Multidrug-Resistant Corynebacterium spp., Escherichia coli, and Klebsiella spp. Isolated from the Lungs of a Six-Year-Old Arewa Stallion

INTRODUCTION

Diagnostic microbiology plays a crucial role in veterinary medicine by enabling the identification of pathogens responsible for infectious diseases, thus guiding appropriate treatment strategies.^[1] In equine practice, respiratory infections are a significant concern due to their impact on animal health and performance.^[2] The emergence of multidrug-resistant (MDR) bacteria poses a severe challenge to treatment efficacy, thus making timely and accurate microbial diagnosis even more critical.^[3] This report highlights the complexities of managing a case of MDR bacterial infection in a 6-year-old Arewa stallion, emphasizing the importance of diagnostic microbiology in devising effective therapeutic interventions.

This case report provides critical insights into the diagnostic challenges posed by MDR bacterial infections in equine respiratory diseases. The findings align with current trends in veterinary microbiology, thus highlighting the need for comprehensive diagnostic protocols, including necropsy, culture, biochemical testing, and antibiotic susceptibility testing. By presenting this case, we aim to shed light on the evolving scope of antibiotic resistance in equine respiratory diseases and the pivotal role of diagnostic microbiology in addressing these challenges, to ultimately improve animal health outcomes.

CASE REPORT

A lung sample was obtained from the carcass of a 6-year-old Arewa stallion presented to the Necropsy Unit, Veterinary Teaching Hospital, and submitted to the Bacteriology Laboratory, Department of Veterinary Microbiology, Ahmadu Bello University Zaria, Nigeria. History revealed that the nail was removed from the left forelimb, and tetanus toxoid was administered 3 weeks before the presentation of the stallion to the Large Animal Clinic. The clinical signs observed were hemoglobinuria, sweating, high temperature (41°C), congested ocular mucous membrane, dilation of the nostrils, restlessness, recumbency, and generalized edema around the head and neck. The medications administered were imidocarb dipropionate, benzyl penicillin, intravenous dextrose saline, xylazine, diclofenac, and multivitamin, and the feed were hay and bran (wheat offal).

The stallion died 2 days of after being sick and recumbent, and the carcass was transported to the Necropsy unit for postmortem examination. The gross findings were a slightly enlarged heart with a chicken fat clot in the right ventricle; congested, hemorrhagic and hepatized lungs; hemorrhagic trachea with frothy exudates; enlarged and congested liver; enlarged spleen; severe hemorrhagic gastroenteritis with helminths in the colon; and congested kidneys.

Outcome of culture and isolation

Three organisms were isolated from the lungs of the horse. The first isolate appeared as small, grayish, non-hemolytic colonies on blood agar and did not grow on MacConkey. The isolate was gram-positive rods with a Chinese letter-like appearance [Figure 1], presumptive for Corynebacterium spp.

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Figure 1:

Gram-positive rods with the shape of Chinese letters, presumptive for Corynebacterium spp (Grams stain, X100 Magnification).

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The second isolate appeared as medium-sized, smooth, raised, whitish non-hemolytic colonies on blood agar, which was pinkish on MacConkey (lactose fermenter), produced a greenish metallic sheen on EMB [Figure 2a], gram-negative rods [Figure 2b], suggestive of E. coli. The third isolate appeared as gray, small, mucoid, non-hemolytic colonies on blood agar, pink, mucoid colonies on MacConkey [Figure 3a] and Gram-negative short rods [Figure 3b] presumptive for Klebsiella spp.

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Figure 2:

(a) Colonies with greenish metallic sheen on eosin methylene blue agar; and (b) Gram-negative rods, presumptive for Escherichia coli (Grams stain, X100 Magnification).

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Figure 3:

(a) Small size, pink, mucoid colonies on MacConkey agar; and (b) Gram-negative short rods, presumptive for Klebsiella spp (Grams stain, X100 Magnification).

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Biochemical test results

The biochemical test identified the second isolate as E. coli and the third as Klebsiella spp. [Table 1 and Figure 4].

Table 1: Outcome of biochemical test on isolates presumptive of Escherichia coli and Klebsiella sp.

S/N	Biochemical test	Isolate II	Isolate III
1	TSI	Acid/acid+gas	Acid/acid+gas
2	Indole	Positive	Negative
3	Citrate	Negative	Positive
4	Urea	Negative	Positive
5	Methyl red	Positive	Negative
6	Voges Proskauer	Negative	Positive
7	Motility	Motile	Non-motile
		Escherichia coli	Klebsiella spp.

TSI: Triple sugar iron

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Figure 4:

(a) Biochemical test outcome of Escherichia coli (b) Biochemical test outcome of Klebsiella spp. From L-R (a & b): Triple Sugar Iron, Indole, Urease, Citrate, Methyl Red, Voges Proskauer and motility.

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DISCUSSION

The isolation of Corynebacterium spp., E. coli, and Klebsiella spp. from the lungs of the stallion highlights the complex interplay of bacterial pathogens in severe equine respiratory infections.^[4] The case presented a rapid progression of clinical symptoms, including systemic inflammation (generalized edema), severe respiratory distress, and multisystemic involvement, ultimately leading to mortality within 2 days of onset. The observed history of tetanus toxoid administration following the removal of a nail from the forelimb suggests that the animal was already immunologically challenged, potentially predisposing it to secondary infections. In addition, the clinical signs such as hemoglobinuria, fever (41°C), restlessness, and recumbency point toward systemic toxemia and organ failure.^[5,6]

Postmortem examination revealed gross pathological changes consistent with a severe, multifocal bacterial infection. The findings of hepatized lungs, frothy exudates in the trachea, and congested organs, combined with hemorrhagic gastroenteritis and helminth infestation, suggest a state of overwhelming sepsis. Isolation and identification of the pathogens confirmed their involvement in the disease process. Corynebacterium spp., characterized by its unique Chinese letter-like appearance and catalase positivity, is known for its opportunistic pathogenicity in immunocompromised animals. The presence of E. coli and Klebsiella spp., both facultative pathogens, indicates possible aspiration or hematogenous spread, contributing to severe pulmonary infection and systemic dissemination.^[7,8]

Antibiotic resistance profiles of the isolated bacteria further complicated the therapeutic management of the case. The resistance of all three isolates to multiple antibiotics, including commonly used agents such as ciprofloxacin, gentamicin, and amoxicillin, underscores the growing challenge of antimicrobial resistance in veterinary medicine.^[9] It reiterates the importance of infection control and supportive care in managing systemic infections.^[10]

CONCLUSION

This case emphasizes the critical challenge of MDR bacterial infections in equine medicine, highlighting the need for prompt diagnosis, effective treatment, and antimicrobial stewardship. It reiterates the need for ongoing surveillance and research into resistance patterns in veterinary pathogens.