Clostridium tetani
Life history
Clostridium tetani is the bacteria responsible for the often fatal disease tetanus. The word tetanus comes from the Greek word tetanos which means to ‘stretch’. Tetanus is characterized by rigidity and convulsive spasms of skeletal muscle. It wasn’t until 1884 that the cause of the disease was discovered by Carle and Rattone. In 1889, Kitasato (one of Koch’s pupil) isolated the organism and showed that it produced disease when injected into animals.
Spores of C. tetani are everywhere and are found in soil, dust, animal intestines, and animal feces (including humans). Its occurrence is worldwide but is most commonly found in dense populated regions in hot, damp climates with soil rich in organic matter.
Transmission is primarily by contaminated wounds. The typical example is stepping on a rusty nail. Tetanus is not contagious from person to person but is infectious as acquired through environmental exposure. Only vaccine-preventable disease that is infectious but not contagious.
Microbiological characteristics
C. tetani is an anaerobic gram-positive bacillus that forms spores. The bacteria have been described to look like a drumstick, racket or club. This organism is sensitive to heat and cannot survive in the presence of oxygen. However, the spores are very resistant to heat and usual antiseptics.
(from: http://www.bact.wisc.edu/Bact330/nfC.tetani.jpeg)
Disease it causes
C. tetani causes tetanus and there are 4 clinical types. Incubation period ranges from 3-21 days, with an average of about a week. It has a fatality rate of 30%.
Generalized – (80%) Most common type. Toxins get distributed via lymphatic and vascular system and spread more widely and affect more nerves. First symptom is the characteristic lock jaw. It spreads and begins to affect the rest of your muscle, starting with the neck and moving to your back. This generalized muscle rigidity comes with reflex spasms as your body tries to respond to various stimuli. These spasms can cause fractures, tendon rupture and respiratory failure. Death from tetanus results from respiratory failure and cardiovascular instability. Other symptoms caused by autonomic dysfunction may include fever, sweating and high blood pressure. Recovery can take months but is usually complete unless complications occur.
Localized – Very uncommon. Patients with this clinical type experience muscle rigidity close to the site of injury. These contractions can persist for many weeks before disappearing. 1% fatality rate.
Cephalic – Form of localized disease that affects cranial nerves. It can happen after ear infections or head injuries. It affects cranial nerves so it can affect the muscles in your face (eyelid, tongue, lips, etc).
Neonatal – Form of generalized tetanus that occurs in newborn infants. Usually happens when the umbilical cord is cut with an unsterile instrument. There are some cultures where it is ritualistic to apply cow dung to the already cut umbilical cords of newborn infants. 90% fatality rate. Very common in developing countries.
(from: http://wwwuser.gwdg.de/~hbruegg/ct/tetanus2.gif)
Virulence factors
C. tetani spores germinate in proper anaerobic conditions, and one ideal medium are wounds with dead cells. When the conditions are right, they will germinate and produce two toxins: tetanolysin and tatanospasmin. Tetanolysin is a hemolysin with no recognized pathogenic ability while tetanospasmin is the peptide responsible for tetanus.
Tetanospasmin is a 150kD peptide made out of a heavy chain (B) and a light chain (A) joined by a disulfide bond. The heavy chain specifically binds to neuronal cells (disialogangliosides). The light chain , a zinc endopeptidase, attacks the vessicle associated membrane protein and blocks the release of inhibitory neurotransmitters (GABA & Glycine).
Inhibitory nerutransmitter produce inhibitors that bind to receptors on excited neurons. The binding of the inhibitor blocks the neuron from releasing the acetylcholine that is responsible for muscle contraction. As a result, the muscle relaxes. If these neurotransmitter are blocked (as the case when these toxins are present), there is nothing stopping the release of acetycholine from the excited neuron. As a result, the muscle will stay contracted.
Tetanospasmin is used to create the toxoid used in immunization vaccine. Tetanus toxoid was first produced in 1924 and consists of a formaldehyde-treated toxin.
Epidemiological data
Frequency
United States: Reported Cases
1940s - .4 cases per 100,000 population
1970s - .05 cases per 100,000
2002 - .01 per 100,000
(from: http://www.cdc.gov/nip/publications/pink/tetanus.pdf)
Global
Reports show about 1 million cases of tetanus every year, most of them in third world countries. About half of these tetanus related death is a result of neonatal tetanus. (from:http://www.emedicine.com/med/topic2254.htm)
Injuries and Conditions that Cause Tetanus
Puncture – 37%
Laceration – 24%
Chronic wound – 11%
Abrasion – 6%
Injection drug user – 5%
Other – 17%
(from: http://www.cdc.gov/nip/publications/pink/tetanus.pdf)
Links
Animation: Relaxation of a Muscle by an Inhibitory Interneuron
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/bacpath/relax.html
Animation: Mode of Action of the Tetanus Exotoxin
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/bacpath/tetexo.html
List of Sources
http://www.vaccineinformation.org/tetanus/qandadis.asp
http://www.fact-index.com/t/te/tetanospasmin.html
http://www.emedicine.com/med/topic2254.htm
http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/tetanus.pdf