Heartworm 103 - Those Pesky Cats

Cats are not just small dogs, and feline heartworm disease is a prime example. The first two articles of this series dealt with how heartworms cause disease and preventative/diagnostic strategies primarily in relation to dogs. Today's article will outline specifically how heartworms effect cats; in many ways they are a more frustrating and elusive problem in cats, made all the more dangerous because many owners don't realize feline heartworm is a serious concern.

Accidental Guests: Heartworm Disease in Cats

Cats are not the natural host for heartworms. Unfortunately, no one seems to have explained this to the mosquitoes that spread infection or the heartworm larvae that cause it. Consequently, these uninvited and unadapted parasites really, really tick off the cats immune system. The first consequence of this is positive - cats seem much better than dogs at killing off larvae before they develop into adults. Studies suggest only about one-third to one-half of naturally infected cats develop adult worms, though it's tough to nail down accurate percentages due to the difficulties in testing that we'll go into later. 

Hey, human. Indoor cats get
heartworms too.

The second consequence of cats' super-miffed immune system is a negative one, however. Killing off the larvae within the lungs results in significant inflammation. Vessels and lung tissue gets thickened, making blood flow and air exchange less efficient and resulting in the classic signs of respiratory disease - coughing, rapid or labored breathing, and general malaise. This is now referred to as heartworm-associated respiratory disease (HARD), and it is a major reason why I've started recommending all cats receive monthly preventatives. Depending on the studies you look at, perhaps as many as 15% of cats in some areas are exposed to heartworms and thus at risk for HARD - even if they never develop adult worms. And if you think your kitty is free and clear because it's indoor only, think again - in one published series of cases, 27% were indoor-only cats.

What about the ones who do develop adult worms, though? Whereas dogs may have dozens of adults, cats rarely have more than two or three - again, their immune system is pretty good at whacking those larvae. This means many cats with adults have such a low burden they may show none of the cardiovascular problems typical in dogs. The caveat to this is that when those adults die off after a year or three, the dead worms are big enough to block off major lung vessels - and between that and the cat's once-again huge inflammatory response it can be enough to quickly kill them. Indeed, most veterinary texts list "sudden death" as a major "sign" of feline heartworm infection. That's not the way I like to diagnose things.

Diagnosing Heartworms in Cats...

...is really, really frustrating.

In the second article we talked about how the main test used to diagnose heartworm in dogs looks for a protein produced by adult female worms. This test works equally well in cats, if there is an adult female worm. In one study on naturally infected shelter cats over 50% had only a single worm, and none had more than four. This means cats are fairly likely to have male-only infections, and thus come up negative on this test - even though a single male could potentially kill them. 

Cats aren't good at tests.

Even more importantly, a lot more cats will develop the serious respiratory disease discussed above (HARD) without ever developing adult worms. On the bright side, there are antibody tests available that check for exposure to heartworm larvae. These would be great for diagnosing HARD or even male-only adult infections...except studies suggest they aren't that sensitive and miss many exposed cats, maybe more than 50% of them. 

Not surprisingly, I don't spend much time screening outwardly-healthy cats for heartworm. I'll run both tests when I suspect a cat has HARD or adult worms...but if they're negative, I don't cross heartworm off the list. Chest x-rays or ultrasound of the heart can be useful, but diagnosing feline heartworm can be elusive even when you're sure that's the problem.

Treating Feline Heartworm

Here's a quick topic. Remember how one dead adult may be enough to gunk up the lungs and cause sudden death? We can kill the worms easily enough, but we're just as likely to kill the cat. Yeah...we don't treat feline heartworm. "Treatment" is pretty much relegated to using anti-inflammatory drugs to control the signs and hoping for the best when the worms do die off. It's pretty frustrating when you find yourself crossing your fingers and wishing a freeloading parasite a long, healthy life.

Heartworm Preventatives for Cats

Most veterinarians used to be wishy-washy at best about recommending monthly preventatives for cats because adult infections were relatively uncommon, and serious complications (like death) even moreso. But the respiratory complications - which can potentially be life-threatening themselves - are looking more and more common. Complicate this by the fact we don't have a great diagnostic method  (meaning we likely are underestimating prevalence) and hopefully you're starting to understand why I've started recommending all cats stay on a monthly preventative. 

As of this writing, there are two topical products and two oral products on the market, and all are effective for heartworm prevention. Both topicals are great products, covering fleas and some common intestinal parasites while sparing you the adventure of trying to get a pill into a cat. Neither oral product covers fleas, but Interceptor covers several of the more common intestinal parasites. It's a good alternative in areas where fleas aren't a concern as it's a fair bit cheaper than the topicals.

I've covered a lot of ground in these three articles, and in a good bit of depth. If you're not 100% convinced that heartworm is a serious risk that warrants monthly preventatives, I'm happy to address any lingering questions or concerns in the comments section below!

Heartworm 102 - Prevention & Testing

Click here for Part I.

In the last article, we talked about what heartworms are and how they affect pets. You might check that one out first if you missed it, as a basic understanding of the life cycle is helpful in understanding today's topics - prevention and testing.

Prevention Strategies

Yeah, Joe was skeptical too.

I'm forever the skeptic, so I'll admit if someone told me I needed to buy pills for my pets every month for life I'd be the first to scan their eyes for dollar signs - veterinarian or otherwise. I certainly understand when the occasional client looks at me incredulously, or asks about just giving during the summer. But we aren't making this stuff up - with a little understanding of preventatives and the biology of disease, you should agree.

First off, it's worth noting that we're generally talking about preventing disease, not infection. The major problem with heartworms are adults (cats are a bit different, but we'll address that in the last article), and it takes about six months for them to mature after infection. The microscopic larvae that cause initial infection are susceptible to low doses of certain anti-parasite drugs, so the idea is that by giving pets a dose at regular intervals we can stop the disease (adult worms) from developing, even if we haven't stopped infection per say. That's good, because as anyone who's been to Louisiana in July will tell you, it's bloody near impossible to keep from getting a bitten by mosquitoes no matter what you do. And it only takes one.

So, why monthly instead of every six months? That's how long it takes to get adults, right? Well, the trouble is that as they age, larvae develop resistance to those relatively safe anti-parasite drugs. Effectiveness gets unpredictable 4-5 weeks after infection, and eventually they become pretty impervious to those drugs. We have to resort to drastic measures to kill adults - in fact, the standard treatment for adult infection is in the arsenic family. So monthly dosing is the safest bet (and a lot easier to remember). On the flip side, it's worth noting that if you miss a dose by a week or two you'll probably be okay. No guarantees, but it makes for easier sleeping when life (or memory) inevitably mucks with your schedule.

And why every month? Mosquitoes don't wear snowshoes, right? I could tell you that "heat islands," warm areas near houses and the like, may create microenvironments where mosquitoes can hatch even in the winter. I could tell you it only takes one infected mosquito to threaten the life of your pet. I could tell you many preventatives have important benefits other than just heartworm protection, and I'll discuss those below. All are true, but I'll level with you. Even if I could guarantee your pet wouldn't see a mosquito until April 1st or after October 31st (I can't), I know most of us - myself included - would forget to start until sometime in May or miss that last dose in the fall. And say you snowbird it somewhere warm for Christmas - what then? It's just easier to be consistent if you're on a consistent schedule. And where heartworm is concerned, consistency can save pets' lives.


Types of Preventatives
The currently available preventatives fall into three broad categories - monthly oral or topical products, and one injectable product that's given every six months. All of these are valid, effective choices for heartworm prevention; the main difference between products is what they cover in addition to heartworm.

Most oral and topical products have additional anti-parasite drugs that worm your pets monthly for GI parasites they're likely to get into, such as roundworms and hookworms. This is actually another big argument for monthly preventatives, as those are parasites people can get - specifically, those small people who rarely wash their hands and like to suck on their fingers after playing with the dog. Roundworms would likely survive a nuclear fallout, so they can certainly infect your pet in the middle of winter, and that monthly "heartworm pill" is a good way to keep them from infecting your three-year-old in turn.

Certain products will cover additional parasites such as tapeworms, whipworms, and even fleas. To my knowledge no single product currently covers everything, so it's worth talking to your veterinarian about what product or combination of products best covers the risks specific to your area and your pets lifestyle. I should also point out that's the big drawback to the "heartworm shot" - it doesn't provide monthly protection against all that other disgusting stuff pets tend to get into, just heartworm. It will kill off hookworms, but only every six months at the time the shot is given. For most dogs I recommend an oral heartworm product in addition to a topical flea-and-tick; for cats I usually recommend a topical product since getting an oral product in them can be a feat in itself.

Heartworm Testing
Yes, your dog should be tested for heartworm yearly even if you give monthly preventatives. Nothing is 100% effective, and disease caught early and treated early does far less harm. There are many scenarios that might lead to treatment failure, such as:

• You miss a dose for whatever reason. This is by far the most common situation - heck, I've  missed doses.
• Your pet has some indigestion and throws up the pill, unbeknownst to you.
• Your pet goes swimming shortly after a dose, or another pet grooms it off them.
• The medication has been mishandled and has lost its effectiveness. Heat can destroy most drugs - say, you forgot and left it in the car for a few summer days, or you bought it from somewhere other than your veterinarian and it was inappropriately handled (that's a whole other can of worms, and the subject of a future article). 

There has also been some concern over possible resistance developing to the drugs we use; at this point reports of "treatment failures" are few, and I'm fairly sure most could really be traced back to one of the above reasons. However, realistically we have to expect that some degree of resistance will develop eventually. 

It's worth noting the standard heartworm test looks for the presence of adult female worms. Think back to the first article where we discussed the parasite's life cycle and you will realize this means the test won't be positive until at least six months post infection. Likewise, if there are only a few worms and all are male, the test would be negative as well - I don't generally recommend regular testing for cats partially due to this, but we'll come back to that in the last article. It's complicated.

So, to summarize - all pets should get monthly preventatives (or the six-month shot for dogs), and dogs should be tested annually. Hopefully it's clear now why that's in their best interest. Please comment with any lingering questions and I'll gladly address them. In the final article, we'll talk specifically about heartworms and cats. They have some unique concerns and complications - as a professor liked to remind us back in school, "they aren't just small dogs."

Heartworm 101 - Life and Times of a Freeloading Parasite

I have a tendency to be long-winded, so I'm going to simplify this for the impatient folks out there because the message is very important. Heartworm disease kills dogs and cats. Treatment is risky and expensive. Heartworm is (fortunately) easily preventable. Therefore, all dogs and cats should be on year-round monthly preventatives.

It really is that simple. However, if you want more than just my word on that - if you want to know what heartworms are, how infection and disease occurs, and why giving those preventatives monthly is so important - keep reading.

What Are Heartworms?
Heartworm is the common name for Dirofilaria immitis, a large roundworm parasite very similar to those disgusting white intestinal worms that dogs and cats become infected with from time to time. While the commonness of infection varies from place to place, heartworm has been diagnosed in all 48 continental states and many other countries as well. Their life cycle is rather complicated, but a basic understanding goes a long way toward both understanding the disease and making sense of veterinary recommendations about preventatives.

Microscope view of heartworm larvae
in a dog's blood.

Dogs are the natural host for adult worms, which infect the large arteries of the lung, and in heavy infections are found in the heart itself. Males and females mate to produce larvae called microfilaria (i.e., small threadworms). These microscopic worms live in the bloodstream for up to two years, waiting to be sucked up by another disgusting little critter - mosquitos. This triggers the larvae to mature through a couple more stages, eventually winding up back in the salivary gland of the mosquito - just in time to get injected back into another unsuspecting dog. Over the next six months, those microscopic larvae will travel to the vessels of the lungs and mature into adult worms that may be nearly a foot long, ready to mate and start the cycle all over again.

How Do Heartworms Harm Pets?
It doesn't take a medical degree to realize that bunch of worms clogging up the bloodstream is unhealthy. Indeed, when many worms are present they interfere with bloodflow into the lungs and force the heart to work harder, leading to classic signs such as exercise intolerance and coughing. The severity of such problems are largely a function of how many worms are present - pets with three or four worms may show no overt signs, while those with dozens will likely be in rough shape.

Heartworms inside a dissected heart.

Simple mechanical blockage of the heart and vessels is far from the only problem, however. For starters, the immune system is really, really not a fan of freeloading parasites. Significant inflammation can result in the lungs from both adult and juvenile worms; indeed, research is pointing toward juvenile worms as being a major cause of feline asthma syndromes.  In response to the worms, the immune system produces massive amounts of  antibodies - proteins directed at identifying and helping destroy invading microorganisms. Unfortunately, for all its bluster the immune system is pretty impotent to deal with the worms, and winds up doing much more harm than good. The arteries the worms live in thicken from inflammation, and all those antibodies wind up settling out in small vessels - particularly in the kidneys, where they may cause a fair bit of damage.

One of the most dangerous complications of heartworm disease happens when the worms die. Dead adults get pushed downstream until they lodge in smaller vessels of the lungs, forming an embolus that cuts off blood to that part of the lungs until the body can break it down. Enough of those dead worms in the wrong place can be fatal. Furthermore, once adults set up shop in the cadiovascular system they begin pumping those tiny microfilaria into the bloodstream - potentially millions of them. Strangely, these don't seem to cause much trouble while alive; however, they turn the pet into a walking infection source for every mosquito in the area, which in turn may infect other pets. When the larvae die, though, the dead parasites trigger the immune system and cause microscopic abscesses where they land. This probably isn't a big deal when most of them are alive, but it is a major concern when treating pets with the disease, as millions dying at once can potentially send an animal into anaphylactic shock. Both of these are primary reason why infected animals need to be under strict exercise restriction, and why treatment must be undertaken with close monitoring at the hospital.

So, that's what heartworms are and what they do. In the next article, we'll talk about preventatives - what options are out there, why giving them every month is important, and about the benefits they provide aside from heartworm protection. We'll also talk about screening tests, particularly why we recommend yearly testing for all pets - even those on preventatives.

Click here for Part II - Prevention and Testing.