Chronic Wasting Disease on the Rise in Wisconsin Deer; Will it Infect Humans?

Submitted by Rebekah Wilce on

CWD DeerThe rate of chronic wasting disease (CWD) is on the rise among deer in Iowa County, Wisconsin and elsewhere across the state. CWD is a fatal, transmissible spongiform encephalopathy (TSE) similar to what is commonly known as mad cow disease that is caused by twisted proteins, or prions. For hunters, writes outdoors reporter Patrick Durkin, this means the disease might be affecting the herd now. For anyone who eats venison, this means greater chances that the disease could conceivably make the species jump and infect humans

, according to Dave Clausen, a veterinarian whose term on Wisconsin's Natural Resources Board expired in May.

About one out of every three male deer aged 2.5 years and older carries CWD in north-central Iowa County, as does one out of every six yearling male deer (1.5 years old), according to the Wisconsin State Journal, and the rates are climbing at about ten percent a year. As several experts told Durkin, the increasing rates are "unprecedented," "frightening," and "disturbing."

Over 633,000 hunters purchased licenses to hunt white-tailed deer in Wisconsin in 2012, according to the Department of Natural Resources (DNR). The primary deer hunting season (for guns) runs for nine days in late November. An exact number of Wisconsinites who eat hunted venison is not known, although media reports indicate it is large. But testing of these deer for CWD is on the decline, even as infection rates rise. In 2002, over 40,000 deer were tested in Wisconsin, and .51 percent tested positive. In 2012, 6,611 deer were tested, and 5.13 percent tested positive.

As then-Natural Resources Board member Clausen wrote in a white paper on CWD and human health in 2012, the "ever-increasing number of CWD infected deer on the landscape ... and the accompanying exponential increase of environmental contamination with CWD prion will result in increased inter-species, including human, exposure to the CWD prion .... Under our current management strategy[,] human exposure is and will increase."

CWD in WI Deer

Less Testing, More People Eating Infected Venison

One of the reasons why it is possible for CWD to make the species jump to humans is because of insufficient warnings to hunters by the DNR, Clausen says. The DNR website says there is "no strong evidence" that CWD can be passed to humans, but warns hunters to "minimize contact with the brain, spinal cord, spleen and lymph nodes" when processing deer.

But in 2010, the World Health Organization (WHO) changed its definition of infective tissue to include skeletal muscles from CWD-infected deer and elk. And a 2012 Centers for Disease Control and Prevention (CDC) study notes, "CWD prions are present nearly ubiquitously throughout diseased hosts, including in muscle, fat, various glands and organs, antler velvet, and peripheral and CNS [central nervous system] tissue." It concludes that the potential for human exposure to CWD from handling or eating material from infected deer "is substantial and increases with increased disease prevalence." Both the WHO and the CDC recommend that people avoid eating meat from CWD-infected deer or elk.

Unlike the WHO and CDC, Clausen said the Wisconsin Department of Health and Safety (DHS) will not publicly recommend against eating infected venison until there is hard evidence that someone has gotten Creutzfeldt-Jakob Disease (CJD, the human TSE) from eating infected venison. But he believes that the government should be operating on the precautionary principle -- that "if something is plausible, that we should be erring on the side of caution unless we have absolute hard evidence that it's not possible."

But, Clausen adds, the "precautionary principle is bad for business." If people become so concerned about contracting CWD that they stop hunting, it means a potential decrease in DNR revenue; and the federal government has stopped funding CWD testing and research in the last year or two. UW-Madison Professor Michael Samuel has seen federal research funds for studying "disturbing" new trends in CWD dry up. "There's little interest in CWD these days, Wisconsin and nationwide," he told Durkin.

With testing on the decline, the DNR "has tracked hundreds of cases" in which people have eaten infected venison and "knows that there are many more," according to the Wisconsin Center for Investigative Journalism. As the rates of the disease rise -- as the CDC notes -- "the potential for human exposure to CWD by handling and consumption of infectious cervid material ... increases."

CMD founder John Stauber says, "Every dead deer in the state should be tested, and no deer should be butchered, processed or eaten unless it has been tested free of the disease. The current situation is contaminating processing plants with infectious prions."

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Anonymous replied on

The reason the number of test on deer have gone down is the test Is being done on sick deer and deer in infected areas,not just random testing. So therefor positive test will be higher. And cost for testing will have gone down.

Kenneth replied on

The fact that CWD was first discovered in two experimental facilities begs the question.Was CWD created by experiments on deer?Mad Cow Disease was caused by feeding animal byproducts to cattle in England.

Terry S. Singel... replied on

Tuesday, May 28, 2013 Chronic Wasting Disease CWD quarantine Louisiana via CWD index herd Pennsylvania Update May 28, 2013 6 doe from Pennsylvania CWD index herd still on the loose in Louisiana, quarantine began on October 18, 2012, still ongoing, Lake Charles premises. http://chronic-wasting-disease.blogspot.com/2013/05/chronic-wasting-disease-cwd-quarantine.html Wednesday, June 12, 2013 CWD now waltzing into Texas from Pennsylvania CWD index herd, via Louisiana, or Missouri now ? http://chronic-wasting-disease.blogspot.com/2013/06/cwd-now-waltzing-into-texas-from.html Sunday, June 09, 2013 Missouri House forms 13-member Interim Committee on the Cause and Spread of Chronic Wasting Disease CWD http://chronic-wasting-disease.blogspot.com/2013/06/missouri-house-forms-13-member-interim.html Tuesday, June 11, 2013 CWD GONE WILD, More cervid escapees from more shooting pens on the loose in Pennsylvania http://chronic-wasting-disease.blogspot.com/2013/06/cwd-gone-wild-more-cervid-escapees-from.html Thursday, June 13, 2013 Experimental interspecies transmission studies of the transmissible spongiform encephalopathies to cattle: comparison to bovine spongiform encephalopathy in cattle http://transmissiblespongiformencephalopathy.blogspot.com/2013/06/experimental-interspecies-transmission.html kind regards, terry

Anonymous replied on

Your comments do not reflect the reality of actually working with patients infected with prion diseases, nor, do they reflect the possible mutations or variations which may occur spontaneously and shift the transmission rate rather quickly in human or animal populations. Whether writing about early Kuru in Indonesia to current Deer Wasting in Wisconsin, the public should be made aware of all aspects of this terrible disease. Your information presents as if you are employed or wish to be employed by big agriculture. Do you live in Wisconsin? Does your family live in Wisconsin? Would you let your children eat deer that has been hunted and killed in the wilds of Wisconsin? Why? I can not refrain from telling readers that it is very difficult to have humans or animals tested for prion diseases because laboratories can not decontaminate testing items or surface areas. Only temperatures above 450F can kill prions. Most surgical autoclaves do not produce temperatures this high, not alone standard laboratories. For this reason England was using disposable surgical trays to remove certain soft tissues such as tonsils. Is this being done in the United States? Furthermore, if you can only test one deer out of possibly ten deers that you suspect of having a prion disease because of testing challenges, then the public is being exposed to further unnecessary dangers. It can take twenty years for a prion disease to come to fruition, meaning the manifestations of the first symptoms becoming visible to the sheer horror of family, friends and loved ones. I suggest reading "A Deadly Feast" because it is one of the oldest books written about prion diseases before distortion and hubris was added to the mix. For that matter, perform a video search for patients dying of a prion disease and watch in horror. The last time I saw a video showing this on television was over fifteen years ago and you have to ask yourself, why?

Terry S. Singel... replied on

*** The potential impact of prion diseases on human health was greatly magnified by the recognition that interspecies transfer of BSE to humans by beef ingestion resulted in vCJD. While changes in animal feed constituents and slaughter practices appear to have curtailed vCJD, there is concern that CWD of free-ranging deer and elk in the U.S. might also cross the species barrier. Thus, consuming venison could be a source of human prion disease. Whether BSE and CWD represent interspecies scrapie transfer or are newly arisen prion diseases is unknown. Therefore, the possibility of transmission of prion disease through other food animals cannot be ruled out. There is evidence that vCJD can be transmitted through blood transfusion. There is likely a pool of unknown size of asymptomatic individuals infected with vCJD, and there may be asymptomatic individuals infected with the CWD equivalent. These circumstances represent a potential threat to blood, blood products, and plasma supplies. http://cdmrp.army.mil/prevfunded/nprp/NPRP_Summit_Final_Report.pdf The chances of a person or domestic animal contracting CWD are “extremely remote,” Richards said. The possibility can’t be ruled out, however. “One could look at it like a game of chance,” he explained. “The odds (of infection) increase over time because of repeated exposure. That’s one of the downsides of having CWD in free-ranging herds: We’ve got this infectious agent out there that we can never say never to in terms of (infecting) people and domestic livestock.” https://www.avma.org/News/JAVMANews/Pages/121201a.aspx P35 ADAPTATION OF CHRONIC WASTING DISEASE (CWD) INTO HAMSTERS, EVIDENCE OF A WISCONSIN STRAIN OF CWD Chad Johnson1, Judd Aiken2,3,4 and Debbie McKenzie4,5 1 Department of Comparative Biosciences, University of Wisconsin, Madison WI, USA 53706 2 Department of Agriculture, Food and Nutritional Sciences, 3 Alberta Veterinary Research Institute, 4.Center for Prions and Protein Folding Diseases, 5 Department of Biological Sciences, University of Alberta, Edmonton AB, Canada T6G 2P5 The identification and characterization of prion strains is increasingly important for the diagnosis and biological definition of these infectious pathogens. Although well-established in scrapie and, more recently, in BSE, comparatively little is known about the possibility of prion strains in chronic wasting disease (CWD), a disease affecting free ranging and captive cervids, primarily in North America. We have identified prion protein variants in the white-tailed deer population and demonstrated that Prnp genotype affects the susceptibility/disease progression of white-tailed deer to CWD agent. The existence of cervid prion protein variants raises the likelihood of distinct CWD strains. Small rodent models are a useful means of identifying prion strains. We intracerebrally inoculated hamsters with brain homogenates and phosphotungstate concentrated preparations from CWD positive hunter-harvested (Wisconsin CWD endemic area) and experimentally infected deer of known Prnp genotypes. These transmission studies resulted in clinical presentation in primary passage of concentrated CWD prions. Subclinical infection was established with the other primary passages based on the detection of PrPCWD in the brains of hamsters and the successful disease transmission upon second passage. Second and third passage data, when compared to transmission studies using different CWD inocula (Raymond et al., 2007) indicate that the CWD agent present in the Wisconsin white-tailed deer population is different than the strain(s) present in elk, mule-deer and white-tailed deer from the western United States endemic region. http://www.istitutoveneto.it/prion_09/Abstracts_09.pdf PPo3-7: Prion Transmission from Cervids to Humans is Strain-dependent Qingzhong Kong, Shenghai Huang,*Fusong Chen, Michael Payne, Pierluigi Gambetti and Liuting Qing Department of Pathology; Case western Reserve University; Cleveland, OH USA *Current address: Nursing Informatics; Memorial Sloan-Kettering Cancer Center; New York, NY USA Key words: CWD, strain, human transmission Chronic wasting disease (CWD) is a widespread prion disease in cervids (deer and elk) in North America where significant human exposure to CWD is likely and zoonotic transmission of CWD is a concern. Current evidence indicates a strong barrier for transmission of the classical CWD strain to humans with the PrP-129MM genotype. A few recent reports suggest the presence of two or more CWD strains. What remain unknown is whether individuals with the PrP-129VV/MV genotypes are also resistant to the classical CWD strain and whether humans are resistant to all natural or adapted cervid prion strains. Here we report that a human prion strain that had adopted the cervid prion protein (PrP) sequence through passage in cervidized transgenic mice efficiently infected transgenic mice expressing human PrP, indicating that the species barrier from cervid to humans is prion strain-dependent and humans can be vulnerable to novel cervid prion strains. Preliminary results on CWD transmission in transgenic mice expressing human PrP-129V will also be discussed. Acknowledgement Supported by NINDS NS052319 and NIA AG14359. PPo2-27: Generation of a Novel form of Human PrPSc by Inter-species Transmission of Cervid Prions Marcelo A. Barria,1 Glenn C. Telling,2 Pierluigi Gambetti,3 James A. Mastrianni4 and Claudio Soto1 1Mitchell Center for Alzheimer's disease and related Brain disorders; Dept of Neurology; University of Texas Houston Medical School; Houston, TX USA; 2Dept of Microbiology, Immunology & Molecular Genetics and Neurology; Sanders Brown Center on Aging; University of Kentucky Medical Center; Lexington, KY USA; 3Institute of Pathology; Case western Reserve University; Cleveland, OH USA; 4Dept of Neurology; University of Chicago; Chicago, IL USA Prion diseases are infectious neurodegenerative disorders affecting humans and animals that result from the conversion of normal prion protein (PrPC) into the misfolded and infectious prion (PrPSc). Chronic wasting disease (CWD) of cervids is a prion disorder of increasing prevalence within the United States that affects a large population of wild and captive deer and elk. CWD is highly contagious and its origin, mechanism of transmission and exact prevalence are currently unclear. The risk of transmission of CWD to humans is unknown. Defining that risk is of utmost importance, considering that people have been infected by animal prions, resulting in new fatal diseases. To study the possibility that human PrPC can be converted into the infectious form by CWD PrPSc we performed experiments using the Protein Misfolding Cyclic Amplification (PMCA) technique, which mimic in vitro the process of prion replication. Our results show that cervid PrPSc can induce the pathological conversion of human PrPC, but only after the CWD prion strain has been stabilized by successive passages in vitro or in vivo. Interestingly, this newly generated human PrPSc exhibits a distinct biochemical pattern that differs from any of the currently known forms of human PrPSc, indicating that it corresponds to a novel human prion strain. Our findings suggest that CWD prions have the capability to infect humans, and that this ability depends on CWD strain adaptation, implying that the risk for human health progressively increases with the spread of CWD among cervids. PPo2-7: Biochemical and Biophysical Characterization of Different CWD Isolates Martin L. Daus and Michael Beekes Robert Koch Institute; Berlin, Germany Key words: CWD, strains, FT-IR, AFM Chronic wasting disease (CWD) is one of three naturally occurring forms of prion disease. The other two are Creutzfeldt-Jakob disease in humans and scrapie in sheep. CWD is contagious and affects captive as well as free ranging cervids. As long as there is no definite answer of whether CWD can breach the species barrier to humans precautionary measures especially for the protection of consumers need to be considered. In principle, different strains of CWD may be associated with different risks of transmission to humans. Sophisticated strain differentiation as accomplished for other prion diseases has not yet been established for CWD. However, several different findings indicate that there exists more than one strain of CWD agent in cervids. We have analysed a set of CWD isolates from white-tailed deer and could detect at least two biochemically different forms of disease-associated prion protein PrPTSE. Limited proteolysis with different concentrations of proteinase K and/or after exposure of PrPTSE to different pH-values or concentrations of Guanidinium hydrochloride resulted in distinct isolate-specific digestion patterns. Our CWD isolates were also examined in protein misfolding cyclic amplification studies. This showed different conversion activities for those isolates that had displayed significantly different sensitivities to limited proteolysis by PK in the biochemical experiments described above. We further applied Fourier transform infrared spectroscopy in combination with atomic force microscopy. This confirmed structural differences in the PrPTSE of at least two disinct CWD isolates. The data presented here substantiate and expand previous reports on the existence of different CWD strains. http://www.prion2010.org/bilder/prion_2010_program_latest_w_posters_4_.pdf?139&PHPSESSID=a30a38202cfec579000b77af81be3099 2012 Envt.06: Zoonotic Potential of CWD: Experimental Transmissions to Non-Human Primates Emmanuel Comoy,1,† Valérie Durand,1 Evelyne Correia,1 Aru Balachandran,2 Jürgen Richt,3 Vincent Beringue,4 Juan-Maria Torres,5 Paul Brown,1 Bob Hills6 and Jean-Philippe Deslys1 1Atomic Energy Commission; Fontenay-aux-Roses, France; 2Canadian Food Inspection Agency; Ottawa, ON Canada; 3Kansas State University; Manhattan, KS USA; 4INRA; Jouy-en-Josas, France; 5INIA; Madrid, Spain; 6Health Canada; Ottawa, ON Canada †Presenting author; Email: emmanuel.comoy@cea.fr The constant increase of chronic wasting disease (CWD) incidence in North America raises a question about their zoonotic potential. A recent publication showed their transmissibility to new-world monkeys, but no transmission to old-world monkeys, which are phylogenetically closer to humans, has so far been reported. Moreover, several studies have failed to transmit CWD to transgenic mice overexpressing human PrP. Bovine spongiform encephalopathy (BSE) is the only animal prion disease for which a zoonotic potential has been proven. We described the transmission of the atypical BSE-L strain of BSE to cynomolgus monkeys, suggesting a weak cattle-to-primate species barrier. We observed the same phenomenon with a cattleadapted strain of TME (Transmissible Mink Encephalopathy). Since cattle experimentally exposed to CWD strains have also developed spongiform encephalopathies, we inoculated brain tissue from CWD-infected cattle to three cynomolgus macaques as well as to transgenic mice overexpressing bovine or human PrP. Since CWD prion strains are highly lymphotropic, suggesting an adaptation of these agents after peripheral exposure, a parallel set of four monkeys was inoculated with CWD-infected cervid brains using the oral route. Nearly four years post-exposure, monkeys exposed to CWD-related prion strains remain asymptomatic. In contrast, bovinized and humanized transgenic mice showed signs of infection, suggesting that CWD-related prion strains may be capable of crossing the cattle-to-primate species barrier. Comparisons with transmission results and incubation periods obtained after exposure to other cattle prion strains (c-BSE, BSE-L, BSE-H and cattle-adapted TME) will also be presented, in order to evaluate the respective risks of each strain. Envt.07: Pathological Prion Protein (PrPTSE) in Skeletal Muscles of Farmed and Free Ranging White-Tailed Deer Infected with Chronic Wasting Disease Martin L. Daus,1,† Johanna Breyer,2 Katjs Wagenfuehr,1 Wiebke Wemheuer,2 Achim Thomzig,1 Walter Schulz-Schaeffer2 and Michael Beekes1 1Robert Koch Institut; P24 TSE; Berlin, Germany; 2Department of Neuropathology, Prion and Dementia Research Unit, University Medical Center Göttingen; Göttingen, Germany †Presenting author; Email: dausm@rki.de Chronic wasting disease (CWD) is a contagious, rapidly spreading transmissible spongiform encephalopathy (TSE) occurring in cervids in North America. Despite efficient horizontal transmission of CWD among cervids natural transmission of the disease to other species has not yet been observed. Here, we report a direct biochemical demonstration of pathological prion protein PrPTSE and of PrPTSE-associated seeding activity in skeletal muscles of CWD-infected cervids. The presence of PrPTSE was detected by Western- and postfixed frozen tissue blotting, while the seeding activity of PrPTSE was revealed by protein misfolding cyclic amplification (PMCA). The concentration of PrPTSE in skeletal muscles of CWD-infected WTD was estimated to be approximately 2000- to 10000-fold lower than in brain tissue. Tissue-blot-analyses revealed that PrPTSE was located in muscle- associated nerve fascicles but not, in detectable amounts, in myocytes. The presence and seeding activity of PrPTSE in skeletal muscle from CWD-infected cervids suggests prevention of such tissue in the human diet as a precautionary measure for food safety, pending on further clarification of whether CWD may be transmissible to humans. http://www.landesbioscience.com/journals/prion/Prion5-Supp-PrionEnvironment.pdf?nocache=1333529975 Friday, November 09, 2012 *** Chronic Wasting Disease CWD in cervidae and transmission to other species http://chronic-wasting-disease.blogspot.com/2012/11/chronic-wasting-disease-cwd-in-cervidae.html Sunday, November 11, 2012 *** Susceptibilities of Nonhuman Primates to Chronic Wasting Disease November 2012 http://chronic-wasting-disease.blogspot.com/2012/11/susceptibilities-of-nonhuman-primates.html Friday, December 14, 2012 *** Susceptibility Chronic Wasting Disease (CWD) in wild cervids to Humans 2005 - December 14, 2012 http://chronic-wasting-disease.blogspot.com/2012/12/susceptibility-chronic-wasting-disease.html Saturday, March 09, 2013 Chronic Wasting Disease in Bank Voles: Characterisation of the Shortest Incubation Time Model for Prion Diseases http://chronic-wasting-disease.blogspot.com/2013/03/chronic-wasting-disease-in-bank-voles.html Thursday, June 13, 2013 Experimental interspecies transmission studies of the transmissible spongiform encephalopathies to cattle: comparison to bovine spongiform encephalopathy in cattle http://transmissiblespongiformencephalopathy.blogspot.com/2013/06/experimental-interspecies-transmission.html CJD9/10022 October 1994 Mr R.N. Elmhirst Chairman British Deer Farmers Association Holly Lodge Spencers Lane BerksWell Coventry CV7 7BZ Dear Mr Elmhirst, CREUTZFELDT-JAKOB DISEASE (CJD) SURVEILLANCE UNIT REPORT Thank you for your recent letter concerning the publication of the third annual report from the CJD Surveillance Unit. I am sorry that you are dissatisfied with the way in which this report was published. The Surveillance Unit is a completely independant outside body and the Department of Health is committed to publishing their reports as soon as they become available. In the circumstances it is not the practice to circulate the report for comment since the findings of the report would not be amended. In future we can ensure that the British Deer Farmers Association receives a copy of the report in advance of publication. The Chief Medical Officer has undertaken to keep the public fully informed of the results of any research in respect of CJD. This report was entirely the work of the unit and was produced completely independantly of the the Department. The statistical results reqarding the consumption of venison was put into perspective in the body of the report and was not mentioned at all in the press release. Media attention regarding this report was low key but gave a realistic presentation of the statistical findings of the Unit. This approach to publication was successful in that consumption of venison was highlighted only once by the media ie. in the News at one television proqramme. I believe that a further statement about the report, or indeed statistical links between CJD and consumption of venison, would increase, and quite possibly give damaging credence, to the whole issue. From the low key media reports of which I am aware it seems unlikely that venison consumption will suffer adversely, if at all. http://web.archive.org/web/20030511010117/http://www.bseinquiry.gov.uk/files/yb/1994/10/00003001.pdf now, let’s see what the authors said about this casual link, personal communications years ago. see where it is stated NO STRONG evidence. so, does this mean there IS casual evidence ???? “Our conclusion stating that we found no strong evidence of CWD transmission to humans” From: TSS (216-119-163-189.ipset45.wt.net) Subject: CWD aka MAD DEER/ELK TO HUMANS ??? Date: September 30, 2002 at 7:06 am PST From: "Belay, Ermias" To: Cc: "Race, Richard (NIH)" ; ; "Belay, Ermias" Sent: Monday, September 30, 2002 9:22 AM Subject: RE: TO CDC AND NIH - PUB MED- 3 MORE DEATHS - CWD - YOUNG HUNTERS Dear Sir/Madam, In the Archives of Neurology you quoted (the abstract of which was attached to your email), we did not say CWD in humans will present like variant CJD. That assumption would be wrong. I encourage you to read the whole article and call me if you have questions or need more clarification (phone: 404-639-3091). Also, we do not claim that "no-one has ever been infected with prion disease from eating venison." Our conclusion stating that we found no strong evidence of CWD transmission to humans in the article you quoted or in any other forum is limited to the patients we investigated. Ermias Belay, M.D. Centers for Disease Control and Prevention -----Original Message----- From: Sent: Sunday, September 29, 2002 10:15 AM To: rr26k@nih.gov; rrace@niaid.nih.gov; ebb8@CDC.GOV Subject: TO CDC AND NIH - PUB MED- 3 MORE DEATHS - CWD - YOUNG HUNTERS Sunday, November 10, 2002 6:26 PM ......snip........end..............TSS Thursday, April 03, 2008 A prion disease of cervids: Chronic wasting disease 2008 1: Vet Res. 2008 Apr 3;39(4):41 A prion disease of cervids: Chronic wasting disease Sigurdson CJ. snip... *** twenty-seven CJD patients who regularly consumed venison were reported to the Surveillance Center***, snip... full text ; http://chronic-wasting-disease.blogspot.com/2008/04/prion-disease-of-cervids-chronic.html http://chronic-wasting-disease.blogspot.com/2006_12_01_archive.html TSS

Anonymous replied on

oh reeeeally? just foreigners? so, by foreigners, do you mean people who LOOK foreign, or people who were born somewhere else? so does this mean that if you were born in america, no matter your color or language, you don't mix deer meat in with your meat? i wonder how you get this idea?

Prion Pocalypse replied on

Its coming. Decades from now people will wonder what went wrong and why nothing was done to stop it.Right now cwd causes disease in squirrel monkeys and othermsmmals like cattle when the cwd prions r injected into the brains. Its coming. Now is the time to enjoy deer hunting before it becomes to risky of a past time. The prions r in the soil and water and vegetation. Millions r at risk and nothing being done because the science can't catch up to the rate of spread. Very difficult to study and many questions unanswered.