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Letter
Co-feeding Transmission and
Its Contribution to the Perpetuation of the Lyme Disease Spirochete Borrelia
afzelii (In Reply to Randolph and Gern)
Read original letter,
http://www.cdc.gov/ncidod/EID/vol8no12/01-0519.htm
Read Randolph's
letter, http://www.cdc.gov/ncidod/EID/vol9no7/03-0116.htm
Dania Richter,* Rainer Allgöwer,* and Franz-Rainer Matuschka*
*Humboldt-Universität zu Berlin, Berlin, Germany
Suggested citation for this article: Richter
D, Allgöwer R, Matuschka F-R. Co-feeding transmission and its contribution
to the perpetuation of the Lyme disease spirochete Borrelia afzelii
(in reply to Randolph and Gern). Emerg Infect Dis [serial online] 2003
Jul [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no7/03-0342.htm
Although transmission between co-feeding vector ticks may perpetuate
particular tick-borne viruses, this mode of transmission plays no role
in the epizootiology of Lyme disease spirochetes (1,2).
In their letter, Randolph and Gern defend their suggestion that tick-borne
pathogens perpetuate effectively by direct passage from one feeding tick
to another by criticizing our analysis (3). These researchers
mainly address our comparison of the transmission efficiency between simultaneously
feeding ticks with that between ticks feeding sequentially on a persistently
infected rodent. Our experiments demonstrate that approximately six times
as many larvae (85.4%) acquire B orrelia afzelii spirochetes from
a systemically infected mouse than from a mouse on which an infected nymph
is feeding simultaneously (13.6%) (1). In nature, however,
larval ticks rarely co-feed with nymphs on mice or voles; only approximately
one fifth (18.8%) of these hosts harbor both subadult stages simultaneously.
And of the nymphs, only approximately one quarter (26.4%) are infected
by Lyme disease spirochetes. As a result, the natural transmission efficiency
between simultaneously feeding ticks would be only one twentieth (5%)
of that observed in the laboratory. Multiplying the experimentally observed
efficiency of co-feeding transmission (13.6%) by the likelihood of larval
and nymphal ticks co-infesting small rodents, as well as by the prevalence
of infected nymphal ticks, reduces the efficiency of co-feeding transmission
in nature to <1%. Although Randolph and Gern commit several minor mathematical
errors, their calculations support our argument that few larval vector
ticks would acquire spirochetal infection directly from an infected nymph
(3).
Randolph and Gern err, however, by applying the same mathematical modifications
to the transmission efficiency by which larvae acquire spirochetes from
a persistently infected host (3). Whereas the efficiency
of co-feeding transmission observed in the laboratory must be modified
to pay tribute to the rare event of larvae co-feeding with an infected
nymph in nature, the efficiency by which larvae acquire infection from
a persistently infected host is independent of such limiting parameters.
Because a competent rodent host remains infectious to larval ticks throughout
its life, the proportion of hosts infested by particular subadult stages
of the vector is irrelevant. Thus, the transmission efficiency on a persistently
infected host is unchanged in the laboratory and the field. Almost 85.4%
of larvae feeding on mice or voles in nature would, therefore, acquire
spirochetal infection—far more than by co-feeding. We are correct in stating
that natural transmission by sequentially feeding ticks is more efficient
than transmission between co-feeding ticks.
Randolph and Gern suggest that we could have recorded the distance between
the feeding ticks to clarify whether the increase from a 13.6% transmission
efficiency between co-feeding ticks to a transmission efficiency of 85.4%
from a persistently infected host is associated with the development of
a systemic infection. Our experimental observation (Table
1 in our article [1]), as well as a study on the
movement of spirochetes through their host’s skin (4),
conclusively demonstrates that the increase in transmission efficiency
is due to the progressive dissemination of spirochetes from the site of
inoculation. The likelihood of a larva’s acquiring spirochetes from any
site of its host’s skin increases with the passage of time since the infected
nymph attached. To compare the two modes of transmission in terms of efficiency
(Table 2 in our article [1]),
we permitted the larvae to attach randomly to their rodent hosts, just
as they would do in nature.
In the epizootiology of Lyme disease spirochetes, “simultaneous” transmission
between co-feeding ticks (<1%) is some two orders of magnitude less
efficient than sequential transmission between ticks feeding on persistently
infected reservoir rodents (85.4%). The two studies that relied on natural
infestation densities and refrained from using artificial feeding capsules
conclusively demonstrated that transmission of Lyme disease spirochetes
between ticks feeding simultaneously and in close proximity contributes
little to the perpetuation of this pathogen, either in North America or
in Europe (1,2). We are correct in concluding that Lyme
disease spirochetes are maintained in nature mainly by sequential attachment
of ticks to persistently infected reservoir hosts.
References
- Richter D, Allgöwer R, Matuschka F-R. Co-feeding
transmission and its contribution to the perpetuation of the Lyme disease
spirochete Borrelia afzelii. Emerg Infect Dis 2002;8:1421–5.
- Piesman J, Happ CM. The
efficacy of co-feeding as a means of maintaining Borrelia burgdorferi:
a North American model system. J Vector Ecol 2001;26:216–20.
- Randolph S, Gern L. Reply to Richter et al: Co-feeding transmission
and its contribution to the perpetuation of the Lyme disease spirochete
Borrelia afzelii. Emerg Infect Dis 2003;9:893–4.
- Shih CM, Telford SR, Pollack RJ, Spielman A. Rapid
dissemination by the agent of Lyme disease in hosts that permit fuminating
infection. Infect Immun 1993;61:2396–9.
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