Veterinary Blog for Veterinarians and Animal Lovers

Life Cycle

Figure 1.  Diagram of heartworm life cycle from Hill’s Atlas of Veterinary Clinical Anatomy

Canine heartworm is a 23 to 30 cm long worm that resides inside the heart of the dog.  The female produces baby heartworms called microfilariae which spread out through all the blood vessels of the dog.  When mosquitoes feed on infected dogs, they suck in blood and the baby heartworms.  They molt three times inside the mosquito producing L3

larvae.  The environmental temperature must be warm or else they don’t develop.  The mosquito carrying the L3 larvae bites another dog.  The larvae will enter the bite wound, penetrate tissues and molts a fourth time producing L4 larvae after 7 days.  The L4 larvae further migrates into the deeper tissues for 60 to 90 days until they finally molt into the young adult worm.  The young worm reaches the inside of the heart through blood circulation.  The young worms become mature and mate in the pulmonary arteries.  They then settle in the right chambers of the heart and pulmonary arteries where they can survive for up to 7 years.  Birth and release of baby heartworms into the circulation takes about six and a half months.

Figure 2.  From shakervet.com

Diagnostic Trends and Issues

The use of heartworm drugs like Heartgard®, Interceptor®, Sentinel®, Revolution™ and ProHeart® has dramatically decreased infection among the canine population.  However, these drugs have affected the reliability of antigen blood tests used in detecting adult worms in the circulatory system.  So to correctly evaluate the heartworm status of a dog, these antigen blood tests should be backed up with other diagnostic procedures like physical examination, medical history, direct blood examination, x-ray examination and echocardiogram.

Figure 3.  Microfilaria as seen under the microscope surrounded by red blood cells from commons.wikimedia.org

Periodic blood testing for heartworms is a way of monitoring the effectiveness of the above mentioned drugs.  If an area has a lot of infected dogs then the recommendation is to perform yearly testing.  If the pet owner switches from one drug to another, the recommendation is to test at the time of changing and then retest after four to seven months to make sure that the drug is working.  If there is a lapse of more than 8 weeks in the administration of medication, then retesting must be done on a yearly basis to detect possible infection.

Figure 4.  Witness® heartworm antigen test showing both postive and negative results from www.westernmedicalsupply.com

Heartworm treatment Issues

The newest drug used for treating heartworm infection is melarsomine dihydrochloride known in the market as Immiticide®.  Its effectiveness, safety and ease of administration have replaced thiacetarsamide, the original heartworm treatment available to veterinarians for many years.  This drug has a flexible dosing regimen based on how infected the dog is.  The standard dosing regimen consists of two doses of the drug given 24 hours apart.  This is ideal for dogs that don’t show any symptoms or those that are in the early stages of the disease.  The other regimen involves giving the dog a single shot of the drug and then is observed for a month for any allergic reaction or circulatory problem.  When the month goes by without any incident, the dog is given the standard two dose regimen.  This is the regimen being used for dogs in the late stage of heartworm infection or class 3 in veterinary medical terms and for dogs with high amount of worms in their bodies as confirmed by various diagnostic procedures.  This is a more cautionary protocol since the one month interval gives the dog a chance to recover from whatever reaction the dog would undergo from a sudden demise of a whole lot of worms in the circulation.  The two more injections would make sure that all other existing worms and their larvae would be taken care of.  The issue, however, for this protocol is the increased time of treatment and increased cost since the drug is very expensive.  There is also the issue of trying to use this drug on very old dogs suffering from the disease and those suffering from other terminal illnesses.  In these cases, veterinarians are forced to seek other treatment options.  One such option, being circulated around the internet, is the so called “slow kill” treatment.  Heartworm drugs like ivermectin and milbemycin oxime are administered to infected dogs at the usual monthly interval and dosage for one or more years.  Most of the worms would die slowly while the remaining ones become either sterile or structurally abnormal.  This option would be more affordable for most pet owners since the cost of heartworm drugs is much lower compared to Immiticide®.  In addition to the heartworm drugs, the dog is also given an antibiotic doxycycline to kill a bacteria-like organism within the worms called Wolbachia.  This bacterium has a symbiotic relationship with the worm and research has shown that it causes an allergic immune response when the worm dies.  Treating the dog with this drug would kill these bacteria lessening the allergic immune response.

Figure 5.  Sketch showing administration of Immiticide® intramuscularly in the epaxial (lumbar) muscles in the third through fifth lumbar region from www.drugs.com

Another treatment option is surgical removal of heartworms.  The dog is placed under general anesthesia and a flexible alligator forcep is inserted into the main artery leading to the heart.  The adult worms are then grabbed and pulled out of the heart one by one.  This treatment option is usually the preferred option in cases where there are too many worms in the heart and blood vessels or when the dog has signs of heart damage due to the worms.

Figure 6.  A picture of a Fujimon alligator forcep used in pulling out the heartworms through the main artery leading to the heart from Dr. Colin Johnstone’s website Parasite and Parasitic Diseases of Domestic Animals.

Figure 7.  A left side X-ray view of the Fujimon alligator forcep inside the heart from Dr. Colin Johnstone’s website Parasite and Parasitic Diseases of Domestic Animals.

Figure 8.  Adult heartworms extracted from the heart using the Fujimon alligator forcep from Dr. Colin Johnstone’s website Parasite and Parasitic Diseases of Domestic Animals.

Prevention

Figure 9.  Heartgard® packaging from shakervet.com

Heartgard® is known by its generic name as ivermectin.  It is a huge chewable tablet that is quite palatable and is effective against a wide range of external and internal parasites.  It is used as a monthly preventive medication against heartworm and can still provide adequate protection even if you fail to give it for two months straight.  It is safe for all breeds of dogs except for the Collie breeds and Collie crosses which are more sensitive to its toxic effect at very high doses.  So, accidental ingestion of an excess amount of this drug by said breeds usually ends up in death.

Figure 10.  Interceptor® packaging from petplace.com

Figure 11.  Sentinel® packaging from www.serenityanimalhospital.net

Interceptor® and Sentinel® are known by their generic name as milbemycin oxime.  It is a tablet given once a month and is highly effective not only against heartworms but also against other intestinal worms like hookworms, roundworms and whipworms.  Like Heartgard®, it can still provide adequate protection even if you fail to give it for two months straight.

Figure 12.  ProHeart® tablet packaging from petproductsontheweb.com

Figure 13.  ProHeart®6 packaging from www.proheart6dvm.com

ProHeart® and ProHeart®6 are known by their generic name as moxidectin.  It is available in two forms: tablet (ProHeart®) and liquid for injection (ProHeart®6).  The tablet is also given once a month and like Heartgard®, Interceptor® and Sentinel® it can still provide adequate protection even if you fail to give it for two months straight.  The injection provides 6 months protection against heartworms with one single shot.  It is injected under the skin and can still provide adequate protection even if you fail to give it for four months straight.

Figure 14.  Revolution® packaging from 1800petmeds.com

Revolution™ is known by its generic name as selamectin.  It is a liquid drug that is applied once a month on the skin of the neck of the dog in between the shoulder blades where it slowly spreads out and eventually is absorbed into the bloodstream.  It is not only effective in preventing heartworm but can kill fleas and their eggs, sarcoptic mange mites, ticks and ear mites.  It is much safer to use in Collie breeds as compared to Heartgard® and can also provide adequate protection even if you fail to apply it for two months straight.

Preventive medication using these drugs should be started when the dog reaches 6 to 8 weeks of age and it is recommended that the dog must be tested first using the antigen blood test and then followed up by a microscopic examination of the blood for baby heartworms if the result of the antigen blood test is positive.  If medication is missed for more than two months then the medication should be given continuously for a year and an antigen blood test performed 6 months after to see if there is any infection.

Tags: canine, disease, heartworm, ivermectin, microfilaria

This entry was posted on Thursday, February 4th, 2010 at 4:02 PM and is filed under Canine Parasitic Diseases. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.