Module 10: Veterinary Ectoparasites

Module 10.1: Introduction to Ectoparasites


Ectoparasites are generally arthropods that live and feed on the exterior of the host, commonly the integument. In veterinary medicine, the most common ectoparasites that infest/infect our patients are ticks, mites, lice, and fleas.  This module will guide you through the morphological characteristics that distinguish salient differences between ticks, mites, lice, and fleas as inevitably a client, family member, or member of the community will bring you a “bug in a jar” and want you to identify it. As a veterinarian, you are also on the frontlines to identify invasive or non-native parasites that have entered the country via the legal or illegal movement of animals.


Arthropod classification tree
Arthropod classification

General characteristics of insects and arachnids


This class of ectoparasites includes ticks and mites. The major identifying feature of these arthropods in comparison to insects is adult arachnids have 8 legs (4 pairs).  The arachnids (ticks and mites) undergo gradual metamorphism meaning that each stage of development can be challenging to distinguish from another as they all appear similar.  The progression of development includes eggs that are laid in the environment molt into 6 legged larvae, then molt into  8 legged nymphs, and then into 8 legged adults. Remembering that some stages of development have 6 legs is important especially with ticks since not all stages of development occur upon the same animal.

Arachnid Metamorphism: egg, larva, nymph, male, female
Arachnid Metamorphism


Important veterinary insects lice, fleas, and flies.  In this laboratory, we will be focusing on the salient feature of just lice and fleas. Adult insects in comparison to arachnids are adults have 6 legs (3 pairs).  Most insects (including fleas) undergo complete metamorphosis development which following hatching from an egg, the individual changes form radically between molts. In other words, unlike the mites and ticks, we can identify each separate stage of development.  Insects such as lice undergo an incomplete metamorphosis which is when individuals of different ages are of similar form but live as progressively larger instars (larval stages) between molts.  Incomplete metamorphosis is a similar process to gradual metamorphosis EXCEPT for that type of maturation occurs in ticks and mites.

Image shows examples of incomplete and complete metamorphosis
Incomplete vs Complete Metamorphosis

Summary Table

Table 10.1: Ectoparasite Summary Table


Type of Arthropod

Pairs of Legs

Type of metamorphosis

Arachnida 4 pairs of legs (8 legs) Simple (gradual)
Arachnida 4 pairs of legs (8 legs) Simple (gradual)
Insecta 3 pairs of legs (6 legs) Incomplete
Insecta 3 pairs of legs (6 legs) Complete

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