J. Parasitol.. 81(6), 1995, p. 865-869 O American Society of Parasitologists 1995 HOST GROOMING AND THE TRANSMISSION STRATEGY OF HELIGMOSOMOIDES POL YGYRUS Alexander D. Hernandez* and Michael V. K. Sukhdeo Department of Animal Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 ABSTRA~: Grooming behavior may play a part in the transmission of the gastrointestinal nematode, Heligomosomoides po- lygyrus in the mouse host. After infective larvae are placed on individually housed mice, significantly higher numbers of adult worms were recovered from the small intestine of mice that were allowed to self-groom when compared to infection levels in mice that had been fitted with Elizabethan collars to prevent self-grooming. Larvae placed on a single mouse housed with 3 other untreated mice resulted in all mice in the group becoming infected, suggesting that allogrooming may also be important in parasite transmission. A significantly higher percentage of larvae nictate on rough surfaces such as damp peat moss substrate when compared to smooth surfaces such as 0.5% agarose. Mice exposed to larvae placed on peat moss substrate have significantly higher infection levels when compared to mice exposed to larvae on a 0.5% agarose substratum, suggestingthat natural transmission of infective L3 larvae in mice may be dependent on a substratum type that allows nictation behavior. A significantly higher percentage of worms were attracted to mouse urine and mouse and rat epidermal lipids when compared to deionized water controls in an in vitro preference assay, suggesting an attraction to host-specific signals. These results support the hypothesis that transmission of this parasite is an active process involving movement of the infective larvae of H. polygyrus into the host's active space where they are ingested during grooming behavior. Transmission is a critical period during a parasite's life cycle, especially if it involves survival outside the host environment, and any strategies that increase the chances of successful trans- mission are adaptive (Croll, 1966; Ulmer, 1971 ; MacInnis, 1976). Parasite transmission strategies are characterized as either pas- sive or active processes. In passive transmission, the parasite does not utilize specific behavioral responses as part of its strat- egy to come in contact with and ultimately infect its host. In active transmission, or host finding, the parasite exhibits specific behavioral responses to increase the probability of contact with its host, and these often occur in a hierarchical sequence, i.e., responses to environmental cues that attract the parasite to the host's habitat, responses to host-associated cues that identify the host's active space, and responses to the host itself to elicit attachment or penetration behavior (MacInnis, 1976).In several species that utilize active transmission, few deviations from this general strategy have been recorded (king, 1937; Blake, 1962; Rogers and Sommerville, 1963; Klinger, 1965; Croll, 1966; U1- mer, 1971; MacInnis, 1976; Gibson and Everett, 1982; Mc- Gladdery, 1984; Zuckerman and Jansson, 1984; Granzer and Haas, 1991; Grewal and Wright, 1992). Heligmosomoidespolygy~~~ is a gastrointestinalnematode with a direct life cycle and an infective free-living larval stage asso- ciated with feces (Baylis, 1926;Spurlock, 1943;Ehrenford, 1954; Bryant, 1973). This parasite is not species specific; it has been reported from 9 genera and 3 families of microtine and murine rodents (lewis, 1968; Forrester, 1971; Walker, 1975; Gregory, 1992), and thus, it is possible that the transmission strategies of this parasite may not be host specific. It has been extensively studied in and is most frequently associated with, mice, pri- marily Mus musculus. Investigations on the interactions be- tween this parasite and its mouse host have suggested that trans- mission occurs during host feeding (Spurlock, 1943; Ehrenford, 1954; Lewis, 1968). This view is not supported by studies on Received 22 September 1994; revised 26 May 1995; accepted 26 May 1995. * Current address: Department of Zoology, Natural Science Building, Michigan State University, East Lansing, Michigan 48824. the diet of mice, which reveal that many of them feed primarily on seeds and insects for which they forage widely in their habitat, and that the fecal deposition areas may not be near their feeding areas (Goodpaster and HofFmeister, 1954;Whitaker, 1966;Flake, 1973; Walker, 1975; Meserve, 1976). However, several species in this parasite's host range defecate within their nests (Nich- olson, 1941 ;Goodpaster and Hoffmeister, 1954; Walker, 1975), including M. musculus, which seems to defecate everywhere (Southern, 1954). Mouse sleeping behavior (which occurs in the nest) is highly correlated with the level of infection by this parasite (Tanguay and Scott, 1992). The supine sleeping position presumably increases body contact with infected feces, and this suggests that grooming behavior may play a part in the trans- mission of the parasite (Tanguay and Scott, 1992). Grooming is considered an intricate part of an animal's life, for example, rats spend 25-40% of their awake time grooming themselves (Bolles, 1960; Spruijt et al., 1987). Mus musculus may also spend anywhere from 20 to 50% of their time in grooming activity (Dunn et al., 1987; Meisenberg, 1988), but this varies significantly with time of day and environmental conditions (Poirel, 1988). Studies on various vertebrate species indicate that grooming behavior has several benefits, including thermoregulation, cleanliness, and sexual development (Moore, 1983, 1987; Thiessen, 1987), but it is also an important defense behavior against ectoparasitic infection (Borchelt et al., 1976; Hart, 1990, 1992, 1994). In this study we examined the hypothesis that H. polygyrus is transmitted via the grooming behavior of its mouse host. We tested 3 predictions resulting from this hypothesis: (1) H. po- lygym infective larvae on the host are ingested when the host grooms; (2) the infective larvae of H. polygyrus can attach to the host (host attachment); and (3) the infective larvae of H. polygyrus can use host-specific cues to locate the host habitat (response to the host's active space). MATERIALS AND METHODS Parasite maintenance Heligmosomoides polygyrus was maintained in male Swiss-Webster mice weighing 2C-30 g (Hilltop Laboratories, Scottdale, Pennsylvania). Fecal slurries were coarsely strained, centrifuged, and incubated on moist