Pesticides, and particularly rodenticides, can be harmful to humans and other animals that unknowingly consume them or otherwise come into contact with them. In the past, rodenticides ingested intentionally and unintentionally by humans resulted in illness and death. Presently, conventional rodenticides ingested by animals other than the pests (namely, mice and rats) for which they are intended also result in the unintentional killing of those animals. Pet animals and wildlife are both harmed or killed by conventional rodenticide usage.
Vomiting (also called emesis) is a reflexive act caused by coordinated contractions of various muscles that eject stomach contents forcefully through an animal's mouth. Humans and many other animals are capable of vomiting as a natural reaction to purge toxic substances from the body. During vomiting, the muscles of the abdomen and chest contract and the diaphragm spasms downward and inward exerting pressure on the stomach. Next, and nearly simultaneously, the cardiac sphincter, which is a part of the diaphragm surrounding the esophagus, relaxes to assist in opening the esophagus. The longitudinal muscle of the esophagus contracts, which further opens the cardiac sphincter, and the resulting pressure forces contents of the stomach up into the esophagus and out of the animal's mouth.
Rats and many other rodents are unable to vomit, i.e., cannot produce an emetic reflex. Physiologically, rats have a powerful barrier between the stomach and the esophagus but lack sufficient esophageal muscle strength to overcome and open this barrier by force, which is necessary for vomiting. In humans and other animals, the emetic reflex requires that two muscles of the diaphragm contract independently, but rats are unable to dissociate the activity of these two muscles so that they may produce the independent contractions necessary for vomiting. In addition, rats lack complex neural connections that are present within the brain stem and between the brain stem and viscera of humans and other animals that coordinate the numerous muscles that produce the emetic reflex.
While rats are unable to vomit, they do exhibit other behavior-based techniques to avoid consumption of and poisoning by toxic substances. For example, rats learn to avoid certain foods that make them sick. When a rat discovers a new food, the rat consumes a small amount of the food, and if the rat becomes ill after ingesting the food, the rat learns to scrupulously avoid that food in the future. Rats learn to identify the food that is to be avoided by its taste and smell. Rats experiencing nausea also display pica, which is the consumption of clay or other non-food materials. By ingesting clay, some toxins are bound in the rat's stomach, which assists in reducing the effects of the toxin as experienced by the rat.
While rats are unable to vomit, they can regurgitate. Regurgitation is not the same as emesis and does not produce the forceful expulsion of the stomach contents through the esophagus and out of the mouth. In studies, when a rat is fed a diet of bulky food items, when the rat regurgitates stomach contents, the regurgitant is pasty and thick in composition and, as a result of the rat's tongue action, becomes packed as a plug within the rat's pharynx, larynx and esophagus. Because the regurgitant forms a plug, the rat chokes and often dies.
The rat's esophagus includes inner circular and outer longitudinal layers of striated muscle. These two layers of striated muscle become smooth muscle near the esophagus's point of attachment with the rat's stomach. A gastroesophageal barrier separates and closes off the rat's esophagus from its stomach. The gastroesophageal barrier is formed by a crural sling, a lower esophageal sphincter, and intraabdominal esophagus that lie between the crural sling and lower esophageal sphincter. The crural sling is part of the rat's diaphragm and is composed of a U-shaped bundle of fibers that wraps around the esophagus and attaches to the rat's vertebrae so that when the crural sling contracts, the rat's esophagus is pinched closed. The rat's diaphragm is formed by two muscles: the crural sling and the costal muscle, which is attached to the rat's rib cage. The lower esophageal sphincter is a circular muscle that surrounds the base of the esophagus and, at its lower edge, includes muscle fibers that insert into the limiting ridge, as described below.
The stomach of a rat includes two parts, i.e., a forestomach and a corpus. The forestomach is a non-glandular, thin-walled portion that receives the esophagus and serves as an initial holding chamber for food that is consumed. In rats, the forestomach's walls are similar to walls of the rat's esophagus. Unlike the forestomach, the corpus is a glandular, thick-walled section having walls that include secretory glands, which produce mucus and digestive enzymes. In rats, digestion begins in the stomach's corpus. A pyloric sphincter controls movement of stomach contents from the corpus into the intestines. The forestomach and corpus are separated by a limiting ridge (also called the margo plicatus), a low fold of tissue that extends circumferentially from the large curvature of the stomach to the small curvature of the stomach just below the esophagus. At the esophagus, the limiting ridge forms a U-shape that nearly surrounds the esophageal opening into the forestomach and the muscle fibers of the lower esophageal sphincter are inserted into the limiting ridge. Due to this anatomical structure, when a rat's lower esophageal sphincter contracts, both the walls of the rat's esophagus and the sides of the limiting ridge's U-shaped portion are pulled together, which tightly closes the esophageal opening in the rat's stomach thereby preventing stomach contents from being expelled by vomiting.
Pressure at the two ends of a rat's gastroesophageal barrier is much higher than the pressure found in the rat's thorax or abdomen during any phase of the breathing cycle. The strength and pressure of this barrier make reflux in rats nearly impossible although rats may engage in regurgitation. Rats are not capable of vomiting because, for several reasons explained below, they cannot produce the necessary coordinated muscular contractions to overcome this powerful barrier.
Rats are incapable of relaxing the crural sling while simultaneously contracting the remainder of the diaphragm. The rat's esophagus passes through the crural sling, and as explained above, when the crural sling of the diaphragm contracts, the rat's esophagus is pinched tightly closed. Rats are physiologically incapable of dissociating the activity of the crural sling and costal muscle of their diaphragm, and as a result, a rat cannot relax the crural sling while simultaneously contracting the costal muscle. Instead, in rats as opposed to in humans, both diaphragm muscles always contract or relax together. Rats' inability to separately and selectively control its two diaphragmatic muscles render their bodies incapable of producing the pressure on the stomach necessary to open the crural sling so as to allow their stomach contents to be expelled.
Rats are also unable to open their esophageal sphincter to permit the forceful expulsion of stomach contents during vomiting. In rats, the esophagus consists of a thin, weak unstriated, longitudinal muscle at its point of connection with the stomach, which is not sufficiently strong to open the rat's lower esophageal sphincter so as to allow expulsion of the rat's stomach contents.
Unlike emetic animals (including humans), rats and other nonemetics lack neural connections within the brain and between the brainstem and viscera that are necessary to coordinate the numerous muscles that produce the emetic reflex. While the brainstem nuclei and the muscle systems used in vomiting are present in rats and other nonemetics, the complex neural connections between the nuclei or between the brainstem and viscera, which are necessary to produce coordinated muscular contractions required for vomiting, are absent.
A need exists for a rodenticide that is safe and non-toxic if consumed by humans or non-rodent pets and wildlife.