Ticks on the move (Proceedings)

Ticks are increasing in importance in small animal practice because of their ability to cause irritation and tick paralysis and because they vector several diseases. Successful tick control is an important part of small animal practice in many areas.  Tick control can be improved beyond the monthly administration of a topical product, by appreciating which ticks are likely to be found in your area and some basics of tick biology. 

Identification of the genus of common adult ticks is not difficult, particularly with the use of a tick key.  With the increased diagnosis of tick borne diseases, keys and other materials for identification of ticks are readily available from state and federal agencies through the internet.  The California Department of Health has a wallet sized tick identification card that can be downloaded (http://www.cdph.ca.gov/HEALTHINFO/DISCOND/Pages/TickBorneDiseases.aspx ).  Generally, a magnifiying glass will provide enough magnification to identify adult ticks. For most common ticks, the adult stages are the ones most likely to be found on pets and people and published material on tick ID usually specifically relates to adult ticks, with the sizes of immature stages included for comparison.  Nymphs of some tick genera may also encountered and often look similar to adults.  Tick larvae are the most difficult to identify, but knowing the host prevalence of common species and their seasonal abundance helps in making an educated guess about immature stages.

Ticks are divided into 2 families:  the hard ticks (Ixodidae) and soft ticks (Argasidae).  The hard ticks are of much greater importance as parasites of domestic animals and humans and comprise almost all of the ticks encountered by practitioners and clients.  Only 4 genera are found widely distributed in the U.S. and although there may be species variation regionally there are often similarities among species within each genus.

All of the important hard ticks in the US, are three host ticks, leaving the host after each blood meal. Usually these common ticks climb onto vegetation and wait for a suitable host to pass by. The common tick species all emerge from eggs in the environment as six-legged larvae (commonly called “seed ticks”).  They remain on the host for a few days to feed and then return to the environment to molt to the 8-legged nymph stage.  Tick nymphs find a host, feed and again drops off the host into the environment. 

After molting again to the final, adult stage, the male and female ticks find a final host and usually mate on the host (some Ixodes species may mate off the host.  The female tick leaves the host and lays her eggs in the environment.  The life cycle of ticks may take 2 or 3 years to complete and different stages are often most abundant during specific seasons on the year.  An exception is the brown dog tick, Rhipicephalus sanguineous, which can complete its life cycle in buildings in just a few months. 

Dermacentor ticks are found across the U.S.  Common species transmit disease (Table 1), and can cause tick paralysis.  The most widespread species is D. variabilis (the American dog tick), which is found throughout the eastern US to the eastern portions of the Plains states and also along the Pacific coast.   Dermacentor andersoni (the Rocky Mountain tick) is found in western states, while D. occidentalis is common in California and Oregon.  Dermacentor ticks have white markings on the dorsal surface and relatively short mouthparts compared to the other ticks with white markings (Amblyomma spp).  Dermacentor ticks generally prefer small mammals as hosts in the immature stages so these stages are not frequently found on pets and people.  Adult Dermacentor feed preferentially on larger mammals and humans as adults.

Important members of the genus Amblyomma include A. americanum and A. maculatum.  Like Dermacentor, Amblyomma ticks often have white markings on the dorsal surface, but Amblyomma ticks have long mouthparts compared to Dermancentor.  Both species of Amblyomma were tradionally associated with the southeastern US., but appear to be expanding their ranges farther north and west.  Amblyomma americanum, the lone star tick, in particular, is now found throughout most of the eastern U.S.  This tick is not highly specific in its host preferences and all stages can be found on dogs and cats.

Female A. americanum can be readily identified by the white dot in the middle of the dorsal surface, but markings on the back of males are much more difficult to visualize.  Amblyomma maculatum prefers to feed on birds in the immature stages, while adults are usually found in the ears of medium sized to large mammals. Amblyomma ticks can transmit disease and have been reported to cause tick paralysis.  Because of the long mouthparts of the tick, a substantial inflammatory reaction may occur at the site of attachment causing confusion with the “bull's eye rash” of Lyme disease.

Ixodes spp are of the greatest concern to pet owners because they vector Lyme disease:   I. scapularis in the eastern and Midwestern U.S. and  I. pacificus  in the western U.S.  Ixodes is smaller than other common ticks and can be more easily overlooked on pets and people.  Ixodes has long mouthparts like Amblyomma, but lacks the white enameled markings on the dorsal surface. Immature stages, males and unengorged females are dark colored.

Larvae are only about 0.5 mm in length and are very difficult to see.  In all stages of this tick genus a groove called the anal groove runs from the posterior margin of the body to just anterior to the anus and is the identifying characteristic.  Unfortunately, visualizing the groove usually requires a dissecting microscope. Not all Ixodes ticks found by pet owners are I. scapularis, but identification to species is difficult.   Ixodes scapularis, like Amblyomma, has increased its range dramatically in recent years. 

One of the primary reasons for this change in tick distribution is thought to be the increase in the white tail deer herd in the eastern U.S.  Deer serve as hosts for adult Ixodes and Amblyomma.  Larvae of I. scapularis feed on small rodents, the reservoir for Borrelia burgdorferi, as well as lizards and birds.  Nymphs feed on rodents and other smaller mammals and adults feed on medium to large sized mammals.  Ixodes pacificus feeds on a variety of small animals (rodents, lizards, birds, rabbits and some larger animals) in the immature stages, but primarily on lizards.  The preference for reptiles in the immature stages results in lower rates of transmission of Borrelia burgdorferi.

In addition to transmitting Lyme disease, Ixodes also can transmit Anaplasma phagocytophilum and Babesia microti (a rodent species that may infect humans).  Tick paralysis has also been seen with I. scapularis attachment.

Rhipicephalus sanguineous, the brown dog tick, has the most restricted distribution of the common species.  It has a definite preference for warm temperatures.  However, all stages of the life cycle parasitize dogs, so the tick can survive cold temperatures in kennels and homes. Eliminating ticks from these areas can be difficult.  Acaricides need to be sprayed into all cracks and crevices that can harbor ticks and tick eggs.  Rhipicephalus sanguineous is a reddish brown color and has no white markings, but does not have the long mouthparts of Ixodes, the other plainly color tick genus.  The life cycle can be completed in 3 months in warm conditions and numbers of ticks can increase quite rapidly.  Clients complaining of tick infestations in buildings are typically dealing with R. sanguineous. The brown dog tick is the vector of Babesia canis, Ehrlichia canis and possibly Anaplasma platys and Babesia gibsoni.

Ticks should be removed carefully from animal and human hosts.  Because infectious agents could be released from the mouthparts when the tick is removed, it is recommended that tweezers or forceps be used instead of fingers.  Grab the tick at the point of attachment and pull steadily.  If portions of the mouthparts are left in the skin an inflammatory reaction and bacterial infection may occur.  Several products available for monthly application on dogs and cats and a long acting tick collar are. 

Owners may complain that they see ticks on animals when using these products, but it is important to recognize that an occasional tick on a pet may still occur because products are not 100% effective and some time is required to kill the ticks.  In general, it is assumed that killing ticks within 24-48 hours will prevent disease transmission, although some variation may occur.  Two newly introduced products have claims of killing ticks in just a few hours. 

Table 1. Tick borne diseases

Disease

Pathogen

Tick Vector

Granulocytic anaplasmosis

Anaplasma phagocytophilum

I. scapularis, I. pacificus

Babesiosis

Babesia canis, B. gibsoni

Rhipicephalus sanguineus

Hepatozoonosis

Hepatozoon americanum

A. maculatum

Ehrlichiosis

Ehrlicia canis

E. ewingii

E. chaffeensis

R. sanguineus, D. variabilis

A. americanum

A. americanum

Lyme disease

Borrelia burgdorferi

I. scapularis, I. pacificus

Rocky Mountain spotted fever

Rickettsia rickettsii

D. variabilis, D. andersoni

Cytauxzoonsis

Cytauxzoon felis

D. variabilis

Clients should be encouraged to use integrated pest management in areas of high tick populations and disease transmission.  In the northeastern US where I. scapularis  and Lyme Disease, the creation of a “tick free zone” in home yards is recommended.  Practices such as removing leaf litter and brush, discouraging rodent and deer populations, moving woodpiles and creating a mulch or gravel border between the yard and woods or stonewalls have all been recommended.

References

Blagburn BL, Dryden MW.  2009.  Biology, Treatment and Control of Flea and Tick Infestations.  Vet Clinics Small Anim. 39:1171-1200.

Dryden MW, Payne PA.  2004.  Biology and control of ticks infesting dogs and cats in North America.  Vet Therapeutics.  5: 139-154.

Nicholson WL, Sonenshine DE, Lane RS, Uilenberg G.  Ticks (Ixodida) in Mullen G, Durden L. (ed).  2009.  Medical and Veterinary Entomology, 2nd ed. New York. Academic Press. pp 493-542.

Stafford KC III.  2007.  Tick Management Handbook.  Connecticut Agricultural Experiment Station.  Available from the website of the Connecticut Agricultural Experiment Station (http://www.ct.gov/caes/site/default.asp ) and from the CDC website.