QUESTIONS

1. What is a "Heartworm Development Unit?"
    
2. How might a dog come to have heartworms in its heart yet no circulating microfilariae?  List as many scenarios as you can  (I thought of 5.)
    
3. How might a dog come to have circulating microfilariae yet not have adult heartworms in the heart?  (I could only think of 2 scenarios.)
    
4. What species of mosquito is the chief heartworm vector in the Southeast U.S.?
    
5. You are practicing in a highly endemic area for heartworm.  A homeless person comes into your clinic with his dog.  He has saved up enough money for heartworm preventive but not enough for testing.  Would you choose to sell him Interceptor or Heartguard30?
    
6. What is the incidence of feline heartworm infection in an endemic area?
    
7. Upon what life stage does DEC act & why do you have to give it daily?
    
8. How many microfilariae per cc of blood would you expect in a heavily infected dog?
   
   a) 100        b) 1000        c)  5000        d) 50,000        e) 200,000
    
9. How long do adult heartworms live?
    
10. Do heartworm infected cats have a microfilaremia?
     
11. How old do heartworms have to be to be susceptible to carparsolate?
     
12. How long after heartworm treatment should you wait before repeating antigen testing?
     
13. You are looking at a blood smear & see a microfilariae. It has a narrowed tapering nose & a hook on its tail.  What is it?
     
14. Where is the most common non-cardiopulmonary site for aberrant heartworm migration in the cat.
     
15. What is the prepatent period of the heartworm?
     
16. What worms are not detected by antigen testing?
     
17. What would be the best way to go about heartworm testing in the cat?
    
    
    
    
     

ANSWERS

1. The ambient temperature necessary for L1 to begin development to L3 is 57 degrees F. This whole process normally takes a couple of weeks inside the mosquito but actually how long it takes is very temperature dependent. 
   
   A "heartworm development unit"  is every degree that the average daily temp. exceeds 57 degrees.  It takes 234 heartworm development units to actually get L3's in the environment.  Heartworm preventive should be started the following month.
   
   
   
   
    
2. Obviously we are talking about occult heartworm disease which, by the way, accounts for 25-35% of cases.
   
   a)  Single sex infection
   
   b)  Immune destruction of microfilariae (probably the most common reason)
   
   c)  Dog was just treated for adult worms but not yet treated for microfilariae. (It was previously recommended to treat for microfilariae prior to giving adulticide as it was thought this reduced the vessel damage seen after adulticide treatment.  This idea has not borne out and coupled with the fact that gravid females can be left in the heart after adulticide tx, it is now recommended to treat for adults first & microfilariae 4-6 weeks later.)
   
   d)  Dog is on fenthion for fleas (fenthion is a microfilaricide.)
   
   e)  No one knew the dog had heartworms when it was put on preventive treatment.  In approx. 8 months, microfilariae will be cleared & further, preventives will permanently sterilize male adult worms & temporarily sterilize female adult worms.)
   
   
   
   
    
3. This phenomenon accounts for 1-2% of infections.
   
   a) Microfilaria (L1s) are small enough to cross the placental barrier and further more L1s may live up to 3 years.  Imagine a 2 year old dog born to an infected mom.
   
   b)  D-uh.  Dog treated for adult worms but not yet treated for microfilariae.
   
   
   
   
    
4. The species in question is Aedes albipictus (the "tire-breeding" mosquito).  This is of interest because this mosquito didn't even live in the U.S. until 1985. It has since bumped the Culex species that was the prior title-holder.  Unlike the Culex species, this Aedes is an afternoon feeder (meaning it feeds when dogs are likely to be outside - when their owners are at work.)  Culex was a PM feeder.  This Aedes also takes a blood meal 2x as large as what Culex would take meaning that it has the capability of producing 2x as many L3s.
   
   
   
   
    
5. Sorry Novartis but Merial wins out here.  You should sell him Heartguard30 & here's why.   We can figure this dog is probably infected & is getting regularly exposed. This means he has L1s, L3s, L4s, L5s, & adult worms.
   
   We can't kill the adults with either medication so let's move on.
   
   L1s will clear on either drug BUT in heavily infected dogs with tons of microfilariae, those going on interceptor frequently went into shock due to acute death of tons of microfilariae.  This is because Novartis made interceptor with a really high milbemycin dose so that it could kill intestinal parasites.  This dose is so high that the microfilariae die hugely & dramatically.  It is a slower kill with ivermectin.
   
   L3's- well, interceptor wins here. It will kill L3's & ivermectin won't.
   
   L4's- This is where both drugs actually act  mainly.
   
   L5's- Ivermectin will kill young L5's.  This means that if we look at the same dog say in a year, there will be fewer adult worms in the heart.  (There was a study where a some heartworm naive dogs were infected. Four months later, some got no tx, some got heartguard & the rest got interceptor.  The heartguard dogs actually had 95% fewer worms than the controls!  Interceptor could kill some L5's but not as reliably.  The Interceptor dogs only had 41% less adult worms.)
   
   Given all of this, I vote for heartguard - safer if lots of circulating microfilariae & can reduce the number of adult worms that will be present when the guy saves up enough money for treatment :)
   
   
   
   
    
6. The incidence of feline heartworm disease in an endemic area has been found to be 1-10% by necropsy.
   
   
   
   
    
7. DEC works on the L3s in the process of molting to L4s.  L3s hang out in the SQ tissues for approx. 10 days before molting to L4's.  This molting period requires 48 hours afterwhich the L4s hit the circulation & spend the next 4 months migrating to the heart.  If DEC is not on board during that 48 hour molting period, the molt will be successful. (Very tiny window of worm vulnerability.)
   
   
   
   
    
8. Heavily infected dogs have 200,000 microfilariae per cc.  (Gross!)
   
   
   
   
    
9. In the dog adult worms live up to 5 years.  In the cat they live 16-25 months.
   
   
   
   
    
10. Cats do have a microfilaremia but it only lasts about 2 weeks so don't expect to see it happening.  Microfilaria are rapidly banded up by the immune system & removed in the lungs.  Watch for congestion in the right caudal lobe on rads.
    
    
    
    
     
11. Heartwroms have to be 6 months old.  This is one reason to re-treat in a couple of months (some worms have to grow into susceptibility.)  Note melarsomine diHCl kills all the way back to the elder L5s.
    
    
    
    
     
12. At ACVIM in Orlando, it was said to re-test in 4 months.
    
    
    
    
     
13. Was this too obvious a question?  It is Dipetalonema reconditum.  It's intermediate host is, I think, the flea.  Does some one know for sure?
    
    
    
    
     
14. They tend to go to the lateral ventricles of the brain (hate when that happens.)
    
    
    
    
     
15. In the dog the prepatent period is 5-7 months (average 6.5 months).  In the cat, it is 8 months.
    
    
    
    
     
16. Male worms are totally ignored by the antigen tests and females under age 7 months also escape detection.
    
    
    
    
     
17. Here's my erudite recommendation having just finished reading up.  First start with an antigen test.  This test is not very sensitive (in one group of infected cats w/worms in hearts only 38% tested positive.)  If negative, follow with an IFA antibody test (ELISA antibody tests are too sensitive).  A titer > 1:40 is considered positive. A titer of 1:40 requires one to go by clinical signs (cats will have antibodies for up to a year after the death of their adult worms.)