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Veterinary Forum April 2009 (Vol 26, No 4)

Emerging concern for vector-borne diseases

by Marie Rosenthal

    LAS VEGAS — If you feel as if there are more fleas and ticks than ever before, you may be right, Susan E. Little, DVM, PhD, DEVPC, said here at the Western Veterinary Conference.

    "That's an accurate perception. But if you feel like these vectors just came out of nowhere, they didn't. They were always around, but they have certainly increased their numbers dramatically and we have denser populations than we used to have."

    Driven by socioeconomic factors, changing climate and an increase in the numbers and distribution of wildlife hosts, arthropod vectors have successfully spread across the country, changing the landscape of infectious diseases. New species are being introduced to this country, and resident species are being identified in new areas, according to Little, who is professor and Krull-Ewing endowed chair in veterinary parasitology in the department of veterinary pathobiology, Oklahoma State University Center for Veterinary Health Sciences.

    "When I was growing up in Kentucky, mosquitoes did not feed during the day, but all of that has changed. The Asian tiger mosquito, Aedes albopictus, has extended its range up the East Coast as far north as Maine," Little offered as an example of a species infiltrating new areas. Before 1985, A. albopictus was unheard of in North America.

    "Today, this species is the most prominent anthropophilic, human-feeding, day-feeding mosquito we have in the United States."

    In addition to importations, vector populations, such as white-tailed deer and wild turkeys, slowly spread to new areas because of changes in habitat and climate and increases in reservoir populations. If the changes that make a climate hospitable are sustained, reservoir hosts and vectors move into that new area. When the threshold is surpassed, veterinarians notice more patients in their offices, she explained.

    "So it's above the threshold of where we notice it. It's patient after patient coming in with this infection. To get to that point, you have to have a higher proportion of the vectors infected," Little said.

    "We have very healthy wildlife reservoir host populations and that means we have very healthy tick populations," Little added.

    People who live in wooded areas are less likely today to apply pesticides than homeowners of yesteryear, but that means there is a trade-off. With this healthy ecosystem comes the risk for vector-borne disease. "You know, there are no ticks in a Wal-Mart parking lot," Little joked. "We're seeing an increasing distribution and number and activity of vectors and that together can lead to increasing transmission of vector-borne diseases."

    A national survey of more than 2,500 veterinary clinics that were part of a voluntary reporting system found positive test results for Lyme disease, anaplasmosis, ehrlichiosis and heartworm disease in most states. The study was sponsored by IDEXX Laboratories, which reported test results from more than 3 million canine samples collected from 2000 to 2008. Test results were generated from the IDEXX reference laboratory network, as well as in clinics testing with the SNAP 3Dx and 4Dx.

    In the Southeast, 3.9% of dogs tested positive for heartworm antigen, compared with 0.8% in the Midwest, 0.6% in the Northeast and 1.2% in the West. Little said many were surprised to see the statistics from the West. "More than 1% of dogs in the West were heartworm positive, and this was surprising because heartworm is underappreciated as an infection in the western United States.

    "We think that lack of intervention accounts for some of the difference we're seeing in the data, and there were counties in northern California that had a heartworm prevalence rate three times higher than that for the entire state of Florida. The transmission rate, however, isn't as high, and the pressure isn't as high. Yet without protection, without intervention, the pet dogs are at greater risk," she said.

    "When I talk with some veterinarians who practice in California, they tell me they don't have heartworm and they don't even test for it," she said, adding, "Well you have to test for it to find it."

    Overall, positive test results for Lyme disease were highest in the Northeast; positive results for anaplasmosis were highest in the Midwest and positive results for ehrlichiosis were highest in the South.

    "Overall in the United States, about 5% of dogs test positive for antibodies to Borrelia burgdorferi, the organism that causes Lyme disease. That doesn't mean they have Lyme disease, it just means they've been exposed. But in areas where exposure rates are high, you would expect disease rates to be higher than other areas," Little said.

    "We are seeing emergence [of diseases] because the vectors have spread to new areas, and that's what we're watching right now — the encroachment. Diseases are showing up in areas where they were not previously found. So when someone tells me 'I've been practicing (here) for 20 years and I've never seen a case of this,' I believe him, but that doesn't mean he won't see a case next year.

    "If we as practitioners say, 'Well it's not going to get into my town' and we don't put dogs on a preventative, then it will get in."

    The Companion Animal Parasite Council sponsored the session.

    For more information:

    Little S. Vector-borne diseases: an emerging concern. Presented at: The Western Veterinary Conference. Las Vegas, Nev.; Feb. 15-19, 2009.

    NEXT: FORUM FIVE — Focusing on babesiosis


    Did you know... In the 1980s, skeptics denounced the existence of Lyme disease as a discrete infectious disease. Not until the 1990s was the pathogenesis of the human disease understood.Read More

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