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An
Overview of Canine Heartworm Infections
Canine
heartworms, scientific name Dirofilara immitis and members
of the nematode family, were discovered more than a century ago and
were once only a problem in southern climates. Today however they are
recognized as a major global parasite affecting dogs, wolves, coyotes,
foxes, and occasionally cats and ferrets. Because it is such a devastating
disease, veterinarians actively encourage all patients to treat and
prevent heartworm using a wide variety of drugs and medications. Thankfully,
modern drugs are highly effective, but it is vital that every owner
remain current with preventative medication for his or her dog.
Why and how might my dog become infected?
The heartworm
requires two separate hosts in order to complete its six- or seven-month
life cycle – a mosquito and a mammal such as a dog or a cat. Adult
worms in already-infected animals live in the chambers
of the heart and the pulmonary
arteries. The adults reproduce and large numbers of microfilariae
are discharged into the bloodstream. A mosquito ingests microfilariae
when feeding on an already-infected mammal and then serves as an intermediate
host for the larval stage of the worm. The larvae develop in the gut
of the mosquito for one to four weeks and then migrate to the mouthparts
of the mosquito and wait to be injected into a new host. As an infected
mosquito pierces the skin of a potential host, the matured larvae enter
the body, molt,
and move to the chambers of the heart over the course of several months.
Once the larvae reach the heart and/or pulmonary arteries, they mature
into adult worms and may remain alive for up to three years. Heartworms
may reach 12 inches in length, and a single, heavily-infested dog may
harbor up to 250 parasites.
How is a heartworm infection diagnosed?
The first
sign of a heartworm infection is generally a soft cough that can worsen
with exercise but is often mild enough to be easily dismissed. As the
number of adult worms increases however the cough will worsen and the
dog will have more and more difficulty with physical activity. Infected
animals tire easily, lose weight, and become weak and listless. The
once-mild cough may worsen yet again and begin to produce blood. If
heartworms are left untreated even walking will become impossible for
the dog and death will eventually occur as a result of congestive heart
failure.
Vena caval
syndrome may occur when, left to their own devices, a mass of worms
leaves the heart and becomes lodged in the posterior vena
cava. This leads to total loss of appetite, collapse, and
almost certain death.
If suspicion
of a heartworm infection has been aroused, ultimate diagnosis is generally
achieved via blood tests conducted in a diagnostic laboratory. The ELISA
test is capable of detecting antigen even if microfilariae are not yet
present in the bloodstream. If the disease has progressed to such a
stage where microfilariae have been produced and are in the blood, a
Knott’s test
and filtering procedures may be used to isolate the larvae. Radiographs
may also be useful as they can visually detect the presence of adult
worms in the heart.
What are the treatment options for dogs with heartworm infections?
All but
the most severe cases of heartworm can usually be successfully treated.
The goal of treatment is to kill all living worms and all living microfilariae.
Generally, those dogs with only mild symptoms and a lighter worm load
will have the easiest road to recovery. The greater the level of infection
(i.e. the greater the number of worms), the more complications might
arise during treatment procedures. Before any course of treatment is
begun, the individual dog must be examined to determine if he is strong
enough to withstand the trauma of therapy.
Once the
health of the dog has been established, the first step is to kill all
adult worms using an arsenic compound. The drug is administered in two
doses a day for two days, followed by a period of inactivity to allow
the dog’s body a chance to absorb the dead worms. Three to four
weeks after administration of the initial arsenic
compound, the second step is to kill the microfilariae that are still
living in the dog’s bloodstream. This requires a daily dosage
of medication for seven days. At the end of the week, blood tests are
done to see if microfilariae can be detected. If a cure has not been
achieved, the dosage is increased until no traces of the larvae remain.
Follow-up studies are recommended up to a year after initial treatment
to ensure that all worms and larvae have in fact been killed.
Surgical
removal of worms has been indicated in some extremely advanced cases
of heartworm infection, but this is rare and carries inherent dangers.
If your dog is diagnosed as having heartworms, your veterinarian is
the best source of information regarding treatment and follow-up therapy.
What steps can I take to minimize the risk of infection for
my dog?
Modern
veterinary medicine has blessed us with a vast array of drugs to prevent
heartworm infection in our dogs. The products come in many different
forms, including injections, daily or monthly tablets, and monthly topical
medications. Which form of prevention is right for your dog will depend
upon where you live and your individual needs. Your veterinarian will
be more than happy to provide you with the information you need to decide
upon the proper course of prevention. What is most important is that
preventative measures are taken, as heartworms can be devastating to
your dog’s health.
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