Am J Trop Med Hyg. 2004 Sep;71(3):268-71.

Short report: duration of tick attachment required for transmission of
powassan virus by deer ticks.

Ebel GD, Kramer LD.

Arbovirus Laboratories, Wadsworth Center, New York State Department of

Infected deer ticks (Ixodes scapularis) were allowed to attach to naive
mice for variable lengths of time to determine the duration of tick
attachment required for Powassan (POW) virus transmission to occur.
Viral load in engorged larvae detaching from viremic mice and in
resulting nymphs was also monitored. Ninety percent of larval ticks
acquired POW virus from mice that had been intraperitoneally inoculated
with 10(5) plaque-forming units (PFU). Engorged larvae contained
approximately 10 PFU. Transstadial transmission efficiency was 22%,
resulting in approximately 20% infection in nymphs that had fed as
larvae on viremic mice. Titer increased approximately 100-fold during
molting. Nymphal deer ticks efficiently transmitted POW virus to naive
mice after as few as 15 minutes of attachment, suggesting that unlike
Borrelia burgdorferi, Babesia microti, and Anaplasma phagocytophilum,
no grace period exists between tick attachment and POW virus
transmission.

PMID: 15381804 [PubMed - indexed for MEDLINE]

Are you ready for nymphal tick season?
I wrote up a short review on transmission risk for LD that I posted on
calda.intranets.com and also made into a poster. Sorry the footnotes
don't format properly.
Here is the text:

Disease Transmission Via Tick Bite

Nymphal ticks are implicated in most human cases of Lyme disease. 1
Promptly and properly removing embedded ticks are key to preventing
transmission of disease agents. In one study, experimentally infected
Ixodes pacificus nymphs do not EFFICIENTLY [emphasis added] transmit Bb
to mice until ticks have been attached for 3-4 days, HOWEVER 11% of
mice became infected after only 2 days of tick attachment (<=48hrs.) 2
Matuschka and Spielman reported that about 5% of infected I dammini
(i.e. scapularis) nymphs transmit Bb in the first day (24hr), about 50%
after 2 days. 3  J. Piesman et al.  reported that Ixodes ticks
transmitted Lyme disease to 1 of 14 rodents exposed for 24 hours, 5 of
14 rodents exposed for 48 hours, and 13 of 14 rodents exposed for 72 or
more hours.  The authors concluded that prompt removal of ticks is
important. 4  Patmas and Remora reported on a case of Lyme disease that
was transmitted after only 6 hours of attachment by a deer tick.  The
authors concluded that, "The current recommendation against treatment
of short-duration tick bites may need reconsideration." 5
It is impossible to remove a tick promptly if one does not even know it
is there. According to some experts, only 14-32 % of patients who have
Lyme disease recall a tick bite.   Durland Fish, associate professor in
the department of epidemiology and public health at Yale University
wrote this letter to the New York Times (6-14-01):
You report on a new study showing that Lyme disease is very difficult
to catch, even from a deer tick in a Lyme-infested area (front page,
June 13). But the 3 percent chance of getting Lyme disease from a tick
bite cited in the New England Journal of Medicine article that I co-
authored pertains only to people who have found and removed a tick,
which greatly decreases the chance of infection. Most people never
notice the tick that gave them Lyme disease [emphasis added]. About 25
to 30 percent of nymph-stage deer ticks in the Northeast are naturally
infected with the Lyme disease bacterium. Nearly all of these infected
ticks will cause Lyme disease if they are not removed [emphasis added].
Ticks in this stage are about the size of a poppy seed, making them
very difficult to find. People who live in Lyme disease areas should
take the risk of tick bites seriously and do all they can to prevent
them, keeping in mind the 25 percent chance of infection if they miss a
tick, instead of the 3 percent chance if they find one.6
UC Berkeley entomologist Robert Lane reports that one of his team of
researchers was fed upon by infected nymphal ticks for at least 1-2
days after exposure to tick-infested habitat, despite "extreme
personal preventive measures." He advises people who work or recreate
in endemic areas to check themselves frequently and carefully for
several days following exposure, since unfed nymphs are very difficult
to spot and may be easier to detect when partially replete. 7
__________________
  Clover JR, Lane RS. Evidence implicating nymphal Ixodes pacificus
(Acari: Ixodidae) in the Epidemiology of Lyme Disease in California. Am
J Trop Med Hyg. 53(3):237-240, 1995.
  Peavey CR, Lane RS. Transmission of Borrelia burgdorferi by Ixodes
pacificus nymphs and reservoir competence of deer mice (Peromyscus
maniculatus) infected by tick-bite. J Parisitol 1995;81:175-178
   Matuschka FR, Spielman A. Risk if infection from and treatment of
tick bites. Lancet 1993;342;8870:529-30
   Piesman, J, et al. Duration of Tick Attachment and Borrelia
Burgdorferi Transmission. J Clin Microbiol. 1987 Mar;25(3):557-8.
   Patmas, MA, Remora, C. Disseminated Lyme Disease After
Short-Duration Tick Bite. JSTD 1994; 1:77-78
   Nadelman RB, Wormser GP. Recognition and treatment of erythema
migrans: are we off target?
Ann Intern Med. 2002 Mar 19;136(6):477-9.
  Lane, RS, et al. Human Behaviors Elevating Exposure to Ixodes
pacificus (Acari: Ixodidae) Nymphs and Their Associated Bacterial
Zoonotic Agents in a Hardwood Forest. J.Med.Entomol.41(2):239D248
(2004)