The integumentary system of botfly larvae – How the physiology of a specialized mammalian parasite contributes to its ecological success. A project by Edwin Harris for ENT 599, Winter 2023.

Bot flies are obligate cutaneous, gastrointestinal, and nasopharyngeal parasites of mammals [1, 2]. They are one of several dipteran families that engages in myiasis, or the infestation of live vertebrates by maggots or grubs (fly larvae), a form of tissue endoparasitism [1, 2, 3]. With about 160 known species, bot flies are thought to be the most diverse taxon of mammal endoparasites within Diptera and are among the most highly specialized [2, 4]. Like other flies, they are holometabolous, with a life cycle consisting of egg, larval, pupal, and adult stages [1]. Larvae complete three instars within host tissue but pupate after exiting the host [1, 2]. The larval and pupal stages are typically longer than the adult stage; adults are non-feeding and short-lived [2].

The parasite burden imposed by bot flies on their hosts is typically low [2]. Females frequently divide their eggs among several hosts (or vectors), reducing the stress caused to the host and the level of intraspecific competition experienced by developing larvae [2]. Larvae cannot develop in dead hosts, so adaptations such as production of bacteriostatic secretions that reduce the likelihood of secondary infection of host tissues, are advantageous to the parasite [2]. Nevertheless, infestation by bot flies can reduce host fitness by removing water and nutrients, impairing vision or mobility (especially in small mammals), or causing respiratory problems, sinus blockages, and ulcers [1, 2, 5].

Heavy infestation by Gasterophilinae (stomach bots) can perforate the stomach, cause gastric abscesses, inhibit proper digestion of food, and generally debilitate host animals [1]. Introduction to atypical host animals often leads to more severe negative-outcomes for the host, as well as impaired development of the parasite [2, 5]. Host death may result from unusually severe infestations of bot fly larvae or from panic behaviors induced in livestock when female flies (particularly gasterophilids and Hypoderma bovis) attempt to oviposit or larviposit [1, 2]. Some bot flies (particularly Cuterebra spp.) avoid the latter problem by laying their eggs on plant tissue that may be ingested by a suitable host or by attaching their eggs to arthropod vectors, typically blood-feeding insects like mosquitos [1, 5].

Life cycle of *Dermatobia hominis*, illustrated by Geraldo Victorino de Franca Jr. After mating, an adult female captures and attaches eggs to a mosquito or other blood-feeding arthropod. Once the vector approaches a warm-blooded host, the eggs hatch and bot fly larvae burrow into the host tissue. The larva creates an open cyst in the host’s skin, called a warble, through which it breathes. After feeding on the host’s tissue and completing three instars of development, the larva emerges from the warble, wanders to a suitable pupation site, and forms a puparium in the soil. An adult fly emerges after pupation and seeks a mate, allowing the cycle to repeat.

Once introduced into a host, first instar larvae travel to the tissue in which they must complete their development, often using connective tissues to move within the host’s body and minimize exposure to hemopoietic defenses [1]. Dermatobia hominis are an exception: their larvae typically burrow into skin and develop near their hatching site [2]. Larvae feed by scraping host tissues with hook-like mandibles while secreting saliva that contains digestive enzymes to aid in nutrient acquisition [1, 2]. With the exception of Cuterebrinae, larvae overwinter in the host’s body [2].

After completing three instars, larvae exit the host, either through expulsion with feces (in the case of stomach bots), being sneezed out of the nasal cavity (nose bots), or wriggling out of the open cyst, called a warble, that the maggot has created in the host epidermis (skin bots) [1, 2]. Between larval molts, maggots may change feeding sites: for example, larvae of the sheep nose bot fly, Oestrus ovis, move from the nasal cavity into the frontal sinuses between the 1^st and 2^nd instar where they can feed without being dislodged prematurely but returns to the nasal cavity during the 3^rd instar, allowing the larvae to exit for pupation [1].

Subfamily	Hosts	Typical site of egg deposition	Typical site of larval development
Cuterebrinae	Rodents, lagomorphs (Cuterebra), primates (Dermatobia)	On vegetation	Skin
Gasterophilinae	Horses, deer, elephants, rhinoceroses	On host	Digestive system
Hypodermatinae	Mammals, especially bovine livestock	On host	Skin
Oestrinae	Mammals, especially deer and sheep	Larviposition on host	Nasal cavity

Subfamilies of bot flies [1, 2, 5,6].

Ostridae flies generally exhibit a high degree of host and tissue specificity, in contrast to other, less-specialized myiasis-causing flies [2]. Their larvae are adapted to feed and develop within a particular tissue type and a particular host species; they possess several conspicuous physiological traits that make them well-suited for this form of obligate parasitism. The larval body, which is legless and usually conical, possesses a soft, unsclerotized cuticle that is susceptible to desiccation, similar to many other immature dipeterans [1,6]. The composition of the cuticle is, however, thought to defend and disguise the larvae from the host immune system [6]. Compared to non-parasitic counterparts, bot fly larvae possess fewer cuticular sensilla and, in some cases, a thicker cuticle [6]. Additionally, maggots exhibit heavily sclerotized mouth hooks that allow them to attach to and feed on their hosts and sclerotized cuticular spines that aid in attachment to, migration across, and maintenance of position within host tissues [1, 2, 6]. The morphology and arrangement of these sclerotized structures differs among subfamilies.

Stomach bot flies are exposed to an especially challenging environment within the mammalian digestive system and possess unique adaptations compared to larvae that infest other tissue types. They exhibit molecular adaptations to the high-temperature environment in which they develop: the greater stability of guanine-cytosine bonds under thermal stress compared to adenine-thymine pairs has been proposed as an explanation for the high content of G-T pairs in the genome of Gasterophilinae [4]. And while the respiratory systems of most bot fly larvae are structurally and functionally similar to other dipterans, the hemolymph of Gastrophilinae contains hemoglobin, helping them to tolerate the low-oxygen levels present in the host gut [6].

This project will address the physiological adaptations that facilitate the parasitic stage of the bot fly life cycle, with a focus on the integumentary system of bot fly larva.

References:

1. Wall, R., & Shearer, D. (1997). Myiasis. In Veterinary Entomology: Arthropod Ectoparasites of Veterinary Importance (1st ed., p. 197-250). Springer-Science + Business Media, B.V.

2. Scholl, P. J., Colwell, D. D., & Cepeda-Palacios, R. (2019). Myiasis (Muscoidea, Oestroidea). In Medical and Veterinary Entomology (3rd ed., pp. 383–419). Academic Press. https://doi.org/10.1016/B978-0-12-814043-7.00019-4

3. Balashov, Y. S. (2006). Types of parasitism of acarines and insects on terrestrial vertebrates. Entomological Review, 86, 957–971. https://doi.org/10.1134/S0013873806080112

4. Yan, L., Pape, T., Elgar, M. A., Gao, Y., & Zhang, D. (2019). Evolutionary history of stomach bot flies in the light of mitogenomics. Systematic Entomology, 44(4), 797–809. https://doi.org//10.1111/syen.12356

5. Slansky, F. (2007). Insect/Mammal Associations: Effects of Cuterebrid Bot Fly Parasites on Their Hosts. Annual Review of Entomology, 52, 17–36. https://doi.org/10.1146/annurev.ento.51.110104.151017

6. Cowell, D.D., et al. Oestrid Flies : Biology, Host-Parasite Relationships, Impact and Management, CABI, 2006. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/osu/detail.action?docID=289452.