Heartworm
disease
Heartworm
is a serious and potentially fatal disease in dogs. It is caused by
a worm called Dirofilaria immitis, of which the adult lives in the
heart and pulmonary arteries. The female worm is 2.3 to 5.5 cm long
and 5 mm wide; the male is about half the size of the female. One
dog may have as little as 1 or as many as 300 adult worms.
Lifecycle
The
average life span is up to 5 years and, during this time, the female
produces millions of young (microfilaria). These
microfilaria live in the bloodstream, mainly in
the small
blood vessels near the skin. The immature heartworms cannot complete
their life cycle in the dog; a mosquito is required for some stages
of the heartworm life cycle. The disease is not spread directly from
dog to dog. The mosquito, is required for transmission As many as 30
species of mosquitoes can transmit heartworms. The female mosquito
bites the infected dog and ingests the microfilaria during a blood
meal. The microfilaria develop further for 10 to 30 days in the
mosquito and then enter the mouth parts of the mosquito. The
microfilaria are now called infective larvae because at this stage
of development, they will grow to adulthood when they enter a dog. The
mosquito bites the dog where the hair coat is thinnest. Having
long hair obviously does not prevent a dog from getting heartworms.
(The mosquito will just bite the dog on the thinly haired areas,
nose, inner thigh and bellie) When fully developed, the infective
larvae enter the bloodstream and move to the heart and adjacent
vessels, where they grow to maturity in 2 to 3 months and start
reproducing, thereby completing the full life cycle.
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Symptoms
It
takes a number of years before dogs show outward signs of infection.
Consequently, the disease is diagnosed mostly in 4 to 8 year old
dogs. Heartworm cause disease by clogging the heart and the major
blood vessels of the heart. They interfere with the valve action in
the heart causing heart disease and by clogging the main blood
vessels, the blood supply to other organs of the body is reduced,
particularly the lungs, liver and kidneys, leading to malfunction of
these organs. The signs of heartworm disease depend on the number of
adult worms present, the location of the worms, the length of time
the worms have been present, and the degree of damage to the heart,
lungs, liver, and kidneys from the adult worms. The
most obvious signs are: a soft, dry, chronic cough, shortness of
breath, weakness, nervousness, listlessness, and loss of stamina. All
of these signs are most noticeable following exercise, when some
dogs may even collapse.Severely infected dogs may die suddenly during
exercise or excitement.
Listening
to the chest with a stethoscope will often reveal abnormal lung and
heart sounds. In advanced cases, congestive heart failure may be
apparent and the abdomen and legs will swell from fluid accumulation.
There may also be evidence of weight loss, poor condition, and
anaemia.
Diagnoses
In
order to diagnose the presence of heartworm we may need to carry out
some tests. Blood tests for adult antigen or microfilaria, Blood
chemistries: routine blood test to examine the function of abdominal
organs and radiographs and ultrasonography: to examine the changes to
the heart and major vessels if heartworm is present.
Treatment
Treatment
of heartworm is complicated and has risks involved with it, if your
dog is diagnosed with heartworm infection we will discuss the
treatment and risks involved. The treatment can last 1-2 months, and
even then may not be effective. If there is permanent damage to the
heart or abdominal organs this will still be present even after
eradication of the infection.
Prevention
Prevention
is in this case far better than cure. Prevention involves killing the
microfilaria before they develop in to adults. This can be done in a
number of ways.
- Daily
treatment is not used much these days as there are safer and more
effective preventions available.
- Monthly
treatment which are given orally or topically. They will kill the
microfilaria (infective larvae) in the dog before they reach
maturity. If the is a gap of 2-3 months there is a chance that larvae
may have matured into adults and will not be killed by the monthly
treatment.
- Yearly
treatment -Dogs may also be given a yearly Proheart injection to kill
any infective larvae. The Proheart injection is given from 3 months of
age. When given at 3 months of age it will remain at effective levels
for 6 months NOT a year ( this is because the dog is still growing
and will not weight the same at 9 months of age as 3 months of age)
if given from 9 months of age the levels will be effective for a full
12 months.
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