Part 1: Top 30 Zoonotic Diseases

For my own personal use only:

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60% of human pathogens originate from animals.
Brucella melitensis was first isolated in 1887 from a soldier in Malta by Major-General Sir David Bruce.

Anthrax

Cerebrospinal fluid with gram-positive anthrax bacteria

Brown-black eschar that resembles anthracite coal (hence the name anthrax, Greek for coal) on the arm of a man in the country of Georgia

In 2001, powdered anthrax spores were mailed through the US Postal Service
- Classic case:
  - Cattle, sheep, goats, bison, camels, antelope:
    - Sudden death
    - Bloating
    - Bleeding from orifices after death
  - Human:
    - Skin lesions with dark eschars
    - Malaise
    - Gastrointestinal signs
    - Fever
    - Acute respiratory distress
    - Septicemia
- Etiology: Bacillus anthracis
  - Gram-positive aerobic rod-shaped bacteria
  - Bacteria sporulates when exposed to oxygen
  - Endemic in North America
- Zoonotic issues:
  - Routes of human infection:
    - Cutaneous
    - Ingestion (infected meat)
    - Inhalation
  - Precautions: Personal protective equipment (PPE): Respiratory protection
- Pearls:
  - World Organisation for Animal Health (OIE) notifiable disease
  - Bioterrorism agent: In 2001, powdered anthrax spores were deliberately mailed through the US Postal Service, leading to 22 infections in people & 5 deaths
  - Spores persist in environment for decades
  - Outbreaks can occur with heavy rainfall, flooding, or drought
  - Avoid full necropsies (i.e., do not open the carcass) of affected animals as bacteria will sporulate and contaminate environment

Baylisascaris

Contact with raccoon feces can lead to transmission of B. procyonis
- Classic case:
  - Definitive host (raccoons, sometimes dogs or kinkajous): No clinical signs
  - Intermediate host (commonly rodents, opossums, foxes, badgers, sea otters, birds, non-human primates, humans; NOT livestock):
    - CNS signs
    - Ocular disease
- Etiology: Baylisascaris procyonis
  - Intestinal nematode
  - Definitive host infected by ingesting eggs or eating infected intermediate host
- Zoonotic issues:
  - Humans infected by fecal-oral transmission
  - Precaution: Avoid direct contact with dog and raccoon feces
- Pearls:
  - Dogs can also be intermediate hosts and develop clinical signs
  - Keep dogs on monthly heartworm/nematode preventives to minimize risk of intestinal infection

Bovine tuberculosis (TB)

Two 1929 images comparing a healthy bovine udder (left) and an udder from a cow with tuberculosis (right)

Vertebral tuberculosis (arrows) in a human
- Classic case:
  - Cow:
    - Decreased appetite, progressive emaciation
    - Cough
    - Fever
    - Weakness
  - In humans, affects:
    - Lymph nodes
    - Bones
    - Joints
    - CNS
    - Lungs
    - Genitourinary system
- Etiology: Mycobacterium bovis
  - Gram-positive, acid-fast bacterium in Mycobacterium tuberculosis complex
  - Cattle are primary host
  - Many "spillover" hosts (e.g., sheep, goats, horses, llamas, dogs, pigs, ferrets, cats, rodents)
- Zoonotic issues:
  - Routes of human infection:
    - Ingestion of unpasteurized dairy products or undercooked/raw meat
    - Inhalation of aerosolized agent
    - Bacterial contact on broken skin
    - Wildlife and soil are potential sources of infection
  - Precautions: PPE: Respiratory protection
- Pearls:
  - OIE notifiable disease
  - Canada is bovine TB-free
  - Eradication programs in US and Mexico
  - Control efforts in the US & routine pasteurization of milk have decreased M. bovis cases to less than 2% of total human TB cases (the rest caused by M. tuberculosis )
  - Cats are rarely infected with M. bovis and have been suspected of transmitting TB to humans (and vice versa)

Brucellosis: cattle, sheep, goats, dogs

Sheep and dogs are two species that can be infected with Brucella bacteria

Gram-stained B. canis
- Classic case:
  - Bovine, ovine, and caprine:
    - Abortions, usually in second half of gestation
    - Epididymitis and orchitis in bulls
  - Canine:
    - Abortions, stillbirths
    - Orchitis
    - Diskospondylitis
  - Human:
    - "Undulant fever"
    - Drenching sweats
    - Headache
    - Flu-like signs
    - Can be chronic debilitating disease
- Etiologies: Brucella spp. are gram-negative, coccobacillary, facultative intracellular bacterium
  - Brucella abortus : Cattle (also found in bison, elk, and [recently] feral pigs)
  - B. melitensis : Ovine and caprine
  - B. canis : Dogs
- Zoonotic issues:
  - B. abortus and B. melitensis :
    - Routes of infection:
      - Ingestion of unpasteurized dairy product
      - Exposure to Brucella vaccine, infected animals, or in a laboratory
    - Precautions:
      - PPE
      - Handle vaccines with care
  - B. canis :
    - Importance as a cause of disease in humans not well-established
    - People infected by exposure to infected animals and in the laboratory
    - Precautions: PPE
- Pearls: B. abortus and B. melitensis are OIE notifiable diseases
  - Bovine:
    - Eradicated in Canada, nearly eradicated in US
    - Possible bioterrorism agent
    - Poll evil and fistulous withers in horses may be caused by B. abortus
  - Ovine and caprine:
    - Found in Mediterranean, Middle East, and Central America
    - Exotic to Canada and US, endemic in Mexico
  - Canine:
    - Reportable in many US states
    - Owners of Brucella-positive dogs should be advised that the disease could potentially spread to humans and dogs can’t be “cleared” of infection

Bartonellosis (cat scratch fever)

Bartonella henselae is spread by fleas from cat to cat
- Classic case:
  - Cats:
    - Usually asymptomatic
    - +/- Fever
    - +/- Inappetence
  - Humans:
    - Small, reddish-brown papules or pustules at inoculation site
    - Lymphadenopathy
    - Fever
    - Malaise
    - Neurologic signs
    - Psychiatric signs
- Etiology: Bartonella henselae
  - Gram-negative rod bacterium
  - Reservoir host: Domestic cats and other felids
  - Transmitted via flea feces between cats
- Zoonotic issues:
  - Transmitted to humans by cat bites or scratches
- Pearls:
  - Worldwide distribution
  - Most cases of human bartonellosis are mild or asymptomatic and self-limiting; immunocompromised people are more susceptible

Images courtesy of CDC PHIL (anthrax in Georgian man, anthrax in CSF, animated B. procyonis egg, tuberculosis udder, B. canis , and journal cover), Baerni (raccoon), Yale Rosen (spinal tuberculosis), Don DeBold (dog and sheep), Luis Fernández García (flea), and John Cummings (PPE).

Cross-Species