There are some bacteria that spend most of their time in animal hosts, but occasionally infect humans who have close contact with animals. These zoonotic infections include brucellosis (acquired by slaughtering or drinking the milk of cows, sheep or camels infected with Brucella) and cat-scratch disease (caused by Bartonella henselae and acquired by—well, you can probably guess). In other cases, however, 2 (or more) related species or genera of bacteria have adapted to live, and sometimes cause disease, in different hosts, with one species sticking to an animal host and another hanging out with (or in) humans.

Whooping Cough and Kennel Cough: Bordetella pertussis and Bordetella bronchiseptica

Imagine you're a doctor seeing a 3-month-old patient who has come to clinic because of paroxysmal coughing (coughing that occurs in intense, prolonged bouts followed by a deep, gasping inhalation). The patient does not look acutely ill, but is coughing persistently during the exam. You’re concerned about a highly contagious, gram-negative coccobacillus that may be the source of these symptoms, because it often causes particularly severe disease during the first months of life. What organism are you thinking about?

The answer depends on what species of patient you imagined! In humans, whooping cough is caused by infection with Bordetella pertussis and presents with a persistent, often paroxysmal cough with a characteristic "whoop" sound during inhalation. In infants under 4 months of age, pertussis can have severe complications, such as apnea (a temporary pause in breathing) and seizures. In dogs, a different Bordetella species, B. bronchiseptica, is one of the causes of kennel cough, which is recognized by a characteristic cough often described as "honking." (Kennel cough can also be caused by viruses, including canine distemper virus, canine adenovirus type 2 and canine parainfluenza.) B. bronchiseptica can also cause upper respiratory tract infection in other mammals, including rabbits, pigs and cats, whereas B. pertussis is strictly a human pathogen and appears to have derived as recently as a few thousand years ago from a B. bronchiseptica-like ancestor, with many genes lost or inactivated in the process of evolving into a pathogen with a more limited host range.

These 2 Bordetella species have a variety of virulence factors in common, including the adhesin filamentous hemagglutinin. Other virulence factors, such as the aptly named tracheal colonization factor and the exotoxin pertussis toxin, are expressed by only one of the organisms (B. pertussis in this case). B. bronchiseptica expresses a flagellum that confers motility; recent work suggests that B. pertussis, although previously thought to be nonmotile, is also capable of expressing a flagellum, although the significance of this ability in infection remains unclear.

Vaccination is a key element in protection against both whooping and honking: B. pertussis vaccine is included in the routine childhood vaccination schedule in the United States (although immunity wanes over time, and rates of pertussis have been increasing in recent years in spite of widespread vaccination in childhood), and vaccination against B. bronchiseptica is generally recommended for dogs who will be spending time in congregate settings (kennels, doggie daycare, etc.).

Infected Bites and Shipping Fever: Pasteurella multocida and Mannheimia haemolytica & Histophilus somni

Pasteurella multocida is a gram-negative rod with an extraordinarily broad range of animal hosts, including birds (fowl cholera), pigs (atrophic rhinitis), cattle (pneumonia or bovine respiratory disease) and even saiga antelopes. As a commensal organism found in the mouths of domestic cats and dogs, it is a well-known cause of infected animal bites in humans. Thorough cleaning of all animal bites, as well as antibiotic prophylaxis in some scenarios, can help to prevent wound infection after a bite.

Unlike P. multocida, 2 other members of the family Pasteurellaceae, Mannheimia haemolytica and Histophilus somni, are not typically encountered as pathogens in humans. However, along with P. multocida, they constitute the 3 major bacterial causes of bovine respiratory disease (BRD). Also known as “shipping fever” because it commonly affects calves that have recently arrived at the feedlot after weaning, BRD describes a syndrome of upper and lower respiratory tract infections in cattle that is believed to result from viral respiratory infection complicated by bacterial infection, often in the setting of underlying physiologic stresses (e.g., weaning and transport). Prevention and control of BRD is challenging. Vaccines are available for cattle against P. multocida, M. haemolytica and H. somni, as well many of the viruses implicated in BRD. However, vaccination appears to have little effect on reducing rates of BRD, perhaps due to poor response to vaccination by calves undergoing the stresses of weaning and shipping. In contrast, control of stressors (e.g. by weaning prior to shipping) and metaphylaxis (administration of antimicrobials to animals in a group after disease has been diagnosed in some members of the group, in order to control spread of infection) appear to be more effective.

Humans, Pigs, Cows and…Gray Seals?: Streptococci of All Kinds

Infectious disease doctors and clinical microbiologists who confine themselves to human patients are familiar with a long list of streptococci. This genus of gram-positive cocci can be classified (somewhat confusingly) by a variety of different organizational schemes, including the pattern of hemolysis they exhibit when grown on blood agar plates (alpha (partial), beta (complete) or gamma (none)) and the carbohydrate antigens in their cell wall (Lancefield groups A, B, C, etc.). Streptococcal species cause an enormous range of clinical diseases, including pharyngitis (“strep throat”), cellulitis, meningitis, bacteremia, endocarditis and toxic shock syndrome. While some of these syndromes are most often caused by a specific species (pharyngitis and toxic shock syndrome by S. pyogenes, meningitis and pneumonia by S. pneumoniae), almost all streptococci are capable of causing a wide spectrum of clinical manifestations.

When it comes to mastering the list of clinically relevant streptococci, veterinarians have it no easier. Streptococcal species that cause disease in animals are classified by the same systems used for “human” species (hemolysis and Lancefield typing). Although most of these species can infect a variety of hosts, their names often indicate the animal in which they most frequently cause disease: dogs (S. canis), horses (S. equi subsp. zooepidemicus), pigs (S. suis), gray seals (S. halichoeri) and dolphins and fish (S. iniae). Not all streptococci confine themselves exclusively to humans or animals. S. agalactiae (group B strep) is a major cause of invasive disease, including bacteremia and meningitis in human newborns, as well as mastitis in dairy cows. (Its species name, “agalactiae,” meaning “absence of milk,” appears to be a reference to its effects on milk production in infected cows.)

Even among species more classically described in animals, human infection has been described for nearly all streptococci. Zoonotic streptococci usually make the leap to humans when a person is profoundly immunosuppressed and therefore vulnerable to infection by an organism that is not otherwise suited for invasion of a human host. Furthermore, animal pathogens also play a critical indirect role in human disease, because antibiotics used to treat domesticated animals may contribute to the emergence and spread of antimicrobial resistance among human pathogens as well. In the end, it’s best to remember that bacteria have large animals (including humans) hopelessly beat at the game of adaptation, and the border between animal and human pathogen is never entirely impermeable.