Discussion

In this case, the patient was infected with pathogenic Leptospira species from her pet dog. We were able to diagnose her appropriately through detailed medical interviews and by including leptospirosis in the differential diagnosis. We instructed the patient and her family to strictly maintain personal hygiene to avoid zoonotic infections.

Leptospirosis is a neglected zoonotic disease that significantly affects the population in rural and tropical areas with humid climates.4 It is estimated that there are approximately 1 million cases and 58 900 deaths annually due to leptospirosis, with a mean fatality rate of 6.85%.5

The prevalence of leptospirosis depends on environmental and social conditions. Tropical countries account for 73% of the world’s leptospirosis cases.4 The morbidity appears to be related to populations in rural and impoverished areas with poor sanitation and inadequate precautions.2 The highest estimated disease morbidity and mortality rates have been observed in parts of Oceania, Southeast Asia, the Caribbean and East Sub-Saharan Africa.5 Leptospirosis is caused by bacterial spirochetes of the genus Leptospira. They penetrate the human body through abrasions and mucous membranes by direct exposure to infected animals or indirect exposure through contaminated fluids or soil.6 Possible risk factors include water-based recreational activities, overseas travel, natural disasters and some occupations such as farming, abattoir work and veterinary medicine.2 5 7 Although rats are known to be the most important reservoir host, over 150 mammalian species worldwide can also be responsible for the transmission of leptospirosis.3

The pathogen infiltrates the animal’s body and reaches proximal convoluted tubules of their hosts, where it proliferates and is excreted by urinary shedding.2 8 Thus, obtaining information about the patient’s occupation and history relevant to animal contact and water exposure is crucial in diagnosing leptospirosis.9 The diagnosis of leptospirosis can be challenging in some situations, especially in urban areas or in developed countries, partly because it can be perceived as a tropical disease or may be underestimated by healthcare workers. The diagnosis of leptospirosis is often delayed due to a lack of awareness, particularly in non-indigenous regions.9 10 It is vital to be aware of this potentially serious disease and to consider leptospirosis in the differential diagnosis to avoid any delay in treatment.

The severity of leptospirosis depends on the case, ranging from mild to severe.1 2 Patients typically have fever, myalgia, headache and chills, most of which frequently emerge in other bacterial or viral diseases. Myalgia is one of the characteristic symptoms and often affects the calves, lower back and abdominal area.1 2 Conjunctival suffusion typifies leptospirosis contrary to other infectious diseases.2 Mild leptospirosis is self-resolving and can be managed in an outpatient setting.1 2 On the other hand, 10% of patients develop severe leptospirosis, which is called Weil’s disease and is characterised by jaundice, renal failure and haemorrhaging.1 2 6 8 11

Antibiotic therapy with penicillin G, doxycycline or ceftriaxone should be initiated when a diagnosis is made.1 2 11 12 Based on the patient’s organ involvement, additional therapy, including dialysis, mechanical ventilatory support and extracorporeal membrane oxygenation, may be required in fulminant cases.1 10 13

In this case, the patient presented with a 7-day fever and diarrhoea, grasping pain in the whole muscle and suspected septic shock. A detailed medical interview showed that her pet dog had a fever, diarrhoea and vomiting prior to her presentation. The information regarding the fact that her husband owned rice fields was helpful because it suggested that her dog may have been exposed to Leptospira. After a comprehensive review of her medical history, physical examination and laboratory investigation, we determined that leptospirosis was a possible diagnosis and we thus treated her accordingly. However, without a detailed medical history, we would not have been able to make an accurate diagnosis. Therefore, we should put great emphasis on history-taking, such as animal contact, recreational exposure and the environment where patients live their daily lives. We highlight the importance of considering zoonoses when dealing with suspected sepsis patients who have pets. We inferred that the patient contracted leptospirosis from her pet dog for the following reasons. First, both the patient and dog tested positive for L. interrogans serovar Pyrogenes. Second, the pet dog developed similar symptoms to the patient, including fever, vomiting and diarrhoea approximately 1 month before the patient. The veterinarian assessed the dog as moribund and administered antibiotics for 2 weeks. It was uncertain whether leptospirosis was shed in urine during the treatment period. However, in dogs, urinary shedding starts after the first week of illness and it does not stop until it has received several days of antibiotic treatment.14 The maximum incubation period of leptospirosis is 1 month.2 12 The patient appeared to have developed symptoms during the incubation period. Third, among the patient’s family members, the patient mainly took care of the pet dog and had close contact with the dog while it was debilitated. Finally, the patient only went out when necessary, such as to go grocery shopping, had no contact with other animals other than her pet dog and did not participate in activities with a risk of environmental exposure (eg, outdoor activities or travelling).

On the other hand, there is a limit to what we may presume. Molecular methods that amplify Leptospiral DNA from urine, serum, cerebrospinal fluid, culture of Leptospira and serological methods are used to attain a definitive diagnosis of leptospirosis.1 2 6 The MAT, which detects antibodies against Leptospira serovar and contributes to the diagnosis by comparing acute and convalescent titres, is the most commonly used serological method.1 2 6 When performing an MAT, it should be noted that there is a possibility of cross-reactivity with different serogroups of Leptospira.2 3 15 Paradoxical reactions can occur in which non-infecting serovar results in the highest titre instead of the responsible serovar.2 3 Although both the serological tests of the patient and the dog were positive for L. interrogans serovar Pyrogenes, the dog’s serological tests also showed an elevated convalescent titre of 800:1 for L. interrogans serovar Canicola in the MAT. A possible canine Leptospirosis is indicated when the convalescent titre is not less than 800 during a single MAT.3 14 This means the pet dog had a possibility of contracting L. interrogans serovar Canicola as well as L. interrogans serovar Pyrogenes because the titres were equal. Although the MAT results of L. interrogans serovar Canicola were influenced by previous vaccination,14 the dog had no history of vaccination against Leptospira. Thus, the dog had the possibility of contracting L. interrogans serovar Canicola, L. interrogans serovar Pyrogenes or both. However, based on the patient’s history, in which she had no contact with other animals or environmental exposure, we contend that she had been infected with leptospirosis from her pet dog. The fact that she contracted leptospirosis within the incubation period of exposure to the sick dog supports our hypothesis. Pet dogs are considered to be potential sources of leptospirosis, but the transmission of leptospirosis from pet dogs to humans is rare, even among high-risk populations.15 16 Nonetheless, clinicians should be cautious about the bacteria’s potential transmission to owners, which could cause severe symptoms. The Centers for Disease Control and Prevention advocates that immunocompromised hosts should be monitored by physicians.16 We assume her immune system was potentially suppressed because a laboratory test revealed hypozincaemia. Zinc deficiency was involved in the infection, as it affects cell-mediated immune dysfunction, which is adverse to sepsis.17 18 Zinc deficiency reduces innate immunity by decreasing polymorphonuclear cells and phagocytosis.18 Recognition of major histocompatibility complex class I by natural killer cells and the lysis by natural killer cells are modulated by zinc deficiency.18 It should be noted that the plasma concentration of zinc decreases due to the systemic inflammatory response, thus making it challenging to interpret the data.19 During the acute inflammatory phase, inflammatory cytokines lead zinc to shift into cellular compartments, resulting in hypozincaemia.18 Additionally, hypoalbuminaemia can lower the plasma zinc level because most zinc binds to albumin.20 21 It is also noteworthy that the patient had an impaired taste, which was related to her poor appetite and low body weight. We diagnosed her with zinc deficiency as well as hypozincaemia and administered zinc acetate hydrate before her symptoms improved. There have been few reports of canine-to-human transmission of leptospirosis. There is a case report that describes five cases of human leptospirosis in which transmission from immunised dogs occurred.22 That study reported five human cases of leptospirosis and four immunised pet dogs that were seropositive and which had been in areas in which rats had been seen. That study reported that canine leptospirosis can lead to human leptospirosis via urinary shedding, even from immunised animals. Given the number of pet dogs, the recognition of leptospirosis is crucial from the perspective of public health because it can spread through animal reservoirs. It is, therefore, necessary to maintain a high index of suspicion for leptospirosis to prevent it from spreading in the household and to avoid underestimating its incidence.