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« Jim Dean in San Francisco | Main | The Limits of Reason »

20 May 2005

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Gil.

Thank you.

You know how much I hate the tick disease that killed my dog Thunder. And how crazy it makes me when people say things like "healthy dogs won't get it". We all know that when we're, stressed, overworked, physically or emotionally exhausted, how susceptible we are to the common cold, for an example. For a moment in time, the doors are open and sickness marches in. You don't have to be a physical wreck for it to happen, just "not up to par".

It always amazes me how ready people are to believe the zealots for a certain path, any path, who claim to have all the answers if they are loud enough, certain enough, and seem to think that a flat statement from them constitutes proof. I don't really feel sorry for the people who're taken in; they have minds and they have a choice; I feel sorry for the dogs who have no choice in what's done to them.


Runolfr

You could easily argue that the tick would prefer a healthy animal. Ticks are parasites, and they will benefit from a strong host that will be able to sustain them for a long time. If the host dies quickly from a disease born by the tick, the tick will soon be in need of a new host.

I have no data on this, of course. I'm just pointing out that a sickly host isn't necessarily a good host, at least for a tick.

Gil.

"If the host dies quickly from a disease born by the tick, the tick will soon be in need of a new host."

Well, no. Ticks feed only three times in their lives at three stages of growth. Once a tick drinks its fill, it doesn't need the host anymore. So, it falls off and the host is barely in the first stages of infection. Dogs usually don't die quickly from TBD. Rocky Mountain Spotted Fever is the only one that can kill a dog really fast and it isn't anywhere near as prevalent as ehrlichiosis, babesiosis or Lyme (which normally only cripples).

As far as preferring sickly hosts to healthy ones, who knows? It's possible that a dog that isn't well, for whatever reason, gives off more carbon dioxide, an attractant. Or that he has a different, more enticing smell. We just know too damned little.

Christie

Someone sent me this, so I thought I'd post it. Not sure why they didn't want to post it themselves, but... it's relevant so here it goes:

Tick Biology and Tick-Borne Diseases for the Small Animal Practitioner
Tufts Animal Expo 2002
Steven A. Levy, VMD
Durham Veterinary Hospital, PC
Durham, CT, USA

"Environmental conditions and tick survival

Tick habitats are stratified vertically in three distinct layers:

Microclimate: at the soil vegetative interface

Mesoclimate: within the vegetative layer

Macroclimate: above the vegetative layer.

As ticks move upward through these layers they are exposed to increasing temperatures and decreasing humidity, both of which are serious stressors and potential killers. Tick survival is dependent upon the microclimate where vegetation is a buffer against desiccation and freezing. Suitable hosts are often found in the mesoclimate and ticks quest there but must return to the microclimate to rehydrate. While short trips back to the microclimate provide rehydration prior to continued questing, diapause is used to avoid harsh environments such as those brought by seasonal changes. Vertical movement is common among tick species but horizontal movement is variable with some species questing in just a few feet of surface area and others, such as some Amblyomma, traveling horizontally up to 30 feet to acquire a host.

Host seeking strategies

Ticks do not jump, fly, or rappel on silk threads from overhanging vegetation. Ticks acquire their hosts through direct contact, usually brought about by the host brushing against vegetation harboring ticks, by the host returning to a tick infested nest or burrow, or by the tick crawling a limited distance to the host. Non-nidicolous ticks must place themselves in areas which hosts will frequent to increase the chances of host acquisition. The deer tick, Ixodes scapularis, will congregate along deer trails in response to kariamones (attractant pheromones) left on vegetation by passing deer. Waiting ticks use the ambush strategy which is passive and depends upon the host coming to the tick. Host stimuli such a vibrations, odors, body heat and variations in light patterns evoke extension and movement of the tick's forelegs in preparation for clinging to the passing host. An active hunter strategy is used by other tick species which crawl or run to their hosts. Again, host stimuli elicit tick activity in preparation for host acquisition. In some cases ticks will use a mixed strategy. The height at which a tick quests in the mesoclimate will affect the hosts which a tick will encounter. Host odors are the most important stimulus for questing ticks. Carbon dioxide and ammonia attract hungry ticks to hosts and short range stimuli like radiant heat and sweat attractants (lactic and butyric acids) act secondarily at closer range. Visual cues are especially important in hunter ticks which discriminate between dark shapes and lighter background. Sound and tactile stimuli may attract some species.

Feeding and drop off rhythms

Drop off is synchronized with host behavior to disperse (or retain) fed ticks in optimal habitats. This synchronization is the reason that ticks such as Dermacentor variabilis and Ixodes scapularis rarely drop off their hosts inside houses: the environment is not suited to survivability, molting, oviposition or egg hatching and acquisition of the next host. Larvae of 3- host ticks which feed on nocturnal rodents drop off while hosts are foraging outside nests and emerging nymphs are in a habitat which will expose them to new hosts. Nidicolous ticks feed and drop off when the host is in the nest and thus avoid dispersal in the environment which would be disastrous for a tick which is dependent upon a nidicole to provide environmental protection and exposure to hosts.

Host specificity

Host specificity is defined as the actual rejection of an available animal as a food source. Intrinsic (tick) factors such as selective host recognition and the tick's use of specific mechanisms to depress the host's responses aimed against the tick are the basis for true physiologic host specificity. Host selection requires recognition of specific host characteristics, particularly odors. Highly host specific (e.g., monospecific) tick species are usually nidicolous and attack animals which return to a nest repeatedly with-in a season and seasonally year after year.

Host immunity to ticks

Acquired immunity produces localized reactions in sensitized hosts. This reaction results in fewer ticks feeding and developing as-well-as decreased molting or fecundity of ticks which do feed. Immediate hypersensitivity is mediated by IgE. Humoral immune responses are present but host resistance to ticks does not increase with increasing antibody titer. Cell mediated immunity is the most important host response to tick attack. CMI is cross protective among tick species and delayed in onset (approximately 24 hours). In immune hosts the tick feeding cite is edematous, host hemoglobin is diluted, inflammation leads to host grooming behavior, and tick attachment success and engorgement may be decreased. Ticks which feed on resistant hosts suffer damage to midgut stem cells and decreased digestive capabilities. The tick's ability to evade these effects is the basis for host specificity. Saliva is the main means by which a tick may mediate a host immune response through salivary pharmacologic capabilities.

Vector competence

Vector competence is determined by many factors, some of which are intrinsic (tick based) and some which are extrinsic (host, pathogen, and environment based). Intrinsic factors include host seeking behavior which results in contact with an infected reservoir, duration of tick attachment sufficient for infection, and transstadial or transovarial passage of pathogens so that the next host is exposed. Extrinsic factors include the exposure of the host to the vector tick, susceptibility of the host to the pathogen carried by the tick, host ability to develop effective resistance to the vector tick, pathogen virulence sufficient to infect the host, and potential interference phenomenon from co-infecting microbes. Tick vector competence and vertebrate reservoir competence are mutually dependent."

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