Vector-borne diseases

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.

But ticks move as we move pets, production and wild animals — this is a national and an international issue. We're relocating wild animals throughout the United States, and rescue organizations are bringing in dogs from places like Europe, the Caribbean and Iraq. These organizations usually try to screen as best they can for chronic vector-borne infections, but no testing is 100 percent reliable.

In addition, companion animals while on vacation can pick up ticks and tick-borne infections and take them back home to a completely different area of the country. At times, these vectors can then perpetuate a new transmission cycle in a setting in which veterinarians are less familiar with tick-borne illnesses.

DVM: Are there new mechanisms in place to monitor vector-borne parasitic disease? If so, what are they and how effective are they?

Dr. Breitschwerdt: We used to rely on seroconversion tests with samples obtained at the time of suspected infection, then several weeks later looked for the level of antibodies to go from low to high. But now we have access to DNA-based diagnostic tests that can confirm active infection from tick diseases within a matter of hours.

If the test is positive, that confirms the tick-borne cause of the illness and targeted treatment can be initiated. These molecular diagnostic tests are very useful, but have limitations.

The most effective mechanism to monitor for the prevalence of vector-borne diseases is rapid in-house annual screening using approved products such as the 4Dx assay. Annual screening is important for three reasons: first, it allows veterinarians to screen for evidence of infection prior to the onset of illness; second, any change in prevalence of tick-borne organisms can be detected; and third, veterinarians can play a role in public-health monitoring of vector-borne infectious diseases in their local communities, because most of these organisms infect both dogs and humans.

If practitioners are aware of transmission patterns, they can advise pet owners with regard to daily monitoring for tick attachment to family members — how to carefully and completely remove the tick — and, in case of unexplained febrile illness in family members, to notify the family physician of the presence of infection or exposure of a dog in the same household.

In some areas of the country, veterinarians may be more knowledgeable about tick-transmitted infectious diseases than physicians.

Open communication among veterinarians and physicians is the most important thing when it comes to the management of these vector-borne pathogens. We'll see 50 ticks on a dog before the first tick appears on a human sharing the same environment. If dogs are contracting anaplasmosis, ehrlichiosis or Rocky Mountain Spotted Fever (RMSF), then people in the area may be suffering from these same infections.

For example, clinical manifestations in dogs with RMSF are identical in most instances to manifestations reported in human patients. From a public-health perspective, the dog is an environmental sentinel for RMSF; it is therefore important that veterinarians recognize and accurately diagnose RMSF.

Diagnostic confirmation in a dog using DNA testing or seroconversion allows veterinarians to discuss the risk of R. rickettsii transmission in the peri-domestic surroundings. With appropriate treatment, both dogs and humans show rapid improvement within 24 hours. However, selection of an ineffective antibiotic could lead to death of the dog or the human in the same household.

DVM: Describe the practitioner's role in monitoring vector-transmitted disease. How has this role changed in the last five years?

Dr. Breitschwerdt: Veterinarians monitor for vector-transmitted disease in two ways: by diagnosis of active infections in their patients, and by using annual screenings to determine if dogs have been exposed to organisms, especially those that cause chronic infections and, ultimately, disease.

Veterinarians are being more pro-active in the diagnosis of vector-transmitted diseases, and a major change is the recognition among veterinarians of how important flea- and tick-transmitted pathogens are in daily practice, important from the standpoint of both animal health and human heath.

It is more important now to keep fleas and ticks off dogs and cats than it has ever been in the history of the profession.

Ms. Wetzel is a freelance writer in Cleveland, Ohio.

Disclosure: Dr. Breitschwerdt consults with IDEXX Laboratories and research in his laboratory is funded by a number of pharmaceutical companies that produce acaracide products.