1. Lecture 23 - Zoonotic Infections

2. Zoonotic Infections Zoonosis = disease of animals transmitted to man
transmission may occur in several ways:
direct contact with diseased flesh - eg., tularemia
drinking raw or unpasteurized cow’s or goat’s milk - eg., tuberculosis
inhalation of dust particles contaminated with animal excreta or
products - eg., psittacosis
eating insufficiently cooked infected flesh - eg., trichinosis
bite of insect vectors carrying infectious agents - eg., plague
bite of diseased animal - eg., rabies
because of their proximity to humans, domestic animals are the more
common source of zoonoses (domestic animals may become infected by
contact with wild animals or their insect vectors)
occupational risk for some professions - farmers, veterinarians, etc

3. Zoonotic Infections
Methods of control for diseases in domestic animals that are
transmissible to humans:
sacrifice or isolation of diseased and/or infected animals
pasteurization of milk
adequate cooking of meat before eating
vector control
vaccination of susceptible animals
Diagnosis requires either isolation of organisms or appropriate serology;
pt history and clinical signs may be helpful but not sufficient to
make diagnosis

4. 4 Zoonotic Gram-negative coccobacilli
Yersinia pestis - plague
Yersinia enterocolitis - enterocolitis
Yersinia pseudotuberculosis -
appendicitis-like syndrome
Francisella tularensis - ulceroglandular,
typhoidal or oculoglandular disease
Brucella melitensis
Brucella suis
Brucella abortus
Pasteurella multocida - bipolar staining
bipolar
staining
Facultative
intracellular
pathogens;
persist in
macrophages:
CMI important
undulant fever
Extracellular
pathogen;
humoral response
important

5. Yersinia
belongs to Enterobacteriaceae - gram negative rods, which ferment
glucose and are oxidase negative
3 species are pathogenic to man
Yersinia pestis = cause of plague
Y. enterocolitica and Y. pseudotuberculosis = cause entercolitis (also
known as yersiniosis)
Yersinia are small gram-negative coccobacilli which often demonstrate
bi-polar staining (safety-pin) using Giemsa or Wayson stains
Y. pestis = non-motile
Y. enterocolitica and Y. pseudotuberculosis = motile at 25oC but not 37oC

6. Yersinia pestis = the plague bacillus
1. Epidemiology
- infectious bacteria maintained in nature in
sylvatic cycle
- involves rodents & rat flea (Xenopsylla cheopis)
- involves ground squirrels in West, prairie dogs
in the Southwest,and wood rats in South
- urban plague begins when infected fleas of the
sylvatic cycle infect animals living closer
to humans, such as the rat
- starts an urban cycle which increases risk
to man

7. Genetic location and expression of virulence factors in Yersinia pestis.

8. Yersinia pestis = the plague bacillus
2. Antigenic structure
- some of the major antigens are expressed on plasmids, while some on
chromosome
- F-1 antigen = capsule, antiphagocytic, expressed best at 37oC
- V-W antigens and several Yersinia proteins (Yops) are plasmid
gene products coordinately expressed at 37oC in the presence of
low Ca2+ concentrations; imp in resisting phagocytosis (V-W) as well
as intracellular survival and growth within macrophages; Yops
proteins form a type III secretion system for injecting several
Yops proteins into eukaryotic cells which kills them
- coagulase = clots blood in insect gut
- LPS = may cause hemorrhage, vascular collapse and focal necrosis
- Pla gene = plasminogen activator which degrades fibrin clots and
C3b and C5a

9. Yersinia pestis = the plague bacillus
3. Clinical manifestations
- Y. pestis causes human plague when bacteria are introduced into body
by bite of infected flea. Rat fleas acquire organism during a blood
meal on an infected rodent (usually the rat)
- Y. pestis produces a coagulase which, at <27oC and with help of an
enzyme in flea’s gut, clots the blood; this prevents flea from
getting nourishment during the next feeding - regurgitates the
bacteria into bite site
Bubonic plague = spread of bacteria to regional lymph nodes (groin or
axillae) forming buboes (swollen, tender lymph nodes) which may
suppurate and drain; bacteria are found within macrophages in
nodes; septic shock with fatality rate is 75% if untreated
Pneumonic plague = spread to lung can result in respiratory disease
and aerosol dissemination; death due to respiratory failure
(“Black Death”); ~100% fatal if not treated

10. Pathogenesis of Yersinia pestis in plague patients.

11. Plague: enlarged, tender inguinal lymph nodes in a Vietnamese child
with bubonic plague.

12. Plague: advanced stage of inguinal lymphadenitis in bubonic plague. The
nodes have undergone suppuration and the lesion has drained spontaneously.

13. Smear of lymph node aspirate from a case of bubonic plague showing many
bi-polar staining Y. pestis (left, Wayson’s stain). Smear of peripheral blood
of patient with septicemic plague, showing bi-polar staining Y. pestis (right,
van Gieson stain).

14. Yersinia pestis = the plague bacillus
4. Host defenses
- production of specific anti-Fraction 1 antibodies will stimulate
phagocytosis and promote killing by neutrophils
- cell-mediated immunity will activate macrophages and assist in
killing facultative intracellular bacteria
5. Diagnosis
- need to isolate the causative agent - biopsy the bubo and culture
6. Treatment
- streptomycin, tetracycline or chloramphenicol
7. Prevention and control
- formalin-inactivated vaccine available for those travelling to endemic
areas and those in high risk groups (only effective against bubonic
plague)

15. Yersinia enterocolitica
common cause of enterocolitis
rodents, dogs, cats and domestic farm animals are natural hosts
ingestion of contaminated food or water, bacteria have affinity for M
cells in Peyer’s patches, traverse the mucosal membranes of gi
tract thru M cells, then grow in mesenteric lymph nodes
facultative intracellular bacteria
have many of the same genes of Y. pestis as well as invasin gene (inv)
and ail (adherence invasion locus)
enterocolitis = abdominal pain, diarrhea and fever; regional lymph-
adenopathy; watery diarrhea due to heat-stable enterotoxin
which stimulates cGMP
often self-limited; fluid and electrolytes if needed; TMP-SMZ and
aminoglycosides if liver and spleen abscesses occur

16. Yersinia pseudotuberculosis
causes an appendicitis-like syndrome
ingestion of contaminated food or water, bacteria have affinity for M
cells in Peyer’s patches, traverse the mucosal membranes of gi
tract thru M cells, then grow in mesenteric lymph nodes
facultative intracellular bacteria
have many of the same genes of Y. pestis as well as invasin gene (inv)
and ail (adherence invasion locus)
abdominal pain, regional lymphadenopathy; usually no diarrhea; symptoms
of appendicitis
TMP-SMZ, aminoglycosides, cephalosporins, and other drugs have been
used

17. Francisella non-motile gram-negative coccobacilli encapsulated
F. tularensis causes tularemia or rabbit fever - spread to man directly
by ticks and deerflies; most strains which infects rabbits are
highly infectious to humans (could result from skinning rabbit, or
eating infected rabbit meat, or bite of infected tick or deerfly)
facultative intracellular pathogens in macrophages and monocytes

18. Francisella Clinical manifestations:
- after 2-5 d incubation, organisms are multiplying in cells of RES,
forming focal necrosis and small granulomas; signs and symptoms depend
on how disease contracted:
1. Ulceroglandular = most common; bacteria penetrates skin abrasion,
produces local lesion which necrotizes and spreads to local lymph
nodes with regional lymphadenopathy; could also be from tick bite
2. Typhoidal = after ingestion of contaminated meat, severe typhoid-like
disease occurs, with local abscesses in Peyer’s patches and
mesenteric lymph nodes; high fever and severe toxemia noted
3. Oculogladular = infection via conjunctival membrane leads to swelling
of eyelid, ulceration of conjunctiva and regional lymphadenopathy

19. Tularaemia: irregular ulcer
at the site of the initial
lesion.

20. Protection against Francisella tularensis or Yersinia pestis is cell-mediated.

21. Francisella
Diagnosis = direct immunofluorescent antibody staining of bacteria in
infected tissues or blood; isolation and culture is dangerous for
lab; serologic demonstration of rising antibody also used and
is most common test used
Treatment = streptomycin or gentamicin
Control = live attenuated vaccine used for lab workers; protective
gloves and goggles for hunters and trappers

22. Pasteurella 1. Characteristics - small gram-negative coccobacillus
- bi-polar staining
- encapsulated (A, B, D and E types; A most common)
- most common strain is Pasteurella multocida
2. Epidemiology
- rabbits, cats and dogs carry these bacteria as normal flora of naso-
pharynx and gingival crevices
- cattle which carry these strains may develop fulminating hemorrhagic
pneumonia when stressed or dehydrated (shipping fever); also
been seen in flocks of chickens and turkeys
- rabbits gets snuffles (chronic nasopharyngeal infection)

23. Pasteurella 3. Pathogenesis
- extracellular pathogens which resist phagocytosis due to capsule
- LPS may contribute to tissue damage
- pilus serves as adhesin
- after entry thru skin due to animal bite, local colonization is
mediated thru pilus at wound site, with spread to regional
lymph nodes; acute inflammatory response follows but
bacteria resist ingestion due to capsule
4. Clinical manifestations
- P. multocida typically is seen in infected cat and dog bites, with
localized inflammation at the site with abscess formation and
regional lymphadenopathy; osteomyelitis is a serious
complication of this disease

24. Protection against Pasteurella multocida is mediated by opsonic antibodies.

25. Pasteurella
Diagnosis: should suspect this organism in any infected cat or dog bite;
isolate organism from abscess and culture
Treatment: penicillin, tetracycline or chloramphenicol used to treat
abscesses; prolonged treatment needed; abscess should be
drained
Control: no effective vaccine available; ampicillin can be used
prophylactically

26. Brucella
Imp cause of disease of cattle, goats, pigs, etc. usually affecting their
reproductive organs leading to abortion, sterility and decreased milk
production. Humans infected thru contact with diseased animals, cause
an acute febrile disease (undulant fever) or a more chronic process
which may have wide array of symptoms
Characteristics:
- gram-negative coccobacillus, no capsule
- slow growers, aerobic, catalase and oxidase positive
B. melitensis = goats and sheep
B. abortus = cattle
B. suis = pigs
B. canis = dogs (infrequent cause of human infections)
Important source of disease for humans

27. Brucella
Predilection of brucellae for reproductive organs of ruminants due to
presence of erythritol in amniotic and allantoic fluid - leads to abortion
shedding huge numbers of bacteria; bacteria also establish themselves
in mammary glands, infecting milk (animals usually show no symptoms).
Humans become infected by handling these animals or drinking contaminated
milk.
Brucellosis is relatively uncommon in US ( cases/yr due to animal
vaccination) but endemic in many parts of world

28. Sources of Brucella infection.

29. Brucella Pathogenesis:
- brucellae gain entrance thru broken skin, by ingestion or thru conjunctiva;
carried to liver, spleen and bone marrow by lymphatics; are facultative
intracellular pathogens multiplying in RES
- intracellular multiplication induces chronic inflammatory response char.
by tissue lesions with minute granulomas composed of epitheliod cells,
PMNs, lymphocytes and some giant cells, usu. in spleen and bone marrow
Clinical manifestations:
- 2-4 wk incubation
- onset is influenza-like, with fever, severe arm and muscle pain, sweating
and fatigue; disease may continue for weeks with fever and symptoms
rising and falling in 10 d intervals (undulant fever) - most pts recover
in 3-12 months
- some pts develop chronic infections with arthritis, meningitis, etc.
- abortion is not seen in humans

30. Brucella Host defenses: CMI is most important
Diagnosis: isolation from blood, lymph nodes and bone marrow aspirates;
requires special media; serological results are often only test which
makes diagnosis
Prevention/control: live attenuated vaccines for animals; pasteurize milk;
animals which are infected should be sacrificed to protect herd
Treatment: combination of doxycycline and rifampin, doxycyline and
gentamicin, or TMP-SMZ for pregnant women and children

31. “LEPTOSPIRA” Tightly coiled aerobic spirochete
Leptospirosis (or Weil’s disease) is a zoonosis caused by pathogenic species of leptospira, Leptospira interrogans. Saprophytic strains are called L. biflexa.
Characteristics:
Tightly coiled aerobic spirochete
-oxidation of fatty acids
can be grown in medium with rabbit serum (Fletcher’s medium)

32. Leptospira interrogans serotype icterohaemorrhagiae. Silver staining of
organisms grown in culture. Notice the tightly coiled body with hooked ends.

33. Darkfield microscopy of Leptospira.

34. Leptospira Pathogenesis:
- zoonosis of rats, dogs, cats, horses, cattle and pigs
- animals which develop carrier state shed leptospira in urine (even
immunized dogs might shed organisms)
- leptospires colonize kidney tubules of animals to produce a chronic
carrier state
- man becomes infected with contact with urine-contaminated soil
and water (often swimming or ingestion of contaminated food
or water)
- leptospires enter thru mucous membranes or skin abrasions, then
spreads thru blood to all tissues
- tissue damage is to endothelium of small vessels - mech unknown
no LPS or toxins
- greatest risk of infection is to those whose occupation brings them
in most contact with animals

35. Stages of icteric and anicteric leptospirosis.

36. Leptospira Clinical manifestations:
- biphasic disease with involvement of the CNS, kdiney and liver
- 5-13% of aseptic meningitis due to leptsopires
- Weil’s syndrome = severe form of leptospirosis characterized by
jaundice, renal damage and interstitial nephritis

37. Leptospira
Immunity: humoral antibody response is needed for host defense
Diagnosis: serology more common than isolation and culture; a microscopic
agglutination test is done using pool of leptospiral antigens and pt
serum and agglutination determined by microscopy
- can isolate or detect leptospira in blood or CSF early in
disease, while later, most leptospires are present in urine
Treatment: penicillin or doxycycline is effective if given during 1st
phase of disease; animal vaccine is used for most animals at
risk of infection