Epidemiology

 

Lyme disease is the most common tick-borne disease in North America and Europe and one of the fastest-growing infectious diseases in the United States. Of cases reported to the United States CDC, the ratio of Lyme disease infection is 7.9 cases for every 100,000 persons. In the ten states where Lyme disease is most common, the average was 31.6 cases for every 100,000 persons for the year 2005.

Although Lyme disease has now been reported in 49 of 50 states in the U.S, about 99% of all reported cases are confined to just five geographic areas (New England, Mid-Atlantic, East-North Central, South Atlantic, and West North-Central). New 2008 CDC Lyme case definition guidelines are used to determine confirmed CDC surveillance cases. Effective January 2008, the CDC gives equal weight to laboratory evidence from

1) a positive culture for B. burgdorferi

2) two-tier testing (ELISA screening and Western Blot confirming) 

 3) single-tier IgG (old infection) Western Blot. Previously, the CDC only included laboratory evidence based on (1) and (2) in their surveillance case definition. The case definition now includes the use of Western Blot without prior ELISA screen.

The number of reported cases of the disease have been increasing, as are endemic regions in North America. For example, it had previously been thought that B. burgdorferi sensu lato was hindered in its ability to be maintained in an enzootic cycle in California because it was assumed the large lizard population would dilute the prevalence of B. burgdorferi in local tick populations, but this has since been brought into question as some evidence has suggested that lizards can become infected. Except for one study in Europe, much of the data implicating lizards is based on DNA detection of the spirochete and has not demonstrated that lizards are able to infect naive ticks feeding upon them. As some experiments suggest lizards are refractory to infection with Borrelia, it appears likely their involvement in the enzootic cycle is more complex and species-specific.

While B. burgdorferi is most associated with deer tick and the white tailed mouse, Borrelia afzelii is most frequently detected in rodent-feeding vector ticks, Borrelia garinii and Borrelia valaisiana appear to be associated with birds. Both rodents and birds are competent reservoir hosts for B. burgdorferi sensu stricto. The resistance of a genospecies of Lyme disease spirochetes to the bacteriolytic activities of the alternative complement pathway of various host species may determine its reservoir host association.

In Europe, cases of B. burgdorferi sensu lato infected ticks are found predominantly in Norway, Netherlands, Germany, France, Italy, Slovenia and Poland, but have been isolated in almost every country on the continent.

B. burgdorferi sensu lato infested ticks are being found more frequently in Japan, as well as in Northwest China and far eastern Russia.Borrelia has been isolated in Mongolia as well.

In South America tick-borne disease recognition and occurrence is rising. Ticks carrying B. burgdorferi sensu lato, as well as canine and human tick-borne disease, have been reported widely in Brazil, but the subspecies of Borrelia has not yet been defined. The first reported case of Lyme disease in Brazil was made in 1993 in Sao Paulo. B. burgdorferi sensu stricto antigens in patients have been identified in Colombia and Bolivia.

In Northern Africa B. burgdorferi sensu lato has been identified in Morocco, Algeria, Egypt and Tunisia.

Lyme disease in sub-Saharan is presently unknown, but evidence indicates that Lyme disease may occur in humans in this region. The abundance of hosts and tick vectors would favor the establishment of Lyme infection in Africa. In East Africa, two cases of Lyme disease have been reported in Kenya.

In Australia there is no definitive evidence for the existence of B. burgdorferi or for any other tick-borne spirochete that may be responsible for a local syndrome being reported as Lyme disease. Cases of neuroborreliosis have been documented in Australia but are often ascribed to travel to other continents. The existence of Lyme disease in Australia is controversial.

To date, data shows that Northern hemisphere temperate regions are most endemic for Lyme disease.

 

From Wikipedia, the free encyclopedia

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