Your Dog And Infections: Alien Invasion

As long as your dog lives in this world, they can sucumb to an infection. Pathogens are always looking out for an opportunity.

The critical distinction to make is between the presence of a pathogen—exposure and active infection. The formula for disease consists of two variables:

  1. presence of pathogen
  2. failure of immune response

The world is full of pathogens—they are everywhere. Some are rarer to come across than others. And some potential invaders are even naturally present on the dog’s own body.

Examples of symbiotic organisms resulting in an infection

Case in point, most skin infections don’t result from an attack of exotic invaders. Instead, the problem is the over-population of bacteria or yeast that normally resides on the canine skin, such as Staphylococcus pseudintermedius and Malassezia.

For example, if your dog develops a hot spot, something interfered with the ability of the immune system to keep the ordinarily present bacteria under control.

Such as your kids might be perfectly well-behaved when you’re around but throw a party and trash the house when you leave.

Further reading: Acute Moist Dermatitis in Dogs: Hot Spots

The immune system is the gatekeeper who keeps things in check. Therefore, the optimal scenario is that the immune system stops any potential invaders—foreign or domestic—in their tracks.

Your Dog And Infections: Alien Invasion

However, there are times the invading army can win, such as when the immune system is:

  • weakened/suppressed
  • overwhelmed
  • busy doing other things
  • doesn’t recognize the invader

The ability of your dog’s immune system to tackle infections depends on their overall health. Stressful events and underlying conditions, such as chronic inflammation or hormonal imbalances, can negatively influence the fighting force.

If your dog suffers from frequent infections, look for an underlying cause.

Your Dog And Infections: Alien Invasion

What can cause the immune system to fail?


The first line of defense for the dog’s body to prevent infections are mechanical or physical barriers. These include:

  • skin
  • mucus membranes
  • corneas

Physical damage to these tissues increases the risk of infection. Further, these barriers are populated by symbiotic bacteria whose secretions can destroy harmful bacteria. Thus, changes in the population of the friendly bacteria can also negatively impact immune response.

Weakened/suppressed immune system

If the immune system isn’t at its best, it opens the door to infections. Things that can weaken or suppress the immune system in your dog include:

  • hereditary conditions
  • stress
  • poor nutrition
  • existing medical conditions such as hypothyroidism, Cushing’s, diabeter or cancer
  • certain medications and therapies such as steroids, immuno-suppressive drugs, chemo or radiation
Unknown pathogen

Just as the immune system can get confused and attack cells in the body it’s meant to protect. It can also fail to recognize a problem. Unrecognized pathogens can make it through the lines of defense—think Trojan horse.

Overwhelmed immune system

Sometimes an infection can move so fast that the immune system cannot keep up. For example, bacteria infections can blow out of proportions in record time. On the other hand, viral infection can cause such damage to the tissues that it allows for opportunistic bacterial infections.

Further information: Immune System Responses in Dogs

Parasitic infections

Dogs are highly susceptible to parasitic infections. In fact, parasites are one of the most common health issues in dogs.

External parasites

External parasites include ticks, fleas, mange mites, and lice. The most common effects of external parasites are skin and coat-related symptoms, such as itchiness and hair loss. However, they can:

  • lead to secondary bacterial skin infections
  • trigger an allergic reaction
  • transmit systemic diseases such as Lyme disease, tapeworms, ehrlichiosis, and Rocky Mountain Spotted Fever

Prevention is the first line of defense against external parasites. Talk to your veterinarian about which product would be the safest and most effective for your dog.

Note: a tick needs to attach to the skin for several hours before transmitting the disease. Removing ticks promptly will help prevent many tick-borne diseases.

Internal parasites

Internal parasites include gastrointestinal worms such as:

  • hookworms
  • roundworms
  • tapeworms
  • and whipworms

as well as microscopic parasites such as Giardia or Coccidia.

Intestinal parasites can cause nausea, vomiting, or diarrhea, but some dogs may show no clinical signs at all. The main dangers of gastrointestinal parasitism are malnutrition and anemia. Regular microscopic fecal examinations and/or deworming are the best way to protect your dog from intestinal parasites.

As a note of interest, there seems to be a beneficial side to some intestinal parasites. There is research looking into the controlled use of parasitic infection to treat autoimmune diseases and allergies.  It falls under the label of helminthic therapy.

Heartworms are another type of internal parasite, which is spread by mosquitoes. As the name suggests, heartworms travel through the bloodstream and invade a dog’s heart and pulmonary arteries. As worms take up residence in the heart and lungs,s they can restrict blood flow to the point of death. Prevention is definitely the key here. Several different types of easily administered heartworm preventatives are available from veterinarians.

Further reading: Parasites in Dogs That You Should Know

Bacterial infections

Bacteria are microscopic, single-celled organisms that are present virtually everywhere in our environment and our bodies. Many bacteria live in symbiosis with their host and are necessary for your dog’s vital functions, such as digestion. Some friendly bacteria can even assist your dog with fighting off bacterial infections. They can improve immune function and out-compete harmful bacteria.

However, certain types of bacteria can cause infection and illness if they overwhelm your dog’s immune system. Such harmful bacteria include Salmonella, Leptospira, Bordetella, E. coli, and many others. Symptoms will vary depending on which part of your dog’s body has been invaded.

Left untreated, a bacterial infection can cause serious health issues and even lead to death.

The good news is that bacteria are living organisms, and as such, they can be killed. Bacterial infections usually respond well to antibiotics as long as treatment begins before a dog’s condition gets too severe. However, the over-use of antibiotics has led to the evolution of resistant strains that no longer respond to treatment. Not finishing a full course of antibiotics can also cause bacteria to become resistant to a particular drug. Therefore, it is important to use antibiotics wisely and follow through with the full treatment as prescribed by your veterinarian.

Further reading: Bacterial Infections in Dogs: Different Types and Treatment

Viral infections

Unlike a parasite or a bacteria, a virus isn’t really a living organism in the true sense of the word. You cannot kill something that isn’t exactly alive.

A virus is a collection of genetic material that invades living cells and reprograms them for its own purposes. A virus exploits the host cell to reproduce itself and then invade other cells, destroying them in the process. Viral infections are a lot like computer viruses. That’s why they share the same name.

Antibiotics don’t kill viruses.

Some antiviral medications are available. However, these have not been tested thoroughly in dogs and are used infrequently.

Treatment for viral infections is usually limited to supportive therapy. If your veterinarian prescribes antibiotics for viral infection treatment, it is to fight secondary bacterial infections. Still, a dog’s own immune system must be able to suppress or rid his body of the virus to recover.

Many canine viral infections are very nasty, and some of them are deadly. Rabies, distemper, canine hepatitis, parvovirus—these are just some members of the lovely viral family.

A healthy and primed immune system can often ward off a viral infection before serious illness develops. This is why vaccination is imperative. While controversies exist regarding vaccinating our dogs, the question isn’t whether to vaccinate or not, but merely how often.

Further reading: Common Viral Infections in Dogs

Fungal infections

Fungi are plant-like organisms such as mushrooms, molds, mildews, and yeasts, some of which can cause infections in dogs.

The most common fungal diseases in dogs are yeast infections and ringworm. Note, ringworm is not a worm but a fungus. Usually, these infections are limited to the skin. However, some fungi can invade the body and be a potentially life-threatening problem.

Antifungal drugs are available. However, treatment usually needs to continue for a long period of time to eliminate the fungus from the body. Some of the more advanced antifungals can also be very expensive and have some pretty scary side effects.

Of course, prevention is always better than treatment. Keep your dog healthy so that their immune system has the best chance of fighting off the fungi he encounters every day.

Further reading: Fungal Infections in Dogs



a disease-producing microorganism, such as a virus, bacteria, fungus, or protozoa


an invasion of microorganisms in the tissues resulting in disease

mucus membranes

protecting layer of tissue lining the passages in the body that are exposed to external substances, such as the respiratory and digestive tract

Related articles:
Angry Vet On Ear Infections: Deciphering Ear Problems in Dogs
Bladder Infections in Dogs: What Causes Them?
Dogs and Coronaviruses: Do We Share Coronavirus Infections with Dogs?

Categories: ConditionsInfections

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Jana Rade edited by Dr. Joanna Paul BSc BVSc

I am a graphic designer, dog health advocate, writer, and author. Jasmine, the Rottweiler of my life, was the largest female from her litter. We thought we were getting a healthy dog. Getting a puppy from a backyard breeder was our first mistake. Countless veterinary visits without a diagnosis or useful treatment later, I realized that I had to take Jasmine's health care in my own hands. I learned the hard way that merely seeing a vet is not always enough. There is more to finding a good vet than finding the closest clinic down the street. And, sadly, there is more to advocating for your dog's health than visiting a veterinarian. It should be enough, but it often is not. With Jasmine, it took five years to get a diagnosis. Unfortunately, other problems had snowballed for that in the meantime. Jasmine's health challenges became a crash course in understanding dog health issues and how to go about getting a proper diagnosis and treatment. I had to learn, and I had to learn fast. Helping others through my challenges and experience has become my mission and Jasmine's legacy. I now try to help people how to recognize and understand signs of illness in their dogs, how to work with their veterinarian, and when to seek a second opinion. My goal is to save others the steep curve of having to learn things the hard way as I did. That is the mission behind my blog and behind my writing. That is why I wrote Symptoms to Watch for in Your Dog, which has turned out being an award-winning guide to dog owners. What I'm trying to share encompasses 20 years of experience. Dr. Joanna Paul BSc BVSc is our wonderful sponsor and has been kind to edit and fact-check my important articles.

  1. This is great information, especially for those that may be dealing with an infection of some form in their dog for the first time. We are quite familiar with yeast infections in our house – our girl has seasonal allergies and they also end up triggering an ear infection. The best thing we can do is to learn how to best address and mitigate the problem each year, as I can’t remove the environmental allergies from her life entirely.

  2. Thank you for provinding a comprehensive overview of what infections are. For the uninformed an infection is just ‘something wrong’. But, it is important that dog owners understand what is happening with an infection and by understanding the risks look at how they can prevent infections happening.

    The most important message for me? Keep an eye on your dog’s health and you can lessen the chance of an infection taking hold.

  3. Excellent information! I didn’t know that ticks had to attach for several hours to transmit disease. That was particularly great information! I always check my dog for ticks immediately after a hike. But this puts my mind a little at ease. I try to protect my dog’s immune system as possible with pro/pre biotics, good food, vitamins, appropriate medications, and regular check-ups. Thanks for sharing!

  4. Very informative. I do a lot to help my dogs’ maintain strong immune systems and thankfully, thus far, we haven’t had to deal with any serious infections.

  5. This is so scary! Both humans and animals can get life threatening infections, and they can quickly create a dangerous situation. I protect my dogs by paying attention to diet & exercise, getting them the core vaccinations and medication for heartworms, fleas & ticks. I’ve been wondering if I should also give them a pro/pre biotic as well – what are your thoughts on pro/pre biotics for dogs?
    Love & Biscuits,
    Dogs Luv Us and We Luv Them

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