Hot Literature: Lyme borreliosis in man and his best friend
As pet owner awareness of zoonotic diseases increases, so does collaborative research between veterinarians and physicians. A workshop at the Centers for Disease Control and Prevention (CDC) brought together specialists in veterinary parasitology and human internal medicine and vector-borne infectious disease to address Lyme borreliosis. These experts summarized some recent study findings and the similarities and differences between the disease in people and dogs in the United States.
Transmission and clinical signs
The causative agent of Lyme disease, Borrelia burgdorferi, is transmitted by certain species of ixodid ticks. People are usually infected by the bite of an infected tick in the nymph stage of these ticks' three-host life cycle, whereas dogs are primarily infected by the bite of adult ticks. The primary reservoir host is the white-footed mouse, although other small mammals, such as shrews and chipmunks, have recently been suggested as potential reservoir hosts.
Symptoms in people usually develop shortly after infection, often before seroconversion. About 70% to 80% of infected people develop a characteristic rash, called erythema migrans, at the site of the tick bite. This rash can be followed by flu-like symptoms and, chronically, by arthritis, carditis, and neurologic disease. Infected dogs do not develop erythema migrans, and most infected dogs do not develop any clinical signs of Lyme borreliosis. Dogs that do develop clinical signs typically do so weeks or months after infection. Common signs include polyarthritis, anorexia, and lymphadenopathy. Dogs may also develop a glomerulonephritis syndrome. The prognosis for dogs that develop this Lyme nephropathy is poor.
Diagnosis, treatment, and prevention
Serodiagnosis remains the best way to confirm a presumed diagnosis of Lyme borreliosis in a laboratory, but the disease is definitively diagnosed through a combination of obtaining an extensive history that includes travel history; the clinician's knowledge of the disease's epidemiology; and laboratory and clinical findings. Testing people for Lyme borreliosis relies on a two-tier approach. Samples that have a positive or an indeterminate result on an initial IgM or IgG antibody detection test are submitted for Western blot testing. This method has a high sensitivity and positive predictive value in people.
Since most dogs have seroconverted by the time clinical signs are seen, a patient-side assay specific for the C6 peptide of the infectious organism is reliable for diagnosis and for evaluating response to treatment, but it cannot distinguish between dogs with active infection and those that have been exposed but have not developed clinical disease.
The recommended treatment for Lyme borreliosis in dogs and people is doxycycline. Beta-lactam antibiotics are also effective against B. burgdorferi. However, antibiotics in the penicillin family are not effective against other rickettsial pathogens that are common coinfections in dogs and people with Lyme borreliosis. One course of antibiotic treatment is adequate and is most effective when initiated early in the course of the disease. Dogs and people are susceptible to reinfection with subsequent exposure to B. burgdorferi.
No vaccine is available for people, but dogs may obtain an effective level of protection by a program of vaccination in endemic areas. The mainstay of prevention for people and dogs is still considered to be limiting exposure to infected ticks. Environmental and tick habitat management, prompt tick removal, and the use of repellents for people and acaricides for dogs are primary control and prevention recommendations.
Man's best friend
In fact, routinely using acaricides and insecticides on dogs has been shown to decrease the incidence of other diseases, including visceral leishmaniasis in children in endemic areas and ehrlichiosis in dogs. Although this has not been studied for Lyme borreliosis, because acaricides given routinely to dogs target primarily the adult stages of ticks, the reproductively active tick population in the local environment may be reduced, thereby protecting human health by lowering the risk of exposure to infected ixodid nymphs.
In general, wide-spread testing has shown that exposure in dogs correlates to the geographical distribution of exposure in people. The experts at the CDC workshop thought that monitoring antibody responses in dogs may help identify when B. burgdorferi begins to populate nonendemic locations. In this way, they thought that dogs could serve as sentry animals, further helping to protect human health.
Little SE, Heise SR, Blagburn BL, et al. Lyme borreliosis in dogs and humans in the USA. Trends Parasitol 2010;26(4):213-218.
For a link to the abstract, click here.