BOTULINUM TOXINS

SUMMARY

Signs and Symptoms: Ptosis, generalized weakness, dizziness, dry mouth and throat, blurred vision and diplopia, dysarthria, dysphonia, and dysphagia followed by symmetrical descending flaccid paralysis and development of respiratory failure. Symptoms begin as early as 24-36 hours but may take several days after inhalation of toxin.

Diagnosis: Clinical diagnosis. No routine laboratory findings. Biowarfare attack should be suspected if multiple casualties simultaneously present with progressive descending bulbar, muscular, and respiratory weakness. Treatment: Intubation and ventilatory assistance for respiratory failure. Tracheostomy may be required. Administration of heptavalent botulinum antitoxin (IND product) may prevent or decrease progression to respiratory failure and hasten recovery. Prophylaxis: Pentavalent toxoid vaccine (types A, B, C, D, and E) is available as an IND product for those at high risk of exposure. Isolation and Decontamination: Standard Precautions for healthcare workers. Toxin is not dermally active and secondary aerosols are not a hazard from patients. Hypochlorite (0.5% for 10-15 minutes) and/or soap and water.

OVERVIEW

The botulinum toxins are a group of seven related neurotoxins produced by the bacillus Clostridium botulinum. These toxins, types A through G, could be delivered by aerosol over concentrations of troops. When inhaled, these toxins produce a clinical picture very similar to foodborne intoxication, although the time to onset of paralytic symptoms may actually be longer than for foodborne cases, and may vary by type and dose of toxin. The clinical syndrome produced by one or more of these toxins is known as "botulism".

HISTORY AND SIGNIFICANCE

Botulinum toxins have caused numerous cases of botulism when ingested in improperly prepared or canned foods. Many deaths have occurred secondary to such incidents. It is feasible to deliver botulinum toxins as a biological weapon, and other countries have weaponized or are suspected to have weaponized one or more of this group of toxins. Iraq admitted to a United Nations inspection team in August of 1991 that it had done research on the offensive use of botulinum toxins prior to the Persian Gulf War, which occurred in January and February of that year. Further information given in 1995 revealed that Iraq had not only researched the use of this toxin as a weapon, but had filled and deployed over 100 munitions with botulinum toxin.

TOXIN CHARACTERISTICS

Botulinum toxins are proteins of approximately 150,000 kD molecular weight which can be produced from the anaerobic bacterium Clostridium botulinum. As noted above, there are seven distinct but related neurotoxins, A through G, produced by different strains of the clostridial bacillus. All seven types act by similar mechanisms. The toxins produce similar effects when inhaled or ingested, although the time course may vary depending on the route of exposure and the dose received. Although an aerosol attack is by far the most likely scenario for the use of botulinum toxins, theoretically the agent could be used to sabotage food supplies; enemy special forces or terrorists might use this method in certain scenarios to produce foodborne botulism in those so targeted.

MECHANISM OF TOXICITY

The botulinum toxins as a group are among the most toxic compounds known to man. Appendix C shows the comparative lethality of selected toxins and chemical agents in laboratory mice. Botulinum toxin is the most toxic compound per weight of agent, requiring only 0.001 microgram per kilogram of body weight to kill 50 percent of the animals studied. As a group, bacterial toxins such as botulinum tend to be the most lethal of all toxins. Note that botulinum toxin type A is 15,000 times more toxic than VX and 100,000 times more toxic than Sarin, two of the well known organophosphate nerve agents.

Botulinum toxins act by binding to the presynaptic nerve terminal at the neuromuscular junction and at cholinergic autonomic sites. These toxins then act to prevent the release of acetylcholine presynaptically, and thus block neurotransmission. This interruption of neurotransmission causes both bulbar palsies and the skeletal muscle weakness seen in clinical botulism. Unlike the situation with nerve agent intoxication, where there is too much acetylcholine due to inhibition of acetylcholinesterase, the problem in botulism is lack of the neurotransmitter in the synapse. Thus, pharmacologic measures such as atropine are not indicated in botulism and would likely exacerbate symptoms.

CLINICAL FEATURES

The onset of symptoms of inhalation botulism may vary from 24 to 36 hours, to several days following exposure. Recent primate studies indicate that the signs and symptoms may in fact not appear for several days when a low dose of the toxin is inhaled versus a shorter time period following ingestion of toxin or inhalation of higher doses. Bulbar palsies are prominent early, with eye symptoms such as blurred vision due to mydriasis, diplopia, ptosis, and photophobia, in addition to other bulbar signs such as dysarthria, dysphonia, and dysphagia. Skeletal muscle paralysis follows, with a symmetrical, descending, and progressive weakness which may culminate abruptly in respiratory failure. Progression from onset of symptoms to respiratory failure has occurred in as little as 24 hours in cases of foodborne botulism.

Physical examination usually reveals an alert and oriented patient without fever. Postural hypotension may be present. Mucous membranes may be dry and crusted and the patient may complain of dry mouth or even sore throat. There may be difficulty with speaking and with swallowing. Gag reflex may be absent. Pupils may be dilated and even fixed. Ptosis and extraocular muscle palsies may also be observed. Variable degrees of skeletal muscle weakness may be observed depending on the degree of progression in an individual patient. Deep tendon reflexes may be present or absent. With severe respiratory muscle paralysis, the patient may become cyanotic or exhibit narcosis from CO2 retention.

DIAGNOSIS

The occurrence of an epidemic of cases of a descending and progressive bulbar and skeletal paralysis in afebrile patients points to the diagnosis of botulinum intoxication. Foodborne outbreaks tend to occur in small clusters and have never occurred in soldiers on military rations such as MRE’s (Meals, Ready to Eat). Higher numbers of cases in a theater of operations should raise at least the consideration of a biological warfare attack with aerosolized botulinum toxin. Foodborne outbreaks are theoretically possible in troops on normal "A" rations.

Individual cases might be confused clinically with other neuromuscular disorders such as Guillain-Barre syndrome, myasthenia gravis, or tick paralysis. The edrophonium or Tensilon® test may be transiently positive in botulism, so it may not distinguish botulinum intoxication from myasthenia. The cerebrospinal fluid in botulism is normal and the paralysis is generally symmetrical, which distinguishes it from enteroviral myelitis. Mental status changes generally seen in viral encephalitis should not occur with botulinum intoxication. It may become necessary to distinguish nerve agent and/or atropine poisoning from botulinum intoxication. Nerve agent poisoning produces copious respiratory secretions and miotic pupils, whereas there is if anything a decrease in secretions in botulinum intoxication. Atropine overdose is distinguished from botulism by its central nervous system excitation (hallucinations and delirium) even though the mucous membranes are dry and mydriasis is present. The clinical differences between botulinum intoxication and nerve agent poisoning are depicted in Appendix E. Laboratory testing is generally not helpful in the diagnosis of botulism. Survivors do not usually develop an antibody response due to the very small amount of toxin necessary to produce clinical symptoms. Detection of toxin in serum or gastric contents is possible, and mouse neutralization (bioassay) remains the most sensitive test. Other assays include gel hydralization or ELISA. Serum specimens should be drawn from suspected cases and held for testing at such a facility.

MEDICAL MANAGEMENT

Respiratory failure secondary to paralysis of respiratory muscles is the most serious complication and, generally, the cause of death. Reported cases of botulism prior to 1950 had a mortality of 60%. With tracheostomy or endotracheal intubation and ventilatory assistance, fatalities should be less than five percent. Intensive and prolonged nursing care may be required for recovery which may take several weeks or even months.

Antitoxin: In isolated cases of food-borne botulism, circulating toxin is present, perhaps due to continued absorption through the gut wall. Botulinum antitoxin (equine origin) has been used in those circumstances, and is thought to be helpful. Animal experiments show that after aerosol exposure, botulinum antitoxin can be very effective if given before the onset of clinical signs. Administration of antitoxin is reasonable if disease has not progressed to a stable state. A trivalent equine antitoxin has been available from the Centers for Disease Control and Prevention for cases of foodborne botulism. This product has all the disadvantages of a horse serum product, including the risks of anaphylaxis and serum sickness. A "despeciated" equine heptavalent antitoxin against types A, B, C, D, E, F, and G has been prepared by cleaving the Fc fragments from horse IgG molecules, leaving F(ab) 2 fragments. This product is under advanced development, and is currently available under IND status. Its efficacy is inferred from its performance in animal studies. Disadvantages include a reduced, but theoretical risk of serum sickness. Use of the antitoxin requires skin testing for horse serum sensitivity prior to administration. Skin testing is performed by injecting 0.1 ml of a 1:10 dilution (in sterile physiological saline) of antitoxin intradermally in the patient’s forearm with a 26 or 27 gauge needle. Monitor the injection site and observe the patient for allergic reaction for 20 minutes. The skin test is positive if any of these allergic reactions occur: hyperemic areola at the site of the injection > 0.5 cm; fever or chills; hypotension with decrease of blood pressure > 20 mm Hg for systolic and diastolic pressures; skin rash; respiratory difficulty; nausea or vomiting; generalized itching. Do NOT administer Botulinum F(ab’) 2 Antitoxin, Heptavalent (equine derived) if the skin test is positive. If no allergic symptoms are observed, the antitoxin is administered intravenously in a normal saline solution, 10 mls over 20 minutes. With a positive skin test, desensitization is carried out by administering 0.01 - 0.1 ml of antitoxin subcutaneously, doubling the previous dose every 20 minutes until 1.0 - 2.0 ml can be sustained without any marked reaction.

PROPHYLAXIS

Vaccine: A pentavalent toxoid of Clostridium botulinum toxin types A, B, C, D, and E is available under an IND status. This product has been administered to several thousand volunteers and occupationally at-risk workers, and induces serum antitoxin levels that correspond to protective levels in experimental animal systems. The currently recommended primary series of 0, 2, and 12 weeks, then a 1 year booster induces protective antibody levels in greater than 90 percent of vaccinees after one year. Adequate antibody levels are transiently induced after three injections, but decline prior to the one year booster.