Vector-borne diseases

Vector-borne diseases

May 01, 2008

RALEIGH, N.C. — What are some of the newest and most important vector-borne diseases affecting companion animals, and how can veterinarians better monitor them?

DVM Newsmagazine recently discussed those topics with Edward Breitschwerdt, DVM, Dipl. ACVIM, professor of medicine and infectious diseases at North Carolina State University's College of Veterinary Medicine.

DVM: What are the most important emergent vector-borne diseases in the United States? Why?

Dr. Breitschwerdt: The most important vector-borne disease for felines right now is Cytauxzoon felis. It is frequently fatal — the cat appears to be healthy but within days it's dead, which is what makes this disease so devastating.

The area in which C. felis is being recognized is expanding. Initially, cases were seen in Arkansas, Missouri and Oklahoma. Now there clearly seems to be expansion of the distribution through much of the southeastern United States.

The bobcat is the major reservoir host of Cytauxzoon, so as bobcat populations expand the ticks that feed on them are geographically spread. Bobcats are very territorial; when the areas they claim contain domestic cats, ticks can easily spread from the bobcat to these cats.

We now have better diagnostic testing, through DNA, for this tick-borne infectious agent. This is important because a diagnosis of cytauxzoonosis must be made quickly. The treatment is complicated. Aside from the correct drugs to be administered, there is a high level of care that must be provided if the infected cat is to survive; usually this requires more than a week in intensive care.

The newest emerging vector-borne disease in dogs is Anaplasma phagocytophilum. This is an intracellular bacterial organism transmitted by the same ticks that transmit Lyme disease.

What makes A. phagocytophilum so important is that the same organism can induce disease in dogs, cats, horses and humans. It was first recognized in the late 1960s and early 1970s by researchers at the University of California-Davis as an infectious disease of horses.

It was subsequently recognized as a disease-causing agent in dogs and cats, and in the early 1990s infection was recognized for the first time in humans. Two recent studies generated data suggesting that Anaplasma phagocytophilum can cause a persistent infection in dogs. If it can induce chronic infection, this information changes our interpretation of several aspects of this disease.

One of the reasons this particular disease has come to the forefront is that the SNAP 4Dx test (IDEXX Laboratories) has added peptides for the detection of Anaplasma species antibodies to the standard testing for heartworm antigen, Borrelia burgdorferi (Lyme disease) antibody, and E. canis antibody.

Positive antibody test results indicate that an animal has been exposed, but antibody detection cannot confirm an active infection. Veterinarians also must understand that this new test detects antibodies at the genus level. Therefore, it does not differentiate between exposure to A. phagocytophilum and Anaplasma platys.

DVM: According to recent reports, tick species are moving/migrating in the United States. Is this true for most parasites of concern to companion-animal veterinarians? If yes, what is driving this change?

Dr. Breitschwerdt: In other parts of the world mosquitoes are the primary vectors for the transmission of infectious agents, but we probably have more of a problem with ticks than with any other vectors found in the United States.

We have a variety of tick species with substantial variation in their distribution, but with tremendous overlap of several different species in the same location. Also, different ticks carry and transmit different pathogens. So in any location there may five or six species of ticks distributing eight or nine different infectious organisms. And as we experience warmer winters in certain parts of the country, this also influences more abundant tick populations.

Tick movement takes place with the movement of wildlife populations, especially deer. Most authorities concur that the major factor contributing to the spread of ticks is the movement of deer populations throughout the United States.