Those darn allergies!
February 20, 2014
From the desk of Dr. Voorheis
Allergies. They make us sneeze and cough. They make our eyes burn and our skin itch. They give us headaches. They can even cause severe allergic reactions to things like certain foods, medications or even insect bites. They are inconvenient at best and miserable at worst. But did you know that our pets can have allergies too? Indeed they can! The main difference is that when pets are allergic to something, they usually itch instead of cough and sneeze etc. Allergies are one of the most common things we treat at WBAH. We see it day in and day out with so many of the critters entrusted to our care. And just as it is with humans, allergies cannot be cured but they can be successfully treated and managed.
“Allergies” is a massive topic to take on in one blog entry. I think the best approach, so as not to overwhelm anyone, is to provide an overview this week and then concentrate on the specifics over the following 2 weeks. By then you will know everything you ever wanted to know about pets and allergies! It is interesting information and you will be an expert by the time we’re done.
You might recognize some of the terms used in discussing allergies from my entry on vaccinations a few weeks back. Skin allergies are the most common type of allergy in my patients, so my blog will focus on skin allergies over the next two weeks. I will also touch base on allergies that affect the GI tract (gastrointestinal). So let’s get started with that overview!
In the world of an immunologist, an allergy is a type of “hypersensitivity reaction”. Immunologists classify hypersensitivity reactions at type 1, 2, 3 or 4. Hypersensitivity reactions 1 and 4 are concerned with the skin. Type 1 is called an “immediate” hypersensitivity and type 4 is “delayed hypersensitivity”. Unfortunately, knowing something is an “immediate” hypersensitivity does not tell us WHAT that hypersensitivity is caused by. Clinicians like myself classify allergies by type of allergy. Examples are food, atopy (pollens, molds, house dust mites), contact dermatitis or proteins of foreign creatures (fleas, insects etc). To help us decide what categories an apparent allergy falls into, we consider the location on the body where the animal is itching. Here is a more detailed breakdown:
Type 1 hypersensitivity
You’ve heard of people who carry an epinephrine injection where ever they go because they might die if stung by a bee? In veterinary medicine, life threatening allergic anaphylaxis is uncommon. I’ve seen it very rarely and most often I see it associated with a vaccine reaction. With better vaccines available today, it is extremely rare. Many skin allergies, from flea allergies to atopy, fall under this category. So what is Type 1 hypersensitivity? In a nutshell, this type of hypersensitivity is also commonly called an allergy, although it is not the only type of allergy. If an immediate hypersensitivity reaction is life-threatening, it is called allergic anaphylaxis or anaphylactic shock. This is probably a term that is familiar. More about how and why this happens in parts two and three of the allergy blog series.
Type 2 hypersensitivity
I will not be discussing this in detail in the allergy blog. In brief, these reactions are called cytotoxic hypersensitivity. These occur when antibodies (and some other proteins) destroy normal cells. Examples of this are the destruction of transfused red blood cells when administered to a mismatched recipient (blood transfusion gone wrong). Certain drugs and infectious organisms can bind to the red cell membrane and change its shape. Thus, it is no longer recognized as “self” and is now considered “foreign and dangerous”. So the cell is removed from circulation, resulting in hemolytic anemia (low red blood cell count). However, most cases of IMHA (Immune Mediated Hemolytic Anemia) in dogs cannot be linked to a medication or infection.
Type 3 hypersensitivity
This is tough stuff. With this, we are dealing with reactions or immune complex reactions. Complicated immune reactions occur when antigens and antibodies combine to form immune complexes. Immune complexes can trigger severe inflammation when deposited in large amounts in tissues. In a localized form, this can be a cause of “blue eye” which is seen in dogs infected with canine adenovirus type 1 or vaccinated with a vaccine that contains adenovirus type 1. Cases of blue eye from vaccinations resolve spontaneously in dogs. Immune complex disease is also responsible for a type of kidney disease called glomerulonephritis.
Type 4 hypersensitivity
When injected into the skin, some antigens induce a slowly developing inflammatory response called a “delayed” or type four hypersensitivity. These reactions are mediated by T Cells which are a type of white blood cell. A good example of delayed hypersensitivity is the TB test. Some of you may remember how you have to go back to have a TB test “read”. This reaction falls under the category of “delayed” or type IV hypersensitivity. Skin allergies, allergic contact dermatitis, flea allergies and atopic dermatitis can also be delayed.
So as you can see, there can be considerable cross over between type 1 and type 4 allergies. Knowing whether something is type 1 or type 4 doesn’t change much about how we treat it. Nor does it help too much to know the reason behind why an individual animal is “itching”.
Location and Season
Ok, so far we have learned that if we are talking about skin allergies, we are talking about type 1 and type 4 hypersensitivities. We also now know that knowing what type of allergy it is doesn’t help too much in diagnosing what our pets are allergic to. But how about the location of the itching? Or time of year? Do these things help at all in our quest for answers? In general, these things can help anywhere from a little to a lot. It turns out that “location, location, location“ is important in more than just real estate. It’s also important in diagnosing what your dog is allergic to.
It is important to note that these are general guidelines and there is cross-over. This is by no means an exact science. In other words, a dog with a food allergy can look like a dog with atopy (allergy to pollens, molds, house dust mites, etc). And a dog with atopy can look like a dog with a flea allergy. In addition, dogs can have both a flea allergy and atopic dermatitis. They can have a flea allergy along with a food allergy. Some dogs hit the lotto, and have allergies to all three! Tons of fun for mom and dad! What about our kitties? Same holds true for them unfortunately.
So what do I mean by “location“? It turns out that where a dog licks and itches, gives us a big hint as to what is causing the itch. Let’s start with the “front half” of the dog vs. the “back half” of the dog and ask ourselves where the signs/symptoms are appearing. If the dog is itching at the back half meaning the area just in front of the base of the tail and sometimes extending in a V shaped wedge along the middle of the back as well as the back of the thighs and the underbelly then that dog has a flea allergy. Period, end of story. Ever wonder why this particular problem is far more prevalent in warm seasons than winter time? It’s because a flea can rapidly go through an entire life cycle in warm weather. I will go into far more detail about flea allergies in next week’s blog. Fascinating stuff!
What about the “front half” of the dog? In general, dogs that rub their face and ears and lick the tops of their front feet (and in some cases their rear feet as well) are giving us strong hints that they may have atopy or a food allergy. Let me be very clear here. There is no way to tell the difference between an atopic dog and a food allergy dog based on those signs. They look exactly the same. Seasonality may come into play for some atopic dogs (those dogs allergic to certain pollens that are only in high numbers during certain times of the year) but some atopic dogs may be allergic to house dust mites and they have year round signs.
Now, a word on food allergies. Walk into any pet supply store. There are an overwhelming number of dog and cat foods available which I mentioned in the blog on nutrition. There is a multibillion dollar pet food industry in this country determined to get you to feed the “right” food. The implication and sometimes the outright statement by the “oh so friendly clerk” is “Feed this food and your dog will not itch”. And how did this person draw this conclusion? You have to wonder. Veterinary dermatologists have determined that food allergies represent the lowest number of allergic dogs seen in general practice, comprising only about 10% of allergic dogs. Of those dogs, allergies to “grains” are a small portion. There is currently a huge push on “grain free” diets being touted to solve skin allergy problems. While not harmful, grains fall under the category of “probably unnecessary” for most dogs because dogs are most certainly carnivores.
The body is constantly exposed to environmental allergens such as molds, pollen, dust mites and fleas. Both healthy and allergic dogs will produce antibodies (IgE, IgG, IgM) to combat these foreign proteins. The allergic dog is thought to produce significantly higher levels of IgE (considered the most important antibody in producing allergic signs/symptoms) than the clinically “normal” dog. There is no specific IgE that will produce pruritus (itching). Each dog has its own “threshold” (an allergic dog is often thought to have a lower threshold or tolerance, which makes it more susceptible). This becomes important in treatment. For example, let’s say we have a dog that is a known controlled atopic dog (pollens, molds, house dust mites etc.). For this dog with known atopic allergies, flea control becomes even more important because even though the primary allergy is not to fleas, flea bites could cause enough of a reaction to push the dog past its threshold for itching and then the dog becomes symptomatic for its primary atopic allergy.
When the itchy dog walks into my exam room, I think about the 3 most common allergies. Fleas, atopic disease, and food allergies. I must also think of other causes of itching such as:
• Sarcoptic/Notoedric mange (you know it as scabies)
• Cheyletiellosis (walking dandruff mite)
• Demodicosis (another type of mange)
• Pediculosis (lice)
• Dermatophytosis (ringworm fungus)
• Bacterial hair follicle infection
• Malassezia dermatitis (yeast infections)
In a cat, throw in all of the above plus mosquito bite hypersensitivity and eosinophilic granuloma complex. How is all of that figured out? The dermatologist’s tools are 1) detailed history and physical 2) visual and olfactory clues and 3) laboratory testing. These tests include skin scrapings where we visualize directly under a microscope looking for parasites, tape preps and hair plucks. Other tests are the Wood’s lamp (a black light that illuminates certain fungal infections) and smears of skin for cytology (looking for organisms such as yeast or bacteria) or specific cell types that populate certain itchy skin lesions.
The key to solving dermatological issues is patience and lots of it. Diagnosis can be slow and is often done by process of elimination. No matter what you read on the internet, there is NO blood test that will tell you what food your dog or cat is allergic too. Maybe I should state that differently. There are blood tests that CLAIM to tell you what food the dog or cat is allergic to but they are inaccurate and have poor correlation if any with true food allergies. Food allergies are diagnosed by feeding a special hypoallergenic diet for a period of nine weeks. Inhaled allergies are diagnosed by either intradermal skin testing or by specialized blood tests which test for over 64 different allergens.
Tune in next week when we will talk about something that should be important to every pet owner, whether your pet is dealing with allergies or not. I’m talking about the importance of flea control and what happens when you don’t protect your pets from fleas and other parasites. Not pretty.
Until next week………