This case indicates that T. molitor, whether as a storage pest or as a feed additive in the form of unprocessed mealworm larvae poses direct, potentially lethal, threat to farm animals. However, T. molitor larvae are often added to feed as an alternative protein source. Dried and grounded larvae are usually used in the studies on the pig’s nutrition [17, 18]. Mealworm larvae closely matched with fish meal, making it a potentially attractive alternative protein-rich feed ingredient for livestock feed industry . Supplementation of dried mealworm larvae up to 6% in weaning pigs’ diet may improve growth performance and nutrient digestibility without any detrimental effect on immune responses . Ao and Kim demonstrated, that dried mealworm powder can replace fish meal in weaning pigs’ diet without any negative effect on growth performance or nutrient digestibility . Mealworm larvae hydrolysate had higher digestibility compared fermented poultry by-product or hydrolyzed fish soluble meal . Our results warn against using live larvae, in particular for weaned pigs, since weaning immunosuppression could facilitate the infestation. Only processed, dried T. molitor larvae meal will not pose a direct threat to animal health.
We are not able to fully determine how the larvae get into the stomach. Suggesting the size of insects in the stomach, we think that pigs could swallow already hatched larvae. Probably a feed contaminated with eggs had been put into the silo. There, due to unfavorable conditions (low temperature and humidity), larvae did not hatch from eggs. It is possible that the feed was some time outside the silo before animals feeding (e.g. after mixing with water or it was left in the troughs) and during this time the larvae were probably released from eggs. Then mealworm larvae (approx. 2 mm) could be swallowed by pigs. This explains the results in which we found the eggs in the feed obtained from the silo and the live larvae in the stomach. The main problem is the lack of mechanical disruption of the larvae, due to a feeding style (liquid feeding regime) that prevents chewing on them, and the small size of the first larvae. In the stomach we found larvae of different sizes, which indicates that they were of different ages. Assuming that the animals were fed for 2 weeks with contaminated feed, some of the larvae could be digested and some could be located in a non-glandular region by active movement.
According to literature reports, T. molitor may pose a health risk. In many cases, eggs or larvae are eaten by humans and animals along with grain-based food. Usually they are digested or they passed along with the digestive tract and excreted with feces. In some cases, however, they are able to survive and live in alimentary tract. The first reports about the presence of insect larvae in human organs date back to the 19th century, when they were observed in the tonsils, nose and bladder, or in the umbilical cord [20, 21]. Insects, including T. molitor larvae were diagnosed in the gastrointestinal tract of humans, including the stomach and the intestines [22,23,24]. Hinman and Faust  described the occurrence of T. molitor larvae in human organs. They found that the gastrointestinal tract is the most common habitat for parasitizing larvae of this beetle , which was also confirmed by Palmer’s research . Rodriguez-Morales et al.  reported ulcer infestation of T. molitor in an AIDS patient. The last human case of canthariasis caused by T. molitor was described in 2019 by Aelami et al.  and concerned urinary canthariasis due to mealworm larva in a 10 years old boy in Iran. There are few reported cases of live mealworm beetle larvae in the body of animals. In 2013, a case of T. molitor larvae development in a black-legged heron under the skin in the area between the esophagus and the trachea was described . Gastrointestinal canthariasis caused by mealworm beetle larvae is much rarer. In current literature there are no such reports in the case of farm animals. It should also be noted that beetles of the Tenebrionidae family are potentially significant allergens for workers exposed to grains or grain products . Therefore, the presence of these grain pests not only directly threatens animals, but also indirectly threatens the workers on farms and feed factories.
It should be remembered that there is a negligible chance of invasion of beetle larvae in animal breeding using Good Animal Husbandry Practices principles. The disease, although it may not cause clinical symptoms, can be detected by the presence of traces of pests on feed that can be perceived by the owner with the unaided eye. Control of T. molitor at farms is also important because of its role as a potential mechanical vector for pathogens [25, 30]. Mainly, the development of T. molitor results from the negligence of the animals’ owner or feed mill, therefore, in each case, comprehensive disinfestation should be carried out. The occurrence of canthariasis may indicate a lack of welfare, hygiene of animal feed or a lack of proper disinfection of feed silo. The mealworm beetles infestation on the area of the described farm in the spring period is probably correlated with the phenology of the occurrence / migration of these insects in the environment. Other predisposing factors may be bacterial infections or immunosuppression. In addition, the weaning immunosuppression could play a key role in our case. The occurrence of beetle larvae in the gastrointestinal tract could lead to irritation and mentioned gross lesions in stomach. Presence of T. molitor could also affect mucosal-associated lymphoid tissue (MALT) by mechanical stimulation. However, due to the post-weaning immunosuppression, tolerance of larvae, or provoked damage to the gastric mucosa may have occurred. Based on the obtained data, we are not able to determine the relationship between gastric canthariasis and cases of colibacillosis. It cannot be excluded that other pathological factors prevented digestion of the larvae. The actual problem caused by mealworms is still unclear - some gastritis appears not necessarily a reason for the problem. But the fact that correction reduced mortality is strong indication that the larvae were the problem.
The first and most important aspect of prevention of insect larvae infestation in farm animals is to ensure basic animal and feed hygiene and to maintain welfare, because poor sanitary conditions in breeding and feeding rooms are probably the most important factor determining this disease. The feed should be regularly monitored for the presence of adults whose pheromones can attract new individuals. Small / backyard farms can play an important role, where due to cost savings the correct preventive and the hygienic procedures are not applied. Such farms can be an important factor in the migration of storage pests. Forage left outside for a long time or feed residue in warehouses and silos at farms can also attract these pests.
For large farms, individual action plans should also be implemented, which include regular emptying and cleaning of the grain silos, storing of feed in appropriate containers and conditions, and control whether these operations are carried out. The use of Good Hygiene Practice, including pest control, in both livestock farming and the feed industry, can prevent many cases of the canthariasis. For a large population of T. molitor in farm buildings, disinfestation should be considered. These practices should be particularly used in places where infestation with these pests has previously occurred. No treatment for gastric canthariasis caused by T. molitor has been developed in livestock at present. Appropriate antiparasitic agents may help in the treatment of invasion. We believe that in the case of gastrointestinal canthariasis, similar therapeutic procedures can be used as in the treatment of myiasis. In countries where myiasis is endemic, systemic injection or the use of pour-on preparations to prevent flies are allowed. Numerous insecticides are available for treatment, including macrocyclic lactones (doramectin, eprinomectin, ivermectin or moxidectin) in various formulations. Doramectin and ivermectin after systemic administration have a systemic effect which may lead to the elimination of live larvae within the body. Eprinomectin is also effective in controlling the insect larvae invasion, however, in many countries it is only registered for cattle. Oral administration of avermectin may also lead to spontaneous death of larvae in the digestive tract.