Handbook of Equine Parasite Control
Handbook of Equine Parasite Control is an essential guide for equine practitioners, veterinary students, and veterinary technicians dealing with parasites in the horse. Martin K. Nielsen, DVM, PhD, DipEVPC, DipACVM, is Associate Professor and Schlaikjer Professor in Equine Infectious Disease at the M.H. Gluck Equine Research Center in the Department of Veterinary Science at the University of Kentucky in Lexington, Kentucky, USA. Craig R. Reinemeyer, DVM, PhD, DipACVM, is a Founder and President of East Tennessee Clinical Research in Rockwood, Tennessee, USA.
Handbook of Equine Parasite Control
Biology and Life Cycles of Equine Parasites
Life cycles are the road maps that guide parasites to their ultimate goal - propagating a subsequent generation. Some parasites follow a single, direct path to grandma's house, while yet others may travel by convoluted routes, sojourn for protracted periods at some wayside convenience, or even pick up a passenger or two. These differences represent alternate strategies for coping with the vagaries of the environment and of their eventual hosts.
A thorough knowledge of life cycles is not emphasized merely to torment veterinary students. Rather, life cycle details reveal opportunities to control parasites through chemical or management interventions, to exploit unfavorable environmental conditions, or to promote natural enemies that might act as agents of biological control. Taking advantage of these potential control opportunities will be emphasized in individual chapters in this volume.
At the root of all life cycles is a fundamental principle that distinguishes helminth parasites from other infectious agents such as viruses, bacteria, fungi, and protozoa. Through various types of clonal expansion, the latter can all amplify their numbers within a host animal. Literally millions of individual organisms may arise from infective burdens that are orders of magnitude smaller. The reproductive products of nearly all helminths, however, are required to leave the host and undergo essential change in a different location. Defecation is the most common means by which reproductive products exit the host, but a notable exception includes immature parasitic stages that are ingested by blood-sucking arthropods ( e.g ., Onchocerca , Setaria ). Most parasitic products can become infective in the environment, whereas others require intermediate hosts or vectors. Regardless, all of these essential transformations occur "outside the definitive host". Indeed, dramatic biological change is mandatory before a parasitic organism is capable of infecting a new host animal or of reinfecting the original host.
Compared to those organisms that amplify their numbers through clonal expansion, helminth disease is a numbers game. Simply put, as the number of invading parasites increases, greater tissue damage or nutrient loss results, and the range and severity of clinical signs become more extensive.
In this chapter, we propose to describe the basic life cycles of the major helminth parasites of equids. Specific control opportunities may be mentioned in this overview, but these will be discussed more fully elsewhere in the volume.
The members of the Strongyloidea ("strongyles") are moderately sized, stout worms with substantial buccal capsules. The males have a copulatory bursa at the posterior end and females of all species produce eggs that are similar in appearance. Eggs of small strongyles cannot be differentiated microscopically from those of large strongyles, and the only practical method of differentiation (other than molecular approaches) is through coproculture. The strongyloids of horses all have direct life cycles; intermediate or paratenic hosts are never used ( Figure 1.1 ).
Figure 1.1 Strongyle life cycle. The life cycle of strongyle parasites. Parasitic stages can be seen above the horse and preparasitic stages below it. Fertilized eggs are shed by adult females in the cecum and colon, and excreted to the environment in the feces. Here, the eggs hatch and a first-stage larva (L1) emerges. The L1 then molts to L2 in the feces. Another molt gives rise to the L3, which retains its L2 cuticle and thus has a double-layered sheath. The L3 leaves the fecal pat and migrates on to forage, where it is ingested by a horse. Inside the horse, t