Vertical and transstadial transmission of Borrelia burgdorferi in the Ixodes tick vector

The very first sign of systemic infection of ticks with Borrelia, as well as transovarial transmission of Borrelia to tick eggs and larvae, was found in Ixodes ricinus as well as in Ixodes pacificus , by Willy Burgdorfer, who published how he found it, already in 1984:

Discovery of the Lyme disease spirochete and its relation to tick vectors.

Burgdorfer W. Yale J Biol Med. 1984 Jul-Aug;57(4):515-20. Fuld tekst i PMC

Quote from full text on the subject of the microscopic examination of ticks for the Lyme disease spirochete:

"In late September and early October 1981, Dr. Benach provided additional collections of I. dammini from Shelter Island, New York, where Lyme disease was known to be endemic. Again, none of 44 males and females had rickettsial infections. The hemolymph of two females, however, contained large microfilariae that differed morphologically from Dipetalonema (Wehrdickmansia) rugosicauda, a microfilaria detected in several adult 1. ricinus in Switzerland in 1978 (18).

To determine whether these nematodes were present also in the digestive system, I dissected both ticks and prepared Giemsa-stained smears from individual midgut diverticula for microscopic examination. No microfilaria was found. Instead, I encountered poorly stained, rather long, irregularly coiled spirochetes (Fig. 1). Darkfield microscopy of additional diverticula confirmed the spirochetal nature of the organisms, which had rather sluggish and slow movement. Additional tissues, including salivary glands, malpighian tubules, ovary, and central ganglion of either tick, were free of spirochetes.

Subsequently, 124 remaining ticks were dissected and each of their organs was examined for similar organisms. Seventy-five (60 percent) contained spirochetes that were limited to the midgut (Fig. 2). Organisms occasionally seen in preparations of hindgut and rectal ampule may have originated from midgut tissues. All other tissues were free of spirochetes.

Remembering the European literature, I could not dismiss the thought that the microfilariae did lead me to the discovery of the long-sought cause of ECM and Lyme disease.

Needless to say, I shared these observations with several of my colleagues, including Dr. Benach, who not only saw to it that I was well supplied with field-collected ticks from Shelter Island, but also provided sera from patients with clinically diagnosed Lyme disease for preliminary serological identification of the organism, and also Dr. Barbour, who at the time was engaged in a study of the variable major proteins of cultured tick-borne relapsing fever spirochetes and who offered his expertise to culture and immunochemically characterize the organism. [for more on the latter subject see]

The antigenic relatedness of the I. dammini spirochete to the etiologic agent of Lyme disease was established by indirect immunofluorescence as well as by western blot analysis of sera from both Lyme disease and ECM patients [19,20]. Our initial indirect evidence that this organism might be the cause of this disease was subsequently confirmed by isolating from patients spirochetes indistinguishable from those detected in I. dammini and by the microscopic demonstration of spirochetes in skin biopsies of cutaneous lesions of several Lyme disease patients [21—24].

One of our subsequent research objectives was to determine whether I. ricinus, the incriminated vector of ECM in Europe, was also a carrier of spirochetes. Evidence that this was the case was obtained from smears prepared in 1978 from nymphal ticks at the University of Neuchâtel in Switzerland, where I spent several months conducting a tick/rickettsial survey in various parts of that country. Of 135 smears of ticks collected in the Seewald forest on the Swiss Plateau, where according to medical authorities ECM had occurred in the past, 23 (17 percent) contained spirochetes that tinctorially and morphologically appeared similar to those detected in I. dammini.

In the spring of 1982, the U.S. Department of Agriculture gave us permission to import about 600 adult I. ricinus from the Seewald forest (201 of 201 individually examined ticks, 73 (36.3 percent) were infected with spirochetes. The organisms were limited to the midgut in 69 ticks but were found in all the tissues of the other four ticks. [4/73 = 5% were SYSTEMICALLY INFECTED!]
Of an additional 180 females that were fed on rabbits, 39 (21.9 percent) were infected. Two of them [2/39 = 5%] had a generalized infection whereas the other 37 had spirochetes in their midgut only.
Both females with generalized infections transmined spirochetes via eggs to 100 percent larval ticks in one and 60 percent in the other. However, as the larvae developed to nymphs and adults, the degree of spirochetal infection gradually decreased to a level of few organisms in tissues of the central ganglion only. This spirochetal behavior is in sharp contrast to the massive and prolonged development of tick-borne spirochetes and suggests that the growth conditions in the hemocoele of I. ricinus are inferior to those in the midgut.
Morphologically, the I. ricinus spirochete appeared indistinguishable from the I. dammini organisms, and antigenic similarities between the two spirochetes were apparent by direct immunofluorescence and SDS-PAGE protein profiles as well as by indirect immunofluorescence and western blot analysis of sera from ECM and Lyme disease patients (20).

Convinced that ECM of Europe and Lyme disease in the United States are expressions of one and the same etiologic agent, we directed our attention toward the west coast of the United States where the first case of ECM was contracted in Sonoma County, California, in 1975 and where the black-legged deer tick, I. pacificus, had been incriminated as the vector.
In collaboration with Dr. Robert Lane from the University of California at Berkeley and Dr. Robert Gresbrink from the Oregon State Health Department in Portland, we initiated a tick/spirochete survey in south-western Oregon and in north central California — areas where I. pacificus is abundant. The test procedures were similar to those applied to I. dammini and I. ricinus. Adult ticks collected by flagging vegetation in the spring and early summer were dissected individually, and the midgut diverticula were smeared on a microscope slide. After air-drying and after ten-minute fixation in acetone, the smears were treated with FITC conjugates prepared from sera of New Zealand white rabbits that had been immunized with the Lyme disease (Shelter Island isolate) spirochete. [i.e. DIRECT FLUORESCENT ANTIBODY STAIN, as later used by others to stain spirochetes in culture and in blood from Lyme borreliosis sick!].
Ticks with spirochetes in their midgut were further dissected to determine the presence of organisms in other tissues.
Of 645 I. pacificus from Oregon and of 550 ticks from California, 13 (2 percent) and 5 (0.9 percent), respectively, contained spirochetes that morphologically and by fluorescence microscopy appeared indistinguishable from the spirochetes found in I. dammini and I. ricinus. Five of the 13 [5/13 = 38%] infected ticks from Oregon and two of the five [2/5 = 40%] infected ticks from California had a generalized infection; the remaining positive ticks had spirochetes only in the midgut. As yet, we have not succeeded in establishing in modified Kelly’s medium an isolate for immunochemical analysis. The low percentage of spirochete-infected ticks in the West certainly is reflected by the small number of Lyme disease cases reported so far. It is quite possible, however, that there exist, within the distributional areas of I. pacficus, foci with higher infection rates providing a greater potential for human disease.

In concluding my presentation, I would like to emphasize that studies pertaining to the relationship of Lyme disease and ECM spirochetes to their respective tick vectors are still in the initial phases. As yet, little is known about the development of these organisms in ticks and about the mechanics by which they are transmitted to hos animals and man. Nevertheless, the observations and findings discussed above permit the following conclusions:

1. Spirochetes antigenically similar, if not identical, to each other have been found associated with Ixodes ricinus, I. dammini, and I. pacificus, the currently known tick vectors of ECM in Europe and of Lyme disease in North America.
2. Spirochetal development in most ticks is limited to the midgut. In a few ticks, the organisms penetrate the gut epithelium and invade the hemocoele and various tick tissues. Their presence in the ovary may lead to transovarian transmission. Nevertheless, there is evidence that spirochetal infection in filial ticks decreases and becomes restricted to the central ganglion as the ticks develop to nymphs and adults.
3. It is speculated that transmission of spirochetes during the feeding process may occur via saliva by ticks with generalized infection as well as by regurgitation of infected gut content. Contamination by spirochete-infected fecal material cannot be excluded.
4. Finally, in view of the unusual behavior (limited distribution, loss of infection intensity) of the spirochetes in ticks, the possibility that other hematophagous arthropods, such as biting flies, gnats, and mosquitoes, may play a role as mechanical vectors of ECM and Lyme disease cannot be ruled out."

Borrelia transfer by ticks during their life cycle. Studies on laboratory animals.
Stanek G et al. Zentralbl Bakteriol Mikrobiol Hyg A 1986 Dec; 263(1-2): 29-33. PMID: 

The results display that B. burgdorferi is vertically from the female [Ixodes ricinus] ticks to their eggs and transstadially transmitted. The transmission-rate from larval to nymphal ticks is 100%. These findings show the tick itself as a main reservoir of B. burgdorferi..

Ability of transovarially and subsequent transstadially infected Ixodes hexagonus ticks to maintain and transmit Borrelia burgdorferi in the laboratory.

Toutoungi LN et al. Exp Appl Acarol 1993 Aug; 17(8): 581-6
This study shows that B. burgdorferi can be maintained through transovarial and subsequent transstadial transmissions in I. hexagonus.

Infection rates of Borrelia burgdorferi in different instars of Ixodes ricinus ticks from the Dutch North Sea Island of Ameland.

Rijpkema S et al. Exp Appl Acarol 1994 Sep; 18(9): 531-42
. PMID: 

Infection rates of Borrelia varied, in nymphs (n = 347) from 13% to 46% and in adults, (n = 122) from 20% to 43%. The infection rate in larvae (n = 84) collected in 1993 was 21%, showing that transovarial transmission of B. burgdorferi occurs in the I. ricinus population on Ameland.

Infection of field-collected Ixodes ricinus (Acari: Ixodidae) larvae with Borrelia burgdorferi in Switzerland.

Zhioua E et al. J Med Entomol 1994 Sep; 31(5): 763-6
. PMID: 

Larvae were examined for the spirochetes using direct fluorescent antibody assay. Of 652 larvae examined, spirochetes were detected in 20 (3.1%).

Roy Brown - who collected and examined UK ticks for Borrelia burgdorferi

Hull 2001 - LDA conference lecture. HTML  PDF

Found 1 of 8 [12%] larvae was infected with Borrelia burgdorferi by PCR!