Zoonotic Organisms: Exploring Gram-Unfavorable Rods Related to Animal Sources
INTRODUCTION
Zoonoses are human ailments attributable to organisms which might be
acquired from animals. There are bacterial, viral, fungal, and
parasitic zoonoses. Some zoonotic organisms are acquired
instantly from the animal reservoir, whereas others are transmit-
ted by vectors, resembling mosquitoes, fleas, or ticks.
There are 4 medically necessary gram-negative rods that
have vital animal reservoirs: Brucella species, Francisella
tularensis, Yersinia pestis, and Pasteurella multocida (Desk 20–1).
Further data relating to the scientific facets of
infections attributable to the organisms on this chapter is offered
in Half IX entitled Infectious Illnesses starting on web page 589.
BRUCELLA
Illness
Brucella species trigger brucellosis (undulant fever).
Essential Properties
Brucellae are small gram-negative rods and not using a capsule. The
three main human pathogens and their animal reservoirs are
Brucella melitensis (goats and sheep), Brucella abortus (cattle),
and Brucella suis (pigs).
Pathogenesis & Epidemiology
The organisms enter the physique both by ingestion of contami-
nated milk merchandise or by means of the pores and skin by direct contact in
an occupational setting resembling an abattoir. They localize within the
reticuloendothelial system, particularly, the lymph nodes, liver,
spleen, and bone marrow. Many organisms are killed by macro-
phages, however some survive inside these cells, the place they’re pro-
tected from antibody. The host response is granulomatous, with
lymphocytes and epithelioid big cells, which might progress to
kind focal abscesses. The mechanism of pathogenesis of those
organisms isn’t properly outlined, besides that endotoxin is concerned.
No exotoxins are produced.
Imported cheese produced from unpasteurized goats’ milk pro-
duced in both Mexico or the Mediterranean area has been a
supply of B. melitensis an infection in america. The illness
happens worldwide however is uncommon in america as a result of pas-
teurization of milk kills the organism.
Medical Findings
After an incubation interval of 1 to three weeks, nonspecific symp-
toms resembling fever, chills, fatigue, malaise, anorexia, and weight
loss happen. The onset might be acute or gradual. The undulating
(rising-and-falling) fever sample that provides the illness its identify
happens in a minority of sufferers. Enlarged lymph nodes, liver,
and spleen are steadily discovered. Pancytopenia happens. Brucella
melitensis infections are typically extra extreme and extended,
whereas these attributable to B. abortus are extra self-limited. Osteo-
myelitis is essentially the most frequent complication. Secondary unfold
from individual to individual is uncommon.
Laboratory Analysis
Restoration of the organism requires using enriched cul-
ture media and incubation in 10% CO2. The organisms can
be presumptively recognized by utilizing a slide agglutination take a look at
with Brucella antiserum, and the species might be recognized by
biochemical checks. If organisms aren’t remoted, evaluation of a
serum pattern from the affected person for an increase in antibody titer to
Brucella can be utilized to make a analysis. Within the absence of
an acute-phase serum specimen, a titer of a minimum of 1:160 within the
convalescent-phase serum pattern is diagnostic.
Therapy
The therapy of selection is tetracycline plus rifampin. There’s
no vital resistance to those medicine.
Prevention
Prevention of brucellosis entails pasteurization of milk, immu-
nization of animals, and slaughtering of contaminated animals. There
isn’t any human vaccine.
FRANCISELLA
Illness
Francisella tularensis causes tularemia.
Essential Properties
Francisella tularensis is a small, pleomorphic, gram-negative
rod. It has a single serologic kind. There are two biotypes, A
and B, that are distinguished totally on their virulence
and epidemiology. Kind A is extra virulent and located primar-
ily in america, whereas kind B is much less virulent and
discovered primarily in Europe. Kind A is related to rabbits,
whereas kind B is extra generally present in rodents and water
sources.
Pathogenesis & Epidemiology
Francisella tularensis is outstanding in the wide range of
animals that it infects and within the breadth of its distribution within the
United States. It’s enzootic (endemic in animals) in each state,
however most human circumstances happen within the rural areas of Arkansas and
Missouri. It has been remoted from greater than 100 completely different
species of untamed animals, an important of that are rab-
bits, deer, and quite a lot of rodents. The micro organism are transmitted
amongst these animals by vectors resembling ticks, mites, and lice,
particularly the Dermacentor ticks that feed on the blood of untamed
rabbits. The tick maintains the chain of transmission by passing
the micro organism to its offspring by the transovarian route. On this
course of, the micro organism are handed by means of ovum, larva, and nymph
levels to grownup ticks able to transmitting the an infection.
People are unintentional “dead-end” hosts who purchase the
an infection most frequently by being bitten by the vector or by having
pores and skin contact with the animal throughout elimination of the disguise. Hardly ever,
the organism is ingested in contaminated meat, inflicting gastroin-
testinal tularemia, or is inhaled, inflicting pneumonia. Hardly ever,
the individual is contaminated by a penetrating pores and skin lesion in a water
surroundings, resembling a fish-hook damage. There isn’t any person-to-
individual unfold. The primary kind of tularemia in america
is tick-borne tularemia from a rabbit reservoir.
The organism enters by means of the pores and skin, forming an ulcer at
the positioning most often. It then localizes to the cells of the reticu-
loendothelial system, and granulomas are shaped. Caseation
necrosis and abscesses may happen. Signs are prompted
primarily by endotoxin. No exotoxins have been recognized.
Medical Findings
Presentation can range from sudden onset of an influenzalike
syndrome to extended onset of a low-grade fever and adenopa-
thy. Roughly 75% of circumstances are the “ulceroglandular” kind,
by which the positioning of entry ulcerates and the regional lymph nodes
are swollen and painful. Different, much less frequent types of tularemia
embody glandular, oculoglandular, typhoidal, gastrointestinal,
and pulmonary. Illness often confers lifelong immunity.
Laboratory Analysis
Makes an attempt to tradition the organism within the laboratory are not often
undertaken, as a result of there’s a excessive danger to laboratory employees
of an infection by inhalation, and the particular cysteine-containing
medium required for progress isn’t often out there. Essentially the most
steadily used diagnostic methodology is the agglutination take a look at with
acute- and convalescent-phase serum samples. Fluorescent anti-
physique staining of contaminated tissue can be utilized if out there.
Therapy
Streptomycin is the drug of selection. There isn’t any vital anti-
biotic resistance.
Prevention
Prevention entails avoiding each being bitten by ticks and
dealing with wild animals. There’s a dwell, attenuated bacterial
vaccine that’s given solely to individuals, resembling fur trappers, whose
occupation brings them into shut contact with wild animals.
The vaccine is experimental and never out there commercially
however might be obtained from the U.S. Military Medical Analysis
Command, Fort Detrick, Maryland. This and the bacillus of
Calmette-Guérin (BCG) vaccine for tuberculosis are the one
two dwell bacterial vaccines for human use.
YERSINIA
Illness
Yersinia pestis is the reason for plague, often known as the black
loss of life, the scourge of the Center Ages. Additionally it is a up to date
illness, occurring within the western United States and in lots of different
international locations around the globe. Two much less necessary species, Yersinia
enterocolitica and Yersinia pseudotuberculosis, are described in
Chapter 27.
Essential Properties
Yersinia pestis is a small gram-negative rod that reveals bipolar
staining (i.e., it resembles a security pin, with a central clear space).
Freshly remoted organisms possess a capsule composed of a
polysaccharide–protein advanced. The capsule might be misplaced with
passage within the laboratory; lack of the capsule is accompanied by
a lack of virulence. It is likely one of the most virulent micro organism recognized
and has a strikingly low ID50 (i.e., 1–10 organisms are able to
inflicting illness).
Pathogenesis & Epidemiology
The plague bacillus has been endemic within the wild rodents of
Europe and Asia for hundreds of years however entered North
America within the early 1900s, in all probability carried by a rat that
jumped ship at a California port. It’s now endemic within the wild
rodents within the western United States, though 99% of circumstances of
plague happen in Southeast Asia.
The enzootic (sylvatic) cycle consists of transmission amongst
wild rodents by fleas. In america, prairie canine are the
primary reservoir. Rodents are comparatively proof against illness; most
are asymptomatic. People are unintentional hosts, and circumstances of
plague on this nation happen because of being bitten by a flea
that’s a part of the sylvatic cycle.
The city cycle, which doesn’t happen in america,
consists of transmission of the micro organism amongst city rats (the
reservoir), with the rat flea as vector. This cycle predominates
throughout instances of poor sanitation (e.g., wartime), when rats pro-
liferate and are available contact with the fleas within the sylvatic cycle.
The occasions inside the flea are fascinating in addition to important.
The flea ingests the micro organism whereas taking a blood meal from a
bacteremic rodent. A thick biofilm containing many organisms
types within the higher gastrointestinal tract that forestalls any meals
from continuing down the gastrointestinal tract of the flea. This
“blocked flea” then regurgitates the organisms into the blood-
stream of the following animal or human it bites.
The organisms inoculated on the time of the chunk unfold to the
regional lymph nodes, which grow to be swollen and tender. These
swollen lymph nodes are the buboes which have led to the identify
bubonic plague. The organisms can attain excessive concentrations
within the blood (bacteremia) and disseminate to kind abscesses
in lots of organs. The endotoxin-related signs, together with
disseminated intravascular coagulation and cutaneous hemor-
rhages, in all probability had been the genesis of the time period black loss of life.
Along with the sylvatic and concrete cycles of transmission,
respiratory droplet transmission of the organism from sufferers
with pneumonic plague can happen.
The organism has a number of components that contribute to its viru-
lence: (1) the envelope capsular antigen, known as F-1, which pro-
tects towards phagocytosis; (2) endotoxin; (3) an exotoxin; and
(4) two proteins generally known as V antigen and W antigen. The V and
W antigens permit the organism to outlive and develop intracel-
lularly, however their mode of motion is unknown. The motion of the
exotoxin is unknown.
Different components that contribute to the extraordinary pathoge-
nicity of Y. pestis are a gaggle of virulence components collectively
known as Yops (Yersinia outer proteins). These are injected
into the human cell by way of kind III secretion programs and inhibit
phagocytosis and cytokine manufacturing by macrophages and
neutrophils. For instance, one of many Yops proteins (YopJ) is a
protease that cleaves two sign transduction pathway proteins
required for the induction of tumor necrosis issue synthesis.
This inhibits the activation of our host defenses and contributes
to the power of the organism to copy quickly inside the
contaminated particular person.
Medical Findings
Bubonic plague, which is essentially the most frequent kind, begins with
ache and swelling of the lymph nodes draining the positioning of the
flea chunk and systemic signs resembling excessive fever, myalgias,
and prostration. The affected nodes enlarge and grow to be exqui-
sitely tender. These buboes are an early attribute discovering.
Septic shock and pneumonia are the principle life-threatening
subsequent occasions. Pneumonic plague can come up both from
inhalation of an aerosol or from septic emboli that attain the
lungs. Untreated bubonic plague is deadly in roughly half
of the circumstances, and untreated pneumonic plague is invariably deadly.
Laboratory Analysis
Smear and tradition of blood or pus from the bubo is the perfect
diagnostic process. Nice care should be taken by the doctor
throughout aspiration of the pus and by laboratory employees doing
the tradition to not create an aerosol that may transmit the
an infection. Giemsa or Wayson stain reveals the everyday safety-pin
look of the organism higher than does Gram stain. Fluo-
rescent antibody staining can be utilized to determine the organism
in tissues. An increase in antibody titer to the envelope antigen might be
helpful retrospectively.
Therapy
The therapy of selection is a mixture of streptomycin and
a tetracycline resembling doxycycline, though streptomycin alone
can be utilized. Levofloxacin may also be used. There isn’t any sig-
nificant antibiotic resistance. In view of the fast development
of the illness, therapy mustn’t anticipate the outcomes of the
bacteriologic tradition. Incision and drainage of the buboes are
not often mandatory.
Prevention
Prevention of plague entails controlling the unfold of rats in
city areas, stopping rats from coming into the nation by ship
or airplane, and avoiding each flea bites and phone with useless
wild rodents. A affected person with plague should be positioned in strict
isolation (quarantine) for 72 hours after antibiotic remedy is
began. Solely shut contacts have to obtain prophylactic tetra-
cycline, however all contacts must be noticed for fever. Reporting
a case of plague to the general public well being authorities is necessary.
A vaccine consisting of formalin-killed organisms gives
partial safety towards bubonic however not pneumonic plague.
This vaccine was used within the armed forces throughout the Vietnam
Conflict however isn’t really helpful for vacationers touring to Southeast
Asia.
PASTEURELLA
Illness
Pasteurella multocida causes wound infections related to
cat and canine bites.
Essential Properties
Pasteurella multocida is a brief, encapsulated gram-negative rod
that reveals bipolar staining.
Pathogenesis & Epidemiology
The organism is a part of the traditional flora within the mouths of many
animals, significantly home cats and canine, and is transmit-
ted by biting. About 25% of animal bites grow to be contaminated with
the organism, with sutures appearing as a predisposing issue to
an infection. Most chunk infections are polymicrobial, with a vari-
ety of facultative anaerobes, particularly Streptococcus species,
and anaerobic organisms current along with P. multocida.
Pathogenesis isn’t properly understood, besides that the capsule is
a virulence issue and endotoxin is current within the cell wall. No
exotoxins are made.
Medical Findings
A quickly spreading cellulitis on the web site of an animal chunk is
indicative of P. multocida an infection. The incubation interval is
transient, often lower than 24 hours. Osteomyelitis can complicate
cat bites specifically, as a result of cats’ sharp, pointed enamel can
implant the organism below the periosteum.
Laboratory Analysis
The analysis is made by discovering the organism in a tradition of a
pattern from the wound web site.
Therapy
Penicillin G is the therapy of selection. There isn’t any vital
antibiotic resistance.
Prevention
Individuals who have been bitten by a cat must be given ampicillin
to forestall P. multocida an infection. Animal bites, particularly cat
bites, shouldn’t be sutured.
BARTONELLA
Illness
Bartonella henselae is the reason for cat-scratch illness and bacil-
lary angiomatosis. Cat-scratch illness is likely one of the most com-
mon zoonotic ailments in america.
Essential Properties
Bartonella henselae is a small, pleomorphic gram-negative rod.
It’s a fastidious organism and won’t develop on routine blood
agar. It may be cultured on specialised media within the scientific
laboratory.
Pathogenesis & Epidemiology
Cat scratches or bites, particularly from kittens, are the principle
mode of transmission of B. henselae to people. The organism
is a member of the oral flora of many cats. There’s proof
that it’s transmitted from cats to people by the chunk of cat fleas.
Publicity to cat urine or feces doesn’t pose a danger of transmis-
sion. Particular person-to-person transmission of B. henselae doesn’t
play a major function in an infection. Bartonella henselae is a low-
virulence organism, and illness is self-limited in immunocom-
petent people.
The pathogenesis of angiomas that happen in Bartonella
infections in immunocompromised people is unsure.
One present rationalization is that an infection of endothelial cells
by Bartonella induces the synthesis of angiogenesis issue that
causes endothelial cells to proliferate.
Medical Findings
In immunocompetent individuals, B. henselae causes cat-scratch
illness (CSD). This illness is characterised by fever and ten-
der, enlarged lymph nodes, sometimes on the identical facet because the
scratch (Determine 20–1). A papule on the web site of the scratch could
precede the lymphadenopathy. CSD has a protracted course however
finally resolves, even with out antibiotics. A small proportion
of these contaminated develop systemic illness, resembling endocarditis
or encephalitis.
In immunocompromised people, particularly sufferers
with acquired immunodeficiency syndrome (AIDS), B. henselae
causes bacillary angiomatosis (BA). BA is characterised by
raised, cherry-red vascular lesions within the pores and skin and visceral
organs (Determine 20–2). The lesions seem papular or nodular.
Laboratory Analysis
The analysis of CSD is often made serologically. Antibod-
ies towards B. henselae antigens might be detected in a affected person’s
serum by quite a lot of immunologic checks. The organism can
be cultured on synthetic media however takes 5 days or longer to
develop and so isn’t often performed. The analysis of BA is commonly
made by discovering pleomorphic rods in biopsy tissue utilizing the
Warthin-Starry silver stain. Pathologic examination of tissue
from the lesion will distinguish bacillary angiomatosis from
Kaposi’s sarcoma.
Therapy
No antibiotic remedy is often really helpful for CSD. If the
affected person has extreme lymphadenitis, azithromycin is the drug of
selection. Therapy of BA with doxycycline or erythromycin is
efficient. There isn’t any vital antibiotic resistance.
Prevention
Antibiotics aren’t really helpful for individuals who have sus-
tained a cat scratch. There isn’t any vaccine.
SELF-ASSESSMENT QUESTIONS
1. Your affected person is a 10-year-old boy who has a excessive fever and swol-
len, painful axillary lymph nodes on the left facet. His mom says
that he introduced dwelling a useless rat a number of days in the past. You watched he
could have bubonic plague. Relating to the causative organism,
which one of many following is most correct?
(A) It has a really low ID50.
(B) It’s transmitted from rodents to people by ticks.
(C) It’s endemic primarily within the states alongside the East Coast of the
United States.
(D) Its primary virulence issue is an exotoxin that induces interleu-
kin-2 (IL-2) manufacturing by CD4-positive helper T cells.
(E) An infection must be handled with excessive doses of penicillin G
intravenously.
2. Your affected person is a 20-year-old man who was bitten on the hand
when he tried to interrupt up a battle between two cats yesterday. He
now has a purple, scorching, tender, swollen lesion on the chunk web site that has
unfold quickly throughout his hand. Which one of many following bacte-
ria is the more than likely reason for his cellulitis?
(A) Brucella melitensis
(B) Francisella tularensis
(C) Pasteurella multocida
(D) Yersinia pestis
Your affected person is a 30-year-old lady who reviews that she has
had intermittent fever of 102°F, sweating, and fatigue for the previous
one month or so. She has misplaced her urge for food and has misplaced about
10 kilos in that interval. She enjoys consuming unpasteurized goat
cheese. On examination, hepatosplenomegaly is detected. A
blood depend reveals pancytopenia. Which one of many following
micro organism is the more than likely reason for this an infection?
(A) Brucella melitensis
(B) Francisella tularensis
(C) Pasteurella multocida
(D) Yersinia pestis
4. Relating to Bartonella henselae, which of the next is most
correct?
(A) Bartonella henselae is an anaerobic, spore-forming, gram-
constructive rod.
(B) The pure habitat of Bartonella henselae is the cat’s mouth.
(C) Bartonella henselae causes cellulitis in immunocompromised
sufferers resembling AIDS sufferers.
(D) Analysis within the scientific laboratory is determined by detecting anti-
our bodies within the affected person’s serum that can agglutinate cardiolipin.
(E) The drug of selection for Bartonella henselae infections is
metronidazole.
ANSWERS
(1) (A)
(2) (C)
(3) (A)
(4) (B)
SUMMARIES OF ORGANISMS
Transient summaries of the organisms described on this chapter
start on web page 653. Please seek the advice of these summaries for a fast
evaluation of the important materials.
PRACTICE QUESTIONS: USMLE &
COURSE EXAMINATIONS
Questions on the matters mentioned on this chapter might be discovered
within the Medical Bacteriology part of Half XIII: USMLE
(Nationwide Board) Follow Questions beginning on web page 715. Additionally
see Half XIV: USMLE (Nationwide Board) Follow Examination
