Animal Health Issues
Fleas Are Just Around The Corner
There are a few considerations for pet owners for this time of year.
Flea control: Fleas are usually much less of a problem this time of year, as most of the population is in the pupating (cocoon) stage. But once the first hit of warm weather is upon us, all the fleas that have layed dormant as cocoons over winter, hatch at once! You can then suddenly find your pets are seething with them! If you haven’t used flea control over winter, it is worth starting up again early spring/late winter, particularly if you have had serious flea problems in the past, as this means you are likely to have huge numbers of pupating fleas in the house and/or yard. The aim is to kill off the emerging adult fleas (there are currently no preparations that will kill pupating fleas) before they reproduce. Waiting until middle of spring when the first hatching has become adult and produced the eggs is like chasing a runaway truck - the fleas are much harder to get under control once they have reached this stage.
2. Cats coming into season: Cats generally have their first season at six or seven months of age, but the increasing day length speeds up hormonal development and kittens at this time of year tend to come into their first season earlier, sometimes when only four months old. People who buy new female kittens between now and late spring should be aware of this, as it is easy to get caught out with an unexpectedly pregnant cat. Please speak with your veterinary staff members about having your pet desexed!
Canine Heartworm
Heartworm is caused by the nematode (roundworm) Dirofilaria immitis. This is a worldwide disease, more common in tropical climates (where mosquitoes breed year round) and areas of poor mosquito and heartworm control (more reservoirs of infection).
The Heartworm Life Cycle
It is spread between animals by mosquitoes, which ingest the first larval stage (known as an L1 or microfilaria) when they bite an infected host. The L1 larva must moult twice within the mosquito to become an infective L3. They then enter the new host when the mosquito bites the another dog. The larva then moult into an L4 and finally the adult L5 and then migrate via the blood to the pulmonary arteries between the heart and lungs. The maturation process within the canine host takes 5-6 months between initial infection of an infective L3 larva from a mosquito and adult female worms in the blood releasing the L1 microfilariae.
The important point is that mosquitoes are required for a larval heartworm to mature to the adult. Thus dog to dog transmission cannot occur. Although the L1 larva produced from the female heartworms in an infected bitch can cross the placenta to her unborn pups, these will not mature into adult tapeworms in the pups. Likewise if a dog was infected with L1 microfilaria from a blood tranfusion, they will not mature.
Heartworm disease
The disease is caused by the adult worms, which may be up to 20cm in length. The worms preferentially lodge in the pulmonary arteries between the heart and the lungs. (In heavier infestations, worms may invade the chambers of the heart itself.) The presence of the worms causes irritation the inside of the blood vessels, causing the normally smooth inner surface to become thickened, irregular and narrowing the vessel diameter. This causes increase in the blood pressure in the lungs (pulmonary hypertension), decreased blood flow to the lungs and strains the heart. Over time the heart becomes abnormally enlarged, the blood vessels thickened and fibrosis (scarring) of the lungs occurs.
The severity of disease depends on how many adult worms are present, how long the infection has been present and the severity of the host’s immune response. A mild early case may look anywhere from near normal, with slight loss of condition and earlier fatiguing during exercise. Severe cases exhibit weight loss, persistent coughing, gaspy breathing, severe intolerance of even mild exertion and right sided heart failure. The disease is generally slowly progresssive and can be fatal.
Diagnosing heartworm disease
1. The presence of adult heartworms is easily detected using blood tests - these are quick in-clinic tests requiring only a few drops of blood (eg. Witness or snap heartworm test). These only test for adult worms, thus will only detect an infection more than 6-7 months after the dog has been infected.
2. The filtered blood can also be examined for the presence of L1 microfilaria (difil test). For these to be present, there must be both male and female adult worms present, so once again this will only detect an infection more that 6-7 months old.
3. The severity of disease is graded based on the clinical signs and appearance of the chest x-ray (to assess the amount of damage to heart, lungs and blood vessels within the chest).
Heartworm treatment
Heartworm infection can be treated, however it is expensive and long winded. The problem is that once the worms are killed, this can cause a severe and potentially fatal immune response to the degenerating worms. The fragments of dead and dying worms attract the attention immune system and can form fatal blood clots (thromboembolisms). After receiving their medication, these dogs are kept in hospital in strict confinement and close supervision for several days, and restricted exercise for the next 2 months.
Drugs to aid the damaged heart and lungs may also be required - this may be lifelong even after the worms have been killed, as some of the changes in the heart, lungs and blood vessels is irreversible.
Prevention is better than the cure!!
The only foolproof way to prevent heartworm disease is to use heartworm control from puphood. Most people start at the 2nd or 3rd pup vaccination. There are many options - injections (proheart injection-annual in an adult dog) monthly heartworm tablets or chews Sentinel, interceptor, heartgard) or spot ons (revolution). Many of them are combined with intestinal wormer and flea control products. The old fashioned daily heartworm tablets had to be given every day without fail to avoid infection and have basically been phased out.
Controlling mosquito populations also helps. Don't let buckets, pot plant bases etc fill up with water that mosquito wrigglers can live in.
If a dog has been off heartworm control for more that 6 months, then they should be tested before starting on heartworm control again.
Can cats get heartworm?
Yes, although it is less common. The parasite has evolved to a canine host, not a feline one. Thus, cats usually have a lower worm burden, the worms are smaller and have a shorter lifespan, and even when both sexes of heartworm are present, microfilaria will only be produced about 20% of the time. The problem is that the worms are much larger relative to the width of the pulmonary artieries they live in than in the dog, nearly blocking them, and there is a very marked immune response. Thus the chances of problems due to blood clots, particularly in the lungs (pulmonary thromboembolisms) are much higher.
Feline heartworm preventatives include revolution top spot and milbemycin monthly tablets. Preventatives should be used from kittenhood and maintained lifelong.
