International Journal of Food Microbiology, 21 (1994) 155-169 155 © 1994 Elsevier Science B.V. All rights reserved 0168-1605/94/$07.00

FOOD 00666

Salmonella enteritidis Control Programs in the United States

John Mason Salmonella enteritidis Control Program, USDA, APHIS, Hyatts~'ille, MD 20782, USA

A sharply rising incidence of salmonellosis in humans caused by Salmonella enteritidis (SE) in the United States between 1985 and 1989 resulted in a government-sponsored outbreak traceback program which was started in 1990. Layer flocks considered the sources of egg-implicated outbreaks would be tested for SE and the eggs from these flocks would be diverted to pasteurization plants. A program to eliminate SE from primary and multiplier breeding flocks was started at the same time. A subsequent spent hen survey and a survey of prepasteurized liquid egg material revealed that SE was much more prevalent than had been suspected, and that the highest prevalence was found in the northeastern and middle Atlantic states. Since the SE rates did not decrease during the next 2 years, a pilot project was started in 1992 in Pennsylvania, in one of the most highly affected areas. The project was designed to reduce the number of SE outbreaks by diverting eggs from SE-affected flocks and at the same time attempting to delineate the epidemiology and control aspects of the disease. With the results generated during the first 18 months of the Pilot Project, a much larger SE prevention program will start in the same area, in October 1993.

Key words: Salmonellosis; Salmonella enteritidis Control Program; Pilot Project

Introduction

In Apri l 1988, the Centers for Disease Contro l (CDC) in At lan ta , Georgia, r epor ted an a la rming rise in the US in the n u m b e r of ou tbreaks and cases in h u m a n s of salmonel losis caused by Salmonella enteritidis (SE) (St. Louis et al., 1988). Be tween 1985 and 1989, 217 SE outbreaks were repor ted , involving 7370 pa t ien ts of whom 44 died. The n u m b e r of outbreaks increased from 26 in 1985 to 77 in 1989. The ou tbreaks in 1989 were associated with at least 2394 cases and 14 deaths. The n o r t h e a s t e r n and middle At lan t ic states were the areas with the highest prevalence and foods m a d e with fresh shell eggs seemed to be involved in a large p ropor t ion of these incidents . This increase in SE in the Un i t ed States seemed to paral le l what was h a p p e n i n g in Grea t Bri tain and o ther countr ies in

Correspondence address: John Mason, Salmonella enteritidis Control Program, USDA, APHIS, VS 6525 Belcrest Road, Suite 205 Hyattsville, MD 20782, USA. Tel. (301) 436-4363; fax (301) 436-8360.

SSDI 0168-1605(93)E0082-3

156

Europe where SE was causing high mortality in young birds and a large number of human cases.

Because of the fear that what was happening in the Northeast might spread to other parts of the country and perhaps result in a national epidemic of SE, a number of interagency discussions were held in 1988 to determine the appropriate US Government response.

A nationwide mandatory testing program for SE which would include all egg-layer flocks was proposed, with diversion of eggs from SE-positive flocks to pasteurization plants. There was objection to this proposal as being premature, too expensive, and not warranted at that time in view of the lack of information about the number of flocks actually infected with SE. Also, it was considered that there was insufficient technical knowledge about SE in egg-layer flocks on which to base a nationwide program.

As an alternative, a program was proposed based on the follow-up of outbreaks that occurred in humans, as a means of detecting the SE-infected egg-layer flocks. In February, 1990, an SE control program was started to trace back from reported human outbreaks of SE in which eggs were implicated, to the flocks of origin. These flocks would be tested for SE, and if positive, the eggs would be diverted to pasteurization plants until the hens completed their egg cycle.

SE in humans in the United States

A review of available data revealed that the number of SE isolates had increased steadily since 1976 in the New England states, first in Massachusetts and then in Connecticut. Subsequently, sharp increases were recorded in New York, New Jersey and Pennsylvania (Fig. 1).

S. typhimurium has been the most prevalent Salmonella serotype in humans for many years. S. enteritidis, which was second in ranking numerically, has been steadily rising since 1976, and in 1990 surpassed S. typhimurium, which began falling in 1986 (Table I). However, if the incidence of S. enteritidis and S. typhimurium is compared on a regional basis, it can be seen that SE was more numerous only in the New England and Mid-Atlantic regions and less than S. typhimurium in the other regions, but because of the large number of SE isolates in these two regions, S. typhimurium was surpassed nationally in 1990. In 1991, there was a decrease in SE isolates to 7712 and S. typhimurium again regained the lead, with 8878 isolates (Table II).

SE Outbreak Traceback Program

Since the start of this program, 221 SE outbreaks in humans have been reported. Of these, fresh shell eggs have been implicated in 69 outbreaks to date (June 14, 1993). Traceback investigations from these outbreaks led to 33 flocks.

157

12 New England

11 . . . . . . . . . . . . . . . . . . . . . . Mid-Atlantic . . . . . . . . . . . . . . . . . . . . . . E.N. Central . '%

10 ...................................... S. Atlantic • •

9 ' " ' . . . . . .

8

~ 7

" 6

5

4

. . . . . . • " . . - " i ' " "

t :'.':':, . . . . . . . .-" ";i;

o I I I I I I I I I I I I I I I I

1976 '77 '78 '79 '80 '81 '82 '83 '84 '85 '86 '87 '88 '89 '90 '91

Year

Fig. 1. Isolation rate per 100000 population of Salmonella enteritidis, by region (United States, 1976-1991).

Eight of these flocks had been depopulated before the traceback investigations were completed, and environmental tests were not carried out. Two were negative on test for SE. The remaining 23 flocks were tested, and all were found to be positive for SE. These flocks were located one each in Delaware, Colorado and Michigan, four in Indiana, three in Maryland, two in New York, and 11 in Pennsylvania. As a result, more than 2.131 billion eggs from these flocks have been sent for pasteurization rather than being used as table eggs.

The outbreaks have occurred mainly in the Northeastern quadrant of the US, and this is even more evident when the location of the egg-implicated outbreaks is considered, and particularly evident when the location of the actual SE-infected flocks associated with these outbreaks is shown (Figs. 2-5).

A listing of the kinds of settings where SE egg-implicated outbreaks occurred shows that more than 90% occurred in food service establishments. Only 8% originated in the home. It is difficult to estimate how many sporadic SE cases occurred, in the home or elsewhere, because they are not ordinarily investigated and it is impossible to determine what particular foods may have been implicated, as can be done when outbreaks are investigated. The highest numbers of SE outbreaks were seen in the summer months (Figs. 6 and 7).

When the SE phage types found in humans outbreaks are compared with those in the flock of origin of the eggs implicated in the outbreaks, they were determined to be the same in all cases but one (Table III). Phage type 8 has been found to be the most common type found since 1987 (Table III). Phage type 4, the predomi-

158

TABLE I

Human isolates (number and percent of total Salmonella isolates)

Year S. enteritidis S. typhimurium

Number Percent Number Percent

1968 1734 8.8 5133 26.1 1969 1957 9.3 5 412 25.7 1970 2 509 10.3 5697 23.4 1971 2 240 8.7 6 459 25.3 1972 1710 6.5 6521 24.8 1973 1458 5.5 8323 31.3 1974 1439 6.0 7 011 29.3 1975 15 I0 6.5 6 505 28.1 1976 1207 5.2 7 441 32.1 1977 1461 5.4 9 271 34.3 1978 1904 6.6 9 621 33.3 1979 2 694 8.5 9 978 31.4 1980 1884 6.4 10 089 34.4 1981 2532 7.1 11991 33.5 1982 3 322 8.8 12557 33.1 1983 3 256 8.4 12 934 33.3 1984 3 709 10,3 12 550 34.8 1985 5 611 9,9 28 034 49.4 1986 5 980 14,2 10 765 25.6 1987 6 976 15.6 10 364 23.2 1988 6 952 15.9 9 456 21.6 1989 8 346 20.0 8 326 19.9 1990 8 572 21.1 8 219 20.2 1991 7712 19.3 8665 21.7

nant type in Great Britain and Europe, has not been found in commercial poultry flocks in the US, although a number of isolations have been made from human cases, who presumably picked up these infections outside of the US.

The trend in outbreaks shows a high of 77 in 1989, a drop to 67 in 1990, and 67 again in 1991, with 57 in 1992. There have been 30 outbreaks so far in 1993 (August 9).

Testing of egg-layer flocks implicated in human SE outbreaks

Testing for SE in flocks is carried out in two phases. First, environmental swab samples are taken of the manure pits and egg transport machinery in the layer houses. If these samples are positive for SE, the eggs from these houses are sent to pasteurization plants, rather than being sold as table eggs. To confirm the presence of SE in the flock, tissue samples from 60 layer hens in each house are cultured

TABLE II

United States by region; Salmonella isolates from humans (1990-1991)

159

Salmonella Salmonella Total enteritidis typhimurium Salmonella

90 91 90 91 90 91 1. New England 1 125 905 725 598 3986 3561 2. Mid Atlantic 3372 3259 1320 1307 7536 7564 3. East N. Central 1409 924 1479 1291 7509 6160 4. West N. Central 250 231 722 536 2346 2097 5. South Atlantic 1238 1061 1493 1539 6 637 9111 6. East S. Central 243 132 407 384 2533 2306 7. West S. Central 100 124 595 524 2728 2417 8. Mountain 195 241 281 422 1825 1975 9. Pacific 659 829 1 197 2 044 5 678 6 860

Total 8591 7712 8219 8665 40778 40012

1. New England: Connecticut, Maine, New Hampshire, Rhode Island, Vermont. 2. Mid Atlantic: New Jersey, New York, Pennsylvania. 3. East N. Central: Illinois, Indiana, Michigan, Ohio, Wisconsin. 4. West N. Central: Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota. 5. South Atlantic: Delaware, Washington, DC, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia. 6. East S. Central: Alabama, Kentucky, Mississippi, Tennessee. 7. West S. Central: Arkansas, Louisiana, Oklahoma, Texas. 8. Mountain: Arizona, Colorado, Montana, Nevada, New Mexico, Utah. 9. Pacific: Alaska, California, Hawaii, Oregon, Washington.

(from pools of tissues from heart, liver, spleen, ovaries and oviducts). If SE is found in the tissue samples, the restriction on the sale of eggs as table eggs is continued.

Location of Human SE Outbreaks Reported to USDA - 1990 - 1993 (through 8/09/93)

~ ( M A )

\ L, I , 13 F-:--h "~ \ I I I " I '-, ,.P"4 ~ 6 ~ . , ~ 7 (DE)

~ ' ~ 2 ~ 3 \ -20 (MD)

\ ' / ~ H,,w,,i = 1 ~f--. ~ ~ \ Cruise Ship = 3

- \ / - 12\ Puo.oR~co=3 I \ \ Bahamas = 2

Submarine = 1

Fig. 2. Location of human SE outbreaks reported to USDA 1990-1993 (through 8/09/93) .

160

Location of Egg Associated Outbreaks Reported to the SE Task force in 1990 - 1993 (through 8/09/93)

A) • 4 (CT)

3( 1

Fig. 3. Location of egg-associated outbreaks reported to the SE task force in 1990-1993 (through 8/09/93).

Spent hen survey for SE

One of the difficulties with initiating the SE control program in 1990 was the limited information available at the time about the actual number of egg-layer

Location of Commercial Egg Producing Flocks Implicated as Probable Sourcesof Human SE Outbreaks 1990 - 1993 (through 8/09/93)

<1

Fig. 4. Location of commercial egg producing flocks implicated as probable sources of human SE outbreaks 1990-1993 (through 8/09/93).

161

SE Positive F l o c k s (Detected through Outbreak Traceback Program) 1990 - 91 - 92 - 93 ( th rough 8/09t93)

z 1

Fig. 5. SE positive flocks (detected through Outbreak Traceback Program): 1990-91-92-93 (through 8/09/93) .

flocks infected with SE. To gather these data as quickly as possible, two SE surveys were started concurrently in 1991 to supply the necessary information. One was a spent hen survey, and the other a survey of unpasteurized liquid egg material (Ebel et al., 1992, 1993).

The national spent hen survey which started in April 1991, lasted for approxi- mately 3 months, and was designed to determine the regional prevalence of SE infection in US commercial egg laying houses. Ceca were collected from chickens sent to slaughter. A total of 300 ceca were sampled per egg-layer house (shipment)

+0j 40 I'--

30

20

t0

o Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

~ 1 Number of Outbreaks

Fig. 6. United States seasonal distribution of Salmonella enteHditis outbreaks: 1990-1991-1992.

162

Number of outbreaks 20-

15-

10-

J F M A M J J A S O N O J F M A M J J A S O N O J F M A M J J A S O N O J F M A M J J

1990 1991 1992 1993

Month of outbreak

Source: USOA, APHIS, SE Control Program

Fig. 7. Seasonal occurrence in human SE outbreaks: 1990-1991-1992-1993 (through 8/09/93).

and pooled into samples of five ceca each, for a total of 60 pooled samples per house. Seven of the 10 largest spent hen slaughtering plants (representing approxi- mately 80% of the spent hens killed in the US) participated in the study.

As was expected, the results showed marked regional differences. Birds from 406 houses were sampled. In the Northern Veterinary Services (VS) Region 81 of 181 houses (45%) were positive for SE, in the Central and Western Regions 28 of 163 (17%) and in the Southeastern Region two of 62 (3%) (Fig. 8).

Survey of unpasteurized liquid egg material

To obtain information on the occurrence of SE in unpasteurized liquid egg, sampling was carried out at 20 US egg-breaking plants weekly for 52 weeks, starting in February 1991. One 10-12-ounce sample was collected from each full storage vat containing unpasteurized liquid egg. Samples were sent to the National Veterinary Services Laboratories (NVSL) and Ames, Iowa for salmonella cultur- ing, and serotyping of all Group D isolates.

After 52 weeks of collection, with a total of 1002 samples, 13% were found to be positive for SE. The differences by VS Region were considerable, with 20% of the samples from the Northern Region positive for SE, 15% for the Central Region, 6% for the Western Region, and 10% for the Southeastern Region (Fig. 9).

163

TABLE III

Comparison of SE phage types from human cases, environmental samples, and organ tissue (samples studied as a result of USDA, APHIS, Veterinary Services' Outbreak Trace Back Program)

Outbreak Flock Phage types number number Human

isolates Hen house environment Layer hens organ culture

1 1 14B 14B 14B 2 1 14B 14B 14B 3 1 2 14B 14B 4 2 2 2 2 5 3 13A 13A, 23 Not done 6 4 13A 13A, 23, 28 13 7 5 8 8 Negative 8 6 8 8, 23 8 9 7 8 8, 13A, 23, 28 8, 13A, 23

10 7 8 8, 13A, 23, 28 8, 13A, 23 11 8 8 8, 13, 23 8 12 9 8 8, 13A, 23, 28 8, 23, 28 13 10 8 8, 23 Not done 14 11 8 8, 3, 13, 13A, 23 8, 23 15 12 8 8, 13, 23, 14B, 34 8 16 12 8 8, 13, 23, 14B, 34 8 17 12 8 8, 13, 23, 14B, 34 8 18 13 8 8, 13A 8, 2, 23, 13A 19 14 8 Not done 8, 23 20 15 8, 34 8, 3, 34 Not done 21 16 8 8, 2, 23 8 22 17 8 8, 13A, 22, 23, 34 8, 13A, 28 23 17 8 8, 13A, 22, 23, 34 8, 13A, 28 24 18 8 8, 3 Not done 25 18 34v a 8, 3 Not done 26 19 8 8, 23 Not done 27 20 13A 8, 13A, 23 13, 13A 28 20 13A 8, 13A, 23 13, 13A 29 21 24 b 13A Not done

(Organ culture- mice-24, 13A) (Rectal swab° cow-13A)

30 22 8 8 On test

a 34v is considered a variant of 3. b 24 is considered a variant of 13A.

The SE US sanitation monitored program for breeding flocks

B e c a u s e S E t r a n s m i s s i o n was t h o u g h t to be t r a n s o v a r i a l , w i th p a s s a g e o f

i n f e c t i o n pos s ib l e f r o m b r e e d i n g f locks to r e p l a c e m e n t pu l l e t s to c o m m e r c i a l egg

l aye r f locks , a p r o g r a m was s t a r t e d to t es t all e g g - l a y e r b r e e d i n g f locks fo r SE .

F o r t u n a t e l y , a m e c h a n i s m to a c c o m p l i s h this a l r e a d y ex i s ted . T h e N a t i o n a l P o u l t r y

L64

Spent Hen Survey 1991 (vs Regions)

. . . . . . ~ Northern Region L;entra12;/°16v~52t;~? egiOns ~ 81/181 (45 %)

~Southeastern ) '

Fig. 8. Spent hen suwey 1991 (VS regions).

Improvement Plan (NPIP), required the monitoring of breeding flocks for the eradication of pullorum disease, and this system could easily be expanded to include examination for SE. This program became effective July 1, 1989, initially as a voluntary program. It was designed to reduce the prevalence of SE in hatching

SE Survey of prepasteurization liquid egg material Feb, 1991 - Feb. 1992 (Weekly samples - 20 pasteurization plants)

W--,n.0,on ,,, .) O,. oo

j

Fig. 9. SE survey of prepasteurization liquid egg material Feb. 1991-Feb. 1992 (weekly samples, 20 pasteurization plants).

165

eggs and chicks through an extensive testing and sanitation program on the breeder farms and in the hatcheries. In February, 1990, the USDA required that all hatching eggs and newly-hatched chicks from egg-type chicken breeding flocks that move between states be from flocks certified as free of SE. Since the start of the program, of the total 259 multiplier breeding flocks with 2.8 million birds, 27 flocks with 270 000 birds were found to have SE in the environment, and 11 flocks with 150000 birds, had SE isolated from cultures of organs.

Risk of contracting SE from eggs

Estimates of the number of eggs actually containing SE organisms have varied from 1 : 10000 in eggs bought in markets, to 1 : 200 to 1 : 12 in eggs from SE-in- fected flocks. Da ta from egg culture studies indicate that the actual level of contamination will vary according to many factors. However, the chance that contaminated eggs could cause an illness or an outbreak, further depends on the possibility that the particular eggs will not be properly refrigerated a n d / o r cooked thoroughly.

The risk of SE infection from SE in eggs is relatively low. There are about 40000 isolates of salmonella in the United States each year. Some 20% or about 8000 are due to SE. If only one case in 50 is reported there may be some 400000 cases per year. It is assumed that one half or 200000 could be egg-related. Egg consumption in the US is about 190 eggs per person per year (250 million × 190 =

Regional Distribution of U.S. Population 1990 Human SE Cases Total SE Outbreaks and Egg-related SE Outbreaks (1990. 1992)

Rates of Human Isolates per 100,000 population (1990 pop.) Population of Egg-layers.

~ Northern Region ~ ' ~ Central and Western Regions ~ "~ /~ -~ ~ 3, ~o, popu,at,o. \ ~//" )., ~ _I. 14%of.eas / / I "~ ~ . . . . . / 10~o*outbre,ks \ /1 / J~ - ° ' -PLP :_C 7 % of egg-related outbreaks j~ ~ ~ o: ~ ~: ~a=B~

1.3 % (rate of human isolates) f 84 % of Outbreaks ~ - ~ -" 46 % of total egg-layers ( 80 % of egg-rel, outbreaks /

~6,3 % (rate) ~ ~/

/~14 % of caeas 3- /~ lO%ofoutbreaks ~"

I - - - - ~ u t b r e a k s / Sou_t heaster n

Fig. 10. Regional distribution of the US populations of 1990 human SE cases. Total SE outbreaks and egg-related SE outbreaks (1990-1992). Rates of human isolates per 100000 population (1990

pop.)/population of egg-layers.

166

47.5 billion eggs consumed in the US annually). If 47.5 billion is divided by 200 000, on the average there would be one case for every 237 500 eggs consumed. With about 30 egg-associated SE outbreaks per year, there would be one outbreak for every 1.58 billion eggs.

Discussion

The USDA initiated its SE program in February, 1990 in response to the threat of a serious SE outbreak in humans and in commercial poultry flocks. At the time, not much data were available on the actual level of infection in poultry flocks or the degree of risk for the human population. After some 3½ years of experience, a better estimate can be made.

From the results of the spent hen survey, we can conclude that a considerable proportion of egg-layer flocks are infected with SE, with up to 45% in the northeastern quadrant of the US, 17% in the Central and Western Regions, and 3% in the Southeastern Region. These regional differences were also seen in the liquid egg survey results, and were further substantiated by the concentration of human outbreaks and the known infected flocks found on traceback, in the northeastern quadrant (Fig. 10).

The traceback program has identified only 23 infected flocks, of the total 3000 large commercial flocks in the US Certainly there are many more infected flocks than have been detected through the traceback program. This leads to a considera- tion of the actual risk for humans of contracting SE from the many flocks that are infected, and that have not yet been detected and restricted, and what should be done to prevent further SE infection.

There are a number of obvious measures that everyone is in agreement about: improved refrigeration, increased use of pasteurized eggs in institutions, and better food handling in the home and in food service establishments. What is at issue basically is what can be done on the farm to prevent and eliminate SE infection in breeding and commercial egg-layer flocks.

Again, there are a number of factors about which there is little disagreement: the need to insure that breeding flocks are free of SE, the provision of SE-free feed and SE-free pullets, effective rodent and fly control, improved biosecurity and the possible use of an effective vaccine. The main point of contention, however, is the question about testing flocks for SE, whether this should be done on a voluntary or mandatory basis, and what should be done with flocks that are found to be infected with SE.

The compulsory testing of all flocks for SE, with diversion of eggs from the positive flocks to 'breaker plants' would seem to be a simple solution. On analysis, however, this type of program would be very expensive, difficult to administer, very disruptive and damaging to the egg industry, and would increase the cost of eggs considerably. Producer losses alone could range from $30 to $136 million if SE is found in only 13% of egg-layer houses. Because of supply deficits, the consumer

167

price for a dozen eggs might increase by as much as 75%, in this instance (Mason and Ebel, 1992).

A more reasonable approach would be a voluntary program in which the producer participates in an integrated prevention program for SE, with testing of flocks just before the end of laying cycles, approved cleaning and disinfection, intensive rodent control, replacement of the positive flocks with clean pullets, and the possible use of an SE vaccine. This type of program would gradually eliminate the SE-infected birds, and generally lower the risk from SE to a more acceptable level.

SE Pilot Project in Pennsylvania

Since the outbreak traceback program by itself did not promise to lower the SE rate in humans quickly enough, a voluntary SE Pilot Project was organized in Pennsylvania, and started in April, 1992. The project was a cooperative effort between egg producers and State and Federal agencies, and was organized to test the environments of hen houses for SE, then test eggs from positive houses, and have the eggs diverted for pasteurization if positive for SE. At the same time, a variety of control procedures (SE-free feed, SE-free pullets, biosecurity, rodent control, cleaning and disinfection, and use of an SE vaccine) were to be evaluated to determine which were effective, singly or in combination, in preventing or eliminating SE in egg-layer flocks.

The shift to testing eggs instead of birds was based on the belief that if eggs were the immediate concern, then we should try to culture the eggs to determine the level of SE contamination, rather than concluding that if any birds in the flock were positive for SE on organ culture, we should divert all eggs from that flock to pasteurization. It was felt that the laying hens could be positive for SE, as well as the environment and the mice in the house, but that the eggs themselves might not be positive at all times, and could be safely marketed as table eggs, if monitored properly, and found to be negative.

At the present time (August 1993) 97 flocks (5.0 million egg layers) have been or are being monitored. The environments of flocks are tested 3 times during the laying cycle. Eggs are tested every 2 weeks in environmentally positive houses. Mice are trapped and cultured. Chick papers and the environment of pullets being placed in project houses are also tested. A summary of the major findings so far is given below.

1. SE does not appear to be clearly related to specific management practices.

2. The environments of 66% of the flocks in the project were found to be SE-positive.

3. More than 600 000 eggs have been cultured for SE in pools of 10 or 20 eggs. In environmentally positive houses, positive eggs have been found in 44% of the

168

flocks. The egg contamination rate in these environmentally positive houses has been very low: of 562520 eggs, 126 were positive (0.02%). If two flocks which contributed over 50% of the positive eggs are excluded, the egg contamination rate in environmentally positive houses is just over 0.01%. Essentially, the culture of eggs has been found to be a practical monitoring procedure to determine the risk of SE for humans.

4. The ratio of 'blood egg' positives to random regular (nest run) egg positives has averaged 4:1 where comparisons are possible.

5. Mice would appear to be a significant risk factor for SE in a flock, maintaining SE from one flock to the next and transmitting SE within a flock. Mice were found to be SE-positive in 79% of the environmentally positive houses.

6. Actual mouse populations in hen houses are often underestimated. In some situations even professional pest control operatives were unable to achieve ade- quate control. Control of mice requires an intensive and continuing effort. Initial efforts can be frustrating.

7. SE-positive flocks can be found in the absence of significant mouse populations.

8. SE phage types found in hen houses in the environments, mice and eggs are generally the same. The predominant phage types observed have been: environ- ment 8, 23, 3, 13, 13A, 2, 34; mice 8, 23, 3, 13, 13A; eggs 8, 23, 13, 13A.

9. Infected chicks are still occasionally encountered and may be a potential source of infection for commercial flocks.

10. Positive pullets may originate from previously infected chicks or from SE exposure (environment, mice) in the pullet house. Thorough cleaning and disin- fecting between pullet flocks and mouse control are essential. The environmental testing of pullet flocks should be carried out when they are 14-16 weeks of age.

11. Initial cleaning and disinfecting has been successful in completely decontami- nating only a small percentage of environmentally positive houses. In difficult situations the use of formaldehyde gas has been effective, but this procedure is rapidly becoming unavailable.

After one and half years of operation, the Pilot Project has more than met its objectives. More is now known about the risk factors for SE transmission, and no SE outbreak has been traced back to a flock in the project. A broad-scale parallel SE prevention program with a similar protocol for the culture of eggs is being started in Pennsylvania in October, 1993, with coverage of most of the 20 million egg layers in the state. This promises to produce an early reduction in the number of SE cases and outbreaks in the Northeastern quadrant, where about 85% of the SE in humans is found.

169

R e f e r e n c e s

Ebel, E.D., David, M.J. and Mason, J (1992) Occurrence of Salmonella enteritidis in the US commerical egg industry: report on a national spent hen survey. Avian Dis. 36, 646-654.

Ebel, E.D., Mason, M.J., Thomas, L.A., Ferris, K.E., Beckman, M.G., Cummins, D.R., Scroeder-Tucker, L., Sutherlin, W.D., Glasshoff, R.L. and Smithhisler, N.M. (1993) Occurrence of Salmonella enteritidis in unpasteurized liquid egg in the United States. Avian Dis. 37, 135-142.

Mason, J., and Ebel, E.D., (1992) APHIS S. enteritidis Control Program. In: Proceedings of the Symposium on the Diagnosis and Control of Salmonella. USAHA, San Diego, CA. pp. 78-109.

St. Louis, M.E., Morse, D.L., Potter, M.E., et al. (1988) The emergence of Grade A eggs as a major source of S. enteritidis infections. J.Am. Med. Assoc. 259 (14), 2103-2107.