Update on vector-borne zoonoses (Proceedings)

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Update on vector-borne zoonoses (Proceedings)

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Apr 01, 2008

Vector-borne infectious diseases have been identified more frequently in humans and domestic animals in recent years, and include some of the most important emerging and re-emerging infectious diseases. Infectious diseases that may be transmitted to dogs and cats via a vector include viral, bacterial/rickettsial, and protozoal diseases. The vectors most commonly are arthropods such as sandflies, mosquitos, fleas, and ticks. The epidemiology of vector-borne infections depends upon the distribution of the arthropod vector, and travel history is an important consideration when examining dogs with possible vector-borne infectious diseases.

The development of molecular diagnostic techniques has contributed to improved recognition of a number of zoonotic vector-borne infectious agents, including Bartonella and Borrelia. Improved surveillance for these diseases has also improved recognition. Human and animal travel is increasing worldwide and has contributed to emergence of many infectious diseases. Competent vectors for Plasmodium and Leishmania exist within the United States, so these infectious diseases have the potential to become a huge human health problem if they are imported here and the right conditions exist for transmission. Re-emergence can also occur when vector control programs succeed and subsequently fail to be maintained.

Habitat changes, deforestation and urbanization have contributed to emergence of a number of important vector-borne diseases in humans. Reforestation has probably contributed to emergence of Lyme borreliosis in the United States. Climate influences on host behaviour include more people and their pets staying indoors in an air-conditioned environment, which would tend to decrease the risk of vector-borne infectious disease. Humidity, rainfall, wind, and the duration of daylight are also important considerations.

Dogs can also serve as sentinels for human exposure to vector-borne infectious diseases, but in the majority of instances (perhaps with the exception of Bartonella), dogs and cats do not directly transmit these infections directly to people. Because many of these infections may be transmitted by blood transfusion, care should be taken when collecting and handling blood from dogs and cats suspected to be infected with a vector-borne infectious disease.

Coinfection with multiple zoonotic vector-borne infectious agents is a relatively common phenomenon that is being increasingly recognized. Coinfections can complicate the clinical picture, and in some situations, one organism may affect another's pathogenicity. Documentation of exposure to one vector-borne infectious agent should be a cue to search for exposure to other agents transmitted by similar vectors.

Emerging vector-borne zoonotic infectious agents of importance in dogs, cats, and humans include:

Borrelia burgdorferi

Borrelia burgdorferi is the cause of Lyme disease, the most common vector-borne disease in humans. The seroprevalence of Lyme in dogs in the western states is currently poorly understood, and we are currently recruiting canine serum samples in order to determine this more accurately. Although Lyme disease is being increasingly recognized in humans, PCR evidence of Borrelia infection in archival rodent specimens dates back to 1894. Ecological changes, primarily farmland reforestation and residential development in wooded areas, an associated explosion in deer populations, together with close proximity of reservoirs, ticks, humans and dogs, have probably contributed to emergence of Lyme disease.

The vector in the upper Midwest and north-eastern United States is Ixodes scapularis, and in the west it is Ixodes pacificus. The primary reservoir in the upper Midwest and the northeast is Peromyscus leucopus, the white-footed mouse. In the west, recently evidence was published that Western gray squirrels may be an important reservoir in California.

Following inoculation of spirochetes into the host by the tick, the organisms migrate through connective tissue from the bite site, causing fever, inappetence, thrombocytopenia and lameness due to neutrophilic polyarthritis in some dogs approximately 2-5 months after infection. As a result, seronegative Lyme disease is rare in dogs. A small percentage of dogs may develop more severe complications, including chronic, treatment-resistant arthritis or protein-losing nephropathy, especially in Golden and Labrador Retrievers. The vast majority of dogs show no signs of illness.

Specific diagnosis is generally performed using serology for antibodies, and is often difficult because of the widespread subclinical exposure in endemic areas. In dogs, diagnosis usually involves detection of antibodies to the C6 peptide, which can be performed using the SNAP 4Dx test kit, or the quantitative C6 ELISA available through IDEXX. Vaccination with Borrelia burgdorferi vaccines should not interfere with diagnosis using the C6 ELISA assay. Detection of antibodies in conjunction with suggestive clinical signs should prompt quantification of C6 antibody titers using acute and convalescent serology, such that recent infection may be documented. Asymptomatic, seropositive dogs should probably be evaluated for proteinuria and coinfections with other tick-borne pathogens, tick control should be recommended. Treatment of sick dogs is with doxycycline for 4 weeks, but although clinical signs may resolve, complete elimination of the spirochete may not occur. Vaccination is still controversial, but there is evidence from the human literature that use of the recombinant ospA vaccine is safe and relatively efficacious. It should be reserved for dogs in endemic areas that frequent tick-infected areas.