Contents |
General Life Cycles
Life cycles of arthropods involve a series of structural changes known as metamorphoses, the actual sequence of which varies with different parasite groups. Complete metamorphosis begins when adults lay eggs from which larvae hatch. (Figure 1A).
Figure 1A
The larval forms grow and shed their skins, or moult, several times, each time to accommodate their increases in size. Larvae may either live freely or be dependent on their hosts for obtaining nourishment. Eventually, a hard-cased structure called a pupa is formed, which may have the capacity to survive winter. The pupa hatches into the adult parasite, the final stage of metamorphosis. Thus, there are four distinct stages in the life cycle: egg, larva, pupa and adult. Incomplete metamorphosis involves a larva that grows and moults one or more times to become an adult-like form known as a nymph, which in turn grows and moults one or more times to become an adult
(Figure 1B).
Figure 1B.
In this case, there are only three distinct stages — eggs, larvae, and immature adults, or nymphs, that grow to maturity without further change in body type.
Flies
There are a number of fly species which are primarily a nuisance, especially under confinement conditions. The fly season is April-October. The constant buzzing of flies is irritating and can result in reduced foraging that may lead to production losses. Blood loss due to large numbers of feeding mosquitoes, such as those in the Southeast, may lead to anemia, unthriftiness, and weight loss or reduced weight gains. However, these fly problems are not all that common, and control measures are usually not emphasized. There are many insecticides that can be used for control when necessary. Routine disposal of manure and organic materials will help control nuisance flies, and the local mosquito control program will help control mosquitoes.
Lice and Mites
These parasites are relatively permanent residents on the animal. Infestation, commonly called mange when mites are involved, may be seen as intense irritation, with the animal scratching and chewing, creating skin lesions that can become ugly. They thrive and reproduce during the cooler months of October through March. Transmission from animal to animal is by contact, so crowding should be avoided. Control can be accomplished by using appropriate insecticidal products at the onset of cooler conditions and as necessary thereafter.
Ticks
Ticks thrive on blood obtained from the host. They are subdivided into hard and soft ticks, according to structural characteristics.
The bodies of hard ticks are roughtly oval and pointed at the front. The anterior segment is a false head, the structure of which may help to identify them. The structures on the head anchor the tick to the host’s skin and facilitate blood feeding. The abdomen can expand to several times its original size as a tick feeds on its host. This phenomenon, referred to as engorgement, is seen only in females. The patterns of pigmentation on the top side of the tick also helps with identification. A further classification of hard ticks is made based on whether their life cycle involves one, two or three hosts. Ticks have a life cycle incorporating incomplete metamorphosis. Adult ticks feed and mate on mammals. Engorged females drop to the ground and lay eggs. The eggs hatch, reproducing larvae, called seed ticks. The seed tick moults twice, passing through a nymphal stage before reaching maturity. A blood meal must be taken before each moult can occur. Ticks are classified as one-, two- or three-host ticks, depending on how many times they drop off, moult and seek a new animal. A one-host tick remains on the animal from the seed-tick stage to maturity. A two-host tick drops off the initial host to moult from larva to nymph. The nymph seeks a second animal for the final blood meal before final moult to adult. The three-host tick drops to the ground for each moult, after which a new host is sought.
Soft ticks differ from hard ticks in many respects. They have a leathery outer skin rather than a hard cuticle, and both males and females engorge when feeding on the host. Their shapes vary among species, and their false head is located on the bottom side of the tick, near its front, so it is not pointed as in hard ticks. ‘Oobius megnini’ the spinous ear tick, is an example of a soft tick. Only larvae and nymphs of this species are parasitic and can cause swelling of the ear, resulting in scratching and signs of disorientation. Adults live in hidden areas in the environment, such as within cracks in the wood of barns.
Insecticides recommended for other ectoparasites will control ticks. Dipping or high-pressure sprays provide the best results. The spinose tick can be controlled by applying an insecticide directly into the ears.
Diagnostic Methods
In general, most external parasites can be collected with various equipment: nets and aspirators for flying insects; jars, traps, combs and forceps for crawling insects and ticks; skin scrapings for mites. Most external parasites can be seen readily and identified through published descriptions and keys. However, the use of a microscope is usually necessary.