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The Ecology of Parasitism
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2023
Eric S. Loker, Bruce V. Hofkin
Three categories of density-dependent regulation can be considered: decision-dependent regulation, host-death dependent regulation and competition-dependent regulation. In the first case, the parasite avoids a particular host if the host is already infected. In other words, it may seek to establish a separate infrapopulation in a host not already colonized and potentially saturated with parasites. Such behavior is well known among parasitoid wasps. An ovipositing female wasp often has the ability to sense if parasitoid larvae have already infected a particular host. If so, she will move on and find another parasite-free host in which to oviposit. The same is true for entomopathogenic nematodes like Steinernema that can detect and are deterred by volatile chemicals emanating from hosts already infected with and compromised by such nematodes.
Host Defense and Parasite Evasion
Published in Eric S. Loker, Bruce V. Hofkin, Parasitology, 2015
Eric S. Loker, Bruce V. Hofkin
Another example of a collaborative, mutualistic arrangement to achieve infection of an invertebrate host is provided by entomopathogenic nema-todes such as Steinernema and Heterorhabditis (see also Box 7.2). Infective Steinernema larvae actively search for, find, and infect insect hosts. Once in the insect, they release from special areas of their intestine mutualistic bacteria (Xenorhabdus) that then proliferate rapidly in the host insect’s hemocoel, producing a number of factors that damage the host’s hemocytes. The bacteria also inhibit expression of host-produced antimicrobial peptides, such as cecropin, and inhibit prophenoloxidase and thus melanization. Furthermore, the bacteria produce antimicrobial factors that prevent the growth of opportunistic bacteria and release enzymes that degrade molecules produced by the host insect, providing a nutrient soup that favors growth of their associated nematodes. Consequently, both Xenorhabdus and Steinernema proliferate in the host, which is soon killed. Eventually thousands of larval nematodes leave the host insect, each carrying an inoculum of these specialized bacteria to facilitate infection of a hapless new host. Because of their efficiency in killing insects, entomopathogenic nematodes have been used widely as biological control agents.
The relevance of studying insect–nematode interactions for human disease
Published in Pathogens and Global Health, 2022
Zorada Swart, Tuan A. Duong, Brenda D. Wingfield, Alisa Postma, Bernard Slippers
Nematodes capable of infecting and killing insects are known as entomopathogenic nematodes (EPNs) [17–19]. Entomopathogenic nematodes are used as beneficial biological control agents of insect pests, providing an alternative to expensive, broad-spectrum, chemical insecticides [20–22]. Nematodes from the Heterorhabditis and Steinernema genera are frequently used in biocontrol and are therefore the EPN most commonly studied [23,24]. In addition to acting as definitive hosts to nematodes, insects are often involved in the nematode life cycle as intermediate hosts or vectors [25,26]. Dispersal by an insect vector is a characteristic of many animal and some plant-parasitic nematodes.