Botulism is a severe neuroparalytic illness that manifests through distinctive cranial nerve palsies, potentially progressing to bilateral, symmetric, descending flaccid paralysis. This paralysis initiates proximally before affecting distal limb musculature, and in critical cases, may lead to respiratory failure and fatality. The intensity of paralysis directly correlates with the toxin dosage. Patients afflicted with botulism typically remain alert and oriented; however, symptoms such as ptosis, ocular muscle paralysis, voice modulation due to vocal cord paralysis, and gait disturbances from skeletal muscle paralysis can be erroneously perceived as signs of intoxication or altered mental states. Sensory deficits and pain are infrequently reported by patients (3,42,43). Despite these characteristics, botulism diagnosis is often delayed or overlooked.
Diagnostic Challenges in Botulism Cases
While the progression of paralysis in botulism is considered uniquely identifiable, initial clinical encounters often result in misdiagnosis of neurological symptoms and their progression (3,14). Investigations into confirmed botulism cases that were initially misdiagnosed, particularly within outbreak scenarios, have been revealing. Outbreak analyses have underscored instances where botulism was only identified post-discharge, highlighting the risk of delayed or missed diagnosis (35,38) (CDC, unpublished data, 2016). Critical decisions regarding initial treatment and management must rely on clinical assessments. Botulinum antitoxin, the definitive treatment, necessitates prompt administration. Laboratory confirmation may extend over several days; thus, delaying antitoxin administration in cases with moderate to high suspicion of botulism pending lab results can negatively impact patient outcomes (3,35,44,45).
The diagnostic complexities arising from the variable presentation of botulism were evident in a significant foodborne outbreak, where initial misdiagnoses included myasthenia gravis, stroke, and psychiatric disorders (46), despite most patients exhibiting classic botulism signs and symptoms (46). Recent literature reviews and classical case studies indicate myasthenia gravis and Guillain-Barré syndrome as the most common misdiagnoses for botulism (13,14,16,30,31). Differential diagnoses have encompassed a broad spectrum of conditions, from common to rare, such as cerebrovascular accidents, Lambert-Eaton syndrome, meningitis, encephalitis, and tick paralysis (13).
A CDC review of 332 potential botulism cases (1980–2016) revealed that treating physicians considered alternative diagnoses in 83% of cases (274), citing up to six other possible illnesses. Guillain-Barré syndrome (99 cases) and myasthenia gravis (76 cases) were the most frequently considered alternatives. Among 160 botulism cases (2009–2015) consulted by the CDC botulism clinical service, botulism was the primary diagnosis for 90% (144) of cases, secondary for 6% (10), and tertiary for 3% (five).
In pediatric and adolescent cases, differential diagnoses were reported in 22% (79 cases), with myasthenia gravis (28%; 22 cases), poisonings/intoxications (25%; 20 cases), Guillain-Barré syndrome (14%; 11 cases), and poliomyelitis (11%; 9 cases) being the most prevalent (31). Misdiagnosis often occurs because botulism is less common than conditions with similar symptoms, like myasthenia gravis and Guillain-Barré syndrome. Inadequate neurological examinations and failure to identify typical neurological findings can also reduce the likelihood of considering botulism (26). Atypical presentations, such as asymmetric deficits, though rare, may further complicate diagnosis (33,34,38,47).
Recognizing Clinical Signs and Symptoms for Botulism Diagnosis
Reviews and analyses for these guidelines (13,14,16,30,31,36) identified dysphagia, blurred vision, slurred speech/dysarthria/hoarseness, gastrointestinal issues, dry mouth, dyspnea, and diplopia as the most frequently reported symptoms in botulism patients. Descending paralysis, ptosis, and ophthalmoplegia were the most common clinical signs.
Botulism symptoms and signs develop over hours to days. Initial minor visual disturbances or gastrointestinal discomfort (in foodborne cases) may precede progressive cranial nerve palsies, followed by descending flaccid bilateral paralysis. The severity of neurological signs varies, ranging from mild cranial nerve findings like ptosis to extensive paralysis affecting cranial nerve-innervated, respiratory, extremity, and axial muscles. Early gastrointestinal symptoms (nausea, vomiting) are more prevalent in foodborne botulism compared to other forms (13,14,16). Vomiting was reported in 50% (172) of foodborne botulism patients versus 5% (three) in wound botulism cases (14). The etiology of these gastrointestinal symptoms—whether due to botulinum neurotoxin, other clostridial products, or food spoilage substances—remains unclear. It is also unknown if intentional food contamination with purified botulinum toxin would induce gastrointestinal symptoms (14). Constipation is frequently noted as an early symptom in children (31). Infants and young children may struggle to articulate symptoms like double vision; signs are therefore more commonly reported than symptoms in this demographic (31). Neurological manifestations in infants are described differently; hypotonia, weak cry, and poor feeding were reported in infants with foodborne botulism but not in older children (36).
Botulism is classically described as causing symmetric neurological deficits, consistent with the toxin’s circulatory distribution to neuromuscular junctions (12). However, detailed case studies document asymmetric neurological deficits in some patients (47), with larger series reporting asymmetry or unilateral deficits in 6%–15% of cases (13,16). These data are complex to interpret due to reliance on clinical chart abstractions, which may suffer from missing or incomplete data and variable reporting across healthcare providers. Unreactive pupils, expected in botulism, were reported in only 25% of confirmed cases in a large series (13). While rare, fever, nondescending paralysis, and altered mental status have been documented (13,31). Proximal muscles are typically affected before distal muscles, yet equal or greater distal weakness has been reported (13). These less common findings may stem from inadequate neurological examinations, pre-existing conditions, co-infections, or rare variations in botulism presentation.
Respiratory failure without preceding neurological deficits is an extremely rare presenting symptom, suggesting a missed or incomplete neurological examination that would have revealed preceding cranial nerve palsies.
In a series of 72 sporadic botulism cases, most patients presented with symptoms indicative of classical neurological deficits (slurred speech, weakness, dysphagia). However, some initial presentations were less typical, including isolated gastrointestinal symptoms, back pain with mobility issues, altered consciousness, and oral/lingual numbness (CDC, unpublished data, 2016). Delayed diagnosis was more common in patients with atypical initial symptoms (CDC, unpublished data, 2016).
Key Clinical Considerations for Botulism
- Recognize the broad spectrum of botulism signs and symptoms, from limited cranial nerve palsies (e.g., ptosis) to respiratory failure and complete extremity paralysis.
- Be aware of potential early respiratory compromise due to upper airway obstruction from cranial nerve muscle paresis, leading to pharyngeal collapse or secretion pooling, even when diaphragmatic function is initially preserved.
Clinical Recommendations for Botulism Diagnosis
- In cases of suspected myasthenia gravis or Guillain-Barré syndrome, particularly with unexplained symmetric cranial nerve palsies (with or without other muscle paresis), consider botulism in the differential diagnosis.
- Perform thorough, serial neurological examinations to identify and monitor the progression of botulism-related neurological deficits.
- If botulism is suspected, immediately contact local or state health departments for emergency expert clinical consultation and to request botulinum antitoxin from the CDC when indicated.
Ancillary Diagnostic Testing in Botulism
Background on Testing
Routine laboratory tests, such as complete blood counts, cerebrospinal fluid (CSF) analysis, and radiology, are typically normal in botulism patients. Elevated CSF protein is common in Guillain-Barré syndrome, particularly after the first week of illness (48), while mild CSF protein increases are infrequent in botulism (13). Brain imaging can help exclude brainstem strokes presenting with non-localizing symptoms. The Tensilon (edrophonium) test, traditionally used for myasthenia gravis, is generally negative in botulism, although minimal responses have been reported (36).
Electrodiagnostic studies—repetitive nerve stimulation (RNS), electromyography (EMG), and nerve conduction studies (NCSs)—aid in determining the cause of muscle weakness. RNS involves electrical stimulation of a motor nerve at low (2–5 Hz) or high (30–50 Hz) frequencies, recording muscle response. EMG uses needle electrodes in muscles to record electrical activity at rest and during contraction, showing motor unit potentials. NCS involves nerve stimulation and recording electrical responses from sensory nerves (sensory NCS) or muscles (motor NCS) (49). Classical botulism findings include increased compound motor nerve action potential amplitude with high-frequency RNS (30–50 Hz) (50), fibrillation, reduced muscle unit recruitment, decreased duration of muscle unit potentials on EMG, and reduced motor-evoked amplitude on NCS with otherwise normal findings (49). However, early in the disease, these studies may be normal or near normal and thus unhelpful.
EMG, RNS, and NCSs have limitations. They are operator-dependent, technically complex, require specialized training and equipment, are not universally available, can take up to 2 hours, and require expert interpretation. Early botulism cases may yield normal results (except single-fiber EMG) (51,52); abnormalities appear later in the course. EMG requires patient cooperation and can be painful, especially RNS at 30–50 Hz (49). Clinicians should explain the procedure to conscious, paralyzed, and intubated patients. The sensitivity and specificity of these tests for botulism are not precisely known. Electrodiagnostic findings in other neuromuscular diseases (e.g., Miller Fisher syndrome) can mimic botulism (50,51). Single-fiber EMG with jitter measurement is more sensitive but less specific, requiring even more expertise, equipment, and patient cooperation (52). Electrodiagnostic findings must be interpreted within the clinical, epidemiological, and laboratory context.
Electrodiagnostic studies can be valuable in diagnosing suspected botulism across different healthcare settings. During outbreaks, they are less critical for patients with classic cranial nerve palsies and descending paralysis. However, in unclear cases, they can differentiate botulism from other neuromuscular conditions like myasthenia gravis or Guillain-Barré syndrome, especially in sporadic cases. Early diagnostic certainty helps guide treatment decisions, distinguishing between antitoxin for botulism and plasmapheresis/immunoglobulin for Guillain-Barré syndrome. Electrodiagnostic abnormalities may persist for weeks, making these studies useful later in the illness when toxin detection in serum is less likely. In outbreaks with atypical features, electrodiagnostic studies have proven helpful (CDC, unpublished data, 2015). In crisis or contingency situations, the feasibility of these studies decreases.
Recommendation for Ancillary Testing
- Consider electrodiagnostic testing to aid in botulism diagnosis when feasible. Expertly performed and interpreted EMG, RNS, and NCSs can provide valuable diagnostic information.
Exposure Risk Factors in Botulism Diagnosis
Background on Risk Factors
Identifying botulism risk factors in a patient’s history can aid diagnosis. Wound botulism risk factors include injection drug use (especially black tar heroin), while foodborne botulism is linked to home-canned foods (3). However, atypical exposures can also cause botulism, meaning the absence of typical risk factors does not rule out the disease. Multiple suspected cases, especially among related individuals, suggest a common-source outbreak, significantly increasing diagnostic probability (3). Geographically dispersed, unconnected cases do not exclude a widespread outbreak from a seemingly innocuous product. Public health authorities must promptly investigate suspected cases and inform clinicians about potential exposures for patient interviews and outbreak linkage.
Recommendation Regarding Exposure Risks
- Clinicians should inquire about exposures to known botulinum toxin sources, but recognize that the absence of such exposures does not exclude botulism.
Clinical Criteria Tool for Early Botulism Diagnosis
For challenging diagnoses, especially in crisis or contingency settings where botulism probability increases, an evidence-based clinical criteria tool has been developed to assist early identification, and may also be used in conventional settings (Box 1) (36). This tool, developed using data from various botulism cases, compares the frequency of acute botulism signs and symptoms to identify highly sensitive diagnostic criteria (Tables 3 and 4). Modified by expert input and considerations for missed diagnoses, the tool excludes ancillary tests (electrodiagnostic, neuroimaging, Tensilon, lumbar puncture) to maximize objectivity and reproducibility among healthcare workers. Signs difficult to quantify (e.g., sluggish pupils) and early-stage epidemiological risk factors are also excluded. Suitable for adults and children, including pregnant women, and usable by various healthcare workers after brief training in crisis situations (e.g., large outbreaks), the tool is not intended to replace thorough examinations or diagnose botulism. Instead, it guides clinicians to consider botulism, mitigating distractions from atypical findings (36). In conventional settings, it prompts botulism consideration followed by detailed evaluation. In crisis settings, meeting these criteria alone may warrant presumptive botulism treatment. Partially met criteria may indicate medium botulism likelihood, necessitating monitoring (Figures 1 and 2). Meeting criteria is not diagnostic; conditions mimicking botulism, like myasthenia gravis and Guillain-Barré syndrome, may also fulfill them. During outbreaks, “worried well” individuals without objective signs often seek care (53). Managing these individuals efficiently and educating the public about non-emergent symptoms is critical during large outbreaks to prevent treatment delays for genuine cases.
Laboratory Confirmation of Botulism
Clinical findings are paramount for initial treatment and management of suspected botulism. Botulinum antitoxin, the specific therapy, should be administered without delay. Laboratory confirmation can take days, and delaying antitoxin for lab results in high-risk cases can worsen outcomes (3,35,44,45).
Laboratory testing confirms clinical suspicions, verifies antitoxin efficacy against the causative botulinum neurotoxin serotype, and identifies toxin in suspected food sources for safe removal and prevention of further cases. Botulism confirmation in symptomatic individuals requires: 1) botulinum neurotoxin detection in serum, stool, or gastric fluid; 2) identification of botulinum neurotoxin-producing Clostridium species (C. botulinum, C. baratii, C. butyricum) in stool or wound cultures; or 3) botulinum neurotoxin detection in consumed food (3). Environmental testing is not standard in foodborne botulism investigations. Confirmation is performed by select public health laboratories and CDC’s National Botulism Laboratory, offering free emergency specimen testing and detailed collection/shipment instructions.
Types of Botulism Laboratory Tests
The mouse bioassay remains the gold standard for botulinum neurotoxin identification in specialized public health labs (54). It requires mouse colonies and expertise in recognizing botulism signs in mice. Specimens are injected into mice with and without antitoxin, observed for 96 hours by trained technicians. Results may be available within 24 hours for high toxin levels, but lower levels causing human illness may not be detectable in mice. While FDA-approved, other methods can support clinical diagnosis.
Real-time polymerase chain reaction (PCR), available in reference labs, detects bont genes A–G and identifies toxin-producing Clostridium species. As PCR detects DNA, toxin production confirmation necessitates methods like mouse bioassay. Mass spectrometry (Endopep-MS) offers high sensitivity and specificity, differentiating serotypes A, B, E, and F within hours (55), but is limited to CDC and select public health labs.
Non-reference labs typically cannot confirm botulism due to limitations in detecting botulinum neurotoxin or differentiating toxigenic from non-toxigenic Clostridia. Misidentification of C. sporogenes as C. botulinum by non-reference labs has been noted (CDC, unpublished data, 2018).
Specimen Collection and Transportation for Botulism Testing
Serum specimens must be collected pre-antitoxin (BAT) treatment, as it neutralizes the toxin, potentially yielding false negatives. Laboratory confirmation relies on clinical acumen, prompt consultation with public health departments, and timely collection/transport of appropriate specimens (Table 5; Box 2) (2,3,28). Early specimen collection is crucial to detect toxin before it irreversibly binds within neurons and serum levels decline below assay detection limits. For adults, collect enough whole blood to yield 10–15 mL serum (20–30 mL whole blood); smaller volumes are acceptable for children, with a 4 mL serum minimum for mouse bioassay.
Collect 10–20 g stool if possible; smaller amounts or rectal swabs (infants/young children) are acceptable. For constipation, use sterile non-bacteriostatic water enemas and non-glycerin suppositories; tap water is not recommended. Stool can be collected post-BAT treatment as Clostridium may persist even after serum toxin neutralization (17); BAT treatment should not be delayed for stool collection. Suspect foods should be sent in original containers for expert testing. Even dried food remnants have yielded positive results (CDC, unpublished data, 2016). If necessary, use sterile, unbreakable containers. Refrigerate all specimens immediately (36°F–46°F [2°C–8°C]) and maintain refrigeration during transport; do not freeze. Testing asymptomatic exposed individuals is generally not recommended due to likely undetectable toxin levels; exceptions include high-toxin laboratory exposures where specimens and BAT can be administered pre-symptomatically.
Confirmation in sporadic cases is valuable for differential diagnosis, treatment direction, and prognosis. Negative test results are common despite strong clinical suspicion, often due to delayed specimen collection post-peak toxin levels. Wound botulism may not always yield positive Clostridium detection, especially after antibiotic use.
Recommendations for Laboratory Testing and Treatment
- Treat suspected, symptomatic botulism with botulinum antitoxin based on clinical findings, without awaiting laboratory confirmation, as results may be delayed or negative despite botulism. (See Allergic Reactions and Other Side Effects of Botulinum Antitoxin for BAT risks and benefits.)
- Discuss specimen collection with expert consultants from CDC or local/state health departments.
- Collect specimens for laboratory confirmation as early as possible post-suspicion, as toxin levels decrease over time (Table 5; Box 2). Obtain serum pre-BAT administration.
- Store and transport specimens refrigerated (36°F–46°F [2°C–8°C]); do not freeze.
