Microbiology, Lec 26.
an example of how epidemiology is done (compiled by Ruth Ley, grad student in the Schmidt lab)
On January 28, 1981, Frank Holtzhauer, Epidemiologist, Ohio State Department of Health, called David N. Taylor, M.D., E.I.S. Officer, Enteric Diseases Branch, Bacterial Diseases Division, Center for Infectious Diseases, Centers for Disease Control, to discuss an outbreak of Salmonellosis occurring in Jefferson County, Ohio.
Beginning in mid-December 1980, and continuing over a period of eight weeks, a total of 36 cases of Salmonella muenchen (group C2) enteritis had been reported to the Ohio Department of Health from the two acute-care hospitals in Steubenville, Ohio, additional cases had been reported at the acute-care hospital in Weirton, West Virginia, just across the Ohio river from Steubenville. After consultation with John Brough, Director of Preventive Health Services, West Virginia State Department of Health, it was agreed that an investigation of this outbreak should be undertaken. On January 29, Dr. Taylor and Emmet V. Schmidt, Ph.D., epidemiology-elective student, left for Steubenville to assist in the investigation.
Salmonellosis is the most frequent single cause of food infections in the USA. It accounts for at least 25% of the outbreaks of nausea and vomiting, abdominal cramps and diarrhea every year. The usual cause of human salmonellosis is contaminated (and insufficiently cooked) beef, lamb, pork, chicken and particularly eggs. There are hundreds of serotypes of Salmonella,and their names often indicate where they were first isolated (e.g. S. moscow, S. panama, S. senegal). Some are still found in their place of discovery, most have become cosmopolitan. S. muenchenis one of the most reported serotypes in the U.S. - so in this outbreak, its name wasn't much of a clue.
When they arrived in Steubenville, Taylor and Schmidt (no relation to Dr. S.K. Schmidt) found it to be a very run-down old mill town. They settled into the Holiday Inn and went round to meet the Steubenville Health Commissioner, two state epidemiologists, and a microbiologist who they'd be working with. It was the microbiologist who'd determined the cause of the outbreak. The first thing they learned was that the outbreak was still going on (the total was 36 cases in the end). They were briefed about age, sex, date of onset of the cases. 80% of the children infected were sick enough to be hospitalized. The range of the cases was enormous, from a one-month old boy to a seventy three year old woman. The cases were scattered all over town, and only a few of them knew each other. In such outbreaks, there is always a common denominator: an eating place, a dinner party, a church social. But this outbreak was a total mystery.
They formed two teams, and started visiting households of the patients. Taylor was particularly interested in the first two cases reported, the 5 month old girl, and the 25 year old man. This was because the baby's father, Frank, was friends with the second patient, Gene. Case No. 3 was 27 year old Paul, friend of Frank and Gene. One was unemployed, the other two worked on a municipal garbage truck. The thinking was: young guys, what do they do together? Taylor interviewed the baby's mother. Her house was spotless, she seemed embarrassed that her baby had become sick. Taylor asked about formulas, Christmas or holiday foods, about restaurant meals. All dead ends. The family couldn't afford to eat out much, for Christmas Frank had shot a deer and dressed it himself. They rarely went out. Sometimes they played cards with Gene and Paul, sometimes they just hung out. So Taylor went to see Gene and his family, and Paul. Same story, except that they'd had ham for Christmas. Everybody around town seemed to eat ham around Christmas-time. The two teams went on from house to house, and a couple of things started to take shape. The cases seemed to fall into two groups, blue-collar cases down in the valley, and middle class cases up on the hill. This didn't help.
The second thing was the age breakdown. The largest single age group was between 20 and 29. This group accounted for 28% of cases. The national average is 12% for this group. Why the prevalence of young adults? Was there a preponderance of young adults in Steubenville? To find out, they selected at random 32 houses in the neighborhood, and called them up. This survey revealed that it was not usual in Steubenville to find that many young adults in every family. So the next question was, what do young adults do? They went back to visiting houses and asking questions, nothing came of it, these people still didn't have anything in common.
They were stuck, and in the meantime, S.muenchen was occurring in Lansing, Michigan, with the same onset dates, and the same lack of a pattern. By this time, the local health commissioner was getting antsy, because he had all the eating places on his back. Taylor and Schmidt were getting tired and homesick. They tried the only thing they could think of, the ham.
They went on a tour of the city's meatpacking plants and meat markets. They picked up samples of meat in each place, and had them tested. It was a pretty dull tour, the owners were all a bit nervous because they didn't want to be involved with food poisoning. In one place, a young woman came in, crying, smeared make-up, bedraggled. She was acting strange, spaced out. Stoned. The proprietor soothed her, and made a comment to Taylor and Schmidt about unemployment and drugs and alcohol.
They had dinner with the microbiologist. She had preliminary results of the meat tests: negative. She had been monitoring the stools of all the patients, and one had become negative. The patient was Frank's wife, and she has also just recently become pregnant. Maybe that had something to do with it? They chatted, and started talking about the woman in the meat store, and about drugs. That got Taylor thinking of tranquilizers and Quaaludes and things like that, drugs in capsules that could get contaminated in processing.
They went back to see Frank and his wife in the morning. They beat around the bush a while and inventoried the kitchen, and then they asked if anybody in the family took any kind of drugs. She too beat around the bush for a while, and then after a silence said that she and Frank and most of their friends did smoke some pot once in a while. But she'd stopped, because of her pregnancy. Taylor had been thinking capsules, something to swallow, but pot, that was something you smoked. Taylor asked Frank if he could borrow some pot. He was pretty suspicious at first, thinking that Taylor and Schmidt were feds. He tried to give them a joint, to implicate them in the bust or whatever it was. Finally he just shrugged and laughed and gave them a few grams. Then Taylor asked if Frank would help break the ground to get his friends to give them a little pot. He agreed and made a few phone calls. By the end of the day they had 3 samples. All three were contaminated with S. muenchen. They'd found a new vehicle for the spread of salmonella infections.
The health commissioner was thrilled: all the eating places were calling every day, and his lab had been tied up for weeks. Illicit drugs were no responsibility of his, he was off the hook. So they wanted a press conference right away. Taylor told the assembled press that the source of the outbreak wasn't food or water or an eating place, but a non-commercial product with an unknown distribution. He told them that having diarrhea didn't mean a person was a user: most people were infected indirectly from silent carriers. The Salmonellawas in the marijuana, when a person rolled a joint, his hands became contaminated, when he put the joint to his lips, his lips did. A kiss or any sort of contact (feeding the baby) spread the infection. Then came the tricky part: he told them that to substantiate the theory, they needed to test more samples. They asked the community to help: the police would look the other way while people dropped off their marijuana anonymously for testing. It turned out the marijuana was VERY contaminated, from a million to ten million cells per gram.
To really make sure they had the culprit though, they needed to set up a case-control study. They did this in Michigan, where the source of the Lansing outbreak was still unknown. They selected 34 control households to match the 17 households with cases. The controls matched for social and economic status, and prevalence of children and young adults. In the control households, there had been no diarrhea for the last 2 months. They interviewed everyone in person (this builds more trust), asking about foods, alcohol, and marijuana. Case households used marijuana three times as often as control households. The marijuana tested positive for S. muenchen. The icing on the cake was that when plasmids from marijuana-S. muenchen were compared with plasmids from S. muenchen not from marijuana, the plasmids from the marijuana S. muenchen were identical. That confirmed that the S. muenchen that caused the outbreak was from a common source. The marijuana had probably been treated with manure to make it heavier prior to sale.
adapted from Berton Roueche's article "Annals of Medicine: A contemporary touch" New Yorker Magazine, and Taylor, D. N. et al. 1982. Salmonellosis associated with marijuana, a multi-state outbreak traced by plasmid fingerprinting. New England Journal of Medicine. vol 306: 1249-1253.
4) Some links that might be of interest:
Center for Disease ControlMorbidity and Mortality Weekly Report
Divisions of HIV/AIDS Prevention (CDC)
U.C.S.F.'s list of epidemiology links