaging.

Question 18

A dog presents with vena caval syndrome. What is your treatment plan?

Answer 18

This is an emergency situation.

Surgical removal (usually through the jugual vein) of the worms is mandatory.

Start adulticidal therapy 2-4 weeks after surgery.

Question 19

In feline heartworm disease, what number of infections are single-sex infections?

Answer 19

1/3 of infections

Question 20

Describe this condition.

Answer 20

Abberantly-located Dirofilaria immitis

Question 21

Compare feline heartworm disease to canine heartworm disease.

Answer 21

In endemic areas, infection rate is 5-15% of that seen in dogs.

Fewer L3 mature to adults.

HW mature more slowly.

Shorter adult lifespan (2-5 years).

Typically have <6 adult worms.

Question 22

Why do cats tend to have uncommon or transient microfilaremia in heartworm infections?

Answer 22

Immune mediated clearance of microfilariae

Suppression of microfilariae production

Question 23

Describe the 2 stages of feline heartworm infection

Answer 23

• Stage 1 - heartworm-associated respiratory disease (HARD)
  
  • worms arrive 3-4 months post-infection and may die
  • symptoms similar to asthma
• Stage 2 - death of mature worms
  
  • pulmonary inflammation, thromboembolism
  • chronic: persistent tachypnea and coughing

*A lower number of worms in cats can produce a larger inflammatory response than in dogs.

Question 24

Heartworm antigen tests have a higher incidence of false negatives in cats than in dogs. Why is this?

Answer 24

Fewer adult heartworms are present in feline infections.

Recall that the antigen test responds to an ovarian antigen produced by adult female worms. Fewer females means fewer antigens being produced.

Question 25

Your heartworm antigen test comes back for your feline patient. You know that this test in feline has a high incidence of false negatives, so you decide to run an antibody test, which comes back positive. Is your patient heartworm positive?

Answer 25

Possibly.

Antibodies against heartworm suggests that the cat has been exposed at some point in time.

Question 26

Melarsomine is the go-to adulticidal treatment for heartworm in dogs. Is this the case in cats?

Answer 26

NOPE

Melarsomine is potentially toxic in cats, and does not increase survival time.

Question 27

How can you prevent feline heartworm disease?

Answer 27

Monthly oral or topical macrolides

Consider this for all cats living in heartworm-endemic areas

Question 28

True or false:

Humans can be infected with Dirofilaria immitis

Answer 28

TRUE