Canine Arthropods: Class Insecta

William Stich, BAS, MS, PhD & I. Craig Prior, BVSc, CVJ

Characteristics that distinguish arthropods from other invertebrates include a hard exoskeleton and pairs of jointed appendages. Different arthropod taxa undergo different degrees of metamorphosis as they develop from immature to adult stages, which can be key to effective control of arthropod ectoparasites.

INSECT CHARACTERISTICS

Several characteristics distinguish insects from other arthropods (Table 2).

All members of an insect order share a common maturation process, or metamorphosis. Although entomologists recognize several forms of metamorphosis, when it comes to insect control, only simple versus complex (or complete) metamorphosis is considered.

• Simple metamorphosis—in which immature stages are similar to the adult stage—are usually amenable to similar control measures at all life stages
• Complete metamorphosis—in which immature and adult stages are very different in appearance and, usually, habitat and food source—often require different measures for comprehensive control of different life stages.

TRUE FLIES (ORDER DIPTERA)

Diptera (di meaning two, and ptera meaning wings) usually have a single pair of wings (some, such as sheep keds, are wingless). True flies can be difficult to control, partially because of complete metamorphosis that necessitates integrated control measures.¹

The 3 large dipteran groups, distinguished by antenna morphology, are also distinguishable according to control measures:

1. Cyclorrhaphans (filth flies, several biting flies, and myiasis-producing flies) (Figure 1)
2. Brachycerans (horse flies and deer flies) (Figure 2)
3. Nematocerans (mosquitoes, black flies, sandflies, and midges) (Figure 3).

Figure 1. Cyclorrhaphan maggots are usually found in organic matter, whether decaying (filth flies) or living (myiasis-producing flies).

Figure 2. Brachyceran larvae are often found along waterways lined with vegetation and mud.

Figure 3. Among the Nematocera, immature blackflies (B) are found in rapidly running water; immature mosquitos (M) are found in still waters associated with flooded areas, permanent pools of water, or temporarily filled containers; and immature sandflies (S) are found in crevices or burrows that offer a dark microenvironment with high relative humidity and moderate temperature.

Cyclorrhaphans

Cyclorrhaphans, including houseflies (Musca domestica), that affect domestic animals are often called “filth flies” because larval and pupal stages are associated with decaying organic material. Some filth flies, such as stable flies (Stomoxys calcitrans), are hematophagous as adults.

Flesh flies and bottle flies can be associated with facultative myiasis that is often secondary to poor host condition. Some muscoid flies undergo obligate myiasis, which requires a living host for their larval stages. Among these, rodent bots (Cuterebraspecies) are myiasis-producing flies found in U.S. companion animals (Figure 4).

Figure 4. A rodent bot (Cuterebra species) in the neck of a cat. Female Cuterebra flies deposit eggs around burrows of their natural hosts (rodents), but the larvae also invade predators that visit these burrows.

Brachycerans

Adult horse flies and deer flies are voracious blood feeders that can serve as mechanical vectors of blood-borne pathogens, such as Francisella tularensis. Brachycera breed near slow-moving water with muddy banks and vegetation.

Nematocerans

Hematophagous female mosquitoes, sandflies, midges, and black flies are important pests themselves; however, they are even more problematic as vectors of pathogens and parasites, including:

• Filarial nematodes, such as Dirofilaria (mosquitoes)
• Protozoa, such as Leishmania (sandflies)
• Bacteria, such as Bartonella (sandflies)
• Arboviruses, such as West Nile virus (mosquitoes).

Larval and pupal stages require standing water (mosquitoes), rapidly moving water (black flies), or a microenvironment with high humidity (sandflies).

Control Strategies

Immature and adult fly stages often require different control strategies.¹

• Adults can be mechanically controlled with traps, screens, and fans, and chemically controlled with sprays for knockdown and repellants to protect hosts.
• Larvae and pupae are controlled through their environments, such as sanitation to remove decaying organic material (filth flies) and standing water in containers (mosquitoes). Some flea and tick preventive agents containing neonicotinoid class pesticides are also labeled for controlling biting flies, mosquitoes, and sandflies.²

FLEAS (ORDER SIPHONAPTERA)

Fleas are small (≤ 1/8 inch), wingless, laterally flattened insects that are exceptional jumpers. In dogs, pruritus from adult fleas is common and exacerbated by flea allergy dermatitis. In addition, heavy infestations can cause anemia. Certain flea species have also been incriminated as pathogen vectors.^3,4

Classification

Fleas are often ecologically adapted to their hosts, which means they may opportunistically infest aberrant hosts that venture into the habitats of their natural hosts. Several flea species parasitize various mammalian and avian hosts; those associated with dogs include:

1. Echidnophaga gallinacea (sticktight flea)
2. Tunga penetrans (chigoe or jigger), which embeds in the skin (usually on or near the bottom of the foot)
3. Pulex irritans (human flea)
4. Pulex simulans, which infests rodents, cats, coyotes, dogs, foxes, opossums, raccoons, and other animals
5. Ctenocephalides canis (dog flea), which is rare
6. Ctenocephalides felis (cat flea). which is the most common ectoparasite of dogs and cats in North America. Cat fleas, however, are uncommon in areas where relative humidity remains below 50% (eg, desert Southwest and Rocky Mountains).

Diagnosis

Adult fleas and flea dirt (ie, flea feces) can be removed from the host with a flea comb; flea dirt (Figure 5) is identified when placed on a moist paper towel, which allows the blood-laden feces to dissolve into visible red spots.

Figure 5. Laterally flattened Ctenocephalides felis fleas feed and lay eggs that fall off the host, allowing the caterpillar-like larvae and pupae to develop in host bedding and other parts of the environment. Flea feces (flea dirt) are shown on the pale skin and fur of a heavily infested dog.

Flea larvae may occasionally be found on bedding. These tiny, caterpillar-like larvae have poorly developed heads, 13 segments, and sparse setae (insect “hairs”).

Control Strategies

Fleas undergo complete metamorphosis, making their control particularly challenging,⁵ and most fleas in an infested environment are eggs, larvae, and pupae.

Effective comprehensive flea control requires:⁴

• Removal of adult fleas on pets
• Elimination of environmental stages
• Prevention of subsequent reinfestations.

Preventives. Prevention and elimination efforts have changed dramatically over recent decades due to availability of monthly flea preventives.⁶ Notably, in addition to killing adult fleas, some preventive agents also contain insect growth or development inhibitors that can control immature flea stages.

Because infested wildlife or stray companion animals can introduce fleas, the most effective control regimens involve flea preventives from soon after birth (see label claims) throughout the pet’s life. However, there are substantial geographic differences in distribution and seasonality of cat fleas and other flea species; therefore, tailor prevention to the individual pet and client.

Environment. Although once critical, multiple environmental treatments at 2-week intervals may not be required if pet owners can tolerate emergence of new adults from flea pupae, which must be attracted to treated pets before they can be killed.

LICE (ORDERS MALLOPHAGA & ANOPLURA)

Canine lice are small wingless insects that differ from fleas in several respects; they:⁷

• Are highly host specific
• Usually cause little irritation except in heavy infestations
• Undergo simple metamorphosis
• Are dorsoventrally flattened.

Louse nits (eggs glued to hair shafts), nymphs, and adults are all typically found in the same region on the host.

Figure 6. The dorsoventrally flattened chewing (Trichodectes canis, A) and sucking (Linognathus, B) lice glue eggs (nits, C) to host hairs, which remain on the host throughout their entire life as they undergo simple metamorphosis.

Classification

The 2 major louse groups have distinct head morphologies that reflect feeding habits (Figure 6):

1. Mallophaga (chewing lice) have large mandibles resulting in wide heads. Trichodectes canis is a chewing louse of dogs and rare vector of Dipylidium caninum.
2. Anoplura (sucking lice) have narrow heads due to piercing—sucking mouthparts; sucking lice also have larger claws adapted for grasping fibers (eg, hair). Linognathus setosus is a sucking louse occasionally found on dogs.

Diagnosis

Signs of louse infestation are irritation and damage caused by rubbing, scratching, or biting of infested areas. Heavy infestations may cause severe pruritus, restlessness, intense scratching, ruffled or rough matted coat, and sometimes alopecia.

Differentiation of louse head morphology, host species, and sometimes location on the host are often sufficient for identification of lice species.

Control Strategies

Canine lice are transmitted between dogs through direct contact or by fomites (eg, bedding or brushes) contaminated with hair from infested hosts.

• Quarantine and treat infested dogs before contact with other dogs, and place treated animals in disinfected living spaces.
• Some topical monthly flea preventives may also control lice.²
• Because nits are not susceptible to most treatments, they require repeated treatment after 1 week to kill recently hatched nymphs.

Nits, nymphs, and adults on fomites eventually desiccate and die.

TRUE BUGS (ORDER HEMIPTERA)

True bugs have needle-like, piercing—sucking mouthparts. Some members of this order are plant feeders that deliver painful defensive bites. However, medically important true bugs—bedbugs and kissing bugs—are adapted to acquire relatively painless feedings from slumbering hosts at night (Figure 7). Some irritation and itching can occur after these feedings

Figure 7. Hemiptera (true bugs), which include Cimex species (bedbugs, A) and Triatoma species (kissing bugs, B), feed on dogs (and human beings). Both possess piercing sucking mouthparts and acquire rapid blood meals from slumbering hosts. The bugs are dorsoventrally flattened, allowing them to reside in cracks and crevices between blood meals.

Identification

Kissing bugs, which have 2 pairs of wings, and wingless bedbugs undergo simple metamorphosis and are dorsoventrally flattened. They are found in cracks and crevices during the day during all of their life stages.

Trypanosoma cruzi Vector

Trypanosoma cruzi, the etiologic agent of Chagas disease, has adapted to kissing bug feeding habits, resulting in transmission if kissing bugs infected with T cruzi are eaten or their feces rubbed into vector feeding lesions in the hosts’ skin. Dogs are natural hosts of both kissing bugs and T cruzi.⁸

Control Strategies

Although a few studies have assessed the efficacy of molecules used in flea and tick preventives, such products have not been sufficiently tested for kissing bug control. Thus, kissing bug and bedbug control relies on preventing establishment of home infestations and killing the bugs where they hide during the day. The services of a qualified exterminator are recommended.

IN SUMMARY

Additional information about parasitic insects, insecticides, and specific products to control these and other parasites can be found through the CAPC resource library.² Additional, in-depth, and regularly updated parasitic disease control guidelines can also be found on the CAPC website. Read additional Parasite Protocols for Your Practice articles at tvpjournal.com.

FIGURE CREDIT

Figures for this article provided courtesy of the Oklahoma State University National Center for Veterinary Parasitology (ncvetp.org).

References

1. Foil LD, Hogsette JA. Biology and control of tabanids, stable flies and horn flies. Rev Sci Tech 1994; 13:1125-1158.
2. CAPC Resource Library. Parasite product applications for dogs. Available at: capcvet.org/resource-library/parasite-product-applications-for-dogs.
3. Dryden MW. Flea and tick control in the 21st century: Challenges and opportunities. Vet Dermatol2009; 20:435-440.
4. Blagburn BL, Dryden MW. Biology, treatment, and control of flea and tick infestations. Vet Clin North Am Small Anim Pract 2009; 39:1173-1200.
5. Rust MK, Dryden MW. The biology, ecology, and management of the cat flea. Annu Rev Entomol1997; 42:451-473.
6. Little S, Starkey L. Conquering fleas: Preventing infestations and limiting disease transmission. Today Vet Pract 2012; 2(6):33-39.
7. Arther RG. Mites and lice: Biology and control. Vet Clin North Am Small Anim Pract 2009; 39:1159-1171.
8. Barr SC. Canine Chagas’ disease (American trypanosomiasis) in North America. Vet Clin North Am Small Anim Pract 2009; 39:1055-1064.

I. Craig Prior, BVSc, CVJ, is the owner and medical director of Murphy Road Animal Hospital, in Nashville, Tennessee, and a member of the CAPC Board of Directors. He shares his knowledge through speaking engagements throughout the country and guest appearances with various media outlets. Dr. Prior received his veterinary degree from University of Queensland (Australia). 

William Stich, MS, PhD, is a professor of parasitology at University of Missouri, Department of Veterinary Pathobiology. He received his MS and PhD in veterinary parasitology at Oklahoma State University. Dr. Stich served as president for the Conference of Research Workers in Animal Diseases, and as a councilor for the Society of Tropical Veterinary Medicine. He currently serves on the CAPC Board of Directors and as Editor in Chief of Animal Health Research Reviews.