This study confirms that both SFG rickettsioses and ST are found in a geograpically restricted manner in Sri Lanka. Serological methods were used in the confirmation of clinical cases. Both serological tests were interpreted according to manufacturers' recommendations as location based seroprevelance studies have not been conducted for most areas of Sri Lanka .
When the epidemiological features of the seropositive groups were compared, two differences were noted. The mean age of the ST seropositive patients were less than that of the SFG. This could have been due to greater participation by paediatricians than physicians from areas endemic for scrub typhus than an actual higher incidence of disease in children. Secondly, in the SFG seropositive group, the commonest occupation of adults and commonest parental occupation in paediatric patients was work in the estate sector. In the current study, the majority of SFG samples were received from BH Nawalapitiya and BH Gampola which drain patients from the surrounding estates. For the ST seropositive group, the main occupation of adult patients and of parents of paediatric patients was farming. Both farming and working in estates involve activities that bring patients in contact with ticks and mites that act as vectors of the disease. An interesting observation, previously noted by Nanayakkara et al. (2010) is that the majority of both SFG and ST seropositive female patients were housewives .
Rash was detected in 10-36% of ST seropositive patients while it was present in a very high proportion (87-97%) of the SFG seropositive patients. The opposite was true for the presence of eschar as 55-67% of the ST seropositive patients had an eschar while only 1-4% of the SFG seropositive patients had an eschar. When the proportions of rash and eschar were compared between these two groups in cohort 1 and 2, the differences were statistically significant. It is noteworthy that eschar was not seen in 1/2 to 1/3 of patients seropositive for ST, this may be due to differences in detection or due to different strains of Orientia.
Prevalence of rash, eschar and other clinical features within SFG rickettsioses depends on the implicated organism. ST is associated in about 80% cases with an eschar . Ogawa et al. described the presence of eschar in 87% of patients diagnosed with ST in Japan . In 1998. Liu et al. gave an 88.5% prevalence of skin eschars in a study conducted in China from 1995-2006 . The prevalence of eschar in ST seropositive patients was slightly lower and could be attributed to the difficulty in detection due to the relatively darker complexion of Sri Lankans.
Rash had been noted in varying proportions in ST. While Ogawa described a rash in 93% of the study population in Japan in 1998, a more recent study conducted from 2000-2003 in Thailand described presence of rash in only 8.9% of patients [6, 8]. Similar findings had been reported from China, where Liu et al. found that rash was present in 90.4% of their study population and compared it with another study in China with a 37.2% presence of rash . In the current study rash was seen in 10-36% of patients. This was in accordance with some of the above mentioned studies.
Closer to home, in a study conducted in South India, a rash was seen in 22% of patients and an eschar was noted in 4% of patients with ST . While, it was obvious that in comparison with spotted fever the presence of rash was less in ST patients from Sri Lanka, the relatively lesser prevalence in comparison to studies from Thailand and China may be due to the darker complexion as mentioned above.
Headache was found in 39-88% of ST seropositive patients and 37-58% of SFG seropositive patients. Myalgia was seen in 33-39% of the ST seropositive patients and 53-68% of the SFG seropositive patients. The proportion of ST seropositive patients who had arthralgia was 22-43% while it was 67-71% for the SFG seropositive group. Other clinical features studied were hepatomegaly, splenomegaly, conjunctival injection, and localized and generalized lymphadenopathy, prevalences of which were similar to those reported from other countries [8, 10–12]. As the exact causative organisms for SFG and the strain for Orientia were not identified, a detailed comparison was not attempted.
Clinical features of seropositive groups were not compared with those of the seronegative groups as the chance of false negativity due to early sample collection was present.
A major drawback was the non availability of certain clinical data in the accompanying request forms. In some instances it was possible to call the hospitals and get the information whereas in others it was not.
Samples were received from 23 hospitals representing 8 provinces in Sri Lanka with the exception of the Northern Province. However, the number of samples received from all hospitals was not similar. This may be due to the variation in the number of patients presenting to each hospital with suspected rickettsial disease or the interest of the healthcare staff. The hospitals that sent samples constantly and regularly were Base hospitals Gampola, Nawalapitiya, Embilipitiya, and Kuliyapitiya, and General hospital Kurunegala.
ST was the main rickettsioses found in patients presenting to Base hospitals Kuliyapitiya, Dambadeniya and General hospital Kurunegala in the North Western province, General hospital Anuradhapura in the North Central Province, Base hospital Embilipitiya in Sabaragamuwa province and General hospital Matara in the Southern province of Sri Lanka.
Of these areas, Kuliyapitiya and Dambadeniya are located in close proximity in the Kurunegala District. These two areas are predominantly agriculture based areas with intermediate rain fall. General hospital Kurunegala is the tertiary care hospital in the district capital and receives patients from the city limits and patients transferred from nearby Base hospitals including Kuliyapitiya and Dambadeniya. Thus, the finding of ST as the main rickettsioses present in patients presenting to Base hospitals Dambadeniya, Kuliyapitiya and General hospital Kurunegala indicate the widespread nature of the disease in the Kurunegala district. Orientia tsutsugamushi is transferred mainly by the larval stages of mites of genus Leptotrombidium commonly known as chiggers. Agriculture, particularly rice cultivation, forestry and oil palm and rubber plantation associated work has been shown to pose risk of acquisition of ST . The climatic and weather conditions of this area situated in the intermediate dry zone of Sri Lanka would provide a favourable niche for the potential vectors to thrive. General hospital Anuradhapura, which is one of the two main tertiary care hospitals in the North Central province of Sri Lanka, provides treatment to patients from areas which are predominantly agriculture based. Anuradhapura is in the dry zone of Sri Lanka and scrub jungles are plentiful, especially in areas where the primary forest gardens are destroyed. Base hospital Embilipitiya is situated in the southern most area of the Sabaragamuwa province in the Udawalawa basin. This is a relatively dry area with plenty of scrub jungles. Matara is also in the intermediate rainfall zone of Sri Lanka situated in the Southern province. Though the number of samples received from General hospital Trincomalee of the Eastern province of Sri Lanka was small, the predominant rickettsioses found there was ST. Previous studies have shown ST is the main rickettsioses found in patients presenting to hospitals in the Western Province of Sri Lanka . Nagalingam in 2006 has shown the presence of ST in Jaffna district and in a few other places in Sri Lanka, but the number of samples tested was limited . Therefore, this is the first time where an Island wide representative number (with the exception of the Northern and Western Provinces) of samples were tested over a period of time and serological proof of disease established for ST.
A few studies conducted prior to the ones cited were not included in the discussion as confirmation of the diagnosis in these studies was by the Weil Felix test which is now regarded as an obsolete test as sensitivity and specificity are around 50% . In addition, this test has not been validated for most rickettsial infections.
Base hospitals Nawanapitiya and Gampola were the two hospitals providing the most number of samples for the study. These 2 hospitals are situated within a 1 h distance from University of Peradeniya and about 30 min from each other in the central hills of the country. Samples received from both hospitals were predominantly SFG seropositive. TH Peradeniya situated in a neighboring are has reported SFG rickettsioses during the last few years [4, 17]. The presence of SFG rickettsioses in paediatric patients presenting to Base hospital Nawalapitiya has already been reported . The current study's findings were in agreement with these findings. Teaching hospital Kandy which is situated within a 10 min driving distance from Teaching hospital Peradeniya interestingly receives far less number of patients with suspected rickettsioses. Teaching hospital Kandy drains patients mainly from the eastern slope of the central hills and is the referral centre for patients transferred from the same areas. Thus, it appears that the eastern slope of central hills have a lesser number of patients with rickettsioses. General hospital Matale, situated along the eastern slopes of the central hills, also reported very few patients with SFG rickettsioses.
Base hospitals Diyatalawa and Monaragala, two hospitals in the Uva province and General hospital Badulla, the provincial general hospital in the Uva Province also showed a preponderance of SFG patients. These areas, unlike the western slope of the central hills are situated in the intermediate and dry zone of Sri Lanka. The weather tends to be mostly dry but cold.
Other hospitals have forwarded less than 3 samples and are thus not discussed.
This information would be useful to commence health educational activities in the given areas. Mapping of rickettsioses in this manner is helpful to diagnose the disease in internal travel related infection too. In depth studies could be commenced in these areas in order to identify vectors and reservoirs.
Most vector borne diseases have seasonality. This holds true for rickettsial infections as well . Though Sri Lanka is a small island, it is obvious that different rickettsioses are found in different areas of the country. Thus, to establish seasonality of the disease each needs to be studied over a period of time in the endemic areas. Graphical analysis of the chronology of patient presentation in Gampola, Nawalapitiya, Kuliyapitiya and Embilipitiya hospitals demonstrates a seasonal variability. When data for 2009 and 2010 were compared in the first three hospitals, similar patterns were noted. Comparison of pooled data for Kandy, Kurunegala and Ratnapura districts with data from Epidemiology unit also show similar patterns indicating that the study received representative samples. The minor differences could be explained by the non participation by some hospitals, erratic reporting of cases to Epidemiology unit and differences in the areas of residence of patients.
Notification data and laboratory based data, as shown by the number of samples received and seropositivity show remarkable congruence. Apart from concluding that the sampling procedure in the study appears to be representative of the distribution of rickettsioses in Sri Lanka, these results also demonstrate the remarkable clinical diagnostic accuracy of this infection. The specificity of clinical features for diagnosis is relatively uncommon in infections presenting with fever and rash. Algorithms using clinical features could be used for diagnosis with laboratory confirmation as a backup for atypical presentations and further research. These data therefore need to be made available to clinicians in the given hospitals. At the same time, location based field and laboratory studies need to be conducted to assess tick activity and weather patterns in these areas simultaneously to define the risk associations and establish preventive measures.