Botulinum poisoning: what it is and how the infection develops

Two deaths in a matter of hours from botulism. In Sardinia, a 38-year-old woman lost her life. She had eaten guacamole at a kiosk in Monserrato, in the Cagliari hinterland. A few hours earlier, in Diamante, in the province of Cosenza, a 52-year-old man died after feeling ill after eating a sausage and broccoli sandwich purchased from a food truck on the seafront.

Foodborne infections are always possible, but the risk increases in summer.

Even consuming minimal amounts of food contaminated with botulinum toxins can be enough to cause botulism, which blocks the transmission of nerve signals to muscles, a condition that can be fatal.

The contamination

Italy consistently ranks among the European countries with the highest incidence rates of foodborne botulism. Contamination by Clostridium botulinum, the causative agent of botulism, one of the most feared diseases due to its potentially fatal effects, is virtually undetectable because it does not change appearance, taste, or odor.

Botulinum spores—an anaerobic microorganism, a bacterium that grows in the absence of air—are also widespread in the environment, where they can survive for decades, waiting for the optimal conditions to develop the toxin, one of the most powerful in nature.

The foods most commonly implicated in botulism cases in Italy include canned vegetables in oil (47.7%), canned vegetables in water/brine (25.5%), canned meat (7.8%), canned fish (7.8%), ham (4.6%), salami and sausages (3.3%), canned cheese (2.0%), and macrobiotic foods (1.3%). The foods most frequently implicated in foodborne botulism cases in Italy are home-produced canned mushrooms in oil. Canned olives and turnip greens are also very common.

One of these optimal conditions is the absence of air, combined with the low acidity of a preserved food product. It's no coincidence that reported cases of botulism are often linked to homemade preserves in oil, in which the vegetables have not been properly acidified, or to pesto, and to oils flavored with spices or garlic, rosemary, and chili pepper, which must be acidified with equal amounts of water and wine vinegar, preferably white, before being immersed in oil. Homemade canned meat and fish can also be at risk. Indeed, the name botulism comes from the Latin word botulus (sausage) because its description was associated with the consumption of homemade sausages. In Italy, cases of botulism have also been reported from black olives in water, canned turnip greens, vegetables such as eggplant or mushrooms in oil, and canned meat and fish (especially tuna).

Industrial products are only sporadically associated with botulism cases, as industrial technology has reached very high levels of standardization and safety.

How to make homemade preserves

Italy - according to the Italian National Institute of Health, which has a National Reference Center for botulism - has the highest prevalence of cases in the European Union.

When preserving vegetables in oil, the vegetables must first be blanched in equal parts water and vinegar for a few minutes, then drained and left to dry on a clean cloth or paper towel. Finally, they must be jarred, taking care to keep them covered with oil (there are special products to keep them flat) and leaving a couple of centimeters of air between the lid and the contents. In addition to the acidity of the product and the meticulous hygiene of the containers and lids, boiling the jars in water in a pot with a lid is essential.

Ideally, pasteurization should be done at 121°C for at least 3 minutes, which can only be done on an industrial scale. At home, the pasteurization time must be significantly extended to allow the heat to reach the core of the preserve: for 350-400 g jars, at least 20 minutes of treatment may be sufficient.

The disease: causes and symptoms

The most common cases are foodborne botulism—90% worldwide—but the disease can also be caused by infected wounds, the incorrect administration of botulinum toxin for medical purposes, or inhalation.

The incubation period lasts from a few hours to 72 hours after consuming the contaminated food. Symptoms include visual disturbances, such as blurred or double vision, dilated pupils, difficulty speaking, swallowing, and breathing, and even paralysis of the involuntary and respiratory muscles, and death in the most severe cases.

The diagnosis is clinical and is based on the symptoms and the patient's report of what he or she has eaten.

Obviously, laboratory tests must be started immediately: a search for botulinum toxins in the blood, feces, and food residues consumed by the patient.

The treatment

If detected early, the disease resolves, although often over a long period of time. Treatment involves the administration of a hyperimmune serum containing botulinum antitoxins, which should be administered as soon as possible, even before laboratory results are available. Ventilatory support is also provided with the administration of activated charcoal to decontaminate the intestine. In the most severe cases, assisted ventilation and intensive care admission are required.

Mild symptoms

The disease generally presents with mild symptoms. Only 17.8% of patients require ventilatory support for respiratory failure. During the observation period (from 1986 to October 30, 2023), 3% of subjects with laboratory-confirmed botulism died. The Italian mortality rate is lower than the average reported by industrialized countries with a botulism surveillance system (5%). Over the last decade, our mortality rate has further decreased and now stands at approximately 2.6%.

Responsible food

The food responsible for the poisoning is identified through laboratory tests in 36.4% of confirmed cases. In a further 28%, the food is not analyzed because it is no longer available, but the epidemiological investigation reveals the consumption of a canned food that supports the development of botulism and is therefore compatible with the disease. In cases where the food source is identified, 80% of it is related to home canning; in the other cases, it is industrial products. The canned foods most implicated in botulism cases in Italy are canned vegetables in oil and water, which account for approximately 73% of cases (mushrooms are also included in this category).

Foods at risk

Other food categories include: canned meat (15.6% of cases), canned fish, especially industrially produced tuna (9.1%). The three types of canned food most associated with botulism in Italy are: canned mushrooms in oil, olives, and canned turnip greens. Homemade chili pepper-flavored oil is potentially dangerous because botulism, a ubiquitous environmental microorganism, can be found on the outer surface of chili peppers.

This microorganism is generally found in the environment in the form of spores, which are highly resistant to hostile environmental conditions and heat treatments used to stabilize food. If the spore is found in an oxygen-free environment where there is sufficient water and nutrients, it can grow and produce the botulinum toxins that cause the disease. In this case, the oil provides a barrier between the chili pepper and the oxygen-rich atmosphere, creating conditions that support the growth of the microorganism.

The numbers

Botulism is a rare disease worldwide, including in Italy, where it has been subject to mandatory reporting since 1975. Since 1986, the surveillance system (coordinated by the National Reference Center for Botulism at the ISS) has consolidated systematic data collection. From 1986 to October 30, 2023, 697 suspected cases of foodborne botulism were reported, involving patients presenting with symptoms clinically compatible with botulism. Of these, 452 cases were confirmed by laboratory tests. The average age of patients is 45, and 60% are male. The most affected age group is 25-65.