| HEARTWORMS |
| Heartworms (Dirofilaria immitis) Holly Frisby, DVM, MS Veterinary Services Department, Drs. Foster & Smith, Inc. |
Heartworms were first identified in the United States in 1856 and occurred most frequently on the seacoast in
the southeastern United States. In recent years, heartworm disease has been found throughout the United
States. Living in the Northwoods of Wisconsin, we know there is no shortage of mosquitoes in the northern United
States. So it was not for lack of mosquitoes that heartworm disease was not found here earlier. The movement of
infected animals that could serve as sources of infection for others is probably a significant contributing factor to
heartworms spreading across North America. The actual number of infected dogs and cats in the United States is
unknown.
In 1973, the American Veterinary Medical Association first published guidelines on the treatment and prevention
of heartworm disease in dogs. These are revised periodically. Since the first publication of guidelines, the public's
awareness of heartworm disease has increased dramatically. Diagnosis, prevention, and treatment of this disease
have come a long way.
What are heartworms?
Heartworms belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that
is where the similarity ends. Heartworms spend their adult life in the right side of the heart and the large blood
vessels connecting the heart to the lungs.
Heartworms are found in dogs, cats, and ferrets. They also occur in wild animals such as California sea lions and
black bears. They have rarely been found in people.
How do dogs become infected with heartworms?
the southeastern United States. In recent years, heartworm disease has been found throughout the United
States. Living in the Northwoods of Wisconsin, we know there is no shortage of mosquitoes in the northern United
States. So it was not for lack of mosquitoes that heartworm disease was not found here earlier. The movement of
infected animals that could serve as sources of infection for others is probably a significant contributing factor to
heartworms spreading across North America. The actual number of infected dogs and cats in the United States is
unknown.
In 1973, the American Veterinary Medical Association first published guidelines on the treatment and prevention
of heartworm disease in dogs. These are revised periodically. Since the first publication of guidelines, the public's
awareness of heartworm disease has increased dramatically. Diagnosis, prevention, and treatment of this disease
have come a long way.
What are heartworms?
Heartworms belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that
is where the similarity ends. Heartworms spend their adult life in the right side of the heart and the large blood
vessels connecting the heart to the lungs.
Heartworms are found in dogs, cats, and ferrets. They also occur in wild animals such as California sea lions and
black bears. They have rarely been found in people.
How do dogs become infected with heartworms?
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Adult heartworms lay very tiny larvae called microfilariae. The microfilariae can live up
to 2 years in the dog's bloodstream. These microfilariae enter a mosquito when it
sucks blood from an infected animal. In 2-3 weeks, the microfilariae develop into
larger larvae in the mosquito and migrate to the mosquito's mouth.
to 2 years in the dog's bloodstream. These microfilariae enter a mosquito when it
sucks blood from an infected animal. In 2-3 weeks, the microfilariae develop into
larger larvae in the mosquito and migrate to the mosquito's mouth.
When the mosquito bites another animal, the larvae enter the animal's skin. The larvae grow and after about
three months finish their migration to the heart, where they grow into adults, sometimes reaching a length of
14 inches. The time from when an animal was bitten until adult heartworms develop, mate, and lay microfilariae
is about 6-7 months in dogs and 8 months in cats. (Remember this – it is important when we talk about diagnosis.)
Severely infected dogs can have up to several hundred heartworms in their hearts and vessels. Adult worms in
dogs usually live up to 5-7 years. Thirty to eighty percent of infected dogs have microfilariae, and the
microfilariae can live up to 2 years.
What damage do heartworms cause?
The adult worms can obstruct the different chambers
of the heart and the various large blood vessels leading
from the heart to the lungs. First, the right pumping
chamber (ventricle) of the heart and the large pulmonary
artery leading from it to the lungs becomes enlarged as
the worms take up space. If worms die, they may follow
the flow of blood into smaller vessels in the lung and
obstruct those vessels.
In severe infections, the worms can also block the large vein (vena cava) bringing blood to the right side of the
heart. As the blood backs up, the liver becomes enlarged and damaged.
What are the signs of heartworm infection and how is it diagnosed?
Dogs with heartworm infections may show decreased appetite, loss of weight, and listlessness. Often, the first
sign of the disease is a cough. Animals with heartworm disease will start to show lack of endurance during
exercise. Some will accumulate fluid in their abdomen (ascites) that makes them look pot-bellied. In rare
situations in which animals have many adult worms, the animals may die of sudden heart failure.
Antigen testing is performed to identify dogs infected with D. immitis. Because blood tests are not always
accurate, we need to interpret test results in relation to the history and the symptoms the animal is showing.
Radiographs (x-rays) are often taken to look for typical changes in the heart and lungs caused by D. immitis.
Changes include enlargement of the pulmonary artery and the right ventricle. Certain types of cells (eosinophils)
may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all
help support the diagnosis.
What tests are available to detect D. immitis infection in animals?
There are several blood tests used to detect heartworm infection. In the 1960's, before more sophisticated
tests were available, heartworm testing involved looking for the microfilariae in a drop of blood on a microscope
slide. A better test, the Knott's test, was developed to concentrate the microfilariae from a larger portion of
blood through centrifuging it. This gave veterinarians a better chance of finding the microfilariae.
Later, filter tests became available. In these tests, the blood cells in the blood were lysed (broken) by a
special agent that did not affect the microfilariae. The resulting liquid was then put through a very fine filter.
The microfilariae were concentrated on the filter. The filter was stained and examined under the microscope
for microfilariae.
Veterinarians soon recognized that some animals could have heartworm infections without having microfilariae
in the blood. This occurs if only male worms are present or if the females are not laying microfilariae at the
time of the test. It was obvious that better tests were needed.
Serologic tests were developed to identify antigens (small protein and carbohydrate components) of
heartworms in the bloodstream. There are different varieties of this test. Some test kits run one sample at a
time and can be done right in your veterinarian's office. Others are designed to test multiple samples in large
batches. This batch-type of test is generally performed by outside laboratories to which your veterinarian
sends your dog's blood.
Although the antigen tests were much better than the filter test, we still could not identify all cases of
heartworm infection because antigen tests will only be positive if female worms are present, since the antigen
detected is from the worm's uterus. If the heartworms were not fully mature, or there were only male worms
present, the antigen test result in infected animals would be falsely negative. This means the test result is
negative when the animal is really infected.
Serologic tests have now been developed to detect antibodies (proteins to fight off the 'invaders') the animal
makes against heartworms. This is the test most commonly used in cats. This test will be positive even if only
one male worm is present. But this test has a downfall, too. Although it is very good at giving positive results
when an infection is present, false positive tests are more common with this test than the antigen test. A false
positive result means the test result is positive when no infection is present.
What is included in a good heartworm prevention program?
The best program for prevention of heartworm infection includes using preventives, performing routine
heartworm testing, and reducing exposure to mosquitoes.
Heartworm preventives
Medications used to prevent heartworm infections are called preventives. The first thing to remember is that
preventives are NOT used to kill the adult worms. Special drugs called adulticides must be used to kill the
adults. These drugs will be discussed in the treatment sections. Some preventives can cause severe problems
if given to animals with adult heartworms or microfilariae. Follow the recommendations of your veterinarian and
the manufacturer of your preventive in regard to testing prior to giving the preventive.
A number of monthly heartworm preventives for dogs are on the market. There is also an injectable preventive
that is given once every 6 months. Some heartworm preventives, or drugs that are combined with them, will
control other parasites.
When determining when to give monthly preventives, remember, they kill larvae the animal was exposed to in
the preceding thirty days. For instance, a monthly preventive given on July 1, will kill larvae the animal acquired
from June 1 to July 1. The monthly preventives do NOT work forward in time. A monthly preventive given July 1,
will NOT have an effect on larvae the animal acquires later in July.
In some areas of the country where mosquitoes are present year-round, preventives must be given year-round.
In colder climates, preventives are started 2-3 weeks after the first mosquitoes appear and are given until
after the first hard frost (when the mosquitoes are killed or hibernate).
The most commonly used heartworm preventives for dogs in the United States.
Active Example Dose Given Mode Minimum age
Ingredient of at which Additional Additional Comments
Action preventive Ingredients Effects
can be given
Ivermectin Heartgard M; oral LA 6 weeks None N/A * See
below
Iverhart
Ivermectin Heartgard Plus M; oral LA 6 weeks Pyrantel Controls
Iverhart Plus roundworms * See below
and
hookworms
Milbemycin oxime Intercept M; oral LA 4 weeks, but Controls
puppies or dogs None hookworms,
must weigh 2 roundworms,
pounds or more and whipworms
Milbemycin oxime Sentinel M; oral LA 4 weeks, but Controls
puppies or dogs Lufenuron hookworms,
must weigh roundworms,
2 pounds or more and whipworms;
prevents flea eggs
from developing
Selamectin Revolution M; topical LA 6 weeks None In dogs, controls
fleas and ear
mites; treats
sarcoptic mange,
and controls the
American Dog Tick.
Moxidectin ProHeart 6 Every 6 months; LA 6 months None Treats hookworm
injectable infections
M = Monthly
LA = Kills larvae accumulating in host during preceding month; LI = Kills larvae as they are introduced
*Collies and related breeds are known to have a sensitivity to ivermectin. The dose of ivermectin in a monthly
heartworm preventive is relatively low. The manufacturers of Heartgard, Heartgard Plus, Iverhart, and
Iverhart Plus state the products should be safe in these breeds if used as recommended.
A preventive should be given to all dogs. Remember that mosquitoes can get indoors, so even though your dog
may not go outside, the dog is still susceptible.
Testing
When and how often pets should be tested for heartworm infection is also a matter of debate. In making a
decision on when to test, we must consider how common heartworm disease is where the pet lives, what
heartworm preventive the pet is receiving, and how long the mosquito season lasts.
The American Heartworm Society advises all adult dogs being started on a heartworm preventive for the first
time should be tested. After that, periodic but not necessarily annual retesting should be performed. The
frequency of testing should take into account such variables as the prevalence of heartworm disease in the
area, the possibility of missed or late doses of preventive, and the dog's lifestyle. Consult with your
veterinarian to determine the appropriate testing schedule for your dog.
Because of the heartworm's life cycle, it is advised that for dogs on a monthly seasonal preventive, any testing
should be performed no earlier than 6.5 months after the last date the preventive was given. In areas with
very short mosquito seasons, and for clients who carefully administer the monthly preventives and do not miss
any doses, testing at 2-3 year intervals is sufficient. It is recommended, however, that a test be performed
after the first year the dog has been on the preventive.
For a dog that receives a monthly heartworm preventive year-round, the American Heartworm Society
suggests that the interval between tests could be longer than one year. This is only if there were no missed or
late doses of preventive. If a dose was missed or late, the dog should be tested on an annual basis. It is also
recommended that a test be performed after the first year the dog has been on the preventive.
Testing puppies: In areas where heartworm preventives are given seasonally, a puppy that was not alive during
the previous mosquito season would not have to be tested. For instance, in northern Wisconsin, puppies born in
January do not need to be tested prior to starting them on heartworm preventive in the spring. Remember, any
puppy less than 6.5 months old is probably going to have a negative antigen test, since any larvae the pup was
exposed to would not have had time to mature and be detected by the antigen test.
Mosquito control
Reducing the exposure of a pet to mosquitoes can help prevent them from even being exposed to heartworm
larvae. For detailed information on mosquito control, see Mosquito Control and Preventing Diseases They
Transmit.
How is heartworm infection treated?
The first adulticide (drug to kill the adults) for dogs that was developed was thiacetarsamide sodium
(Caparsolate®), which contains arsenic. It has to be given in the vein through a catheter. If any drug gets
outside of the vein, severe tissue damage could occur. Some animals become quite ill from this drug, and
therapy sometimes has to be stopped. Almost all animals have to be hospitalized for the several days of
treatment.
Melarsomine (Immiticide®) is a medication that also contains arsenic. It is given by injection deep in the
muscles of the back instead of intravenously. It is less likely to cause side effects than thiacetarsamide and is
more effective. The treatment protocol depends on the severity of infection. Dogs with mild cases are treated
once daily for two days. For more severe infections, the dog is given one injection, then thirty days later is
treated once daily for two days. Four months after the treatment, the dog should be tested for heartworms
using the antigen test. Some animals may need to undergo a second round of injections at that time. It is
recommended that dogs also be on a monthly heartworm preventive during the treatment.
Regardless of which drug is used, when the adult heartworms die, they can obstruct blood vessels to the lungs
(these are called pulmonary embolisms). If only a small part of the lung is involved, there may be no clinical
signs. However, if the vessels to a large portion of the lung, or a small area of an already diseased lung are
blocked, severe signs may result. These include fever, cough, coughing up blood, and even heart failure. Because
of the risk of these embolisms, any dog being treated with an adulticide must be kept very quiet during
treatment and for 2-4 weeks thereafter.
In very severe infestations, adult worms are removed from the heart surgically.
Can humans be infected with heartworms?
Yes, there have been instances of heartworm infection in people. Instead of migrating to the heart, the larvae
migrate to the lungs in humans. There the larvae can block vessels causing an infarction. At the site of the
infarction, a nodule develops which can be seen on radiographs. Usually, the person has few, if any signs of
infection. Surgical removal of the nodule is sometimes necessary.
References and Further Reading
American Heartworm Society. Recommended Procedures for the Diagnosis, Prevention, and Management of
Heartworm (Dirofilaria immitis) Infection in Dogs. Batavia, IL; 1995.
Atkins, CE. Comparison of results of three commercial heartworm antigen test kits in dogs with low heartworm
burdens. Journal of the American Veterinary Medical Association. 2003;222(9):1221-1223.
Blagburn, BL. Important heartworm basics for the practicing veterinarian. DVM Best Practice. 2003;March:12-
15.
Calvert, CA. Heartworm Disease. In Birchard, SJ; Sherding, RG (eds.) Saunders Manual of Small Animal
Practice. W.B. Saunders Co. Philadelphia, PA; 1994;487-493.
Datz, C. Update on canine and feline heartworm tests. The Compendium on Continuing Education for the
Practicing Veterinarian. 2003;25(1):30-41.
Georgi, JR; Georgi, ME. Canine Clinical Parasitology. Lea & Febiger. Philadelphia, PA; 1992;193-199.
Griffiths, HJ. A Handbook of Veterinary Parasitology. University of Minnesota Press. Minneapolis, MN; 1978;60-
62.
Hendrix, CM. Diagnostic Veterinary Parasitology. Mosby, Inc. St. Louis, MO; 1998;126-128, 265-272, 296-298.
Lok, JB; Knight, DH. A review of the treatment options for heartworm infections. Supplement to Veterinary
Medicine. June 1997:15-25.
Lok, JB; Knight, DH. Heartworm infections: Strategic timing of testing and prevention. Supplement to
Veterinary Medicine. June 1997:4-14.
Losonsky, JM. In Thrall, DE (ed): Textbook of Veterinary Diagnostic Radiology. W.B. Saunders Co. Philadelphia,
PA; 1994; 323-325.
Sousby, EJL. Helminths, arthropods and protozoa of domesticated animals. Lea & Febiger. Philadelphia, PA; 1982;
307-312.
Wright, JC; Hendrix, CM; Brown. Dirofilariasis. In: Zoonoses updates from the Journal of the American
Veterinary Medical Association. American Veterinary Medical Association. Schaumburg, IL; 1995:56-61.
three months finish their migration to the heart, where they grow into adults, sometimes reaching a length of
14 inches. The time from when an animal was bitten until adult heartworms develop, mate, and lay microfilariae
is about 6-7 months in dogs and 8 months in cats. (Remember this – it is important when we talk about diagnosis.)
Severely infected dogs can have up to several hundred heartworms in their hearts and vessels. Adult worms in
dogs usually live up to 5-7 years. Thirty to eighty percent of infected dogs have microfilariae, and the
microfilariae can live up to 2 years.
What damage do heartworms cause?
The adult worms can obstruct the different chambers
of the heart and the various large blood vessels leading
from the heart to the lungs. First, the right pumping
chamber (ventricle) of the heart and the large pulmonary
artery leading from it to the lungs becomes enlarged as
the worms take up space. If worms die, they may follow
the flow of blood into smaller vessels in the lung and
obstruct those vessels.
In severe infections, the worms can also block the large vein (vena cava) bringing blood to the right side of the
heart. As the blood backs up, the liver becomes enlarged and damaged.
What are the signs of heartworm infection and how is it diagnosed?
Dogs with heartworm infections may show decreased appetite, loss of weight, and listlessness. Often, the first
sign of the disease is a cough. Animals with heartworm disease will start to show lack of endurance during
exercise. Some will accumulate fluid in their abdomen (ascites) that makes them look pot-bellied. In rare
situations in which animals have many adult worms, the animals may die of sudden heart failure.
Antigen testing is performed to identify dogs infected with D. immitis. Because blood tests are not always
accurate, we need to interpret test results in relation to the history and the symptoms the animal is showing.
Radiographs (x-rays) are often taken to look for typical changes in the heart and lungs caused by D. immitis.
Changes include enlargement of the pulmonary artery and the right ventricle. Certain types of cells (eosinophils)
may increase in the blood or secretions from the lungs in heartworm infections. These additional findings can all
help support the diagnosis.
What tests are available to detect D. immitis infection in animals?
There are several blood tests used to detect heartworm infection. In the 1960's, before more sophisticated
tests were available, heartworm testing involved looking for the microfilariae in a drop of blood on a microscope
slide. A better test, the Knott's test, was developed to concentrate the microfilariae from a larger portion of
blood through centrifuging it. This gave veterinarians a better chance of finding the microfilariae.
Later, filter tests became available. In these tests, the blood cells in the blood were lysed (broken) by a
special agent that did not affect the microfilariae. The resulting liquid was then put through a very fine filter.
The microfilariae were concentrated on the filter. The filter was stained and examined under the microscope
for microfilariae.
Veterinarians soon recognized that some animals could have heartworm infections without having microfilariae
in the blood. This occurs if only male worms are present or if the females are not laying microfilariae at the
time of the test. It was obvious that better tests were needed.
Serologic tests were developed to identify antigens (small protein and carbohydrate components) of
heartworms in the bloodstream. There are different varieties of this test. Some test kits run one sample at a
time and can be done right in your veterinarian's office. Others are designed to test multiple samples in large
batches. This batch-type of test is generally performed by outside laboratories to which your veterinarian
sends your dog's blood.
Although the antigen tests were much better than the filter test, we still could not identify all cases of
heartworm infection because antigen tests will only be positive if female worms are present, since the antigen
detected is from the worm's uterus. If the heartworms were not fully mature, or there were only male worms
present, the antigen test result in infected animals would be falsely negative. This means the test result is
negative when the animal is really infected.
Serologic tests have now been developed to detect antibodies (proteins to fight off the 'invaders') the animal
makes against heartworms. This is the test most commonly used in cats. This test will be positive even if only
one male worm is present. But this test has a downfall, too. Although it is very good at giving positive results
when an infection is present, false positive tests are more common with this test than the antigen test. A false
positive result means the test result is positive when no infection is present.
What is included in a good heartworm prevention program?
The best program for prevention of heartworm infection includes using preventives, performing routine
heartworm testing, and reducing exposure to mosquitoes.
Heartworm preventives
Medications used to prevent heartworm infections are called preventives. The first thing to remember is that
preventives are NOT used to kill the adult worms. Special drugs called adulticides must be used to kill the
adults. These drugs will be discussed in the treatment sections. Some preventives can cause severe problems
if given to animals with adult heartworms or microfilariae. Follow the recommendations of your veterinarian and
the manufacturer of your preventive in regard to testing prior to giving the preventive.
A number of monthly heartworm preventives for dogs are on the market. There is also an injectable preventive
that is given once every 6 months. Some heartworm preventives, or drugs that are combined with them, will
control other parasites.
When determining when to give monthly preventives, remember, they kill larvae the animal was exposed to in
the preceding thirty days. For instance, a monthly preventive given on July 1, will kill larvae the animal acquired
from June 1 to July 1. The monthly preventives do NOT work forward in time. A monthly preventive given July 1,
will NOT have an effect on larvae the animal acquires later in July.
In some areas of the country where mosquitoes are present year-round, preventives must be given year-round.
In colder climates, preventives are started 2-3 weeks after the first mosquitoes appear and are given until
after the first hard frost (when the mosquitoes are killed or hibernate).
The most commonly used heartworm preventives for dogs in the United States.
Active Example Dose Given Mode Minimum age
Ingredient of at which Additional Additional Comments
Action preventive Ingredients Effects
can be given
Ivermectin Heartgard M; oral LA 6 weeks None N/A * See
below
Iverhart
Ivermectin Heartgard Plus M; oral LA 6 weeks Pyrantel Controls
Iverhart Plus roundworms * See below
and
hookworms
Milbemycin oxime Intercept M; oral LA 4 weeks, but Controls
puppies or dogs None hookworms,
must weigh 2 roundworms,
pounds or more and whipworms
Milbemycin oxime Sentinel M; oral LA 4 weeks, but Controls
puppies or dogs Lufenuron hookworms,
must weigh roundworms,
2 pounds or more and whipworms;
prevents flea eggs
from developing
Selamectin Revolution M; topical LA 6 weeks None In dogs, controls
fleas and ear
mites; treats
sarcoptic mange,
and controls the
American Dog Tick.
Moxidectin ProHeart 6 Every 6 months; LA 6 months None Treats hookworm
injectable infections
M = Monthly
LA = Kills larvae accumulating in host during preceding month; LI = Kills larvae as they are introduced
*Collies and related breeds are known to have a sensitivity to ivermectin. The dose of ivermectin in a monthly
heartworm preventive is relatively low. The manufacturers of Heartgard, Heartgard Plus, Iverhart, and
Iverhart Plus state the products should be safe in these breeds if used as recommended.
A preventive should be given to all dogs. Remember that mosquitoes can get indoors, so even though your dog
may not go outside, the dog is still susceptible.
Testing
When and how often pets should be tested for heartworm infection is also a matter of debate. In making a
decision on when to test, we must consider how common heartworm disease is where the pet lives, what
heartworm preventive the pet is receiving, and how long the mosquito season lasts.
The American Heartworm Society advises all adult dogs being started on a heartworm preventive for the first
time should be tested. After that, periodic but not necessarily annual retesting should be performed. The
frequency of testing should take into account such variables as the prevalence of heartworm disease in the
area, the possibility of missed or late doses of preventive, and the dog's lifestyle. Consult with your
veterinarian to determine the appropriate testing schedule for your dog.
Because of the heartworm's life cycle, it is advised that for dogs on a monthly seasonal preventive, any testing
should be performed no earlier than 6.5 months after the last date the preventive was given. In areas with
very short mosquito seasons, and for clients who carefully administer the monthly preventives and do not miss
any doses, testing at 2-3 year intervals is sufficient. It is recommended, however, that a test be performed
after the first year the dog has been on the preventive.
For a dog that receives a monthly heartworm preventive year-round, the American Heartworm Society
suggests that the interval between tests could be longer than one year. This is only if there were no missed or
late doses of preventive. If a dose was missed or late, the dog should be tested on an annual basis. It is also
recommended that a test be performed after the first year the dog has been on the preventive.
Testing puppies: In areas where heartworm preventives are given seasonally, a puppy that was not alive during
the previous mosquito season would not have to be tested. For instance, in northern Wisconsin, puppies born in
January do not need to be tested prior to starting them on heartworm preventive in the spring. Remember, any
puppy less than 6.5 months old is probably going to have a negative antigen test, since any larvae the pup was
exposed to would not have had time to mature and be detected by the antigen test.
Mosquito control
Reducing the exposure of a pet to mosquitoes can help prevent them from even being exposed to heartworm
larvae. For detailed information on mosquito control, see Mosquito Control and Preventing Diseases They
Transmit.
How is heartworm infection treated?
The first adulticide (drug to kill the adults) for dogs that was developed was thiacetarsamide sodium
(Caparsolate®), which contains arsenic. It has to be given in the vein through a catheter. If any drug gets
outside of the vein, severe tissue damage could occur. Some animals become quite ill from this drug, and
therapy sometimes has to be stopped. Almost all animals have to be hospitalized for the several days of
treatment.
Melarsomine (Immiticide®) is a medication that also contains arsenic. It is given by injection deep in the
muscles of the back instead of intravenously. It is less likely to cause side effects than thiacetarsamide and is
more effective. The treatment protocol depends on the severity of infection. Dogs with mild cases are treated
once daily for two days. For more severe infections, the dog is given one injection, then thirty days later is
treated once daily for two days. Four months after the treatment, the dog should be tested for heartworms
using the antigen test. Some animals may need to undergo a second round of injections at that time. It is
recommended that dogs also be on a monthly heartworm preventive during the treatment.
Regardless of which drug is used, when the adult heartworms die, they can obstruct blood vessels to the lungs
(these are called pulmonary embolisms). If only a small part of the lung is involved, there may be no clinical
signs. However, if the vessels to a large portion of the lung, or a small area of an already diseased lung are
blocked, severe signs may result. These include fever, cough, coughing up blood, and even heart failure. Because
of the risk of these embolisms, any dog being treated with an adulticide must be kept very quiet during
treatment and for 2-4 weeks thereafter.
In very severe infestations, adult worms are removed from the heart surgically.
Can humans be infected with heartworms?
Yes, there have been instances of heartworm infection in people. Instead of migrating to the heart, the larvae
migrate to the lungs in humans. There the larvae can block vessels causing an infarction. At the site of the
infarction, a nodule develops which can be seen on radiographs. Usually, the person has few, if any signs of
infection. Surgical removal of the nodule is sometimes necessary.
References and Further Reading
American Heartworm Society. Recommended Procedures for the Diagnosis, Prevention, and Management of
Heartworm (Dirofilaria immitis) Infection in Dogs. Batavia, IL; 1995.
Atkins, CE. Comparison of results of three commercial heartworm antigen test kits in dogs with low heartworm
burdens. Journal of the American Veterinary Medical Association. 2003;222(9):1221-1223.
Blagburn, BL. Important heartworm basics for the practicing veterinarian. DVM Best Practice. 2003;March:12-
15.
Calvert, CA. Heartworm Disease. In Birchard, SJ; Sherding, RG (eds.) Saunders Manual of Small Animal
Practice. W.B. Saunders Co. Philadelphia, PA; 1994;487-493.
Datz, C. Update on canine and feline heartworm tests. The Compendium on Continuing Education for the
Practicing Veterinarian. 2003;25(1):30-41.
Georgi, JR; Georgi, ME. Canine Clinical Parasitology. Lea & Febiger. Philadelphia, PA; 1992;193-199.
Griffiths, HJ. A Handbook of Veterinary Parasitology. University of Minnesota Press. Minneapolis, MN; 1978;60-
62.
Hendrix, CM. Diagnostic Veterinary Parasitology. Mosby, Inc. St. Louis, MO; 1998;126-128, 265-272, 296-298.
Lok, JB; Knight, DH. A review of the treatment options for heartworm infections. Supplement to Veterinary
Medicine. June 1997:15-25.
Lok, JB; Knight, DH. Heartworm infections: Strategic timing of testing and prevention. Supplement to
Veterinary Medicine. June 1997:4-14.
Losonsky, JM. In Thrall, DE (ed): Textbook of Veterinary Diagnostic Radiology. W.B. Saunders Co. Philadelphia,
PA; 1994; 323-325.
Sousby, EJL. Helminths, arthropods and protozoa of domesticated animals. Lea & Febiger. Philadelphia, PA; 1982;
307-312.
Wright, JC; Hendrix, CM; Brown. Dirofilariasis. In: Zoonoses updates from the Journal of the American
Veterinary Medical Association. American Veterinary Medical Association. Schaumburg, IL; 1995:56-61.
