Heartworm Life Cycle
Adult heartworms live in the arteries that go from the heart to the lungs (called pulmonary arteries).  Once mature, the females will then produce larval heartworms (called microfilaria) that circulate within the host animal's bloodstream.  Mosquitoes then come and take a blood meal from the host, where they will ingest some microfilaria from the bloodstream.  The microfilaria grow in the mosquito until they become infective.  When the mosquito bites another host animal, these infective larval heartworms (termed L3) will be deposited into the skin of the host animal.  The L3 larvae live in the skin until they mature to L4 and then L5 larvae, which usually takes about 3 months.  Then these L5 larvae will travel to the pulmonary arteries, become reproductively active adults, and the cycle begins again.  The entire cycle takes about 6 months.

Key points to remember

• Mosquitoes transmit heartworm disease
• It takes 6 months after your pet is infected with larval heartworms for adult heartworms to be present in the pulmonary arteries
  

Is my dog at risk?
We see mosquitoes year-round in Texas, so any dog that is not on a monthly heartworm prevention (even in winter months) will be at risk for contracting heartworm disease. 


What signs should I look for?
Many dogs remain asymptomatic for heartworm disease until permanent damage has been done to the heart and lungs.  Common symptoms for heartworm disease in dogs include:

• Coughing
• Exercise intolerance
• Abdominal enlargement
• Fainting
• Trouble breathing
• Sudden death in advanced cases

How do the heartworms cause damage?
The heartworms cause damage to the heart and lungs in several different ways:

1. The mere presence of the worms in the pulmonary arteries causes inflammation and thickening of these blood vessels, which makes the heart pump harder to move the blood through the lungs.  This, over time, leads to stretching of the heart muscle with subsequent congestive heart failure if left untreated.
2. The worms are foreign to the body, which causes inflammation in the lungs, leading to chronic coughing and bronchitis.  Pieces of the worms can also dislodge and break off, or can cause blood clots to form that can lead to pulmonary thromboembolism (blockage of blood flow to points in the lungs) and sudden respiratory distress.
3. If too many heartworms are present in the pulmonary arteries, they can back up into the heart and restrict blood flow coming back to the heart from the veins in the abdomen.  This is called caval syndrome and it is a sign of very advanced, end stage heartworm disease characterized by abdominal enlargement, sudden collapse, or sudden respiratory distress or death

Diagnosing heartworm disease in your dog
The most common test we run on dogs is the heartworm antigen test.  Our dogs receive this test during their yearly wellness exams.  This test detects adult female heartworms that live in the pulmonary arteries.  If this test is positive, your dog has an active heartworm infection.  If this test is negative, there could be several reasons why:

• Your dog is truly heartworm negative
• Your dog has been infected within the last 6 months and there are not yet adult heartworms in the pulmonary arteries
• Your dog has a small heartworm burden or has a predominantly male heartworm infections (since this test only detects females)

We also use a microfilaria screening test on any dog that we suspect has heartworms.  We will look at a drop of blood under the microscope.  If we see circulating microfilaria (larval heartworms) then we know your dog is heartworm positive because these are only seen when adult heartworms are present.  We may not see microfilaria for several reasons:

• There are none in the small drop of blood we looked at
• There are certain times of the day when microfilaria do not circulate in the bloodstream
• If your dog is on a heartworm preventative this will kill the microfilaria so we may not be able to detect them

We like to perform chest x-rays in any dog that tests heartworm positive to see the extent of damage the heartworms have caused.  We can look at the size of the heart, see any pulmonary artery enlargement, see any signs of inflammation in the lungs and see any congestive heart failure that can occur secondary to the heartworm infection.


What if my dog has not been on heartworm prevention and tests negative?
In this case, we will place your dog on heartworm prevention and re-test in 6 months.  This will ensure that the life cycle is complete and any heartworms are living in the pulmonary arteries, and no new heartworms could have infected your dog since he/she was on heartworm prevention.  If your dog tests negative 6 months after starting heartworm prevention and has not missed any doses, your pet is considered heartworm negative and should stay that way as long as heartworm prevention is continued properly.


Treatment for heartworm disease

• Treating the microfilaria and larval heartworms - in most dogs, we try to wait at least 3 months from the time of diagnosing heartworm disease until treating the adult infection.  The medication we use to kill the adult heartworms will not kill any juvenile worms (L3-5).  Waiting 3-6 months gives us a chance to place your dog on continuous heartworm prevention (so no new heartworms can infect him/her) and give any larval heartworms time to migrate to the heart where the medication can work.
• Treating for Wolbachia - this is a parasite that lives within the heartworms and seems to be beneficial to the adult heartworms.  These organisms are killed by an inexpensive antibiotic which we use during the first month in addition to a heartworm preventative.  We see much less side effects and problems with heartworm treatment if we treat for Wolbachia.
• Treating the adult heartworms 

1.  Iverheart/Doxycyline: with this protocol, we give Iverheart (or other Heartgard equivalent - a heartworm preventative) every 2 weeks for a year.  This prevents new heartworm infections by killing the larval stages of heartworms and it weakens the adult heartworms to some degree.  Doxycyline is given during the first 30 days to kill Wolbachia and further weaken the adult heartworms.  Though this is a fairly new protocol, early results of studies done have shown that between 80-85% of dogs will be heartworm negative after 12 months.  This is a less ideal treatment, however, as during this time the heartworms can still cause permanent damage to the heart.  Also, heartworm resistance is starting to emerge in several states so we like to kill any adult heartworms quickly.
   
2. Immiticide: this is the medication that kills adults heartworms.  We use this when possible to kill the heartworms quickly and prevent damage to the heart from long-term infections.  This is an arsenic based compound that kills adult heartworms, but has no effect on larval stages of heartworms.  Using this drug is more expensive and riskier than Method 1 and can have some serious potential side effects, including death.  We will fully discuss Immiticide treatment, its risks and benefits with you if your dog needs heartworm treatment.

How can I make sure my dog doesn't get heartworms?
Because heartworm disease is 100% fatal if not treated, and because the treatment is risky and expensive, preventing your dog from ever getting heartworm disease is much better!  Heartworm prevention comes in either a chewable tablet or topical liquid form and is given once a month throughout the entire year for the rest of your dog's life.  You must purchase this medication from a veterinarian - it is not available without a prescription!  Given appropriately, your dog will never have to worry about heartworm disease and its consequences. 


Remember: in Texas, we have mosquitoes all year round, so it is important to keep your dog on a heartworm preventative even during the winter months!