Veterinary Medicine - October 2007 - (Page S2)

ADVANCES IN COMPANION ANIMAL TICK CONTROL Table 1 - COMMON TICKS FOUND ON DOGS OR CATS Tick Amblyomma americanum Amblyomma maculatum Dermacentor variabilis Common name Lone star tick Gulf coast tick American dog tick Geographic distribution Southeast, Midsouth, coastal Northeast Coastal South, but expanding northward (OK, KS, KY) Eastern U.S., including Northeast, Southeast, Midwest, also coastal western U.S. Northwestern U.S., northern Rocky Mountain states Seasonal activity Spring to fall Summer through fall Spring through fall; greatest activity in summer Spring through summer Dermacentor andersoni Ixodes scapularis Ixodes pacificus Rhipicephalus sanguineus Rocky Mountain wood tick Eastern black-legged tick Eastern U.S., including Northeast Nymphs in spring; and Southeast, also Upper Midwest adults in fall Nymphs in spring; adults in fall Throughout year; greatest activity spring through fall Western black-legged tick Coastal western U.S. Brown dog tick Throughout U.S. from the final host to lay eggs. It does so in a sheltered environment to protect eggs from climatic extremes and predators. Larvae hatch from eggs, and the cycle begins again. Certain ticks, such as R. sanguineus, can complete their life cycle in weeks, while others, such as Dermacentor and Ixodes, can require two years or more. Most hard ticks find their hosts by questing on vegetation.As hosts pass nearby, questing ticks literally grab them and search for a suitable attachment site.Ticks attach to hosts first by cleaving to the host’s skin with a component of their mouthparts called chelicerae.They then insert an anchoring device called the hypostome to ensure that they remain tenaciously attached during feeding. Some ticks produce a cement-like substance that also aids in attachment. Feeding and engorgement usually require several days for each stage and are enhanced by substances in their saliva that inhibit host immune responses, inhibit coagulation of blood, and exert local anti-inflammatory and analgesic responses.3 Engorged larvae, nymphs, and adults drop from the host to either molt to the next stage or produce eggs in the case of engorged adult female ticks.1 Ticks as vectors of disease Ticks are efficient transmitters of disease for several reasons: they require relatively large blood meals and may feed on hosts for prolonged periods; they are not easily removed during feeding because of their hypostomes and cement-like attachment substances; and they produce numerous substances in their saliva that promote feeding.As the different stages feed, they ingest pathogens that may be circulating in the host’s blood. Ingested pathogens replicate in the tick and are disseminated to a variety of sites, including the salivary gland.Tick-borne pathogens enter the dog, cat, or person when the subsequent stage (after molting) feeds on a susceptible host.This is referred to as trans-stadial or stage-to-stage transmission (i.e., a larva or nymph acquires the pathogen, a nymph or adult transmits it).Transmission can also occur via the tick’s joint fluids and regurgitated gut contents. Some pathogens, such as Rickettsia ricketsii, the cause of Rocky Mountain spotted fever, are passed from female ticks to their ova.This is referred to as transovarial or female tick-to-ovum transmission. Larvae that emerge from those ova can then infect a new host with the pathogen when the larvae feed.4 A tick’s competence as a disease vector depends on several factors, such as size of blood meal, capability of the pathogen to infect the tick and replicate within it, successful dissemination of the pathogen throughout the tick’s body, trans-stadial or transovarial transmission of the pathogen, and interactions between multiple microbes infecting the same tick. Selected vectorborne diseases of dogs and cats and their tick vectors are included in Table 1. A more detailed summary of human and animal vector-borne diseases is presented in the article on page 6. 2

Table of Contents Feed for the Digital Edition of Veterinary Medicine - October 2007

Veterinary Medicine - October 2007 Contents Leading Off The Veterinary Medicine Interview: Dr. Kevin Fitzgerald Toxicology Brief Idea Exchange Animal Sheltering in the United States: Yesterday, Today, and Tomorrow The Euthanasia Problem Offer Basic Behavior Counseling for Every Pet at Every Visit Promote and Perform Early Spaying and Neutering Assist Owners in Selecting the Best Pets for their Lifestyles Champion Animal Welfare in Your Community CE Form Product Review Advertiser Index Marketplace/Classifieds Mind Over Miller

Veterinary Medicine - October 2007

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