Haemophilus influenzae
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Haemophilus influenzae, is also called Pfeiffer's bacillus or Bacillus influenza
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It is a Gram-negative, coccobacilli bacterium first described in 1892
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A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative
anaerobe.
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H. influenzae was the first free-living organism to have its entire genome sequenced.
Diseases
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Most strains of H. influenzae are opportunistic pathogens
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They usually live in their host without causing disease, but cause problems only when other
factors (such as a viral infection, reduced immune function or chronically inflamed tissues, e.g.
from allergies) create an opportunity.
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They infect the host by sticking to the host cell using Trimeric Autotransporter Adhesins (TAA).
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Naturally acquired disease caused by H. influenzae seems to occur in humans only.
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In infants and young children, H. influenzae type b (Hib) causes bacteremia, pneumonia, and
acute bacterial meningitis.
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On occasion, it causes cellulitis, osteomyelitis, epiglottitis, and infectious arthritis. In
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Due to routine use of the Hib conjugate vaccine in the U.S. since 1990, the incidence of invasive
Hib disease has decreased to 1.3/100,000 in children.
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However, Hib remains a major cause of lower respiratory tract infections in infants and children
in developing countries where the vaccine is not widely used.
Diagnosis
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Clinical features may include initial symptoms of an URTI mimicking a viral infection, usually
associated with fevers, often low grade.
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The URTI then may progresses to a LRTI in a few days with features often resembling a wheezy
bronchitis.
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Sputum is often grey to creamy in colour.
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The cough may persist for weeks without appropriate treatment.
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Clinical diagnosis of H. influenzae is typically performed by bacterial culture or latex particle
agglutinations.
Culture
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Bacterial culture of H. influenzae is performed in chocolate agar plate with added X (hemin) and
V (nicotinamide adenine dinucleotide) factors at 37°C in a CO2-enriched incubator
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Blood agar shows a satellite phenomenon around other bacteria.
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In Gram Stain, Gram-negative, rod shaped, with no specific arrangement.
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It is both catalase and oxidase test positive.
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Fildes agar is the selective medium for isolation.
Latex particle agglutination
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The latex particle agglutination test (LAT) is a more sensitive method to detect H.
influenzae than culture.
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Because the method relies on antigen rather than viable bacteria
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It also has the added benefit of being much quicker than culture methods.
Treatment
Haemophilus influenzae produces beta-lactamases enzymes.
In severe cases, cefotaxime and ceftriaxone delivered directly into the bloodstream are the elected
antibiotics,
For less severe cases, an association of ampicillin and sulbactam,cephalosporins of the second and third
generation, or fluoroquinolones are preferred.
Rickettsia rickettsii
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Rickettsia rickettsii is a unicellular, Gram-negative coccobacillus
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It is most commonly known as the causative agent of Rocky Mountain spotted fever (RMSF).
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It is an obligate intracellular parasite that survive by an endosymbiotic relationship with other
cells.
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R. rickettsii is a non-motile, non-spore forming aerobic organism.
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They lack a distinct nucleus and membrane bound organelles.
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Their outer membrane is composed mostly of lipopolysaccharides.
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RMSF is transmitted by the bite of an infected tick while feeding on warm-blooded animals,
including humans.
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Humans are considered to be accidental hosts in the Rickettsia–tick life cycle
Pathogen life cycle:
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The life cycle of Rickettsia rickettsii is considered to be a complex one.
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Survival is dependent on both an invertebrate vector, (the hard tick- Family Ixodidae) and a
vertebrate host (including mice, dogs, rabbits).
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Humans are considered to be accidental vectors
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Rickettsia rickettsii mostly affects canines and humans.
Transmission in arthropod vectors
Three arthropod vectors in the United States have been identified in transmitting R. rickettsii to humans,
causing the potentially fatal disease Rocky Mountain Spotted Fever. TheAmerican dog tick (Dermacentor
variabilis), the Rocky Mountain Wood Tick (Dermacentor andersoni), and the Brown dog
tick (Rhipicephalus sanguineus) can acquire Rickettsia rickettsiiin a number of different ways.
Transmission in mammals
An uninfected mammal can become infected with Rickettsia rickettsii when eating food that contains
the feaces of the infected tick. They can also be infected through the bite of an infected tick.
Humans acquire Rickettsia rickettsii infection from infected vectors. After getting bitten by an infected
tick, rickettsiae are transmitted to the bloodstream by tick salivary secretions or, as mentioned
previously, through contamination of broken skin by infected vector feces.
All these modes of transmission ensures the survival of Rickettsia in nature.
Rocky Mountain Spotted Fever.
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During the initial stages of the disease, the patient will experience fever, nausea, vomiting, and
loss of appetite.
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The classic RMSF rash occurs in about 90% of patients and develops 2 to 5 days after the onset
of fever.
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Rash appear as small, flat pink macules that develop peripherally on the patient's body, such as
the wrists, forearms, ankles, and feet.
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Diarrhea, abdominal and joint pain, and pinpoint reddish lesions (petechiae) are observed
during the late stages of the disease.
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Patients with severe infections may require hospitalization. They may
become thrombocytopenic, hyponatremic, experience elevated liver enzymes, and other more
pronounced symptoms.
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It is not uncommon for severe cases to involve the respiratory system, central nervous system
Laboratory confirmation
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Rocky Mountain Spotted Fever is often diagnosed using an indirect immunofluorescence
assay (IFA) test
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The IFA will detect an increase in IgG or IgM antibodies.
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A more specific lab test used in diagnosing RMSF is polymerase chain reaction or PCR which can
detect the presence of rickettiae DNA.
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Immunohistochemical (IHC) staining is another diagnostic approach where a skin biopsy is taken
of the spotted rash; however, sensitivity is only 70%.
Antibiotics
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Doxycycline and Chloramphenicol are the most common drugs of choice for reducing the
symptoms associated with RMSF.
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Failure to receive antibiotic therapy, especially during the initial stages of the disease, may lead
to end-organ failure (heart, kidney, lungs, meningitis, brain damage, shock, and even death)