Porcine enteric colibacillosis

1.   Introduction

Porcine infection caused by Escherichia coli (E. coli), the so-called swine colibacillosis, is responsible for a wide range of problems, such as neonatal diarrhea, post-weaning diarrhea (PWD), edema disease (ED), septicemia, polyserositis, coliform mastitis, and urinary tract infections [1]. Escherichia coli is a gram-negative peritrichously flagellated bacteria belonging to Enterobacteriaceae [2]. The major pathotypes of E. coli include enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic Escherichia coli (EHEC), enteroaggregative Escherichia coli (EAEC), enteroinvasive Escherichia coli (EIEC), diffusely adherent Escherichia coli (DAEC), Vero- or Shiga-like toxin-producing Escherichia coli (VTEC or STEC) and enterotoxigenic Escherichia coli (ETEC) [3]. E. coli is classified into 150 to 200 serotypes or serogroups based on 3 antigens: somatic (O) or cell wall antigen, capsular (K) antigen, and flagellar (H) antigen. Seventy-five types of the H or flagellar antigen, 173 types of O or somatic antigens, and 103 types of the K or capsular antigens have been recognized [4,5] (Figure 1). Additionally, the role of adhesins and surface proteins called fimbriae is to enable the adherence of ETEC to specific receptors on the brush borders of the small intestine enterocytes [6]. Regarding fimbriae, there are five common antigenically different types found in pigs: F4 (K88), F5 (K99), F41, F6 (987P), and F18 [7]. It should be noted that the most diffuse etiological agent responsible for neonatal diarrhea and PWD in piglets is ETEC, which displays the fimbriae F5 (K99) and F4 (K88) [8] (Table 1). Colonization arises in the small intestinal mucosa, which leads to the production of enterotoxins, including α-hemolysin, enteroaggregative heat-stable enterotoxin (EAST-1), heat-labile toxins (LT), heat-stable toxins (ST; variants STa and STb), and Shiga toxin (Stx2e) [9].

Figure 1. Antigenic structure of E. coli [5]. 

Table 1. Pathotypes, adhesins, and enterotoxins of porcine pathogenic E.coli responsible for neonatal and post-weaning colibacillosis (modified from [10,11]).

EAST-1: enteroaggregative heat-stable enterotoxin; EPEC: enteropathogenic E. coli; ETEC: enterotoxigenic E. coli; Stx2e: Shiga toxin type 2e; STa: heat-stable toxin a; STb: heat-stable toxin b; LT: heat-labile toxin.

 2.     Etiology and pathogenesis

2.1 Infection route

Sows, especially those with uterine or mammary gland infections, can shed pathogenic E. coli in their birth canal, feces or milk, exposing newborn piglets to the bacteria, which infects them with neonatal diarrhea [12,13]. Piglets are particularly susceptible to colibacillosis during the first few days of life, especially from 0 to 4 days [10]. The routine cleaning and disinfection are insufficient to break the infection cycle by ETEC because E. coli can be found in fecal-contaminated feed, water, soil, and the environment of the pig barn for a long period [6,14]. Low temperatures and enough moisture, among other factors, achieve long environmental survival times. Some studies have shown that ETEC can survive in a porcine barn environment for up to 4 weeks [15]. Prior research reported that low feed intake over the first week after weaning correlates with the risk of disease occurrence over the post-weaning period [16]. Underfeeding during weaning reduces the growth performance of piglets, contributes to intestinal inflammation, and adversely affects villous height and crypt depth [17]. This morphological disruption of the intestinal mucosa creates an ideal environment for E. coli infection because E. coli and enterotoxins can cross the epithelium to enter piglets [18] (Figure 2).

Figure 2. The procedure in the pathogenesis of post-weaning diarrhea in pigs [19].

2.2 General pathogenic mechanism of E. coli (Figure 3):

(1) The adhesions of E. coils adhere to the epithelial cell of the gastrointestinal tract enables E. coli to deliver enterotoxins (LT, STa, and STb), which activate cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) systems [20].

(2) LT increases the secretion of sodium, chloride, and hydrogen carbonate ions from crypt cell, while STa and STb reduce the absorption of liquid and salts by villus [21].

(3) LT, STa, and STb result in hypersecretion of water and electrolytes into the small intestine, exceeding the absorptive capacity of colon [22].

(4) This process leads to diarrhea, dehydration, poor appetite, reduced nutrient digestibility and growth performance, as well as potential death for piglets [23].

Figure 3. Pathway for post-weaning diarrhea by the heat-labile toxins (LT) or heat-stable toxins (ST; variants STa and STb) of E. coli [21].

2.3 Neonatal enteric colibacillosis:

(1) Enterotoxigenic E. coli causing neonatal colibacillosis enters the newborn piglets through ingestion and proliferates in the intestine [10,24].

(2) Adhesins (F4, F5, F6, and F41) allow ETEC to adhere to specific receptors on the enterocytes of the small intestine [25].

(3) Enterotoxigenic E. coli strains of neonatal colibacillosis produce STa, which stimulates cGMP system, leading to electrolyte and fluid secretion and causing dehydration [10,25].

2.4. Post-weaning colibacillosis:

(1) E. coli causes PWD by entering through ingestion, proliferating, and causing disease with specific virulence factors [6,10].

(2) Enterotoxins produced include STa, STb, LT, and EAST-1. The mechanism of action for STa is similar to neonatal colibacillosis [26].

(3) STb has a different mechanism, inducing the duodenal and jejunal secretion of water and electrolytes, and stimulating bicarbonate secretion [27].

(4) LT activates adenylyl cyclase, which leads to hypersecretion of electrolytes and water and causes dehydration in piglets [28].

(5) EAST-1 is also involved in diarrhea caused by ETEC, but its role is not fully understood [29].

(6) Post-weaning ETEC strains possess fimbriae F4 and F18 [30].

3.     Clinical signs and gross lesions

3.1 Neonatal diarrhea

Neonatal diarrhea due to ETEC is observed mostly in newborn piglets aged from 0 to 4 days of life, and in general, litter from first-parity sows could be more involved due to a lack of protection by passive immunity [31]. When ETEC sustains neonatal diarrhea, enormous quantities of watery to creamy consistency scour are sensitive, with a distinctive smell and often white to yellow in color [10,32] (Figure 4). The mortality can reach up to 70% in neonatal piglets with severe watery diarrhea [33] (Table 2).

Figure 4. Neonatal diarrhea causing by E. coli [34].

Table 2. Clinical characteristics of E. coli on neonatal diarrhea and post-weaning diarrhea (PWD) in piglet (sorted out form [10,35,36])

EPEC: enteropathogenic E. coli; ETEC: enterotoxigenic E. coli.

3.2 Post-weaning diarrhea

Post-weaning diarrhea due to ETEC (main pathogenic bacteria) and EPEC is commonly observed from 2 to 3 weeks after weaning, and sometimes, it can be recorded between 6 and 8 weeks after weaning (Table 2) [10]. The cases of post-weaning colibacillosis due to ETEC are usually characterized by yellowish, grey, or slightly pink watery diarrhea with a characteristic smell, generally lasting one week [10,31] (Figure 5A, Table 2). Affected pigs are usually depressed with a reduced appetite and a rough, sticky, wet hair coat. Sudden deaths can occur, particularly at the start of the outbreak, and dead pigs are usually dehydrated with sunken eyes. The small intestine is usually dilated, slightly oedematous, and hyperemic (Figure 5B). The stomach is usually dilated and full of clotted milk or dried feed in neonatal or post-weaning colibacillosis, respectively, and shows hyperemia of the fundus (Figure 5C). The mesenteric lymph nodes are enlarged and commonly hyperemic. These lesions, even if not pathognomonic, suggest enteric colibacillosis [10,31]. The mortality is 1.5% to 2% in post-weaned and grow-finish pigs with moderate diarrhea and up to 25% in untreated pigs with severe to moderate diarrhea (Table 2) [37].

Figure 5. (A) Diarrheic feces of pigs suffering from enterotoxigenic E. coli (ETEC) post-weaning diarrhea (PWD). (B) The intestine of a pig suffering from ETEC PWD appears dilated, oedematous, and hyperemic. (C) The stomach of a pig suffering from ETEC PWD. Gastric fundus shows severe hyperemia (organized data from [10]).

 4.     Prevalence

Enterotoxigenic E. coli strains are present in 16.6% of non-diarrheic pigs during the suckling period of piglets, 66% in the nursery phase, and 17.3% in the finisher population [33]. Furthermore, ETEC strains can be shed in the feces of healthy pigs, such as sows. Swine colibacillosis infection is widespread in industrialized and developing countries of temperate, subtropical, and tropical climates [38]. Table 3 lists currently known prevalence rates of ETEC in the porcine industry in some countries.

Table 3. Prevalence of enterotoxigenic E. coli (ETEC) in porcine industry of different countries.

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