Community-based prevalence of rheumatic heart disease in rural Ethiopia: Five-year follow-up.

BACKGROUND: As little is known about the prevalence and clinical progression of subclinical (latent) rheumatic heart disease (RHD) in sub-Saharan Africa, we report the results of a 5 year follow-up of a community based, echocardiographic study of the disease, originally carried out in a rural area around Jimma, Ethiopia.
METHODS: Individuals with evidence of RHD detected during the baseline study as well as controls and their family members were screened with a short questionnaire together with transthoracic echocardiography.
RESULTS: Of 56 individuals with RHD (37 definite and 19 borderline) in the original study, 36 (26 definite and 10 borderline) were successfully located 57.3 (range 44.9-70.7) months later. At follow-up two thirds of the definite cases still had definite disease; while a third had regressed. Approximately equal numbers of the borderline cases had progressed and regressed. Features of RHD had appeared in 5 of the 60 controls. There was an increased risk of RHD in the family relatives of borderline and definite cases (3.8 and 4.0 times respectively), notably among siblings. Compliance with penicillin prophylaxis was very poor.
CONCLUSIONS: We show the persistence of echocardiographically demonstrable RHD in a rural sub-Saharan population. Both progression and regression of the disease were found; however, the majority of the individuals who had definite features of RHD had evidence of continuing RHD lesions five years later. There was an increased risk of RHD in the family relatives of borderline and definite cases, notably among siblings. The findings highlight the problems faced in addressing the problem of RHD in the rural areas of sub-Saharan Africa. They add to the evidence that community-based interventions for RHD will be required, together with appropriate ways of identifying active disease, achieving adequate penicillin prophylaxis and developing vaccines for primary prevention.

Introduction

RHD remains a major health care problem, especially in unprivileged populations, with regional differences in prevalence, severity and course of the disease. However, the regional prevalence, potential contributing factors, severity and rate of progression and/or regression are poorly understood, especially in rural areas.[1,2]

In 2017, we reported the results of a community-based prevalence study among children and young adults living in a rural area of Jimma, south-west Ethiopia [3], which showed a high prevalence of different degrees of RHD in the community. We here present a follow-up study of the original cohort of the affected patients as well as matched controls from the community in order to clarify the course of the disease and analyse the rate of progression and/or regression of subclinical latent valvular disease over time.

Methods

Ethics statement

The Jimma University ethical review board approved the project and written, informed consent was obtained from each participant.

Fieldwork

The original study [3] was carried out in the Jimma Zone of Oromia regional state, south-west Ethiopia and used a multi-stage, cluster-sampling technique to derive a sample of 987 participants, aged 6 to 25 years. The prevalence was 37.5 cases per 1000 increasing to 56.7 cases per 1000 when borderline cases were included.

In the current study, all the cases previously identified as having definite or borderline RHD were contacted together with a group of controls of similar age and gender composition to the cases, which had been screened during the first study and did not have any features of RHD. Controls (1–2 per case) were the next available individuals in the survey matched by gender and age (within two years). All subjects that accepted our invitation for follow-up were revisited. A questionnaire was verbally administered in Oromifa (the local language) for inclusion in the registry updating the data previously collected on demographic, social and medical history, together with details of adherence to penicillin prophylaxis. It was followed by a brief clinical examination and echocardiography.

The composition of the households and, specifically, first degree relatives of subjects in the cohort (definite, borderline and a sample of controls) were registered and all first-degree relatives invited to attend the screening clinic. They also completed a brief questionnaire and clinical examination before screening by transthoracic echocardiography.

Echocardiography

A transthoracic echocardiogram was carried out with a CX50 Phillips portable echo machine to obtain parasternal long-axis views, parasternal short axis, apical four chamber, apical long axis and five chamber views. Abnormalities in valve morphology and the presence of valve regurgitation with Doppler interrogation were classified using the current World Heart Federation criteria to ascertain whether definite or borderline disease was present. [4] Definite RHD was defined as the combination of pathological regurgitation with at least two morphological features of RHD or mitral stenosis with a mean gradient ≥4 mmHg or borderline disease of both the aortic and mitral valve. Morphological features of RHD for the mitral valve were anterior mitral leaflet or chordal thickening, restricted leaflet motion or excessive leaflet tip motion during systole; for the aortic valve the features were irregular or focal thickening, a coaptation defect, restricted leaflet motion or prolapse. Borderline disease was defined as having either (a) at least two morphological features of RHD of the mitral valve without regurgitation or stenosis or (b) pathological mitral or aortic regurgitation. Images from both the case series and controls were reviewed by a second experienced cardiologist.

Statistical analysis

Data were double entered before analysis with SPSS version 26. Data are presented as cross-tabulations or mean +/- SD values and comparison between groups carried out with a χ2 test. Relative risks, confidence intervals and p-values for the analysis of the family data were calculated according to the method of Altman. [5]

Results

The follow-up study was carried out at a median of 57.3 (range 44.9–70.7) months after the original study when the participants had a mean age of 17.0 (SD 3.6) years.

Of the individuals who were originally diagnosed as having definite (n = 37) or borderline (n = 19) disease we were able to locate 26 (70%) and 10 (53%) respectively. One of the participants had died, this was due to non-cardiac causes. Eight had migrated to another area (largely due to marriage). The remainder were unavailable mainly as a result of work commitments. Table 1 shows the progression or regression of echocardiographic features in the participants who were followed up according to their original diagnostic category. Of the 26 cases with definite disease, 9 (35%) had regressed with six subjects no longer showing valvular disease while 17 (65%) continued to have definite lesions of RHD. Of the 10 borderline subjects, 4 showed evidence of regression and 3 progressed to have definite features.

Table 1

Progression or regression of echocardiographic changes in the Ethiopian subjects followed up a mean of 4.7 years after original screening.

	Original echocardiogram classification
	Normal	Borderline	Definite
No of subjects	60	10	26
Gender M/F	30/30	7/3	13/13
Age at follow up, yr (SD)	16.6(3.4)	15.3(2.5)	18.3(4.0)
Duration of follow-up, yr (SD)	4.8(0.3)	4.7(0.6)	4.6(0.5)
Echocardiogram classification at follow-up
Normal (%)	55(92)	4(40)	6(23)
Borderline (%)	2(3)	3(30)	3(12)
Definite (%)	3(5)	3(30)	17(65)

Of the 23 cases identified with definite RHD at follow-up, 17 had mitral valve disease, three had aortic valve disease and three mixed valve disease. Three had an audible murmur, 13 described having palpitations although all were in sinus rhythm when examined, and 12 complained of shortness of breath on exertion with two having demonstrable ankle oedema (at the original examination only three of the 23 subjects complained of breathlessness while four had noted palpitations). None of the subjects had required cardiac intervention and no cardiac mortality was reported.

A total of 60 controls were also successfully located over the same follow-up time. Of the controls 5 (8.3%) developed borderline (n = 3) or definite (n = 2) disease (Table 1). The majority of these (4 out of 5) reported episodes of sore throat, but no other features specific for ARF.

Participants who had shown evidence of regression of lesions (n = 13) had a similar age/sex distribution to those whose echocardiographic status was unchanged (n = 20) or showed evidence of progression (n = 8). They also had similar parental educational and occupational status (S1 Table).

Compliance for penicillin prophylaxis was reported in 2 out of 36 individuals (5.6%) with definite or borderline lesions, although all definite and borderline cases were advised to do so. In both cases a parent worked within the health service as a health extension worker.

Of the 23 participants who had definite RHD at follow-up, 20 agreed to have their families screened and of a potential total of 94 first degree relatives 66 (70%) attended for echocardiographic examination. Screening rates were similar for the families of borderline cases and controls. Table 2 shows the numbers of cases detected in households according to proband status and the nature of the relationship of the affected individuals.

Table 2

RHD case detection among the first degree relatives according to proband status in 39 households.

Status of proband	No of households	Number of first degree relatives examined	Relationship	Mean age (yr)	Echo diagnosis of relatives
					Normal (%)	Borderline (%)	Definite (%)
Normal	13	43	All		40(93)	1(2)	2(5)
			Sibling	12.2	23	1	1
			Parent	44.7	17	0	1
			Child	-	0	0	0
Borderline	8	20	All		15(75)	3(15)	2(10)
			Sibling	11.8	7	1	0
			Parent	42.6	6	2	2
			Child	-	0	0	0
Definite	20	66	All		47(71)	6(9)	13(20)
			Sibling	16.8	30	4	7
			Parent	39.5	15	1	5
			Child	10.3	2	1	1

Among 66 individuals screened from 20 households where the proband had definite RHD, 13 (20%) had definite disease, a further six (9%) borderline disease while the remaining 47 subjects were normal. In comparison, of 43 subjects from families of normal probands only 2 (5%) had definite RHD and a further one borderline disease (χ2 = 7.7, p = 0.02). Table 3 shows the relative risks for RHD according to the status of the proband. For probands who had definite RHD, the risk of definite disease in the family relatives was 3.1 times greater and for the combination of borderline and definite disease 2.3 times greater. Where the proband had borderline or definite disease, the risks of RHD in the family relatives were higher (3.8 and 4.0 times respectively). Most of the increased risk was in the siblings; the numbers of cases in children or parents was too small to derive stable risk estimates.

Table 3

Risk of RHD in family relatives according to the status of the proband.

		Risk of RHD in relatives
Proband status	Relatives evaluated	Definite		All
		RR(CI)+	p-value	RR (CI)	p-value
Definite RHD	All family:	3.1(1.1–9.0)	0.04	2.3(1.1–4.8)	0.03
	Siblings:	5.6(0.7–43.5)	0.09	3.0(0.9–9.7)	0.08
Any RHD*	All family:	3.8(0.9–15.7)	0.07	4.0(1.3–12.5)	0.02
	Siblings:	3.6(0.5–27.4)	0.20	3.1(0.7–12.6)	0.12

*Any RHD: definite or borderline disease in proband.

+Relative risk and 95% confidence interval.

Discussion

This study serves to define the rate of progression and/or regression of valvular lesions in a rural area of Sub-Saharan Africa previously shown to have particularly high prevalence of RHD [6,7,8]. Valvular lesions persisted in the majority (65%) of children or young adults originally diagnosed as having definite disease. Although none of the subjects had yet required specific cardiac treatment and no cardiac deaths were reported, a significant number of the patients identified with definite disease reported clinical symptoms and may require intervention in the future. The study was based on the screening of a large number of children and young adults who were carefully selected to be representative of the rural population in this area of Oromia regional state in southern Ethiopia. While the study is small, locating patients in remote rural areas is difficult and very time-consuming and our overall tracing rate of 67% after nearly five years is therefore a major achievement.

Our study does raise questions as to the usefulness of active community screening programs especially in remote rural areas. Apart from the major disadvantage of needing expensive equipment and a high degree of training, many patients identified in our study had mild disease with most remaining asymptomatic during the course of the study. Also the survey data were unable to predict which subjects had continuing or progressive disease. In addition, as has been reported widely [9,10] there was considerable heterogeneity of the echocardiographic appearances over the course of the study with evidence of both progression and regression of valve lesions and the appearance of new disease in 8% of people who had normal echocardiograms during the initial screening.

Disappointingly, although all the definite and borderline cases were advised to receive regular penicillin, almost all reported that they did not. Poor compliance is a widespread problem,[11] and adequate penicillin prophylaxis compliance is extremely hard to achieve. This is likely due to factors common to similar remote rural regions, including lack of training of nurses and health officers in local health centres, poor patient or family understanding of the necessity of penicillin, poor access and high relative costs of accessing health centres and obtaining antibiotic treatment, supply problems and the discomfort associated with i.m. injections. These findings highlight the problems faced in addressing the problem of RHD globally and the need for better primary prevention through the development of an effective vaccine. [12,13,14]

Early observations that RHD exhibits familial clustering have been amply confirmed although these studies failed to reach a consensus on the nature of the underlying susceptibility.[15] Undoubtedly, the very strong links with socioeconomic disadvantage and risk factors shared by members of the same household are likely to contribute towards household clustering. Although our dataset is small we show that in 20 households where the proband had definite RHD, almost 30% of the first degree relatives screened also had borderline or definite RHD. This was much greater than the control group where only 7% of screened relatives had evidence of disease. Our results (Table 3) may be compared with a study in Uganda, which showed that children with any RHD were 4.5 times as likely to have a sibling with definite RHD. [16] The increased risk of RHD in family members does however encourage the possibility of screening first-degree relatives once an affected person has been identified especially in endemic areas where health care resources are limited.

In summary, we show the persistence of echocardiographically demonstrable RHD in a rural sub-Saharan population. Although we found evidence of both regression and progression of the disease, the majority of individuals who had definite features of RHD had evidence of continuing RHD lesions. This study emphasises the fact that there are still many unknown contributing factors for the development and progression of RHD valvular lesions, and that efforts should be made to better understand the mechanisms related to all types of susceptibility and develop effective primary prevention in the form of vaccines.

Age/Sex and parental education/occupation in subjects whose cardiac lesions regressed, were unchanged or progressed during the five years of the study.

(DOCX)

Click here for additional data file.

3 Aug 2021

Dear Dr Kotit,

Thank you very much for submitting your manuscript "Community-based prevalence of Rheumatic Heart Disease in rural Ethiopia: five-year follow-up" for consideration at PLOS Neglected Tropical Diseases. As with all papers reviewed by the journal, your manuscript was reviewed by members of the editorial board and by several independent reviewers. In light of the reviews (below this email), we would like to invite the resubmission of a significantly-revised version that takes into account the reviewers' comments.

Thank you for the opportunity to consider your manuscript, "Community-based prevalence of Rheumatic Heart Disease in rural Ethiopia: five-year follow-up." The outcome of screen-detected RHD is indeed of importance, and natural history studies, though inherently flawed in many ways, contribute meaningfully to our growing pool of evidence around this condition. Below you will see reviewer comments from 3 experts in this field, who raise important issues that would strengthen the presentation of these data. Please consider these comments when preparing your revision.

Sincerely,

Dr. Andrea Beaton

We cannot make any decision about publication until we have seen the revised manuscript and your response to the reviewers' comments. Your revised manuscript is also likely to be sent to reviewers for further evaluation.

When you are ready to resubmit, please upload the following:

[1] A letter containing a detailed list of your responses to the review comments and a description of the changes you have made in the manuscript. Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

[2] Two versions of the revised manuscript: one with either highlights or tracked changes denoting where the text has been changed; the other a clean version (uploaded as the manuscript file).

Important additional instructions are given below your reviewer comments.

Please prepare and submit your revised manuscript within 60 days. If you anticipate any delay, please let us know the expected resubmission date by replying to this email. Please note that revised manuscripts received after the 60-day due date may require evaluation and peer review similar to newly submitted manuscripts.

Thank you again for your submission. We hope that our editorial process has been constructive so far, and we welcome your feedback at any time. Please don't hesitate to contact us if you have any questions or comments.

Sincerely,

Liesl Joanna Zuhlke

Deputy Editor

PLOS Neglected Tropical Diseases

Liesl Zuhlke

Deputy Editor

PLOS Neglected Tropical Diseases

***********************

Thank you for the opportunity to consider your manuscript, "Community-based prevalence of Rheumatic Heart Disease in rural Ethiopia: five-year follow-up." The outcome of screen-detected RHD is indeed of importance, and natural history studies, though inherently flawed in many ways, contribute meaningfully to our growing pool of evidence around this condition. Below you will see reviewer comments from 3 experts in this field, who raise important issues that would strengthen the presentation of these data. Please consider these comments when preparing your revision.

Sincerely,

Dr. Andrea Beaton

Reviewer's Responses to Questions

Key Review Criteria Required for Acceptance?

As you describe the new analyses required for acceptance, please consider the following:

Methods

-Are the objectives of the study clearly articulated with a clear testable hypothesis stated?

-Is the study design appropriate to address the stated objectives?

-Is the population clearly described and appropriate for the hypothesis being tested?

-Is the sample size sufficient to ensure adequate power to address the hypothesis being tested?

-Were correct statistical analysis used to support conclusions?

-Are there concerns about ethical or regulatory requirements being met?

Reviewer #1: No

Reviewer #2: Yes. Objectives, study design are well described. Sample volume was not applicable since it is a follow up study of RHD patients.

Reviewer #3: The authors present 5-year follow-up data on patients with subclinical RHD from Ethiopia. Theycompare progression and regression rates with a similar number of controls. It would be useful for the authors to provide details on how they chose the controls. Were they chosen randomly? What was the sampling population? Was any matching performed? What was the rationale for chosing the number of subjects that they did?

--------------------

Results

-Does the analysis presented match the analysis plan?

-Are the results clearly and completely presented?

-Are the figures (Tables, Images) of sufficient quality for clarity?

Reviewer #1: No

Reviewer #2: Results are described in detail. Authors should also mention the number of cases that were clinically diagnosed based on auscultation initially.

Reviewer #3: The reported estimates of progression and regression are consistent with the published literature. For the subjects who showed progression, readers may wish to know more details regarding how the disease progressed. What were the lesions? How many valves were affected? Were the lesions moderate or severe?

--------------------

Conclusions

-Are the conclusions supported by the data presented?

-Are the limitations of analysis clearly described?

-Do the authors discuss how these data can be helpful to advance our understanding of the topic under study?

-Is public health relevance addressed?

Reviewer #1: No

Reviewer #2: Conclusions are supported by the data. Limitations described but not in detail. The loss of follow up could affect the results of the study, since the numbers are significant.

Reviewer #3: The greater frequency of subclinical RHD among siblings may reflect the effect of common exposures rather than genetic influences. Therefore, the authors may wish to place less emphasis on the reasons for this finding, and instead discuss the practical implications for screening programs in greater detail.

--------------------

Editorial and Data Presentation Modifications?

Use this section for editorial suggestions as well as relatively minor modifications of existing data that would enhance clarity. If the only modifications needed are minor and/or editorial, you may wish to recommend “Minor Revision” or “Accept”.

Reviewer #1: (No Response)

Reviewer #2: Minor modifications are mentioned in above sections.

Reviewer #3: (No Response)

--------------------

Summary and General Comments

Use this section to provide overall comments, discuss strengths/weaknesses of the study, novelty, significance, general execution and scholarship. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. If requesting major revision, please articulate the new experiments that are needed.

Reviewer #1: In this study, the authors describe a study that followed up children diagnosed with latent RHD in 2017.

After 57 months of follow up, two thirds of 36 patients who they were initially identified as definite still remained in the definite category, while a third regressed and some had started developing symptoms of clinical disease.

Comments

While it is impressive that the authors were able to track the study participants five years after the initial screening, there are several issues to address:

What was the rationale for the follow up? Was it a natural history study? What was the sample size estimation and did it account for the changes in valve morphology at study end? Were these participants followed up between year 1 and year 5, were there any strategies for surveillance for worsening including recurrences of acute rheumatic fever? How was equipoise regarding BPG administration to latent RHD participants handled?

This study reflects a general approach to latent RHD screening that was done 2010 and 2015. Typically, cross sectional screening for latent RHD identified children with various grades of latent RHD according to the 2012 WHF and were then followed up. Most of the earlier similar studies have been cited by the authors.

The earlier studies, including this one, had several limitations including lack of follow up to detect episodes of acute rheumatic fever recurrence that could have led to progression of the valve lesion, lack of standardised administration of benzathine penicillin, no proper sample size estimation to allow for advanced statistics to objectively measure the stated outcomes such as progression and regression of valve lesions. Consequently, these designs have been over taken by more recent studies with better planned sample size and follow up plan that allow for proper estimation of outcomes including changes in valve lesion morphology. Secondly, until we have evidence on the role of BPG on latent RHD outcomes, blinded screening and follow up may not be useful as this will generate information already known from the natural history of RF/RHD.

Reviewer #2: It is one of several papers available on follow up of subclinical or latent RHD. However authors have in addition studied the family members of both RHD cases and controls.

Reviewer #3: (No Response)

--------------------

PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Anita Saxena

Reviewer #3: No

Figure Files:

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org.

Data Requirements:

Please note that, as a condition of publication, PLOS' data policy requires that you make available all data used to draw the conclusions outlined in your manuscript. Data must be deposited in an appropriate repository, included within the body of the manuscript, or uploaded as supporting information. This includes all numerical values that were used to generate graphs, histograms etc.. For an example see here: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001908#s5.

Reproducibility:

To enhance the reproducibility of your results, we recommend that you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option to publish peer-reviewed clinical study protocols. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

18 Aug 2021

Submitted filename: Response_to_reviewers 160821.docx

Click here for additional data file.

21 Sep 2021

Dear Dr Kotit,

We are pleased to inform you that your manuscript 'Community-based prevalence of Rheumatic Heart Disease in rural Ethiopia: five-year follow-up' has been provisionally accepted for publication in PLOS Neglected Tropical Diseases.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow up email. A member of our team will be in touch with a set of requests.

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

Should you, your institution's press office or the journal office choose to press release your paper, you will automatically be opted out of early publication. We ask that you notify us now if you or your institution is planning to press release the article. All press must be co-ordinated with PLOS.

Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Neglected Tropical Diseases.

Best regards,

Andrea Beaton, MD

Guest Editor

PLOS Neglected Tropical Diseases

Liesl Zuhlke

Deputy Editor

PLOS Neglected Tropical Diseases

***********************************************************

The authors have addressed most of the major critiques from round 1 of reviews. The manuscript has been sufficiency strengthened with appropriate acknowledgment of areas of limitations and with reasonable conclusions based on these limitations. We now find this manuscript acceptable for publication. I will point out that there are remaining concerns around data availability that should be addressed - please see the PLOS policy: "The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, except in cases where the data are legally or ethically restricted (for example, participant privacy is an appropriate restriction)."

30 Sep 2021

Dear Dr Kotit,

We are delighted to inform you that your manuscript, "Community-based prevalence of Rheumatic Heart Disease in rural Ethiopia: five-year follow-up," has been formally accepted for publication in PLOS Neglected Tropical Diseases.

We have now passed your article onto the PLOS Production Department who will complete the rest of the publication process. All authors will receive a confirmation email upon publication.

The corresponding author will soon be receiving a typeset proof for review, to ensure errors have not been introduced during production. Please review the PDF proof of your manuscript carefully, as this is the last chance to correct any scientific or type-setting errors. Please note that major changes, or those which affect the scientific understanding of the work, will likely cause delays to the publication date of your manuscript. Note: Proofs for Front Matter articles (Editorial, Viewpoint, Symposium, Review, etc...) are generated on a different schedule and may not be made available as quickly.

Soon after your final files are uploaded, the early version of your manuscript will be published online unless you opted out of this process. The date of the early version will be your article's publication date. The final article will be published to the same URL, and all versions of the paper will be accessible to readers.

Thank you again for supporting open-access publishing; we are looking forward to publishing your work in PLOS Neglected Tropical Diseases.

Best regards,

Shaden Kamhawi

co-Editor-in-Chief

PLOS Neglected Tropical Diseases

Paul Brindley

co-Editor-in-Chief

PLOS Neglected Tropical Diseases