General information

Pigs colibacteriosis

Colibacteriosis (colibacillosis, colidiaera) piglets (Colibacteriosis suum) - acute infectious disease of piglets accompanied by diarrhea, symptoms of toxemia, less septicemia and high mortality.

The causative agent is enteropathogenic serovars of E. coli serogroups: O8, O9, O20, O137, O138, O139, O141, O142, O147 and O149. Most of them contain peel antigens K88,987r and have hemolytic properties. Often they produce both thermolabile and thermostable exotoxin. The K88 antigen is almost always found in hemolytic cultures and most often in the strains of serogroup O149 and O141. Sometimes during colibacteriosis, piglets secrete strains containing the K99 antigen, for example: O9: K35: K99: H, O101: K: 30: K99: H.

Epizootic data. Newborn piglets, piglets, suckers of 2-4 weeks of age and piglets in the first two weeks after weaning, develop colibacillosis. The disease is enzootic, often stationary. Colibacteriosis is especially common in large industrial complexes, where, during the period of mass farrowing, the pathogen quickly spreads from one litter to another, covering a large number of piglets on the farm. The source of colibacillosis is patients who have had gilts and colibacillosis and enteropathogenic Escherichia bacteria carrying carriers. The main route of infection is alimentary, less often aerogenic. There are cases of intrauterine infection of piglets. Predisposing factors of the disease are unsatisfactory conditions and inadequate feeding of pregnant sows. Factors of transmission of the causative agent of colibacillosis can be contaminated animal care products, dishes, food, clothing, and vectors can be various insects and service personnel.

Pathogenesis. When the general resistance of the organism of the newborn is weakened, enteropathogenic Escherichia penetrate into the mucous membrane of the small intestine and, multiplying rapidly, release endo- and exotoxins. Thermolabile toxin (produce almost all of the escherichia) is adsorbed on the villi of the epithelial cells of the small intestine, stimulating adenylate cyclase, which leads to a sharp focal increase in the concentration of cyclic adenosine monophosphate. This mediator causes hypersecretion of electrolytes and additional diffusion of water by young low-differentiated epithelial cells of the intestinal mucosa into the lumen of the small intestine and inhibits sodium reabsorption. The lumen of the intestine becomes full of fluid, peristalsis increases, and diarrhea occurs. As a result of increased secretion, water and electrolytes are secreted from the body of the piglets, which leads to acidosis and, in severe cases of the disease, may cause a dihydro shock.

Because of the reduced protective function of the intestinal wall in sick piglets, the Escherichia enter the lymph, then into the blood and tissues, causing septicemia. Sometimes the development of septicemia followed by a fatal outcome is ahead of signs of diarrhea. Additionally, as a result of the action of endotoxin and hemolysin, inherent in the pathogen of colibacillosis of piglets, there is a violation of the porosity of blood vessels, which leads to edema with subsequent disruption of the function of the nervous system.

Clinical signs. Colibacteriosis in piglets occurs in three forms: septic, enterotoxemic (edematous) and enteritic forms. The course of the disease is hyperacute, acute, subacute and chronic.

The septic form of the disease is inherent in newborn piglets and suckling piglets. Its over-sharp and acute, accompanied by high body temperature and high mortality. Clinically, the disease is manifested by refusal of food, depression, impaired coordination of movement, and sometimes diarrhea.

When enteritic form register profuse diarrhea, depression and lethargy.

The enterotoxemic form is characterized by toxic effects and collapse in suckling piglets and edema in weaned piglets. In an edematous form of the disease, a short incubation period is noted - from 3 to 12 hours. Edematous form of colibacillosis is usually sick well-fed piglets. They refuse to feed, excitable, they have a precarious gait, twitching of the head and limbs. On external examination, we note the blueness of the skin of the ears, piglet, abdomen and limbs. The disease is acute, and a few hours after the onset of symptoms, most animals die.

Pathological changes. Pathoanatomical changes in the supersharp course of colibacillosis in dead piglets are absent. In piglets fallen from acute and super-acute colibacillosis, we note the anemia of the mucous membranes, the posterior surface of the thighs and the anus is dirty with fecal masses. The gastric mucosa is hyperemic with a gelatinous infiltrate, the mucous membrane of the small intestine is swollen, covered with mucus, there are signs of catarrhal - hemorrhagic inflammation. Regional mesenteric lymph nodes are enlarged, hyperemic. The lymphatic apparatus of the rectum (Peyer's patches, solitary follicles) is hyperplastic and hyperemic. Spleen dark gray, slightly enlarged, dense, nodular. The liver and kidneys are anemic; under the kidney capsule there are very small punctate hemorrhages. In the liver, muscle of the heart, kidney expressed degenerative changes. The cerebral vessels are filled with blood, the brain tissue is edematous. In an edematous form of the disease, we find edema of the subcutaneous connective tissue and the walls of the stomach. In the stomach we find a large amount of feed solid consistency. The intestinal mesenteric lymph nodes are swollen, enlarged, juicy, hyperemic, and marbling coloration is observed. Yellowish or dark red mucous or watery fluid in the small intestines. Light swollen.

Diagnosis colibacteriosis is established taking into account the epidemiological, clinical data, pathological changes and bacteriological findings. Fresh corpses of piglets or parenchymal organs (liver with gall bladder, spleen, kidney, heart, mesenterial nodes, tubular bone, brain or whole head, sections of the affected small intestine and stomach (from 2-3 bodies)) are sent to the veterinary laboratory. in watertight containers. Parts of the organs are sent in clean dishes (the stomach and intestines separately from other organs) no later than 4 hours after the death of the animal, and the material is sent from animals that have not been treated with antibiotics. corpses are sent feces from 3-4 sick pigs who have not been treated.A positive bacteriological diagnosis in the study of pathological material from piglets is given in the following cases:

  • excretion of enteropathogenic E. coli cultures from pigs who died from the first days of life to weaning and in the first days after weaning,
  • determination of the pathogenicity of isolated culture of Escherichia coli,
  • serological typing by a set of type-specific whey serum.

The order of bacteriological research is regulated by the instruction approved by the General Directorate of Veterinary Medicine of the USSR Ministry of Agriculture.

Differential diagnosis. The pigs' colibacteriosis must be differentiated from viral (transmissible) gastroenteritis, anaerobic enterotoxemia, salmonellosis, dysentery and dyspepsia.

Immunity. Immunity in colibacillosis in piglets is passive, acquired with colostrum, the severity of which is the stronger, the more immunoglobulins are contained in the sow's colostrum and milk.

The resorption of immunoglobulins from colostrum in the intestines of newborn piglets lasts 72 hours, but is most intense during the first days. The veterinary specialists achieve a high concentration of immune bodies in the colostrum by subcutaneous and intramuscular immunization of deep-cemented sows. The optimal timing of vaccination - 30-15 days before farrowing.

Specific prevention. Of the specific vaccination products, a polyvalent GOA formolthimersal vaccine is used, containing 9 strains of different serogroups of the Escherichia, some of which synthesize the adhesive antigens K88as, K88av and produce TL and TS exotoxins and hemolysins. The vaccine is administered to pregnant sows 1.5-2 months prior to farrow intramuscularly, twice with an interval of 10-15 days in doses of 4-5 and 5-6ml, as well as to piglets 10-20 days before weaning 1-1.5 and 1 , 5-2ml. In addition, for prophylactic purposes, polyvalent serum against colibacillosis (colibacillosis) is administered intramuscularly to piglets up to the age of 5 days in doses of 8-10 ml over 5 days -10-15 ml.

Treatment. The most effective antibiotics for colibacillosis are neomycin, monomitsin and chloramphenicol. They are recommended to be administered to young animals in higher doses per 1 kg of weight than adults (neomycin inside piglets up to 6 months - 30 mg, adults - 20 mg). To prevent the resistance of microflora to antibiotics they are used in combination.

Antibiotic treatment of sick piglets should be carried out simultaneously with the struggle to dehydrate the body, by intraperitoneal and oral administration of electrolytes (Ringer's solution). Intraperitoneal use of the drug Dufalac in a dose of 10 ml per piglet gives a good result.

Based on the fact that antibacterial drugs in the body of sick piglets inhibit the growth of Escherichia coli, while depriving the producer of B vitamins, during the treatment of colibacillosis, it is necessary for veterinary specialists to use vitamin B drugs (B1 B12 and others) during treatment.

The use of immunomodulators is recommended (Lidium 2mg / kg mass). Good results are obtained by the use of probiotics (Bio Plus 2B, Lacto-Sacc).

For the piglets who are sick at a later time (older than 15 days), it is sufficient to use a solution of potassium permanganate (1:10 000), a decoction of oak bark or sulfanilamide preparations for therapeutic purposes. When treatment is recommended to use PABK, as well as infusions and decoctions of yarrow, St. John's wort and horse sorrel.

Prevention and control measures. Pregnant sows for 10-15 days before farrowing should be transferred to previously cleaned and disinfected individual cells of the pigsty - mother liquor.

Before transferring, sows are cleaned, contaminated areas of the body are washed with thermal water, the lower parts of the limbs are disinfected with a 0.5% Lysol solution or 5% formalin solution. Preventive sanitation machines pigs-liquors carried out the method of irrigation disinfectants and compliance.

In industrial pig-breeding complexes after mechanical cleaning of the premises, the most polluted objects (floors, slotted grids, feeders, lower parts of walls, enclosing structures, etc.) irrigate once hot, not less than 70 ° C with 2% sodium hydroxide solution or dump at the rate of 0 , 5 liters per 1 m² of surface area (for irrigation, hot 5% soda ash solution can be used, which is applied twice with an interval of 30 minutes). After 25-30 minutes, not allowing the treated objects to dry, they carry out the final cleaning and washing of the entire premises with a high-pressure water jet using mobile or stationary installations.

For the disinfection of the premises, one of the following means is used: 4% hot solution of caustic soda, 2% solution of chloramine, or hypochlor, containing 3% of active chlorine. Consumption of solutions - 0.5 liters per 1m². Exposure -1 hour. After disinfection, the remains of disinfectants in areas accessible to animals are thoroughly washed with water, the rooms are aired and dried.

5 days before farrowing, the ration of sows is gradually reduced so that the day before farrowing they receive food in an amount of not more than half the daily norm. When signs of farrowing appear, the back part of the sows torso is washed with a disinfecting solution (0.5% chloramine solution, furatsilin 1: 4000 solution or potassium permanganate 1: 1000) or wiped with a clean cloth moistened with one of the indicated disinfectants.

A worker taking pigs during farrowing should put on a clean robe and apron, thoroughly wash and sanitize hands. At the born piglet, the umbilical cord is clamped at a distance of 5-6 cm from the skin of the abdominal wall, then cut off by twisting or tied with a disinfected thread. Cut with scissors below the ligature and the stump is immersed in a glass with 5% tincture of iodine. With a clean towel (napkin), the mouth, nostrils and piggy ears are cleaned of mucus, they wipe the torso dry and put (before the beginning of feeding) in a previously cleaned and disinfected box (cage).

After farrowing, the contaminated sites of the sow are washed with warm water and the litter is replaced. Separated afterbirth, trimming the umbilical cord is placed in a galvanized box and destroyed in a specially designated place. The nipples and skin of the mammary gland of the sow are wiped with a towel (napkin) moistened with a disinfecting solution, and then transplanted to the newborn piglet for feeding.

After farrowing, the females are watered with fresh water, and after 4-6 h they are fed with a talker (500-700g) from concentrated feed. From the second day after farrowing, the sows gradually increase their ration and, by the fourth day, bring it to full normal.

All dairy products supplied for feeding piglets, are subjected to heat treatment (boiling, pasteurization) directly on the farm, regardless of their processing in dairy plants.

After weaning piglets from sows, they are kept in a separate room (section) in isolation from adult pigs and piglets of older age groups. Prior to filling the premises, it is cleaned (from manure, etc.) and disinfected.

When a diagnosis of colibacteriosis is established, first of all take measures to restore the veterinary-sanitary order in the premises and isolate patients. The most radical measure in an outbreak of colibacillosis is the break of the epizootic chain - the change of the site for farrowing. The farm stops placing pregnant queens on farrowing in dysfunctional pigsty. Subsequently, the group for farrowing is formed in a separate room or camp. If there are no conditions for this, the redevelopment time of the premises during the “all empty” period is 2-3 days longer than the technological one. Urgent measures are taken to improve the feeding of deep-sprout and nursing females, while suspicious feed quality is replaced by benign ones.

In case of acute course of colibacillosis, forced medical prophylaxis is carried out. To do this, from the first day of life, the inside of the piglets is given one of the antibiotics or sulfonamides, 1–2 times a day, on a physiological solution, specific serum, globulin or oat broth. In addition, one-day piglets can be administered a single injection of trivitamin with dibiomycin in a dose of 30-40000 U / 1 ml of the drug intramuscularly. With a decrease in the effectiveness of drug prevention, it is necessary to change the antibiotic used.

Sick piglets are treated with antibacterial agents. With the appearance of colibacillosis among a significant number of piglets in the same room (section), patients are treated on the spot, taking measures to prevent the spread of the pathogen.

Recovered piglets that have reached weaning age are transferred to rearing or fattening, forming separate groups from them.

Pregnant sows are immediately vaccinated with a vaccine against colibacillosis, according to existing guidelines. If necessary, we vaccinate and piglets older than 10 days of age.

What is swine colibacteriosis?

Enteropathogenic Escherichia coli, which belong to the Enterobacteriaceae group, cause the disease. Escherichia in the breeding process emit a large number of toxins. Diarrhea is a response to the penetration of pathogens. With its help, the pig's body is trying to get rid of harmful components.

Important! The likelihood of colibacillosis infection increases in the autumn-winter period. The infection quickly affects the entire herd.

The source of infection is Escherichia, which is present in soil and water. E. coli can survive for 1-2 months. Pathogenic microorganisms are most active at 37 degrees.

Young individuals are especially vulnerable to infection. Microbes enter the body of piglets through dirty water or feed. E. coli may be present in manure. It is necessary to regularly change litter, as contaminated feces and urine of animals fall on it. The situation is aggravated by the fact that adults are often carriers of a dangerous infection.

Symptoms and signs

A sick animal suffers from dehydration. Active reproduction of harmful bacteria leads to severe intoxication. The temperature of an infected pig rises to 42 degrees. Escherichia affect the important organs of the animal. Infection penetrates to the surface of the intestinal mucosa.

Experts distinguish several stages of the disease:

  1. A sign of the subacute stage is diarrhea. Свинья достаточно быстро теряет большое количество жидкости. Вместе с испражнениями организм лишается необходимых витаминов и минералов.
  2. В дальнейшем у свиньи развивается острый этап, которые приводит к резким болям в брюшной полости. У свиньи западают глазные яблоки, начинаются судороги.
  3. При отсутствии лечения болезнь переходит в сверхострую стадию. Свинья отказывается от корма. Colibacteriosis is accompanied by impaired coordination of movements. The animal's temperature rises sharply. Usually a pig dies within 1-2 days.

On the surface of the intestinal mucosa, experts identify inflamed areas. Colibacteriosis causes deformation of the tissues of the liver and kidneys. In pigs, the membranes of the brain swell.

A characteristic sign of the disease is a change in the color of the spleen. She gets a dark gray shade. In the course of the examination, veterinarians find coagulated milk in the piglet’s stomach. In adults, gastroenteritis is observed, which occurs in catarrhal form.

Important! Even in the case of recovery experts do not exclude the occurrence of relapses.

Causes and pathogens

The cause of colibacillosis is pathogenic Gram-negative movable bacillus Escherichia coli. At the moment, about 9 thousand serological variations of Escherichia are known, which determine their type. But, only some of the species infect the body of animals. Enterotoxigenic (antibiotic resistant) and entero-invasive strains of Escherichia are widely distributed.

Bacteria are very stable in the external environment, they retain their viability in soil and water for up to 60-80 days, in feces for up to 30 days. When boiling death occurs immediately, when heated to 80 degrees stick dies within 15 minutes.

The source of coli infections are sick and sick pigs. Often, the outbreak of the disease is created by sows, carriers of the pathogenic strain of colibacillosis, secreting bacteria from feces, urine, and colostrum. It occurs immediately after farrowing - susceptible young are exposed to infection in the first few days after birth.

The pathogen is also transmitted through water, common household items and contaminated feed. In this case, the infection occurs after the transition to independent nutrition.

Rodents can cause the development of colibacillosis in farms, because they are carriers of pathogenic strains.

As provoking factors can be identified:

  • unbalanced diet
  • micronutrient deficiencies,
  • lack of exercise and ultraviolet.

Forms of the disease

Like any infection, this pathology, depending on the state of the immune system and the degree of pathogenicity of the malicious agent, can occur:

Unformed stools in newborns often speak of infection and are accompanied by the following general symptoms:

  • cyanosis of the skin,
  • in the nasal passages revealed mucous secretion
  • swelling is detected in the following areas: eyelids, neck, submandibular space and occiput. In the back of the body, puffiness is observed in rare cases.

The symptomatology of colibacillosis also varies on the basis of the clinical form and course of the disease:

1 Septic form

It is observed in only born and sucking piglets. The disease is acute and over-acute. Mortality is fast and high (for 24-48 hours). The development is due to the penetration of bacteria into the bloodstream, followed by reproduction. Externally manifested:

  • refusal to eat
  • general oppression
  • high persistent temperature, then - lack of coordination,
  • respiratory failure (cyanosis of the mucous membranes), and as a consequence of a violation in the work of the heart,
  • conjunctivitis,
  • severe diarrhea or dehydration and the absence of diarrhea (feces first liquefy, becomes grayish-white, streaked with blood, and then becomes watery and foams),
  • cramps
  • septicemia.

2 Enterotoxemic form

It is celebrated in the post-withdrawal period, it is acute and manifests itself after a period of 3 to 12 hours. Deaths occur in almost 50% of cases. It is characterized by the penetration of coli into the small intestine and is manifested by toxic and neurotoxic symptoms:

  • loss of appetite,
  • central nervous system lesion - increased excitability, an acute reaction to touching, head twitching and passing lower jaw cramps, walking in a circle,
  • diarrhea,
  • collapse.

Diagnostics

It is possible to diagnose infectious colibacteriosis after evaluating and analyzing the clinical symptoms, pathological changes and epizootic situation. The bacteriological analysis of material obtained from dead piglets is also important.

At occurrence of mass diarrhea, without losing time, they send to the laboratory such materials taken from a dysfunctional economy:

  • corpses of pigs,
  • pathological materials: liver lobe and part of the gallbladder, brain, vessels, lymph nodes, tubular bone, fragment of the small intestine,
  • For bacteriological analysis, it is necessary to take fresh fecal masses from less than five piglets (1-2 g per head).

For accurate bacteriological diagnosis using materials from sick animals not treated with antibiotics.

The diagnosis of colibacillosis in piglets is considered established in the case of:

  1. Isolation of bacterial cultures from spleen, bone and brain.
  2. Identified during the pathological examination of the piglets after the case of the following changes:
    • in the pericardial region, there is a certain amount of serofibrinous effusion with inclusions,
    • respiratory system - the lungs are filled with fluid, mixed with blood,
    • under the pleural region are spotted inclusions in a small amount,
    • the intestines are hyperemic, there are hemorrhages, fibrin filaments are found among the loops,
    • lymph nodes at a section are edematous, there are hemorrhages, the size is increased,
    • parenchymal organs (liver, kidneys) are flabby,
    • there are stagnation in the brain and vessels
    • skeletal muscles emaciated, pale.

And also after exclusion in milk pigs:

  • rotavirus enteritis,
  • dysentery,
  • viral gastroenteritis,
  • make faces,
  • the plague
  • Aujeszky's disease.

Older individuals are excluded:

  • Streptococcosis
  • salmonellosis,
  • pasteurellosis,
  • coronovirus infection,
  • adenovirus infection.

Since the disease is complex and spreads rapidly, treatment measures begin as early as possible. The scheme of use of drugs and their dosage is prescribed by a veterinarian, based on the state of each individual, weight and age of the animal.

Treatment of colibacillosis symptomatic and based on:

  • restoration of water-salt balance - a solution of 0.9% sodium chloride, Ringer's solution,
  • normalization of the gastrointestinal tract - the use of probiotics (Emprobio), biovit 40-80-120, also enveloping drugs to relieve inflammation from the mucous membrane,
  • stabilization of the heart and the elimination of oxygen starvation - caffeine benzoate,
  • elimination of toxicity - adsorbents (activated carbon, Enterosgel, Carbitox),
  • immunity enhancement - consists in the use of specific antibodies against the causative agent of colibacillosis (polyvalent serum at a therapeutic dosage of 15-20 ml for five-day pigs, serum from the blood of animals to be slaughtered, and serum from sows vaccinated against escherichiosis). Also, the natural resistance increases the increased amount of vitamin C that pigs receive from food,
  • restoring appetite
  • suppression of infection - only drugs to which the bacteria have a sensitivity. These include prolonged antibiotics (Amoxicillin, Cobactan), as well as Nitoks, Neopen, Gentavet - with enteritis.

Sick pigs must be kept separate from healthy stock during treatment.

An important factor in the treatment of intestinal pathologies is a hungry diet that lasts up to 12 hours. She is appointed from the first day of illness. Colostrum is replaced with a warm solution of sodium chloride (this measure will reduce the degree of dehydration). Then you can add to the solution up to 50% of the daily rate of colostrum. They feed the piglets every 6 hours.

During this time period the exercise is organized for the piglets, they increase the nutritional value of the feed with minerals and vitamins. It is preferable to give fermented milk products and juicy food to food.

Prevention

Often, colibacillosis is a scourge of unfavorable farms with signs of unsanitary conditions. In the event of an outbreak of the disease, it is of paramount importance to improve the sanitary situation in the pig houses in order to exclude the possibility of moving the infection inside the farm.

For the most favorable outcome of the disease, or its prevention, a preventive strategy is based on an integrated approach that affects:

  • the source of infection is sick youngsters and pregnant sows just before the birth,
  • transmission routes - animal rooms, inventory, rodents,
  • susceptible individuals - newborn piglets and piglets after weaning.

Vaccination

It is very important that piglets acquire passive immunity obtained with colostrum. To do this, pregnant sows are administered a polyvalent GOA vaccine, in which 9 strains of different bacterial serogroups are present.

  • administered to the sow twice 4-6 weeks before farrowing 5 ml of the solution intramuscularly, with a 2-week interval between vaccines,
  • active immunity in sucking piglets is produced at the age of one week,
  • The same vaccine is used in piglets at a dose of 2 ml for 2-3 weeks before weaning.

The disadvantages of this procedure are the fact that biofactory vaccines have only O-serogroups, their effectiveness is considered low.

In the case of animal disease with a serotype of colibacillosis, which is not in the vaccine, it is more efficient to replace the brand-name drug with autovaccine. In order to increase the efficiency in this case, it is recommended to exercise control over the serogroups of Escherichia circulating in the household.

For five-day-old chicks, polyvalent anti-escherichiosis serum is administered intramuscularly, in a prophylactic dose of 8-10 ml (10-15 ml for piglets older than five days). For treatment, the same serum is administered in doses of 15-20 ml and 20-30 ml, respectively, by age.

Full feeding and timely vaccination - the key to strong immunity and animal health.

Disinfection

In the room with piglets, it is important to control the temperature (up to 34 degrees), to monitor the purity of the water.

It is also recommended to timely clean the floors, dishes and equipment. For this use:

  • For flooring and the bottom of the wall - hot sodium hydroxide solution in the form of irrigation at the rate of 0.5 liters per square meter of surface. The solution is left for 30 minutes and then washed off with water under pressure.
  • For disinfection of premises, solutions of chloramine or hypochlorite of 2% concentration are effective. The contact time in this case is 60 minutes. Fluid flow rate is similar. After the surface is washed with water, ventilate the room.
  • Sows preparing for farrowing are transferred to pre-prepared and disinfected queen cells (separate for each pig).
  • Before the pen, each uterus is cleaned of impurities, and the limbs are washed with a solution of 0.5% Lysol or 5% formalin.
  • To the beginning of farrowing, the rear part of the body of the sow is washed with a solution of potassium permanganate or furatsilina.
  • Personnel accepting newborn piglets should be dressed clean. Hands are thoroughly disinfected.
  • After the end of the generic activity, the pig is washed, the nipples are cleaned with a clean cloth with disinfecting solution, and only then the piglets are brought in.
  • During weaning, the youngsters are kept separately, after the complete treatment of the room (manure cleaning, cleaning surfaces), is transferred to more adult animals.

In the pig industry, it is especially important to adhere to the recommendations for the prevention of infectious diseases, observing sanitary standards. Only in this way the risk of escherichiosis in the household will be reduced to a minimum and will save the economy from economic losses.

Conclusion

Colibacteriosis in pigs develops due to infection with Escherichia. Pathogenic microorganisms cause attacks of diarrhea in animals, which lead to rapid dehydration. Lack of treatment can cause mass death of pigs. Piglets are especially sensitive to Escherichia, as they have not yet formed an immune system.