Small ruminants’ diseases and genetic disease resistance in Africa: A review

Abstract

This review was initiated by ICARDA to collect and summarize the available research evidences on small ruminant disease and their impact, diseases resistant traits among sheep and goat, and the extent of selective breeding for traits identified in Africa,. To this end, a guideline for literature selection and spreed sheet template for data extraction was developed by the principal author. The guideline was a milestone to retrieve articles from online sources and assesse the standard of the manuscripts. Likewise, the template was meant to help identify specific qualitative and quantitative data and maintain uniformity in course of double entry data extraction. Both the guideline and the data extraction template were commented and enriched by subject matter experts in the review team before use. Once such platform was in place, the review proper commenced with electronic search for published articles from online sources written in English language. Google scholar and Yahoo search engines were used to retrieve manuscripts from scientific databases including Medline, Web of science, Sicincedirect, AJOL, and CabDirect. Altogether, over 660 published manuscripts were retrieved from 42 African countries. Five hundred eighty six of them were on parasitic and infectious diseases occurrence, their management and impact. The remaining 74 articles contained 89 studies on diseases resistance profile in small ruminants both within and between breeds. To be clear, this review may not be exhaustive; however, it is exploratory and indicative of the study status in the continent. Much of the reports retrieved were from east Africa mainly, Kenya, Ethiopia, Uganda, Sudan, and Tanzania followed by West African countries i.e. Nigeria, Togo, Burkina Faso, Ghana, Cameron and the Gambia. The next highest report was obtained from South Africa, Zimbabwe, Mozambique, Madagascar and few North African countries including Egypt, Tunisia, Morocco and Algeria. Reports from Central and south West Africa were very much limited. Quantitative and qualitative summary were made based on the nature of the data and the findings were eventually organized under four major headings. The first part is about the type and origin of retrived articles, second on the magnitude and distribution of diseases, followed by diseases impact and the last part is on genetic studies for diseases resistance traits in small ruminants and possibility for selective breeding.

Accordingly, over 15 parasitic diseases were reported from 37 countries. Categorically, the diseases reported belong to gastrointestinal nematodes (GINs), flukes, lung worms, haemoparasites, myiasis, tick infestation, mangemites and intestinal protozoans. In the nematode category, Haemonchus contortus was the most widely distributed GIT parasite and highly pathogenic agent reported from all the countries where the report came from. Haemonchus contortus along with other GINs were found responsible for serious health problems in small ruminants of the continent. The other important parasitic diseases documented were trypanosomosis followed by tick-borne diseases and skin diseases due to mangemites. In the infectious diseases category about 11 bacterial and viral diseases were reported; diseases like pest des petitis rumins (PPR), Contagious Caprine Pleuropneumonia (CCPP), Sheep and Goat pox, hemorrhagic septicemia, leptospirosis, Maedi-visina, rift valley fever, bluetongue, brucellosis and contagious-ecthyma were among important diseases noted. Nevertheless, PPR and CCPP were considered to be the leading cause of morbidity and mortality in small ruminants in the continent.

In terms of economic impact, the evidences available on financial losses associated with specific diseases were limited and where available they were given in local currencies. In fact, in countries like Zimbabwe, Nigeria and Ethiopia financial estimates in hard currency have been captured for few parasitic diseases. In this connection Zimbabwe lost 5.6 million dollars in 10 years’ time due to cowderiosis alone in small ruminants. For Ethiopia, a loss of 250,000 tons of fresh meat has been documented from seven million helminth-infected sheep slaughtered annually. Likewise in Nigeria, liver fluke alone caused organ condemnation worth 77,900 dollar per annum. At a larger scale, small ruminant mortality up to 40% was estimated due to helminth in tropical Africa in the absence of treatment intervention. For CCPP the estimated mortality was 50 % in Tanzania and as high as 65% in Eritrea due to seasonal outbreaks. Likewise, for PPR mortality of 49 % in sheep and 64.4% in goats were reported in Nigeria. In Gabon, the case fatality of PPR was observed to reach as high as 98%. From this, one can easily speculate how much impact PPR alone inflicts as it is one of the preponderance viral diseases reported from east to western corner of the continent. Apart from specific diseases impact due to morbidity, mortality, reproductive inefficiency, or any other, the presence of these and other trans-boundary diseases like Rift Valley fever, sheep and goat pox have limited the export of live animals and meat to markets in the west due to sanitary and phytosanitary restrictions. Condemnations of whole or part of carcasses and edible organs due to some of the disease causing agents residing in apparently healthy animals presented for slaughter have also been shown to encur huge financial losses to many abattoirs. Likewise, pathogens such as mange, ticks and other micro-predators inflict serious damage on the skin causing considerable financial loss to the leather industry. In this regard the available evidence in Ethiopia indicated a damage as high as 35% to sheep skin and 56% to goat skin every year. The other impact noted in this review is the presence of multiple drug resistance reported for antitripanocidal drugs, antihelmintics, antibiotics and occasionaly to acaricides. Twelve countries have reported the presence of resistance to one or more drugs or chemicals used in animal health interventions.Pertinat to residue in the small rumints product, nothing much could be retrived.

The third chapter of this report is about studies on resistance of small ruminants to diseases and identifying the source of variation both within and between breeds that may be exploited for selective breeding. In this regard, of the 54 African countries studies on small ruminants’ resistance to various diseases were obtained from only 12 countries. Much of these reports were again from eastern part of the continent followed by West and southern Africa. The studies employed various approach including, natural observation, clinical control trial and fully experimental approach. Despite the presence of several parasitic and infectious diseases in Africa, studies on selection for genetic resistance in small ruminants mainly focused on three diseases. These are gastro intestinal nematodes (mainly H. contortus), trypanosomosis and tick infestation along with few tick-borne diseases. In 73% of the studies, phenotypic traits such as, fecal egg count (FEC), packed cell volume (PCV), worm burden (WB), parasitemia, body condition score (BSC), degree of infestation, and immunological responses were used as proxy-indicator for resistance traits. In the remaining 27% of the studies, the approaches were quantitative genomics and techniques that include quantitative trait loci (QTL), Genome wide association studies (GWAS) and whole-genome sequencing. In both phenotypic and genomic approaches, the heritability reported for resistant traits were low to moderate level. This fact has been substantiated by polygenic nature of the diseases resistance traits in small ruminants.

Among the 19 pure sheep breeds and their crosses investigated in 61% of the studies, nine of them were reported to have resistance traits for one or the other type of diseases. Red Maasai sheep from Kenya, and Djallonke from West Africa were the two sheep breeds for which multiple reports were obtained about their resistance for both GIN and trypanosomosis. Sabi sheep from Zimbabwe was resistant for GIN as well. Namaqua Africaner from South Africa, queue fine de l’Quest and Barbarine from Tunisia were breeds reported to have resistance for ticks and some tick-borne diseases. The Ethiopian Horro and Sudanese Garag and watesh sheep breeds were noted for moderate resistance to fasciolosis and malignant ovine theileriosis, respectively. Pertinent to goat, the number of breeds compared was the same, however, evidence of resistant traits were reported in 6 of them. The small east African Goat from Kenya, Uganda and Tanzania, the West African dwarf goat in countries of West Africa, the Red Sokoto from Nigeria, Xhosa and Nuguni from South Africa and Mubende goat breeds from Uganda were noted to have resistance for GIN. For Trypanosomosis, the West African dwarf goats from Nigeria and Small east African goats from Kenya were reported to have resistance/tolerance profile over the comparison breeds. Xhosa goats were the only goat breed reported to have resistance for tick infestations. When resistance to diseases is compared within each breed in 35% of the studies, most of the resistant sheep and goat breeds reported were noted to have individual variations on resistance profile that can be exploied to develop nucleus elite flocks via selective breeding. The remaining 3 % of the studies compared the difference between species, i.e. Djallonke Sheep verses WAD Goats. The available evidence on diseases resistance traits selective breeding was limited to H. contortus in single farm from South Africa on Dhone Merino flock.

The last part of this report summarized conclusive remarks on the findings of each topic and suggested the way forward accordingly. The conclusion is that small ruminant production in Africa is constantly challenged by plethora of diseases affecting their welfare and seriously undermining the economic return expected from the sector due to mortality, reduced production and trade buns. This is further aggravated by poor management, inadequate focus on small ruminants as important driver of the rural economy and food security; consequently exposing the sector to huge financial losses. Therefore, the need for capacity building on animal health and genetic disease resistance traits research, the importance of strengthening small ruminants’ disease survey and surveillance in all African countries, developing locally adapted strategy for disease control, the need for expanding and strengthening research on genetic markers of diseases resistance traits among indigenous small ruminant breeds and integrating this functional traits in breeding programs and developing comprehensive breeding policy before any breeding intervention on indigenous small ruminant breeds and many other key areas were proposed for future intervention.