Dogs can transmit a wide range of infectious agents to humans. Bite wounds are the most important route of exposure, but scratches, contact between saliva and mucous membranes or broken skin, and exposure to urine or feces can also result in infection. Indirect transmission can play a role as well.

As a result, the differential diagnosis extends beyond bacterial infections to include viral, parasitic, and fungal diseases. An individual’s risk depends largely on the type of exposure, travel history, and underlying risk factors such as immunosuppression, asplenia, or chronic liver disease.

Rabies: A Medical Emergency

One of the most serious viral infections transmitted by dogs is rabies. Although rare in Europe, it remains a major concern because the disease is almost always fatal once symptoms appear. Transmission typically occurs through virus-containing saliva introduced during a bite, and the risk is higher in travelers to endemic regions of Asia, Africa, and Latin America.

The incubation period is usually 1-3 months but can vary considerably. Initially, nonspecific symptoms such as fever and malaise, as well as paresthesias at the site of entry, frequently occur. As the disease progresses, neurologic symptoms develop, such as agitation, hydrophobia, aerophobia, seizures, or flaccid paralysis. Once clinical symptoms appear, the disease is almost always fatal.

For diagnosis, reverse transcription-polymerase chain reaction (PCR) detection of viral RNA from saliva or nuchal skin biopsy specimens can be used. Serology alone is not sufficient to confirm the diagnosis. In practice, however, diagnostic testing should never delay immediate postexposure prophylaxis after a significant exposure. According to the Robert Koch Institute in Germany, a person who has not previously been vaccinated and is bitten by an animal suspected of having rabies is generally given both rabies vaccine and rabies immunoglobulin.

Capnocytophaga canimorsus: Life-Threatening

While travel and exposure history are the primary guiding factors in rabies cases, even a seemingly insignificant dog bite can have serious consequences in cases of bacterial infections. One example is C canimorsus. The bacterium is part of the normal oral flora of many dogs and is primarily transmitted through bites. However, infections have also been reported following scratch wounds or contact of saliva with broken skin or mucous membranes.

People with asplenia, alcohol dependence, liver cirrhosis, or immunosuppression are particularly at risk. Severe cases can, however, also occur in immunocompetent individuals.

The disease often begins with fever, chills, and flu-like symptoms but can rapidly progress to fulminant sepsis. Possible complications include disseminated intravascular coagulation, purpura fulminans, multiple organ failure, or meningitis.

A further challenge is that microbiological diagnosis can be delayed by the organism’s slow growth and the need for specialized culture conditions. Once the pathogen has been isolated, molecular methods and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry can speed identification, but these tools are not always readily available.

This makes early treatment all the more important when there is clinical suspicion of the disease. Initially, for severe infections, intravenous beta-lactam/beta-lactamase inhibitor combinations such as piperacillin/tazobactam or carbapenems are appropriate. Milder infections can be treated with amoxicillin/clavulanic acid, among other options. Further treatment should be adjusted based on pathogen identification and resistance testing.

Pasteurella multocida: Dog-Bite Pathogen

In addition to Capnocytophaga, another important bacterial pathogen in infected bite wounds is P multocida, which is frequently isolated and can also be transmitted when saliva contacts preexisting skin lesions. Infected bite wounds are often polymicrobial, with Pasteurella joined by staphylococci, streptococci, anaerobes, and other organisms.

A typical presentation is the rapid onset of local inflammation, with pain, erythema, swelling, and, when present, purulent drainage often developing within 12-24 hours. If untreated, the infection can progress to abscess formation, septic arthritis, or osteomyelitis. Systemic infection is also possible, especially in high-risk patients.

The diagnosis is based on the clinical presentation and microbiological examination of deep wound material or aspirates. If a systemic infection is suspected, blood cultures are also indicated.

Amoxicillin/clavulanic acid is considered first-line therapy for infected bite wounds. In more severe cases, surgical wound debridement and intravenous antibiotics may also be required.

Leptospirosis: Urine-Spread Infection

Not all dog-associated infections are transmitted through bites or saliva. In leptospirosis, the main route of transmission is different: infected dogs can shed leptospires in their urine, and the organisms enter the human body through small skin breaks or mucous membranes. Exposure to contaminated water, mud, or damp soil is especially important epidemiologically, with dogs representing only one of several potential sources of infection.

The clinical spectrum is broad. It ranges from self-limiting flu-like illnesses to severe organ involvement with acute renal failure, liver involvement, pulmonary hemorrhages, aseptic meningitis, or Weil disease.

Diagnostic options also depend on the stage of the disease. In the early phase, polymerase chain reaction can enable direct detection of the pathogen, particularly in blood and, if necessary, cerebrospinal fluid. As the disease progresses, serologic methods become more important. The microscopic agglutination test is considered the gold standard serologic method, while enzyme-linked immunosorbent assays are used primarily for screening. Mild cases can be treated with doxycycline. Severe cases require intravenous antibiotic therapy, such as penicillin G or ceftriaxone, as well as intensive care measures if necessary.

Dermatophytes: Animal-Contact Fungal Infections

In addition to viral and bacterial pathogens, dogs can also spread fungal infections. One important zoonotic dermatophyte is Microsporum canis, which is found primarily in cats but can also be transmitted from dogs to humans. Infection can occur through direct contact with affected animals or indirectly via contaminated items such as brushes and blankets.

Clinically, the condition usually presents as sharply demarcated, ring-shaped, scaly, and itchy skin lesions with central healing. If the scalp is affected, severe inflammatory kerion-like forms may occur.

Diagnosis includes microscopic examination of skin scales or hair, fungal culture, and, increasingly, PCR-based methods. For M canis, examination with a Wood’s lamp may also be helpful. Localized skin infections are usually treated topically.

By contrast, extensive dermatophytosis or involvement of the scalp typically requires systemic therapy. For tinea capitis caused by M canis, griseofulvin or itraconazole is especially appropriate, while terbinafine may also be an option depending on the causative organism and the site of infection.

Echinococcosis: A Rare Dog Tapeworm Zoonosis

Parasitic diseases are also part of the spectrum of dog-associated zoonoses. In cystic echinococcosis caused by Echinococcus granulosus, infected definitive hosts — especially dogs — shed tapeworm eggs in their feces. Humans can become infected by ingesting the eggs, such as through contaminated food, water, or soil, or through hand-to-mouth transfer after contact with contaminated surfaces or animal fur.

After ingestion, cysts grow slowly over the course of years, most often in the liver and lungs. As a result, the infection may remain asymptomatic for a prolonged period. As the cysts enlarge, however, patients may develop pressure-related pain, cough, or impaired function of the affected organs. Cyst rupture is especially concerning because it can provoke severe allergic reactions, including anaphylactic shock.

Diagnosis is based primarily on imaging studies such as ultrasound, CT, or MRI, with serologic testing used as an adjunct. Treatment is guided by the location, size, and stage of the cysts. Depending on the case, options may include surgery; puncture, aspiration, injection, and re-aspiration; albendazole; or, for appropriately selected inactive cysts, a monitored watch-and-wait approach.

Toxocariasis: Larval Migration Disease

Another parasitic pathogen associated with dogs is Toxocara canis. Infection occurs through oral ingestion of infectious eggs, typically from contaminated soil or from soiled hands and food. Notably, eggs freshly passed in dog feces are not immediately infectious; they must first undergo embryonic development in the environment.

Adult worms do not develop in humans. Instead, the larvae migrate through various organs, leading to clinical presentations that vary by site of involvement. Many infections remain asymptomatic. When symptoms occur, visceral larva migrans may present with fever, hepatomegaly, and eosinophilia. Ocular involvement can also occur and, in severe cases, may result in permanent vision loss.

Diagnosis is typically based on serologic testing, along with evidence of eosinophilia and, when needed, imaging studies. Albendazole and mebendazole are the mainstays of treatment. In cases with severe inflammatory or ocular involvement, corticosteroids may also be necessary.

Multidrug-Resistant Pathogens: Pet Reservoirs

In addition to classic zoonotic pathogens, dogs can also be colonized by multidrug-resistant bacteria and serve as potential reservoir hosts. This includes methicillin-resistant Staphylococcus aureus. Transmission between humans and animals is particularly possible during close physical contact.

Such colonization or infection may be clinically significant, particularly in patients with chronic wounds, implanted foreign material, or immunosuppression. Detection is made using microbiologic swabs, followed by resistance testing. When infection is confirmed, treatment is guided by the pathogen and susceptibility results; severe cases should be managed in consultation with infectious disease specialists or microbiologists.

What Matters in Clinical Practice?

The variety of pathogens and transmission routes indicates that, in cases of suspected dog-associated infections, the clinician should not focus solely on whether a bite injury occurred. Rather, a structured exposure history, the timing of contact, travel history, and the vaccination status of both the animal and the patient is crucial.

Following bite injuries, the initial focus should be on adequate wound care and assessing the need for a tetanus booster. If there is a possibility of rabies exposure, it must be determined immediately whether postexposure prophylaxis is necessary.

For high-risk wounds, prophylactic antibiotic administration may also be indicated. This applies in particular to injuries to the hand, deep puncture wounds, possible involvement of joints or the periosteum, as well as to high-risk patients, such as those with immunosuppression, asplenia, or advanced liver disease. If a wound infection is already present, however, therapeutic antibiotic treatment is required.

This story was translated from Medscape’s German edition.