Pet Pathogens: What Dog Owners Need to Know

Image of the cover of the Fall 2021 issue of Microcosm magazine.

From the Winter 2021 issue of "Microcosm."

It's no secret that humans love their pets. The American Veterinary Medical Association (AVMA) estimates 57% of U.S. households own at least 1 companion animal, with nearly 150 million dogs and cats in the U.S. as of 2016. Pet ownership has steadily increased by 20% over the last 2 decades, rising again amid the pandemic. Dogs lead the pack as the most popular traditional pet (sorry, cats), with 38% of U.S. households owning at least 1 dog. While fewer households own cats (25%), those that do are more likely to include multiple. Exotic pet ownership has also grown 25% in the last decade, with more people than ever caring for fish, ferrets, reptiles, hamsters, rabbits, birds, amphibians and the like.

Human Care

Although the modern veterinary profession began in the 1760s with the establishment of the first school of veterinary medicine in France, animal care has been practiced for thousands of years and likely preceded human medicine. Ancient civilizations relied on healthy animals to sustain their food supply, and some cultures, like the Egyptians, revered animals on a spiritual level. Early veterinary science drew on lessons learned from human cases of tuberculosis, typhoid and cholera to prevent diseases in livestock, and has expanded in the last few decades to focus on the preventive and specialized care of companion animals.

A woman and her pet dog relax on a sofa.

The average pet-owning household will spend more than $1,100 on their pets each year, which is not surprising, since over 90% of owners consider them to be family members. Over 37% of the $95 billion pet industry is spent on medical and veterinary care, and 40% is spent on food and diet. Some owners report spending more on their pet's medical expenses than they do on their own health care costs, and the novel-but-trending pet insurance industry is estimated to be worth more than $1 billion. Proper pet care is essential to keeping animals and human caretakers healthy, and this includes understanding the microbes that live on and within our canine companions.

Vets, Viruses and Vaccines

Much like humans, preventive care for dogs should begin from a young age and include regularly scheduled examinations. Newborn puppies derive some maternal immunity at birth and, assuming they nurse appropriately, through their mother's milk. Maternal immunity begins to wane at around 6 weeks, and puppies should receive their "core" DHPP (distemper, adenovirus/hepatitis, parainfluenza and parvovirus) vaccinations every few weeks up until 4 months of age to ensure an effective immune response. Early vaccination is crucial: Dogs are most susceptible to infection at between 6 weeks and 6 months, especially to parvo, a highly contagious viral disease that causes acute gastrointestinal illness and, if left untreated, has a 91% mortality rate. During the first year, veterinarians may recommend other common optional vaccinations for canine influenza, leptospirosis (caused by a bacteria found in urine) and bordatella ("kennel cough"). These vaccinations may be suggested for dogs that are in close contact with other dogs in kennels, and they may be required for boarding and grooming.

A color-coded U.S. map showing rabies laws by state.
(Click for a larger image.)

One vaccine is not optional: Dogs must be vaccinated against rabies in many states. Rabies lyssavirus is a mammal-specific disease that results in brain inflammation, neurological symptoms, behavioral changes and sometimes death. Although occurrences of rabies in the U.S. have decreased dramatically since the development of a vaccine, the inflammatory brain disease affects hundreds of thousands of animals and kills an estimated 50,000 people in developing nations each year. Ninety percent of rabid animal cases in the U.S. occur in wildlife, marking a dramatic change since 1960, when the majority of cases were in domestic animal species, primarily dogs. All but 9 states have some kind of regulation or requirement regarding rabies vaccination, though some allow medical exceptions.

Despite the importance of vaccination, there are gaps, and the Humane Society estimates that 69% of the 19 million pets living in underserved communities have never seen a veterinarian. To combat this, many organizations offer free rabies clinics and low-cost veterinary services. Meanwhile, there is a reported increased rate in vaccine hesitancy among pet owners: Britain's People's Dispensary for Sick Animals (PDSA) found that in 2018, about 25% of dogs — 2.2 million of them — had not had their necessary vaccinations as puppies, primarily because their owners deemed it "not necessary."

Diagnosing and Treating Infections

A veterinarian treats a small dog.

Fortunately, for vaccinated animals, diseases are relatively benign. The main reasons for a vet visit include skin conditions, stomach issues, urinary tract infections and ear infections, according to Healthy Paws Pet Insurance's third annual Cost of Pet Health Care 2018 report. Common infectious agents include staphylococcus and Escherichia coli, often following a superficial injury like a bite or scratch. Dogs, with their curiosity for garbage scraps and others' feces, can also pick up infections while exploring unhygienic places. Largely, these are cases of bacteria "in the wrong place at the wrong time," says University of Pennsylvania Assistant Professor and Veterinarian Dr. Stephen Cole.

Dr. Cole's work focuses on the clinical and molecular epidemiology of carbapenem-resistant Enterobacteriaceae and other cases of antibiotic-resistant bacteria in animals, which has been increasing in recent years. The issue can be attributed mainly to the overuse of broad-spectrum antibiotics and is particularly problematic in facilities where a large number of animals or multiple species are housed together (i.e., kennels, shelters and pounds.) Because the antibiotics used to treat canine infection are the same medications used in humans, it is especially important to exercise discretion when using or prescribing antibiotics, which may require more time and resource-intensive, culture-based diagnostics.

A rapid alternative to growing bacteria in culture is the use of next-generation DNA sequencing, which can help identify novel bacteria or those that do not grow well in culture, says Dr. Janina Krumbeck of private testing facility MiDog. This technique uses a fecal, blood or saliva sample to genetically identify both pathogenic and benign bacteria present in the canine microbiome by comparing the results to a database curated from previous samples from healthy and infected dogs. Dr. Krumbeck says accurate species-level identification can also reveal which antibiotics a sample may be resistant to, and thus it is a powerful tool in prescribing proper treatment. NGS is not widely used in the veterinary field yet, a key barrier being cost, but Dr. Krumbeck has high hopes that the technology may be used more widely to complement bacterial cultures, especially as MiDog extends to more host species. "Successful treatment begins in accurate identification," she said. "We need to use all the tools available."

Parasites, Pets and People

Microscopic view of a virus or pathogen.

Dog owners and their pets will likely be familiar with routine parasite prevention. A number of over-the-counter methods are available to discourage parasitic insects and arthropods, including flea collars, topical ointments and shampoos that work by interfering with the parasite's nervous system. While fleas are mainly just a nuisance, ticks may jump from animal to human and are known to carry a multitude of pathogens, including viruses, protozoans, and bacteria Borrelia ssp (which causes Lyme disease) and Rickettsia spp (which causes Rocky Mountain Spotted Fever). The risk of disease transmission varies among tick species, seasonality and geography, says L. Rainer Butler, a Ph.D. student at the University of Maryland Medical School studying immune responses to tick-borne diseases. Ticks are most active in the warmer months, but they tend to prefer amphibians and reptiles as hosts in regions where the temperature is mild year-round; in fact, Lyme disease is relatively absent south of the Virginia border. The best practice is to check your pet frequently and remove ticks within 24-48 hours to prevent disease transmission, she says.

Dogs are natural hosts for heartworms (Dirofilaria immitis), and over a million pets in the U.S. harbor the parasite, according to the American Heartworm Society (AHS). Heartworms are transmitted by mosquitoes and reside in the dog's lungs, spreading to the heart and causing long-term damage if left untreated. Because prevention in the form of chewable tablets is easier and more affordable than months of medication, AHS recommends year-round heartworm preventatives, which veterinarians can prescribe. Many dogs will also harbor intestinal parasites — roundworms, hookworms or tapeworms — at some point in their lives, although dogs are most vulnerable in puppyhood when nursing. Dogs with parasites may lose weight, vomit or pass diarrhea; these infections are diagnosed following a fecal examination for microscopic eggs in the dog's stool. Owners can mitigate the risk of an asymptomatic dog passing on the parasite by visiting the vet for annual checkups.

A healthy diet can address many health issues, and some dog food companies are now creating probiotic and prebiotic formulas specifically suited to the canine microbiome. Studies indicate mixed results from a raw-meat diet: Although dogs fed raw meat may harbor a more diverse microbiome, they are also at higher risk for parasitic or bacterial infection. Dogs with allergies or other underlying immune conditions may benefit from a hypoallergenic diet, as inflammation is often associated with an unbalanced microbiome. More research is needed to fully understand the potential of nutrition as a treatment measure.

Pets Love Us

A dog and its owner shown against the setting sun.

We love our pets, and they love us. But can this connection go too far? The relationship between dogs and their owners is intimate; they eat together, play together, even snuggle together on the couch. Over half of all pet owners report kissing their pets more often than their significant other! Yet such closeness can increase the risk of pathogen transmission between dogs and humans, and vice versa. It may be difficult to determine whether a pet is ill, especially because microbes manifest differently in dogs than in humans, and dogs have an evolutionary instinct to hide signs of illness or injury from potential threats.

Can you hug, kiss and love your pets despite the risk of zoonotic transmission? As Dr. Cole says, "It's like kissing a dumpster vs. kissing a toilet seat." Such is the importance of proper health care, not just for humans, but for the pets with whom they share their homes. This builds a safer environment for all, one in which our love of dogs can continue with minimal risk to us, or our pets. And yes, this includes hugs and kisses.


Author: Ashley Jones Robbins, MELP

Ashley Jones Robbins, MELP
Ashley Jones Robbins, MELP is the Advocacy Communications Coordinator at the American Society for Microbiology.