fighting infections ( particularly those that are bacterial in origin ) requires rapid protein breakdown to supply the high energy and raw material requirements ( e . g . amino acids ) for antibody production and other anti - infection response . certain isotopic ratios of proteins , fats , and carbohydrates stored in body tissues differ from the isotopic ratio of metabolized food , and when a catabolic state is induced by infection the body begins to consume stored tissue , which ( due to the different ratios in the stored tissue ) results in a change in the ratios in breath ( e . g . exhaled 13 co 2 content ). in particular the heavy isotope to light isotope ratios become more positive in the presence of infection , such that the “ delta ” representation thereof ( as defined below ) becomes more negative in response to infection . the change is believed to be a direct reflection of the increasing use of body tissue for “ fuel ”. absent extreme stress such as an infection , the lighter isotopes are normally preferentially oxidized for energy and appear in breath . the heavier isotopes then are left behind and stored in body tissues . we refer to this phenomenon more broadly as atom fractionation . during bacterial infection immune responses ( acute and involving the innate immune system ) to the invasion are marked by a sudden release of catabolic cytokines within the first hour of infection . these cytokines consist of tumor necrosis factor ( tnf ), and interleukins ( il ) 1 and 6 . these three cytokines result in the rapid redistribution of body nutrients , such as amino acids . for example , tnf and il - 1 will induce the degradation of skeletal muscle resulting in a release of amino acids that can be used to make immune and inflammatory acute phase protein or use as fuel . the step of fractionation of the carbon flow of molecules occurs as amino acids flow toward new proteins , they are burned instead as fuel . in contrast , the cytokines associated with early stages of viral infections cause the release of different cytokines , namely the interferons , which fail to elicit an acute phase response . hence carbon fractionation yields yet a different pattern , a pattern typically more linked to the reproductive stage of the virus . we have learned that there is a distinct change in isotope ratio amounts within about 2 hours after bacterial challenge begins , followed by relatively stable , albeit changed from the unchallenged animals , ratios for a prolonged period . in contrast , a virally challenged host exhibits a series of periodic modifications , beginning significantly later ( e . g . 2 - 3 days ). we expect a similar pattern for isotope ratios with respect to other atom pairs present in breath . importantly , we have also learned that in the case of bacterial infection , the breath becomes “ lighter ” at a greater rate of speed than during a viral infection . hence , the speed of change , as well as the presence or absence of the repeated spikes in the ratio pattern indicative of a viral life cycle , are indications of the type of infecting agent . in sum , bacterial immune stimuli create a very rapid and strong response by the immune system that involves a rapid protein ( typically muscle ) breakdown . in contrast , viral infections involve an invasion of the cells in the body by the viruses , a takeover of the cellular machinery to make new viral particles ( which can take considerable time ), and then ultimately a rupture of the infected cells to release replicated viruses for another invasion process . this cycle is repeated periodically , at generally regular intervals . for many viruses the cycle is about a 24 - hour interval after each rupture . for humans , we propose in a preferred case to obtain a base line reading for a patient at an annual physical or at another opportunity when the patient is otherwise at a medical facility and not complaining of symptoms of illness . alternatively , the first reading could be when a patient first complains of infection type symptoms ( e . g . particularly elevated temperature ). then repetitive readings can be taken , preferably over a several day period . with respect to a hospitalized patient , this might be done every hour for the first day , followed by twice a day thereafter . however , the frequency of monitoring will change depending on patient condition and early results . for confined non - human animals , or populations of such animals , regular hourly monitoring could be conducted on a continuous basis . alternatively , similar principles could be applied with respect to monitoring the health of humans in an apartment building on an overall basis . we have infected diverse types of animals with bacterial ( endotoxin ) and viral ( attenuated virus ) challenges , and monitored via cavity ringdown spectroscopy the changes in stable isotope ratios in their breath . the endotoxin is a glycolipid component of the cell wall from gram - negative bacteria , which induces a strong immune response . the virus was in the form of an attenuated viral vaccine . we expect a similar host response process for live whole bacteria and virus . for example , we have showed in our laboratory &# 39 ; s earlier experiments ( using mass spectrometer measurements ) that live bacteria and isolated bacterial cell walls ( lps ) each generate indistinguishably similar responses in mice with respect to isotope ratios . this isotope ratio is being reported herein as a “ delta ” value which is expressed in parts per mil ( parts per thousand ) in accordance with the following equation : measurements of isotopic ratios at concentrations ranging from 0 . 5 % to 3 % are typical in breath . fig1 illustrates the time course for delta values for a healthy mammal versus one that was given a bacterial cell wall component ( lps ) as an immune stimulus . the data illustrates that the carbon isotope ratios presented as delta values becomes more negative significantly below controls within about two hours of the time the lps immune stimulus entered the organism . we would expect similar results for other isotope pairs . at time zero mice ( n = 5 per group ) were injected i . p . with either bacterial cell walls ( lps at 1 mg / kg body weight ) or sterile saline ( vehicle ), and placed in a metabolic chamber . continuous airflow through the chamber was sampled approximately every 15 minutes by a cavity ring - down spectrometer ( crds ) and the isotopic ratio of 13 c / 12 c ( delta ) was measured . results were analyzed using sas proc mixed accounting for autocorrelation of repeated measures . stars represent statistically significant differences between treatments at a given time - point with p & lt ; 0 . 05 . fig2 shows that the bacterial cell wall immune stimulus response is similar between the example 1 mice experiments and the example 2 chickens . it confirms that a chicken given the same kind of bacterial cell wall injection also has a significant negative change in breath stable isotope delta values within two hours of the administration of the bacterial cell wall immune stimulus . the chickens serve as their own control with the normal pre - injection stable isotope breath variation between approximately − 7 and − 9 delta . fig3 shows that the isotope ratio response to a viral infection is much slower than to a bacterial infection , and it has different , and distinguishing , characteristics . it takes well over a day ( e . g . often about 80 hours ) for the first appreciable viral response in the isotope ratio to occur . this is contrasted with the much quicker isotope ratio response ( e . g . about 2 hours ) for bacteria . in this experiment food was initially withheld to make the chick hungry and thirsty . a viral agent was administered in drinking water . after the chick had consumed the water with the viral agent , food was restored and the experiment continued . approximately 80 hours into the experiment the first significant decline in delta values occurred . a partial recovery period then ensued . a sudden more negative result and recovery pattern was then periodically repeated . for human clinical use we would propose that a small cavity ringdown spectroscopy machine ( e . g . available from picarro or tiger optics ) provide measurements of a patient &# 39 ; s breath initially every 15 minutes or less after monitoring begins by sampling from a flow - through mask . where a baseline value is available from the patient &# 39 ; s records , the readings could be compared to that . if not , the pattern of the readings could be evaluated to look for a reduced level from average levels in the population , and / or the characteristics of a viral infection pattern developing over a several day period . the normal variation of breath delta values of carbon stable isotopes in carbon dioxide is approximately plus or minus one delta value for healthy mammals . the onset of bacterial infection , i . e . catabolic state , results in breath carbon isotope delta values quickly dropping beneath the normal range by up to three delta . this is a significant decline below the normal range of variation , and thus could help identify a bacterial infection even when the source of infection occurred a day or two before monitoring begins . for other isotope pairs a similar differential in delta values is expected . as an additional example we propose to establish an enclosed chicken coop with a confined population of growing chickens . the air inside the coop could be refreshed at a constant rate and periodically sampled from a consistent location , with changes in isotope ratios being monitored . again , one would look for the characteristic bacterial or viral indicia in the pattern of the ratios to distinguish bacterial from viral infection . however , here the population as a whole is monitored . as an additional example we propose to monitor the nitrogen ratios in human breath . as data is collected for various specific types of viral infections or bacterial infections , we expect to be able to tell from that data more information characteristic of specific viruses or bacteria . for example , some specific types of viral infections are expected to have particular characteristic amplitudes for the periodic pattern . moreover , the combination of the ratio information , with other marker or other information which becomes available to the medical practitioner will help further characterize the nature of the infection . further , the present invention should allow a practitioner to watch the progress of an infection as dynamic changes occur during the infectious response . this may include monitoring the effectiveness of drugs or other therapies . for example , as the body recovers from an infection it is expected that the ratios would return to normal levels . hence , the invention is not to be limited by the specific embodiments described herein . thus , the claims should be looked to in order to judge the full scope of the invention . the invention provides methods of monitoring animal health , with particular capability for non - invasively testing to distinguish bacterial from viral infections .
methods are disclosed for distinguishing whether an animal is experiencing a bacterial infection or a viral infection . one monitors breath taken from the animal over time to measure the relative amount of a first breath stable isotope to a second breath stable isotope therein over time . a quick change in the isotope ratios within several hours from the likely infection is indicative of a bacterial infection . a delayed change in the isotope ratios , followed by periodic repeated alterations in the ratios , is indicative of viral infection . the methods are particularly efficient when using cavity ringdown spectroscopy for the monitoring . they may be used for monitoring a patient already admitted to a hospital , or for monitoring a patient initially complaining of adverse symptoms , or for triage , or for collectively monitoring a population of animals .