Plasmodium knowlesi infection in a returning German traveller from Thailand: a case report on an emerging malaria pathogen in a popular low-risk travel destination

Despite considerable reductions in morbidity, malaria remains to be a relevant public health concern for Southeast Asia. The WHO World Malaria Report 2016 estimates 52,000 malaria infections for the year 2015 for Thailand among a total population of 68.2 million [1]. At the same time, the region represents an important tourist travel destination. The total number of incoming tourists to Thailand has increased almost fivefold in the past 20 years, from 7 million per year in 1995 to 33 million in 2016 [2]. In 2015 more than 760,000 travellers from Germany visited Thailand, the majority for leisure purposes [3]. Thailand is in most parts considered a low risk destination concerning malaria infections in travellers, thus current preventive algorithms recommend exposure prophylaxis through use of insecticide treated bed nets and repellents for most major tourist destinations, without a general recommendation of antimalarial chemoprophylaxis. For longer trips that include stays in rural areas, German and Swiss guidelines, and the recent update of UK guidelines of 2017 recommend provision of an emergency standby medication [4, 5]. The CDC and the previous UK national guidelines advise taking antimalarial chemoprophylaxis for some rural areas, e.g. the forested areas bordering Cambodia, Myanmar and Laos [6, 7]. Recommended antimalarials for chemoprophylaxis are atovaquone/proguanil or doxycycline due to concerns over possible mefloquine resistance.

The incidence of malaria across all Plasmodium species in travellers to Thailand from 12 European countries and the USA has been reported by Behrens et al. at 0.60 cases per 100,000 visits for the year 2008 [8]. The species distribution for malaria infections in Thailand is reported by the World Malaria Report 2016 at 41.8% for P. falciparum and at 58.2% for P. vivax [1]. Plasmodium knowlesi infections are not reported at all in some publications, including the WHO world malaria report of 2016 [1], or as rare events in others. The occurrence of autochthonous infections in Thailand appears to be concentrated around the southern provinces [9‐11].

P. knowlesi has been known as the “fifth malaria species” that naturally infects humans (recently there have been two distinct species identified in P. ovale, increasing the number of human-pathogenic species to six). In contrast to P. falciparum, P. vivax and P. ovale spp. it is mostly a zoonotic parasite that shows a high prevalence in macaques, in particular in Malaysia. The species P. malariae is known as an anthropozoonotic parasite. Different Anopheles species have been described as potential vectors, with divergent feeding affinities towards humans and non-human primates. First infections in humans were reported from Malaysia in 1965, and human infections have been reported as rare events ever since. However, in a retrospective study in Malaysia on samples of patients that had been diagnosed with P. malariae infections, actually 82% of infections were revealed by species-specific molecular analysis as having been caused by P. knowlesi [12] . Also problematic is the reported lack of reliability of common rapid diagnostic tests in detecting P. knowlesi infections [13]. P. knowlesi poses also challenges on malaria prevention efforts, as transmission appears to frequently occur outside homes, rendering bed net use futile [14].

In German travellers returning from Southeast Asia, P. knowlesi infections are analogously considered very rare events. In the available literature, there have been five cases reported in returning German travellers altogether, which have all been visitors to Thailand [15‐19].

As the automated blood count and the CRP rapid test results are conducted in-house at DIDTM, these results were available within the first half hour of presentation, and revealed a marked thrombocytopenia of 79/nl (normal range 140–360/nl) and an elevated CRP (later quantified at 10.6 mg/dl; normal range < 0,5 mg/dl). At the same time the Dengue rapid test and the malaria rapid diagnostic test were negative.

The thin blood smears revealed a Plasmodium infection at an extremely low parasitaemia. In the examination of 100 investigated fields at × 400 magnification only 1 parasite was detected. Consecutively the full thin smear slides were investigated, revealing a total of 3 parasites: one gametocyte and 2 trophozoites (parasite density 0.0002%) (Fig. 2).

The aspects of the identified gametocyte and the trophozoites rather resembled P. vivax. Examination of the thick smear again revealed only 1 structure suggesting a Plasmodium infection without allowing further differentiation. With this finding, treatment was initiated with a 3-day course of atovaquone 250 mg/ proguanil 100 mg, 4 tablets daily, as recommended by the national malaria guidelines of the Association of the Scientific Medical Societies in Germany [21]. Due to the only mildly reduced general condition of the patient the decision for outpatient management was taken. The same day the Plasmodium genus-specific PCR tested positive but the differential PCR revealed negative results for P. falciparum, P. vivax, P. ovale and P. malariae. Both the DIDTM and the national reference laboratory at BNITM returned a positive PCR result for P. knowlesi.

Fever and sweats settled the evening the first dose of atovaquone/ proguanil was given. The patient was seen again at DIDTM on 18 and 23 January, when repeated blood counts and smears were performed. Thrombocyte levels went down to 75/nl on 18 January, and returned to normal levels by 23 January. Blood smears on both follow-up days remained negative for parasites. The serology for P. falciparum and P. vivax remained negative. The patient improved without any signs of adverse treatment effects (Table 1).

In the currently available literature, knowlesi malaria remains a rather rare event outside Malaysia, and particularly rare in travellers returning from Southeast Asia. Of note, the World Malaria Report 2016 does not state any P. knowlesi infection for Thailand, but only P. falciparum and P. vivax infections [1]. However, recent studies are indicating endemicity for P. knowlesi also in other Southeast Asian countries, such as Myanmar and Indonesia [22, 23].

Based on reported incubation periods for knowlesi malaria of about 9 to 12 days [24], it seems likely that the infection in this patient was acquired on the island of Little Koh Chang in the Andaman Sea, which is not to be confused with the much larger island of Koh Chang in the Gulf of Thailand. As macaques are the main reservoir of P. knowlesi, further studies are needed to assess the prevalence of the parasite in the monkey population on this island, which is a hideaway recommendation for tourists that intend to avoid the main crowds on the Thai islands on the gulf side.

As has been described in other publications, knowlesi malaria has to be suspected in situations where commercial rapid diagnostic tests reveal negative results in samples with positive blood smears [13, 17, 18]. The morphology of parasites in smears appears to be heterogeneous, and does not allow for a conclusive diagnosis [25]. Also, parasitaemia at presentation can be very low rendering parasite identification even more challenging, although potential hyperparasitaemia has been reported [25]. In cases where Plasmodium-species differentiation is hampered, it is recommended to do a differential PCR [26]. In the case presented here, the extremely low parasitaemia could even have led to a negative smear result in the first set of specimens. Only one gametocyte that was found in 100 visual fields of the blood smears led to the positive result of a plasmodium infection. The necessity of taking repetitive blood samples in the traveller with fever of unknown aetiology returning from malaria-endemic areas has to be kept in mind. In addition, an infection by P. knowlesi has to be suspected when the commercially available malaria PCR assays reveal a constellation of a positive genus specific sequence together with negative results for species specific sequences, as these usually do not comprise P. knowlesi.

As both artemisinine combination therapies [27] and atovaquone/ proguanil [28] seem to be effective in patients not showing severe illness with both P. knowlesi and P. falciparum infections, these should be considered as first line presumptive treatments where inconsistent laboratory results hamper an instant conclusive diagnosis. In our patient a rapid cessation of fever could be reached with atovaquone/ proguanil within 12 h of the first administered dose.

It is noteworthy that malaria prevention guidelines regarding travels to Thailand do differ between Germany, Switzerland, the UK and the USA. Both the German and the Swiss guidelines do not recommend antimalarial chemoprophylaxis for any region of Thailand, but recommend provision of standby medication for stays in the border regions particularly when outside reach of health care facilities [4, 5]. Contrary to that, the current US and the previous UK (until 2017) guidelines do recommend chemoprophylaxis with atovaquone/ proguanil or doxycycline for stays in border regions [6, 7], such as Ranong Province, bordering Myanmar, where our patient travelled to. The recent update of 2017 of the UK guidelines has now dropped the recommendation on chemoprophylaxis [29]. Adherence to the previous UK and current US guidelines would most likely have prevented this course of a P. knowlesi infection. Nevertheless, the authors are just able to present one case in this report, whereas recommendations for malaria prevention have to be based on risk modelling for geographic regions which are based on larger datasets. However, it has to be kept in mind that Thailand remains a favourite travel destination for German tourists, and diagnostic and treatment algorithms for febrile returning travellers have to take into consideration P. knowlesi infections. Diagnosis can be challenging due to ambiguous morphology of plasmodial parasites, very low levels of parasitaemia and negative results in commercially available rapid diagnostic tests. Treatment with atovaquone/ proguanil showed to be effective with rapid clinical improvement.