Limiting the Zoonotic Risks of Bacterial Infections

The human–animal bond takes many forms: service dog, emotional support animal, companion animal, travel partner, family member. This close relationship puts people at risk of being exposed to zoonotic diseases—that is, diseases that can be transmitted from animals to humans.¹ Many of these diseases are caused by bacteria, some of which are resistant to antibiotics.

Although some zoonotic bacterial infections may have grave consequences for humans and animals, many are preventable. It is therefore important for veterinary nurses to be able to discuss potential bacterial zoonotic diseases with clients, including how to protect their pet from exposure as well as how to protect themselves, other family members, and possibly other pets if exposure has already occurred. It is essential that veterinary nurses know how to protect themselves and prevent transmission within the clinical setting, as well as how to educate owners about the risks of zoonotic bacterial infections and how to prevent them.

Overview of Zoonotic Disease

Zoonotic diseases are found worldwide and account for many newly recognized infectious diseases, as well as existing ones.² They can be caused by bacteria, viruses, fungi, or parasites and are transmitted by direct or indirect exposure, depending on the pathogen. Certain zoonotic diseases can be transmitted back to animals from humans (known as reverse zoonosis or zooanthroponosis), and others may eventually transform to become human-only diseases. Globally, zoonotic diseases are commonplace, with scientists estimating that more than half of infectious diseases in humans can be spread from animals.¹ Most of the bacterial zoonotic diseases seen in the United States are found in both urban and rural settings. Pets can be exposed while indoors or outdoors. Infected pets may not show clinical signs of disease, making it difficult for the veterinary team to identify disease and for the owner to take precautions. While it is important not to create fear around potential zoonotic disease, educating clients about risk factors and proper prevention can help protect both pets and families (BOX 1).

Common Bacterial Zoonotic Diseases

Leptospirosis

Leptospirosis is a zoonotic disease caused by Leptospira spirochetes. These spiral-shaped bacterial pathogens infect a wide variety of mammals worldwide.^4,5 Globally, the most important reservoir hosts are rodents, which account for a worldwide infection incidence of 30%; in some regions, the rate exceeds 80%.⁶ Leptospirosis can affect several organ systems.⁷

Leptospira bacteria colonize the renal tubules of infected animals and are shed in the urine to contaminate soil and water (e.g., lakes, streams, rivers, puddles). In mild, damp environments, they can survive and replicate for up to several months without a host. Places that have increased risk of contamination include homes with backyard poultry, composting areas, ponds/lakes, and areas with poor sanitation and a higher rodent or wildlife population.⁴

Animals can become infected via mucous membranes (including venereal or placental transfer) or abraded or broken skin (e.g., chafing, bite wounds). The most common form of transmission is contact with contaminated water.⁷

All dogs are at risk of leptospirosis, regardless of signalment, geographic location, lifestyle, and time of year.⁴ Dogs with leptospirosis may present with the clinical signs listed in BOX 2. Infection in dogs can cause severe damage to the kidneys and liver and can be fatal. Reports of leptospirosis in cats are rare; however, cats may be disease-resistant reservoir hosts.⁴ In humans, aseptic meningitis occurs in up to 25% of Leptospira infections.⁴

Treatment in dogs can include starting with penicillin followed by doxycycline at 5 mg/kg IV q12h for 2 weeks. Treatment should not be delayed while test results are awaited. Supportive care for blood pressure management and gastrointestinal disturbances should be considered.⁴

Precautions

While caring for dogs with leptospirosis in the hospital, veterinary nurses should use barrier precautions to protect skin and mucous membranes from contact with urine or blood, as these are the main routes of transmission.⁸ Protective personal equipment (PPE) such as gloves, gowns or bunny suits, masks that include eye protection or face shields to protect against aerosolized urine, and shoe covers should be worn.⁸ Dogs should be walked in a designated area often enough that urination does not occur accidentally in the hospital.⁴

For patients that are sent home, the client should be educated to avoid contact with urine and to wear eye protection and gloves when cleaning up urine until after the first 48 hours of antibiotics.⁴

Protection against leptospirosis is provided by offering vaccination for dogs. The initial vaccines are started at 12 weeks of age and consist of 2 vaccines given 4 weeks apart. After this initial series, booster vaccination should be provided yearly.

Client Education

It is important for the veterinary team to remember and to educate owners that all dogs are at risk of leptospirosis.⁴ Changing land use has increased the risk of contact between wildlife reservoir hosts and pets (and subsequently humans) in both city and more rural environments. Owners may not realize how at risk their pets, especially dogs, are for contracting leptospirosis.

In addition, as different organ systems can be affected, the clinical signs of leptospirosis can vary widely. The lack of distinctive clinical features, especially in early stages, makes leptospirosis hard to diagnose. Therefore, it is imperative for veterinary teams to educate clients early and often regarding the dangers of leptospirosis and how to mitigate the risk to their pet and thereby the entire family (BOX 3).

Staphylococcal Infections

Staphylococcus species are gram-positive bacteria in the Staphylococcaceae family.⁹ Although most animals carry at least 1 Staphylococcus species, Staphylococcus aureus and Staphylococcus pseudintermedius are 2 of the species most commonly associated with infection.¹⁰ Antibiotic-resistant strains of both species exist. S pseudintermedius is part of the normal skin flora of dogs.¹¹ S aureus, which is estimated to colonize about 30% of healthy humans (primarily in the nasal cavity), is a major cause of disease in many animal species and can have significant effects on public health and agriculture.^11,12

S pseudintermedius accounts for ~90% of pyoderma cases in dogs.¹³ Additionally, it is associated with antimicrobial resistance in other Staphylococcus species.¹² In dogs, infections with S aureus or S pseudintermedius usually involve the skin. Dermal S pseudintermedius infections tend to occur in dogs with underlying skin disorders or other health conditions.¹⁴ S pseudintermedius infections of the ear or urinary tract, conjunctivitis, prostatitis, and surgical site infections have also been reported.¹⁴ Dermal S aureus infections can look like various skin conditions, including boils or pimples, in both humans and animals; however, the infection can spread to the blood, bones, heart, or lungs and sometimes becomes fatal.^15,16

In cats, S aureus and Staphylococcus coagulans account for 10% of pyoderma cases.¹³ S aureus is more often associated with pyoderma in cats than is S pseudintermedius.¹³

Humans can become briefly colonized with S pseudintermedius, but how frequently this happens is unknown. Human S pseudintermedius infection most often occurs from dog bites. S pseudintermedius can become invasive in humans and cause illnesses such as bacteremia, endocarditis, and device-associated infections.¹⁴ Transmission of S aureus from dogs to humans seems to be uncommon.¹¹

Methicillin-resistant S pseudintermedius infections have become common in dogs, with some estimates of incidence as high as 40%.¹⁴ Some strains are multidrug resistant.¹⁴ Current guidelines for treatment of pyoderma caused by S pseudintermedius infections recommend use of antibiotics and shampoos; however, research into alternative, nonantibiotic therapies is ongoing.¹⁷ Methicillin-resistant S aureus (MRSA) infections have been reported in a variety of companion and exotic animal species. Healthy dogs and cats can be colonized by MRSA, but this colonization is likely transient. Dogs are much less likely to be infected or colonized with S aureus than with S pseudintermedius.¹⁸

Risk Factors

Recent antimicrobial exposure, including therapy with β-lactams or fluoroquinolones, is the primary risk factor for acquiring methicillin-resistant staphylococci.¹⁸ Additional risk factors for MRSA infection in pets include contact with an ill or hospitalized person, intravenous catheterization, and extended hospital stays.¹⁸

Precautions

S aureus can survive in temperatures ranging from 7 °C to 48 °C (44.6 °F to 118.4 °F),¹⁹ and for up to 72 hours on stainless steel surfaces and up to 28 days on dry mops.²⁰ S pseudintermedius can survive on environmental surfaces for several months. Veterinary teams should follow strict hygiene practices to control transmission of methicillin-resistant staphylococci. Practical hygiene practices include:

• Use of PPE, such as disposable gloves, when working with infected patients
• Covering open or draining wounds
• Frequent cleaning of infected patients’ dishes, bedding, cages, and leashes
• Environmental disinfection
• Regular handwashing, especially after handling infected patients and between patients

Client Education

In the home environment, many Staphylococcus infections can be prevented by washing hands thoroughly with soap and water or using an alcohol-based hand sanitizer.¹⁵

Strict hygiene practices are critical to limit transmission of methicillin-resistant staphylococci. Owners should be advised to wear gloves when interacting with an infected pet. In addition, the veterinary team must educate the pet owner to clean the pet’s environment, including bedding, dishes, and collars/harnesses, and remind them to wash their hands before and after every interaction. Owners must understand that their pet cannot be allowed to sleep in bed with them and should be prevented from licking them or items in the environment.

It is imperative for the veterinary team to record client education recommendations in the patient’s medical record. Giving the client recommendations in writing along with a veterinary team member’s contact information can be helpful.

Salmonellosis

Salmonella bacteria are gram-negative, rod-shaped bacteria that belong to the Enterobacteriaceae family. There are 2 main species that have several subspecies and serotypes.²¹ Humans and animals usually become infected by consuming contaminated food or water.

Most animals infected with Salmonella have no clinical signs of illness; however, the most common syndromes associated with Salmonella are systemic septicemia and enteritis. Dogs and cats with Salmonella usually have acute diarrhea with or without septicemia. Sheep, adult cattle, and horses can also develop enteritis. Less common clinical presentations include abortion, arthritis, respiratory disease, necrosis of the extremities, and meningitis.²² In humans, salmonellosis usually results in acute gastroenteritis, which can be severe.²¹

Pet reptiles often have Salmonella in their gastrointestinal tract and can periodically shed the bacteria in feces. Salmonella does not cause illness in reptiles, but bacteria shed by reptiles can cause illness in humans and other animals, either through direct contact with the reptile or from handling or ingesting material contaminated with reptile waste.²³

Precautions

Raw diets have been associated with salmonellosis in dogs and cats because these diets do not undergo a processing step to kill bacteria, including Salmonella.²⁴ Therefore, feeding a raw diet to a cat or dog is not recommended by the U.S. Centers for Disease Control and Prevention (CDC).²⁵ Owners may need to be educated on the risks of raw diets.

Anyone handling a pet with a Salmonella infection should use strict hand hygiene and keep the environment as clean as possible. In the veterinary hospital, it is important not to eat or drink in animal areas and not to kiss high-risk animals such as reptiles. Surfaces should be sanitized.

Client Education

Proper hygiene, including hand washing after interacting with animals, must be reinforced to pet owners, especially if the pet is exhibiting signs of illness (e.g., diarrhea, vomiting) or if the pet is a reptile, amphibian, or a flock of backyard chickens. Children younger than 5 years and people with weakened immune systems should not touch or play with pets or animals likely to carry Salmonella. No one in the household should eat or drink around these animals or their living areas. If possible, the pet’s food bowls, toys, and bedding should be disinfected outside, but cleaning in a dishwasher or with bleach is an acceptable alternative.

Bartonellosis

While several Bartonella species are suspected of causing disease in humans, the primary species of zoonotic concern are Bartonella henselae and Bartonella clarridgeiae. These gram-negative intracellular bacilli are carried by arthropods (commonly fleas), which infect animals through their bite. Cats are the primary reservoir for infection in humans. Humans can also be infected through flea bites, but transmission via a cat scratch or bite contaminated with feces from infected fleas is more common. In humans, the resulting disease (cat scratch disease) can be life-threatening.²⁶

The most common clinical signs of bartonellosis in dogs are indicative of infective endocarditis (e.g., lethargy, fever, heart murmur, cough, tachypnea, lameness). Cats are commonly healthy carriers; however, fever, endocarditis, myocarditis, and transient lymphadenopathy have been seen in cats. Bartonella infection may also lead to other systemic inflammatory disorders.^27,28

In immunocompetent humans, common symptoms of bartonellosis include swollen lymph nodes, fever, and bumps or a rash on the skin. More severe disease, such as bacteremia, endocarditis, and proliferative lesions, can occur in high-risk patients. Children (especially those aged 5 to 9 years) appear to be at greatest risk of cat scratch disease.²⁶

According to the CDC, most cat scratches do not result in disease; however, if symptoms develop within 1 to 3 weeks a physician should be seen for treatment recommendations.²⁹ Treatment of infected cats is only recommended for those with clinical signs of disease, and diagnosis of infection requires culture or  polymerase chain reaction testing.²⁸

Client Education

Flea control is essential to reduce the spread of Bartonella infection among cats, dogs, and humans. Veterinary teams should educate owners regarding proper training and handling of cats and dogs to reduce bites and scratches, especially in households with high-risk people. Avoiding bites and scratches is even more important when dealing with animals likely to be infected (e.g., kittens, feral cats, outdoor cats, cats with fleas). Educating owners about the importance of trimming their cats’ nails should be undertaken to reduce the chance of skin lacerations. It is also important to educate owners to keep cats indoors, as this can reduce the risk of infection.

High-risk households should consider the following recommendations^27,29:

• Adopt healthy cats older than 1 year to reduce the risk of bites and scratches from rough play
• Maintain flea control
• Promptly wash cat bites and scratches
• Prevent cats from licking wounds on people

Antibiotic-Resistant Infections

The continuing emergence of antibiotic-resistant infections is a global public health concern and a focus of One Health efforts.³⁰ One emerging concern is the development of carbapenem-resistant Enterobacterales (CRE), including carbapenem-resistant Escherichia coli and Klebsiella pneumoniae. CRE can infect humans and animals. Although the zoonotic risk to healthy people appears to be low and information about modes of CRE transmission in veterinary settings is currently limited, CRE are difficult to treat; therefore, reducing exposure risk is paramount.³ In addition to the precautions listed in BOX 4, CRE-infected animals in the veterinary hospital should be isolated to protect other animals and veterinary personnel.

Having an antimicrobial stewardship program in place can help reduce the development of antimicrobial resistance.³¹ The AVMA encourages veterinary professionals to implement antimicrobial stewardship to improve antimicrobial prescribing.³² The core principles of such a program include³²:

• Engaging all team members in the stewardship effort
• Advocating for a practice structure to stop common diseases from occurring
• Choosing and using antimicrobial drugs cautiously
• Educating clients about appropriate use of antibiotics

Summary

The human–animal bond between an owner and a pet is mutually beneficial and is essential to the health and wellbeing of both parties. However, pet ownership may also put people at risk for exposure to zoonotic diseases. It is critically important that the veterinary team be aware of potential zoonotic disease, understand how these diseases are transmitted, and educate clients about not only the disease and its potential for transmission but also how to mitigate the risk for transmission.