An Assessment of the Risk Factors Associated with Disease Outbreaks across Tilapia Farms in Central and Southern Zambia
, John Yabe 1,2, Mulemba Tillika Samutela 1, Mwansa M. Songe 3, Alexander Shula Kefi 4, Loziwe Njobvu Chilufya 4 and Martin Sukkel 5Round 1
Reviewer 1 Report
Dear Editor,
I reviewed the manuscript (MS) “An Assessment of Risk Factors Associated with Diseases Outbreaks among Tilapia Farms in Central and Southern Regions of Zambia” from Kunda Ndashe and colleagues. The manuscript is an assessment on the fish welfare and management status in a distinct area of Africa.
Even if the MS is well structured, there are some suggestions to improve the content, to better explain the results and to provide some data useful to fish farmers against disease outbreaks.
Among them, it is important to clarify the fish density parameter, because it is absolutely not applicable to use m2. Another major suggestion is to clarify the discussions and conclusions, too much long and not well related to the reported results.
In my opinion, the manuscript is not acceptable in the current form. It will be considered for publication in Fishes after a Major revision.
Specific comments:
Line 24: Why authors used m2 and not m3 to explain the fish density? m2 is not a useful parameter to evaluate the fish density, representing a criticism for the study. It is not a typing error because it is reported in other parts of the MS. Please clarify this choice or change with a more adequate parameter.
Line 24: Nile Tilapia is too much generic, please specify
Line 32: Please change words already reported in the title
Line 42: Please add some example of disease outbreaks
Lines 45 – 47: The sentence is not too much correct, I suggest to add also technopathy and parasitic diseases
Lines 54 – 56: Please rephrase this sentence to better explain that the biocides are used ONLY against infectious diseases and not to contrast common environmental pathogens.
Line 91: Please remove “(3)” and change “9” to “nine”
Line 158: Please change “discolouration” to “lesions”
Line 162: Please change “remedy” to “treatment” even if it is not considered a drug
Line 182: change bracket order
Line 191: Please change “outbreak episodes” to “outbreaks”, as below reported.
Line 202: Please change “with” to “to”
Lines 206 – 216: Please clarify this paragraph
Line 244: Please change “association” to “correlation”, as well as in other parts of the MS
Line 251: Please rephrase explaining “experimental infection”
Line 256: What the authors mean with “disease pathogens”? Please explain or delete because it is not clear.
Lines 265 – 278: This part is not supported by the results. If the authors did not provide a parasitological evaluation, it is not relevant to discuss that.
Line 276: Please change “infestation” to “infection”
Lines 326 – 328: Please check these data and rephrase to better explain
Line 352: Please change “pathogen” to “pathogens”
Lines 355 – 364: This paragraph is too much confuse, please rephrase
Line 367: Please change “therapies” to “treatments”
Line 410: Please provide correct reference form
Lines 411 – 417: Please clarify what authors mean, why did they report zoonosis? It is not supported in the results
Lines 425 – 427: What did the authors mean? Please better explain to add useful information
Author Response
Response to reviewers’ comments
We are very grateful for the reviews provided by external reviewers of this manuscript. The comments are encouraging and the reviewers appear to share our judgement that this study and its results are important to aquaculture sector. As advised through the reviewer and editor, the manuscript was submitted for extensive English editing and we used the MDPI service for that purpose.
Please see below, in blue, our detailed response to comments.
Reviewer comment: Why authors used m2 and not m3 to explain the fish density? m2 is not a useful parameter to evaluate the fish density, representing a criticism for the study. It is not a typing error because it is reported in other parts of the MS. Please clarify this choice or change with a more adequate parameter.
Authors response: The study included farmers who reared fish in ponds and therefore we used m2 as the measure of fish stocking density instead of m3. Fish stocking density based on volume (m3) maybe used in cages, and tanks. Other publications that have used m2 to describe fish density in ponds include Mmanda et al., (2020), Hossain et al (2022), Glenewinkel, (2007) to mention a few.
Reviewer comment: Please add some example of disease outbreaks.
Authors response: Added text [Outbreaks due to diseases such as tilapia lake virus (TiLV), streptococcosis, and motile aeromonad septicaemia have been reported in several intensive tilapia producing regions around the world]
Reviewer comment: Lines 54 – 56: Please rephrase this sentence to better explain that the biocides are used ONLY against infectious diseases and not to contrast common environmental pathogens.
Authors response: Added text [Globally, biocide and antibiotic treatments are used widely in intensive fish production systems against infectious pathogens that cause disease and are present in the aquatic environments where fish are reared]
Reviewer comment: Lines 206 – 216: Please clarify this paragraph
Authors response: The paragraph explains the relationships total mortality rates on the farm to pond size, fish species, dead fish disposal and control of predator birds. Since their relationship was statistically significant, the odds ratio was used to determine the increase or decrease in number of farmers in each category per unit increase in total mortality.
Reviewer comment: Lines 265 – 278: This part is not supported by the results. If the authors did not provide a parasitological evaluation, it is not relevant to discuss that.
Authors response: the study did not include and clinical or parasitological examination of the fish and therefore the section has been deleted as advised.
Reviewer comment: Lines 326 – 328: Please check these data and rephrase to better explain
Authors response: Text rephrased [High levels of chemicals, such as ammonia, have a toxic effect on fish, leading to their inability to extract energy from feed and lethargy among the fish on chronic exposure]
Reviewer comment: Lines 355 – 364: This paragraph is too much confuse, please rephrase.
Authors response: Paragraph rephrased through the MDPI service [Among the clinical signs observed during mortality episodes, fish gasping for air at the surface of water was the commonest one reported by farmers. This clinical sign is mainly reported in fish in hypoxic conditions, where the dissolved oxygen is insufficient to support respiration. Other clinical signs reported were erratic swimming, reddish lesions of the skin, fin rot or erosion, cotton-like appearance on the skin, lethargy and corneal opacity. These clinical signs reported by the farmers are consistent with those seen in tilapia diseases such as columnaris, streptococcosis, lactococcosis, motile aeromonad septicaemia and saprolegniasis. In Zambia, disease outbreaks caused by Aeromonas spp, Streptococcus agalactiae and Lactococcus garvieae have been reported in a number of farms on lake Kariba rearing Nile tilapia. ]
Reviewer comment: Line 410: Please provide correct reference form
Authors response: Text rephrased (Flores et al. (2015) reported that in Bataan Province in Bangladesh, 58% of the farmers had perimeter barriers around their ponds [61])
Reviewer comment: Lines 411 – 417: Please clarify what authors mean, why did they report zoonosis? It is not supported in the results
Authors response: biosecurity measure of using footdip and handwash stations is meant to reduce the risk of disease transmission through farm workers or visitors. The sentence has been revised to communicate the thoughts of the authors. [Bacterial pathogens such as Streptococcus iniae, Streptococcus agalactiae, Lactococcus garvieae and Mycobacterium marinum can be transmitted mechanically from fish to humans and vice versa through handling [62]. However, the risk of humans spreading fish disease pathogens is low unless they are workers working in handling live fish and the activities are conducted within a few hours of two different rearing units.]
Reviewer comment: Lines 425 – 427: What did the authors mean? Please better explain to add useful information
Authors response: The sentences endeavoured to highlight the though quaternary ammonium compounds and chlorines are extensively used in Zambia, other studies have demonstrated the inefficiencies of these chemicals when constantly used to disinfect tools and equipment on fish farms.
The sentences have now been rephrased through the MDPI service [In a disinfectant susceptibility study, quaternary ammonium compounds and chlorine-based compounds showed mild and poor efficacy towards bacteria isolated from fish farms. Furthermore, the use of chlorine and quaternary ammonium compounds as a disinfectant have been demonstrated to promote the horizontal transfer of plasmids by natural transformation via the exchange of antimicrobial resistance genes across bacterial genera and leading to the emergence of new antimicrobial-resistant bacteria. As farmers in this study commonly used quaternary ammonium compounds and chlorine, a susceptibility assessment is warranted to assess the efficacy of these compounds against bacteria on farms]
References
Mmanda, F.P., Mulokozi, D.P., Lindberg, J.E., Norman Haldén, A., Mtolera, M., Kitula, R. and Lundh, T., 2020. Fish farming in Tanzania: The availability and nutritive value of local feed ingredients. Journal of Applied Aquaculture, 32(4), pp.341-360.
Hossain, M.B., Nur, A.A.U., Sarker, M.M., Banik, P., Islam, M.M., Albeshr, M.F. and Arai, T., 2022. Production Performances and Profitability of Stocking Homestead Ponds with Advanced Carp Fingerlings for Maximizing Family Nutrition and Income Generation. Fishes, 7(5), p.289.
Glenewinkel, H., 2007. Comparison of Two Stocking Densities for Channel Catfish Production in Earthen Ponds. Texas Parks and Wildlife Department, Inland Fisheries Division.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors of the study investigated the management practices that contributed to the occurrence of disease outbreaks of farmed tilapia in Zambia. It was a cross-sectional qualitative study undertaken from January to March 2021; questionnaires were administered to 49 farmers to assess their fish health management and biosecurity competences. Data was analysed using mean, percentage, ratio, and logistical regression. The results showed that majority of farms had high stocking densities, reared Nile tilapia, and sourced water for farming from rivers and streams. Few farmers measured water quality parameters daily, and removed dead fish from ponds daily. The stocking density (p=0.013), fish species (p=0.031), dead fish disposal methods (p=0.023), and control of predator birds (p=0.016) influenced the total mortality recorded on farm, while pond type (p=0.031, p=0.045), water source (p=0.023) and stocking density (p=0.027) influenced the duration of mortality episode. It is evident that some fish health management practices and biosecurity concepts among tilapia farmers in the study area were inadequate and may not contain disease outbreaks or spread of pathogens.
As a whole, the study was properly addressed and provides interesting information that are useful especially for local fish farmers. So, according to this referee, the soundess of this study is mainly focused on the country in which it was conducted.
About the paragraph 3.3 Mortality trends and fish disease management, this referee kindly asks for the possibility to include some names of the diseases that were diagnosed by the experts of the field. Please also provide information about eventual parasitological exams, and tell if they were done or not.
In the discussion you reported that "Since clinical signs of hypoxia were frequently reported by farmers in the present study, we can postulate that mortalities of Nile tilapia might not have been caused by disease pathogens". Please consider that you can postulate this only if parasitological and/or bacteriological exams were made. Rephrase the sentence according to the previous suggestion. It would be interesting to have these information, since you have made a nice introduction and discussion about the most common diseases of Nile tilapia.
Author Response
Response to reviewers’ comments
We are very grateful for the reviews provided by external reviewers of this manuscript. The comments are encouraging and the reviewers appear to share our judgement that this study and its results are important to aquaculture sector. As advised through the reviewer and editor, the manuscript was submitted for extensive English editing and we used the MDPI service for that purpose.
Please see below, in blue, our detailed response to comments.
Reviewer comment: About the paragraph 3.3 Mortality trends and fish disease management, this referee kindly asks for the possibility to include some names of the diseases that were diagnosed by the experts of the field. Please also provide information about eventual parasitological exams, and tell if they were done or not.
Authors response: The questionnaire used to collect information during the study did not include exact diseases diagnosed by field experts that farmers had consulted prior to our study. In that section we concentrated at the clinical signs as reported by the farmers and age of fish when disease outbreaks are reported. The idea was to assess if the farmers were observant of mortality events and clinical signs during high the mortality episodes in their fish rearing units.
Reviewer comment: In the discussion you reported that "Since clinical signs of hypoxia were frequently reported by farmers in the present study, we can postulate that mortalities of Nile tilapia might not have been caused by disease pathogens". Please consider that you can postulate this only if parasitological and/or bacteriological exams were made. Rephrase the sentence according to the previous suggestion. It would be interesting to have these information, since you have made a nice introduction and discussion about the most common diseases of Nile tilapia.
Authors response: the study did not include and clinical or parasitological examination of the fish and therefore the section has been deleted as advised.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I want to thank the authors for the applied revisions, according to my suggestions. In my opinion, the manuscript could be accepted in the journal "fishes"
