2. Toxic Mechanism of Human Foodborne Botulism

(1)

the length of the neurons that innervate the cranial muscles is short, with consequent poor tertiary deposit (acetylcholine reserves are located in the axon or in the soma);

(2)

facial muscles are continuously active (even during sleep), with a prevalence of phasic fibers, high neurotransmitter turnover and rapid internalization of toxin;

(3)

few receptors are present at postsynaptic facial muscular membrane level that, more quickly, suffer from the lack of acetylcholine.

Recovery may occur only by formation of new axon terminals, with the regenerating axon (sprouting) forming contacts at the original synaptic sites. The time of recovery depends on entity of neuromuscular block associated with neurogenic atrophy (chemo-denervation) and on the regeneration speed of nervous terminals and of presynaptic membranes. Central nervous system is not affected, and the toxin do not cross the placenta [15]^[9].

3. Treatment

Key points in the treatment of botulism are (i) the decontamination, (ii) the administration of the specific antidote and (iii) the support of respiratory function if necessary. There are only few differences related to the way of exposure.

3.1. Gastrointestinal Decontamination

Once excluded eventually contraindications, gastrointestinal decontamination should be performed in all case of foodborne botulism, in order to remove the spores and toxin from the gut. Most patients manifest the first clinical signs some days after consumption of contaminated meal; for this reason, gastric lavage (or induced emesis) should be considered only in cases in which the ingestion of possible contaminated food is recent. For all other cases, the gastrointestinal decontamination, must be applied if the constipation due to the anticholinergic effect may cause a permanence of the contaminated food in the gastrointestinal tract. In these cases, even if the efficacy in suspected and confirmed cases of botulism intoxication is not clearly demonstrated, upper and lower decontamination with oro-gastric tube and cathartics/whole bowel irrigation could be performed. In order to obtain an effective catharsis is better the use of sorbitol because magnesium salts may exacerbate neuromuscular blockade. The whole bowel irrigation may be challenging because of the ileus toxin-induced; in some cases, neostigmine may be useful in reversing ileus, inhibiting the enzymatic degradation of acetylcholine [44]^[10]. Activated charcoal administration is also a treatment option in intoxicated patients because it absorbs BoNT/A in vitro [45]^[11].

3.2. Antidotes

The main goal of the antitoxin treatment is to neutralize the free circulating toxins still unbound at presynaptic level of nerve endings. Additional point is that antitoxin reduces the involvement of new nerve endings: usually the clinical symptoms may progress for up to 12 h after antitoxin administration before an effect is observed [46]^[12]. The type-specific antitoxins are not able to counteract any other antigen. As a general approach, antitoxin should be administered as soon as suspect of botulism intoxication is made. The antitoxin is effective also in all the other forms of botulism. For example, animal studies involving inhalation botulism demonstrated that early administration, after an aerosolized release of botulinum toxin (lethal concentration), may be effective [47]^[13]. Equine-derived antitoxin (since 1970), is the unique antidote available. To date, antidote efficacy is well known in experimental studies in animals. In humans, no randomized controlled studies have been performed to evaluate the action of antitoxin therapy (not ethical approach). To date, the efficacy of antidote is based only by case reports and retrospective studies and clinical experiences. Morbidity and mortality studies are difficult to perform because of the rarity of the intoxication and of the late diagnosis. The diagnosis is normally performed when the BoNT is already permanently entered and fixed at presynaptic level: in this step antidote is incapable to reverse the endocellular mechanism of the toxin. Some clinical experience confirmed that the early administration of antitoxin (within 24 h), is more effective in preventing the progression of neurological syndrome and in shortening the duration of mechanical ventilation and intensive care stay [48,49]^[14][15]. Tacket and co-authors analyzed 132 cases of BoNT/A foodborne botulism (1973–1980), considering the effect of the antitoxin therapy on the outcome of patients. Lower fatality rate (10% vs 15%) was registered in patients that received antitoxin within 24 h after the onset of symptoms. In the group of patients that did not receive antitoxin the fatality rate was very high (46%). Patients that received early antitoxin (<24 h) had a median hospital stay of 10 days compared with 41 days for those who received antitoxin >24 h and 56 days for those not treated [48]^[14]. Nonetheless, patients may need respiratory support for long period, usually 2–6 weeks even if in some cases may be longer: 58 days and 26 days for botulism due to BoNT/A and BoNT/B, respectively [18]^[16]. A prolonged rehabilitation program is also needed for some patients with severe intoxication.

 

The safety and improved clinical outcomes was evaluated in patients treated with BAT during the Investigational New Drug (“compassionate use” IND) study period (2010–2013) [54]^[17]. A total of 249 persons aged 10 days–88 years (median, 46 years) were treated with BAT. Of these, 17 (7%) were children (median, 6 years; range, 10 days–17 years). None of the 249 treated patients were pregnant or breastfeeding. Botulism was laboratory or epidemiologically confirmed for 104 (42%) patients. Among the 104 patients, all (n = 33) those treated within 24 h of symptom onset (early treatment) survived, while 90% (64/71) of the treated later survived (not statistically significant). In contrast, early BAT treatment was associated with statistically significant shorter hospital (median, 15 vs. 25 days; p < 0.01) and ICU stays (10 vs. 17 days; p = 0.04) compared with later BAT treatment. Among the 249 patients, 9% of patients experienced at least one adverse effect BAT-related: fever (3%), rash (2%), chills (1%) and agitation, edema, slight hypertension, nausea (1%). Bronchospasm, chest pressure, diaphoresis, erythema, increased respiratory rate, “jitteriness”, leukocytosis, mild hypotension, tachycardia, urinary retention and vomiting were each reported once among adults treated group. In pediatric group (n = 17), single cases of fever, agitation/anxiety and an experience of “hurting all over”, were described. Only one severe adverse reaction occurred in a 10-year-old boy (29 kg body weight) who manifested severe hemodynamic instability characterized by bradycardia leading asystole started 90 min after the BAT infusion and rapidly resolved after epinephrine administration. BAT infusion was restarted and after 30 min a second episode of severe bradycardia occurred, at this point the administration was definitively stopped (an estimated 73% of the recommended dose was administered overall). A single case of serum sickness occurred in a 64-year-old man, which occurred 11 days after BAT administration and physician-reported as mild, self-limited serum sickness characterized by myalgia and arthralgia treated with ibuprofen; the principal investigator also determined it as not serious. In March 2018 an electronic survey was performed through the European Association of Poisons Centers and Clinical Toxicologists (EAPCCT) with the aim to collect epidemiological data and information on the clinical management, the diagnostic capability and the antidote availability in cases of botulism intoxication in Poison Centers/Poisoning treating facilities (PCs) located in different countries. The survey included 19 items on (i) epidemiological data (registered by PCs during 2015–2017) as well as questions on (ii) availability/location of specific laboratory, (iii) clinical management, (iv) type of antitoxin availability (including dosage/adverse drug reaction) and (v) its location. Fourteen PCs answered to the survey (Austria, Belgium, Czech Republic, Estonia, France, Germany, Greece, Iceland, Ireland, Italy, Poland, Slovenia, South Africa and Switzerland). Ireland, Estonia, Slovenia and Poland PCs declared no experience with botulism because managed by Infectious Diseases Services, 10 questionnaires were analyzed. Cases of foodborne botulism, infant and adult intestinal botulism and wound botulism were registered by PCs. Specific Labs for diagnosis were available in seven countries (70%), all located in government services (in two countries operative 24H). All PCs, except two, prescribes antidote before the laboratory confirmation. Trivalent equine antitoxin was the unique formulation available and the dosage varied from one to four bottles: no severe acute adverse reactions have been reported. Antitoxin is stocked in PCs/Hospitals/pharmacies and in six countries in strategic stockpiles. In conclusion, PCs experience on botulism is extremely different: some services manage all cases occurring in the country as reference centers, while others refer to Infectious Diseases Services. During the study period (3 years), all forms of botulism have been observed by PCs (including rare forms such as wound and intestinal botulism). PCR diagnosing testing is not routinely available, and in vivo tests remain the gold standard method, even if, accordingly, turnaround time (TAT) is too long to be useful in the first phase of the clinical management. Trivalent equine antitoxin is available in EU, and the administration is safe. On the contrary, the recommended dose varies significantly among countries. Antidote storage in strategic stockpiles may be useful to manage public health emergencies or unconventional events. These data underline the need of a harmonization of management of botulism between PCs would seem appropriate for the future [55]^[18].

3.3. Supportive Airway Treatment

The milestone of treatment for the cases of botulism intoxication is prompt and supportive care. Because of the high risk of rapid respiratory failure and because respiratory compromise close monitoring of respiration is needed. As suggested by Arnon and colleagues, in mild cases and when a suspicion of botulism intoxication is made the patients should be put in the reverse Trendelenburg position at 20–25° with cervical support; classically, this position enhance diaphragmatic function decreasing the pressure of abdominal viscera and to reduce the risk of aspiration [47]^[13].

3.4. Antibiotic Therapy

The antibiotic therapy is not able to interfere with the toxin mechanism of action. In wound botulism form the antibiotic therapy alone remains insufficient; it is indicated when secondary infections are documented. In all forms, aminoglycoside antibiotics and clindamycin may exacerbate the neuromuscular blockade [56]^[19].

3.5. Experimental Treatments

During the last two decades, several efforts on designing new drugs (e.g., monoclonal antibodies) have been made, especially for blocking the catalytic activity of BoNTs. Efforts have been invested on designing small molecules, peptidic inhibitors, aptamers as well as on testing some natural substances for their anti-botulinum activity [13]^[7]. Studies on specific inhibitors effective in preventing the neuroparalytic action of BoNTs (irrespectively of their serotype and subtype) which could be used in poisoned patients without knowing the particular type of BoNT are underway and appear to be promising for the future [13,14,53,57,58,59]^{[7][8][20][21][22][23]}. Current drug research efforts have mainly focused on BoNT/A and mainly addressed on light chain proteolytic activity. Development of pan-BoNT inhibitors acting independently of BoNT immunological properties and targeting a common step of the intoxication process seems encouraging. In fact, experimental studies on different chemicals or molecules that can interfere with the different stages involving BoNTs mechanisms are available. The new drugs may act as (i) inhibitors of toxin binding, (ii) inhibitors of toxin internalization and trafficking, (iii) inhibitors if toxin translocation, (iv) inhibitors of the toxin disulfide bond reduction, (v) inhibitors of SNARE cleavage by L-chain and (vi) reversal of BoNTs paralysis [53]^[20]. The binding of BoNTs to receptors located at the presynaptic membrane is the first step of the intoxication. Some antagonists of the ganglioside receptors (e.g., quinic, lectins from Limax Flavus and tricum vulgaris, thearubigin) have been identified. Main limitations of these treatments are the serotype specificity and the short therapeutic window. Other drugs interfere with internalization process (mainly mediated by dynamin-dependent endocytic pathways) and intracellular trafficking (e.g., Dyngo-4a, methylamine hydrochloride, bafilomycin A, nigericin, quinolinol). Another option is to interact with specific intracellular toxic mechanism (e.g., adamantanes, lomofungin, chicoric acid botulin, benzimidazole acrylonitrile). Inhibitors of SNARE cleavage by L-chain metalloprotease have been studied, but, despite the promising results, none of these molecules concluded the way to be considered an effective drug [53]^[20]. The last group of molecules considered are those involved in functional recovery of intoxicated nerve terminals. The 3,4 diaminopyridine (3,4-DAP) and analogs, a potassium channel-blocking agent, determine an increase of the presynaptic action potential duration causing an increase Ca²⁺ influx and an increase acetylcholine release [60]^[24]. The 3,4-diaminopyridine does not cross the blood–brain barrier to a substantial extent [15]^[9] and its efficacy in not established [61]^[25]. The only benefit observed regards the improvement in ocular and limb muscle strength, but there has been no benefit on respiratory paralysis [62]^[26]. Guanidine has been administered in the past to enhance acetylcholine release, but its application, evaluated also in placebo-controlled studies, failed to improve the clinical course of the intoxication [63]^[27]. Steroids, immunoglobulins, chloroquine, plasmapheresis, have been tried in single cases with debated benefit.