Dogs and infectious disease

Definition

Dogs were the first animal species to become domesticated. Fossil evidence from fourteen thousand years ago shows that dogs lived among humans as a species physically distinct from their wolf ancestors. Genetic sequencing suggests dogs split from wolves as long as 100,000 years ago, perhaps because they began to live in close contact with humans. However, dogs remain close relatives to wolves; some scholars argue that dogs are simply a subspecies because dogs and wolves (and coyotes and dingoes) interbreed and produce fertile offspring. As canines, dogs belong to a group of social carnivores and scavengers. In practice, however, dogs, even feral dogs, are not effective hunters, depending instead on human provisioning and garbage.

Physically and behaviorally, dogs resemble juvenile wolves, a development called neoteny. Juvenile features and behavior probably make them more appealing to humans and more amenable to human lifestyles.

Natural History and Risk Factors

Dogs have adapted to the ecological niche of human society. They are so well adapted to humans that they understand human signals, such as pointing, gaze direction, and tapping, better than humans’ closest primate relatives. These adaptations support and attest to the intimate nature of the human-dog bond. Although cultural norms and the role assigned to dogs vary, dogs worldwide still share homes, meals and sleeping spaces with humans. Given this, it is surprising that dogs do not form a great public health risk.

Globally, dogs remain the primary transmitters of rabies. They are the primary transmitters of Echinococcus granulosus and, along with cats and wild species, are sources of visceral and ocular larva migrans. Dogs can be one source of infection with Leptospira, and they reportedly occasionally transmit other bacterial and protozoan diseases.

Rabies

Recognition of the role dogs play in rabies transmission, a nearly 100 percent fatal form of encephalitis, is ancient. A four-thousand-year-old Eshnunna civil code sets fines for the owners of “mad-dogs,” or dogs with rabies. Early scholars described a venom, then called a virus, in the saliva of rabid dogs that they believed transmitted the disease. Rabies is actually a collection of viruses, with strains propagating best in a single mammalian species. There are dog strains, fox strains, raccoon strains, bat strains, and others; all can cause disease in any mammal. Although eradicated in developed nations, dog rabies has long been a major threat to humans.

Dog rabies became an epizootic and epidemic threat when humans began to live in cities and keep dogs as pets. Abandoned pets then became feral and depended on garbage to survive. Feeding at single sites brought dogs into close contact, allowing for efficient transmission of rabies.

Prevention of dog-rabies epizootics and human exposures can be achieved through the control of strays, licensing and leash laws, and vaccinations of susceptible dogs. These measures eradicated dog strain rabies from the United States, Japan, and Europe by 2021.

Cystic Echinococcus

Dogs are the natural host for the tapeworm E. granulosus and one of the natural hosts for E. multicularis. The worms mature in the dog’s small intestines, shedding eggs that drop to the ground. Herbivores (especially sheep and goats, but also wild animals and bovines) ingest the eggs, and dogs are reinfected when they eat uncooked organ meats containing larva cysts. Humans can be infected if they ingest water or soil contaminated by dog feces or if they do not practice appropriate hygiene when handling dogs. In the twenty-first century, the highest prevalence of infection with E. granulosus occurs in pastoral societies, particularly in sub-Saharan Africa.

Ingested eggs develop into larvae that migrate to organs such as the liver, lung, and brain, where they form cysts. Cysts may not cause disease unless they are large or if they rupture. Cystic Echinococcus is debilitating but rarely fatal. Prevention includes hygiene, keeping dogs from eating uncooked offal from abattoirs, and regular deworming of dogs.

E. multicularis is a similar tapeworm that uses wild canines, especially the red fox, as its definitive host. Dogs and cats also serve as hosts and can transmit the parasite to humans. E. multicularis most often causes alveolar echinococcosis when cysts develop in the host’s lungs.

Nematode Zoonoses

Dogs are natural hosts for a number of roundworms, including Toxocara canis and the Ancylostomatoidea, or hookworms. Humans have their own set of roundworm parasites, and often people assume dogs are a source; for the most part, dog roundworms do not develop into adult roundworms in humans.

T. canis larvae migrate in human tissues, most often in children, causing abdominal pain, respiratory signs, and allergy-like syndromes called visceral larva migrans. Occasionally, the larvae cause granulomatous lesions (ocular larva migrans) in the retinas of the eyes, which can limit vision and cause blindness. These diseases result from the ingestion of soil contaminated with T. canis eggs. Ancylostomatidae roundworms comprise the group called hookworms and attach to the intestinal wall. These parasites cause anemia in both dogs and humans. Species of canine hookworms cannot mature in human organs and instead cause skin lesions and, rarely, enteritis.

Bacterial Zoonoses

Dogs are one of many reservoir hosts for Leptospira, a spirochete bacterium. There are several pathogenic serovars of Leptospira, some of which can cause disease in humans and dogs. Rodents are the major reservoir. Infective bacteria are excreted with urine into standing water. Leptospira causes a flu-like illness that can lead to liver and kidney failure.

Dogs have been the source of a variety of bacterial diseases in humans, including brucellosis, campylobacteriosis, salmonellosis, yersiniosis, and Helicobacter infections. Dogs may bring humans into contact with ticks and fleas, which can result in the transmission of vector-borne diseases such as Lyme disease, ehrlichiosis, and spotted fever group rickettsiosis. In addition, dog bites can occasionally become infected, most commonly with Staphylococcus, Streptococcus, Corynebacterium, and Bacteroides spp.

Protozoan Zoonoses

Dogs are reservoir hosts for leishmaniasis. The disease, spread by sandflies, is caused by single-celled parasites. Skin lesions, disfigurement, and a rare, potentially fatal visceral syndrome called kala-azar may result.

Impact

In 2024, rabies was estimated to cause the death of around seventy thousand people each year around the world, with domestic dogs being responsible for approximately 95 percent of the transmitted infections. The other diseases transmitted by dogs are not well documented and cause few fatalities. In developed nations, where dogs are controlled and receive veterinary care, public health concerns are limited. Indeed, there is evidence that owning a pet has salutary benefits, such as encouraging owners to exercise and helping to lower stress.

Bibliography

“About Rabies.” CDC, 14 May 2024, www.cdc.gov/rabies/about/index.html. Accessed 19 Dec. 2024.

Macpherson, Calum N. L., et al., Dogs, Zoonoses, and Public Health. 2nd ed., CABI, 2013.

Mertz, Gregory J. “Zoonoses: Infectious Diseases Transmissible from Animals to Humans, Fourth Edition.” Clinical Infectious Diseases, vol. 63, no. 1, 2016, pp. 148–49, doi:10.1093/cid/ciw234. Accessed 19 Dec. 2024.

Morgan, Marina, and John Palmer. “Dog Bites.” British Medical Journal, vol. 334, no. 7590, 2007, pp. 413–417, doi:10.1136/bmj.39105.659919.BE. Accessed 19 Dec. 2024.

Serpell, James, editor. The Domestic Dog: Its Evolution, Behaviour, and Interactions with People. 2nd ed., Cambridge University Press, 2017.

"WHO Expert Consultation on Rabies: First Report." World Health Organization, 2004, www.who.int/publications/i/item/who-trs-931. Accessed 19 Dec. 2024.