Veterinarian to Veterinarian®: Lovesick for dogs? Unfortunately, yes

Dogs offer up their hearts to their humans, along with their germs. Many canine maladies, whether bacterial, viral, or parasitic, can jump from dog to person. When we think about zoonotic risk, some animal villains may come to mind: The cat, whose single swipe might sicken its human victim with the Bartonella henselae organisms that cause cat scratch fever; the lizard, who may leave diarrhea-inducing Salmonella on human hands; and any wild mammal whose bite could make one rabid. However, the “whodunit” label rarely goes to man’s best friend: the dog.

As canines have melded their lives with ours since their domestication began thousands of years ago, so have their pathogens. According to Colin Basler, deputy director of the Centers for Disease Control and Prevention’s (CDC’s) One Health, dogs are behind the spread of several illnesses in their human companions.

Today, dogs still hold rank in our lives. More than 65 million US households include a dog, according to the American Pet Products Association’s 2024 report.¹ The sheer numbers have created a zoonotic bonanza, where dogs and humans trade germs.

Explosive invaders

“The diarrheal ones are the major zoonotic diseases with dogs,” Stephen Cole, VMD, MS, DACVM, assistant professor of clinical microbiology at the University of Pennsylvania School of Veterinary Medicine in Philadelphia, said in an interview.

Campylobacteriosis is a major cause of human bacterial diarrhea worldwide. These spiral, gram-negative bacteria can be acquired through ingestion of contaminated meat and dairy products and by fecal-oral transmission from infected animals. Colonization rates are highest in puppies, particularly kennel stock.² The past decade has seen several outbreaks of multidrug-resistant Campylobacter jejuni infections in people who reported contact with puppies sold in pet stores.³

Dogs are also a reservoir for human infection with Salmonella spp, which can be transmitted by the fecal-oral route. Infected dogs and people can shed the bacteria for 6 weeks or more, during which the clinical picture can include fever, vomiting, and diarrhea, or it can be devoid of any signs of illness. People also have been sickened by direct contact with dry dog food and treats—particularly pig ears—contaminated with Salmonella.^4,5

Bacteria placing humans in the doghouse

Leptospirosis is a rare but debilitating sickness caused by a spirochete found in livestock, pets, and wildlife. Infection in dogs, which can be asymptomatic, typically causes anorexia, vomiting, polyuria/polydipsia, lethargy, fever, and icterus; death follows approximately 10% of the time.⁶

A person can contract leptospires through direct contact with urine or tissue of an infected animal, or exposure to contaminated water or soil. Mucus membranes and broken skin provide entry points for these motile organisms. Human leptospirosis is life-threatening in approximately 1 in 10 patients⁷ and can lead to hepatic and renal dysfunction. Annual canine leptospirosis vaccines protect both dogs and their owners by extension.

Dogs (mainly intact) are the primary host for the zoonotic coccobacillus Brucella canis,⁸ which can be clinically silent or manifest with symptoms such as uveitis, orchitis, and spontaneous abortions. Human brucellosis is generally a mild, flulike syndrome that is mostly linked to other Brucella species spread through unpasteurized dairy products.⁹ Cole explained that human cases associated with B canis are mostly in farmers and kennel workers who are exposed to canine body fluids.

Capnocytophaga canimorsus is a gramnegative bacterium normally found in the oropharyngeal tract of dogs and cats. Transmitted to people mainly through bites, the bacteria can cause severe septicemia, particularly in immunocompromised patients.¹⁰

Bordetella bronchiseptica, another gramnegative bacterium, resides in the upper respiratory tracts of many mammals and travels to humans by aerosol. B bronchiseptica is a major cause of acute tracheobronchitis in dogs,¹¹ and, in rare cases, can cause similar sickness in people, particularly those who are immunocompromised.¹²

Q fever is a febrile illness that may induce pneumonia and endocarditis in people. Most human infection is through aerosol or direct contact with body fluids from ruminants carrying the intracellular bacterium Coxiella burnetii.¹³ However, C burnetii has also been isolated in farm dogs, making them possible vectors to people.¹⁴

There is some evidence that the family dog might serve as a reservoir for other bacteria that can cause problems in people. These include methicillin-resistant Staphylococcus aureus,¹⁵ which the CDC lists as an emerging concern, and uropathogenic Escherichia coli.¹⁶ Dog bites can also deliver a polymicrobial brew that may include various species of Staphylococcus, Streptococcus, and Pasteurella.¹⁷

Some viruses make the leap

When it comes to dog bites, the major concern is rabies. Globally, dog-mediated rabies is responsible for nearly 60,000 human deaths annually.¹⁸ In the US and other developed countries, where rabies vaccination is the norm for pets, humans mostly acquire the disease from wild mammals.¹⁹ Noroviruses are the most common cause of nonbacterial gastroenteritis, and serological data suggest possible transmission from animals to people. In fact, antibodies against canine noroviruses have been found in humans and vice versa.²⁰

SARS-CoV-2, the virus that causes COVID-19, could have limited transmission potential between dogs and people. Benjamin Anderson, PhD, MPH, assistant professor in the Department of Environmental and Global Health at the University of Florida in Gainesville, said that although surveillance data are lacking, he considers the risk to be low.

Canine parasites to run from

Photo: Todorean Gabriel/Adobe Stock

Toxocara cati or roundworms from a cat

Toxocara canis is a roundworm endemic in puppies and many adult dogs throughout the US. The eggs are passed in dog feces and can remain viable in the environment for months to years. Transmission to people is through egg ingestion, usually from contaminated hands or fomites. Children, particularly those with a history of pica, are most prone to infection.²¹ Human toxocariasis, higher in rural and impoverished populations, is characterized by visceral and ocular larva migrans.

Hookworm infection with either Ancylostoma caninum or Ancylostoma braziliense produces the creeping skin eruption cutaneous larva migrans. Hookworm is more prevalent in warm climates,²² where larvae can penetrate human skin after contact with areas contaminated with infected canine feces, such as beaches and sandboxes.

Less commonly, people can acquire Dipylidium caninum and Echinococcus granulosus from dogs. The former is associated with fleas,²³ and human patients can experience gastrointestinal symptoms. The latter occurs mainly in regions where sheep are raised.²⁴ Human echinococcosis, clinically silent in most cases, features lodging of hydatid cysts within organs.

The protozoans Cryptosporidium spp and Giardia spp are ubiquitous within the gastrointestinal tract of dogs and many other animals.²⁵ They are conveyed through oral contact with contaminated water, food, or fomites and cause gastrointestinal distress in people.

Other pruritic zoonoses

Sarcoptes scabiei, the mite behind sarcoptic mange in dogs, can vault to people who contact infected animals or exposed fomites.²⁶ Puppies and debilitated dogs are at higher risk of infection, characterized by intense pruritus and papular dermatitis of the pinnae, face, limbs, and ventrolateral trunk. In humans, skin lesions are excoriated and itchy.

Dermatophytosis, or ringworm, can spill over from dogs to their owners. Microsporum canis has been found in most cases of zoonotic transmission between dogs and people,²⁷ and infection in both species can produce patchy, sometimes alopecic lesions that are crusty and pruritic.

Reverse zoonoses

When it comes to zoonoses, the conversations focus on how animals sicken people, but disease also moves in the opposite direction. “Most people only think about disease transmission from pets to people, but it goes both ways,” Anderson said.

Respiratory viruses can mutate fast and therefore jump species. Human subtypes of influenza A virus have been isolated from dogs, suggesting that owners who are sick with this subtype can also sicken their dogs.²⁸ Researchers have begun setting up surveillance programs in veterinary clinics to better track epidemiological data.

But veterinarians might begin to harvest information now, Anderson added. “For the classical respiratory and enteric infections, it would be helpful to ask the client: ‘Is anyone in your home sick, and did it start before or after your dog became sick?’ ”

Takeaway

The fight against pet-borne zoonoses starts in the veterinary office with thorough annual checkups and appropriate vaccination and parasite prevention, Cole explained. Handwashing between patients, isolation protocols, and personal protection equipment when needed are crucial to “not becoming fomites ourselves,” Cole said. “The best thing we can do to prevent zoonoses in people is to prevent disease in ou animals.”

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