Study on the causes and pattern of sheep mortality under farm conditions in Northern Tanzania

M E Mlimbe, E Hyera, P O Ochanga, A S Nguluma, L J Marwa, A P Rugaimukamu, J D Godfrey, M S Ngabo and E J M Shirima

Tanzania Livestock Research Institute, West Kilimanjaro Centre, PO Box 147, Sanya Juu, Kilimanjaro, Tanzania
musaedward85@gmail.com

Abstract

This observational study was employed to scrutinize the causes of mortality in two sheep breeds raised under semi-intensive management system with the confidence that such information will aid to reduce further losses. A total of 728 sheep consisting of 432 Red Maasai (RM) and 296 Black Head Persian (BHP) died from 2005 to 2017 was summarized from animal health records. The tendencies of mortality in the two breeds were inspected. Descriptive statistics were used to analyze distribution of diseases based on breed, sex, age group and season wise pattern. Trends of mortality showed RM streak persisting on top with the peak losses in 2009. Pneumonia (43.7%), starvation (18.4%) and abscess (15.5%) were the topmost causes of mortality. For breed pattern, mortality was higher in in RM (59.3%) than BHP (40.7%). In both breeds, mortality was low in males compared to females. Also, age group mortality was higher in adults (46.0%) than young (31.3%) and lambs (22.7%). Further, the death loss was highest during winter (52.1%) followed by short rainy (19.2%), hot (18.4%) and long rainy season (10.3%). RM was not naturally tolerant against critical feed shortage and disease cycles as BHP managed and cooler dry season was the peak risky duration. The higher mortality observed in this study directs the need for implementing necessary action regarding management of the herd and disease control measures.

Key words: clinical signs, disease, mortality, pathology, sheep breeds

Introduction

Small ruminants particularly sheep in Tanzania are amongst the commonest farm animal species, which contribute to the livelihoods of smallholders and the rural poor including women headed household situations (ILRI 2011). Sheep population is projected at 5.5 million head, which is equal to 3.4% of the total livestock population in mainland Tanzania (URT 2019). The herd is mainly composed of indigenous animals and small proportion of upgraded, especially with Blackhead Persian (Nziku et al 2016). These animals are possessed by more than 30% of the agricultural households in the rural communities (URT 2012). Their small size, quick maturity, low capital asset per head and quick returns suit them to the needs of poor resource farmers (Winrock International 1983; Chenyambuga et al 2014). Sheep production provides animal protein (meat), immediate cash income, manure, raw materials (hides and skin), capital investments and fulfils socio-economic and cultural obligations (Adane and Girma 2008; Dhaba et al. 2012). In addition, sheep play a unique role in the utilization of available feed resources especially in minimal rainfall regions and rural semi-arid areas where crop production is uncertain and rearing large ruminants is constrained by feed shortage (Kosgey et al 2008; Kumar et al 2010). Thus, sheep make a significant contribution to nutrition and food security especially to the poor in the rural areas (Gupta et al 2018).

Currently, sheep production in Tanzania is confronted with various challenges that greatly affect its economic returns. High incidence of diseases and parasites are considered as major restrictions causing direct losses, such as death and diminished production, and indirect losses, such as export limitations (Pradère 2014; Torsson et al 2017). Also, insufficient feeding is another main limiting factor to sheep production in the country. Nutritional value of available fodder is poor for most of the time owing to the rainfall pattern resulting in poor nutritional status, infertility and extended lambing intervals. Moreover, management practices especially under the semi-intensive and extensive production systems are not ideal resulting in high mortality rates (Ademosun 1998). Study on the incidence of sheep mortality due to various causes in different breeds, sexes, age groups and seasons of the year under a specific management system is important in developing herd health management system (Soundararajan 2014). The present study was therefore made to investigate the causes and pattern of mortality in two sheep breeds under farm circumstances in the northern zone of Tanzania.

Materials and methods

Study area

The study was conducted at the Tanzania Livestock Research Institute, West Kilimanjaro Centre in Northern Tanzania. The Centre is located between latitude 3°S and longitude 39°E, at an altitude of almost 1270 meters above sea level. The climate is semi-arid tropic. Annual average precipitation ranges between 450 – 700 mm, which is not enough to sustain growing of pasture forage resources. The long rainy season extends from late March to May and short rainy season occurs from November to December. Cooler dry season begins from June to October together with frequent cloudiness and strong winds moving with high speed up to 25km/hr. The hot season lasts from January to Early March. Temperature lies between 11.7 ⁰C – 27.8 ⁰C. Dominant soils are silt loam and dark brown in colour. The vegetation is dominated by natural grass and legume pastures.

Study data

Data on the diagnosed causes of mortality in 728 sheep died from 2005 to 2017 was summarized from the existing animal health records of the Centre. The data included 432 Red Maasai (RM) and 296 Black Head Persian (BHP) sheep breeds. All animals were maintained under semi-intensive management system with identical feeding conditions as they were allowed to graze green pasture forages during the day for 8 hours and were provided with water and occasional feed and mineral supplements. Health management practices such as dipping, deworming and vaccination against anthrax were performed on an irregular basis. Peste des petits ruminants (PPR) vaccine was not given to the farm animals. The animals were kept in stable wooden houses each with raised floor and space to accommodate 75 heads. Possible contact with flocks from the neighboring societies was firmly prohibited.

Determination of cause of death

Cause of death was documented based on clinical signs, parasitic features and pathological changes in organs and/or tissues following autopsy investigation. In case multiple diseases or conditions were detected, death was attributed to the infection that caused the most pathology in serious body parts and/or tissues.

Data analysis

Data obtained from the records were summarized using Microsoft Excel spreadsheet. The animals were grouped as lambs (0–4 months), young (4–12 months) and adults (Above 1 year). The seasons were classified as (a) hot season from January to Early March (b) long rainy season from Late March to May (c) Cooler dry season from June to October (d) short rainy season from November to December. Line chart was displayed for trends of mortality. Summarized data concerning to mortality pattern were analysed using descriptive statistics to display distribution of diseases/conditions based on breed, sex, age group and seasonal occurrence. Proportional mortality was calculated as the number of animals died by a specific cause/effect divided by the total mortality due to all causes (Sard 1979). The proportions of recurrence of diseases/conditions were presented in percentages.

Results and discussion

Trends of mortality in sheep breeds showed that RM was more susceptible to mortality risks compared to BHP. In 2009 there was peak mortality in both breeds; mainly in RM followed by BHP. Also, in 2007, 2011 and 2012 there were discrete many deaths in RM compared to BHP. The highest mortality in sheep in 2009 might be related to extreme weather condition and rise of disease incidences in that period (Shemsanga et al 2010). The drought situation might cause lack of sufficient feed at the farm, which might result to malnutrition and increased mortality due to starvation. Besides, stress caused by nutritional imbalance might lower the immune function, which could be the reason for increased mortality from diseases, parasites or accidents that otherwise would have been evaded (Winrock International 1983). The higher trend of mortality in RM indicates that the breed is less resilient to the tropical conditions as compared to BHP.

Figure 1. Trends of mortality in sheep breeds at the farm

Frequent causes of sheep mortality in different breeds and sex are presented in Table 1. Pneumonia was the most common cause of mortality diagnosed in this study with 43.7% of the total cases, followed by starvation (18.4%), abscess (15.5%), hair ball (4.39%), helminthosis (4.26%), plant toxin (3.43%), bloat (3.16%), traumatic injuries (1.92%), snake bite (1.78%), liver cirrhosis (0.82%), pox (0.69%) and salmonellosis (0.69%). Least causes of mortality included enteritis (0.41%), dystocia (0.41%), coccidiosis (0.27%) and obstructive atelectasis (0.14%). For breed pattern, mortality was higher in RM (59.3%) than BHP (40.7%). Also, in both breeds, mortality was low in males compared to females.

The higher fatality cases of pneumonia syndrome observed at this time might be related to poor housing management and husbandry practices such as poor nutrition, incorrect sanitary and air quality practices in herd housing. These situations might have caused acute stress resulting to immune suppression and increased influence of respiratory diseases leading to this outbreak (Goodwin 2006). Also, there are some infectious diseases in sheep such as PPR, contagious caprine pleuropneumonia (CCPP) and ovine progressive pneumonia viral infection, which can be characterized by pneumonia symptoms. Clinically affected sheep showed loss of appetite, weakness, laziness, fever, rapid breathing and gaunt. Postmortem findings showed unusual fluid in the pleural cavity, petechiae in the lungs, swollen bronchi and mediastinal lymph nodes, hemorrhages over serosal surfaces, fibrin deposits over the pleura and adhesions between lung lobes and thoracic wall. But, there were no records of types of pneumonia and specific causative microbes. Pneumonia is ranked as one of the most important health problems that are quite detrimental to small ruminants in the tropics and it causes enormous production losses through morbidity and mortality (Mekbibib et al 2019). Economic losses are due to mortality, diminished production performance, high costs of treatment and trade restriction (Daniel et al 2006).

Starvation was the second most common cause of mortality in this study and it might be occurred due to climatic disasters and lack of improved farm practices according to seasonal variation of feed availability. Some clinical signs seen were wasting due to depletion of body reserves, lying down to reduce metabolic rate and failure to stand up. Starved newborn lambs showed laziness, were less vigorous at the udder for colostrum intake and had severe exhaustion of body reserves. Postmortem results showed dehydrated carcasses, flimsy rumen contents and hyperemia of blood vessels in the abdomen. Starvation resulting from poor forage condition was observed to be among the leading causes of mortality in small ruminants in the tropics (Gachuiri 1987).

Abscess ailment caused by Corynebacterium Pseudotuberculosis was the third most common cause of mortality diagnosed in this study. The disease is commonly known as caseous lymphadenitis (CLA). The higher mortality due to CLA observed in this study might be related to absence of active control measures for the disease at the farm. Sheep died from CLA showed typical sign of swollen lymph nodes at the base of the ear, in front of the shoulder, in the flank and above the udder. At postmortem, the abscesses judged as the cause of death were mostly seen in internal organs such as mesenteric lymph nodes, lungs and liver. Mortality results on pseudotuberculosis in this study indicate that the disease needs great consideration because of its contagious nature and its significance in the contamination of meat and meat products. CLA has been reported as one of the main causes of mortality in small ruminants in some countries and creates major economic impacts through carcasses condemnation or trimming of infected organs at slaughter and reduced milk production, wool growth and quality of the skin (Al-Harbi 2011; Abebe and Sisay Tessema 2015).

Hairballs were a further ailment, which caused momentous fatalities in this study. This condition also referred to as trichobezoar might be due to frequent ingestion of animals’ own hairs, which resulted to retained mass of undigested material within the gastrointestinal tract. This problem in sheep might be occurred due to detention in aa artificial environment, skin irritation and deficiency in trace elements as well as vitamins or proteins. Ingested hairs were balled up in the rumen due to gastric motility and passed into the intestinal tract and caused blockage. Bezoar sheep showed slight distension of the belly, dullness, early satiety, decreased removal of feces, vomiting and significant weight loss. Postmortem findings revealed presence of hair balls in the rumen of 23 animals, which might cause obstruction in the reticulo-omasal opening; whereas, in 9 animals, ingested hairs were found in the abomasal contents and might cause obstruction in the pyloric opening. A rare case of trichobezoar in sheep has been reported in India (Ravi et al 2014).

Helminthosis accounted for 4.26% of the mortalities. The mortality in sheep caused by helminth parasites in this study indicates serious infestation, which might be related to the designated semi intensive management condition, sheep grazing habit and infrequent deworming. The presence of helminths at the farm might be due to low location of the farm and contaminated source of water for the livestock. Some of the helminth infected sheep showed anorexia, abdominal swelling, weakness, pale mucosae of the eyes and mouth, diarrhea and eventually death. The infections ranged from subclinical signs to acute illness with rapid fatalities. Postmortem examinations revealed parasitic infections by Haemonchus contortus with 19(2.61%) cases followed by Fasciola hepatica 9(1.24%) and Cysticercus tenuicollis 3(0.41%). H. contortus, which is the most pathogenic nematode, was found in the abomasum; whereas, Fasciola hepatica was seen in the liver causing acute inflammation in the bile ducts and gall bladder and C. tenuicollis was found in the abomasum, omentum, mesentery and few were attached in the liver. Haemonchosis and fasciolosis are severe infections, which causes large economic losses due to high morbidity and mortality rates and raised costs of treatment and control measures especially in hot, humid climates (Qamar et al 2009). Clinical diagnosis of these parasitic diseases is difficult due to the nonspecific signs and absence of diarrhea and in many severe cases sudden deaths happen (Paddock 2011; Hashemnia et al 2013). Yet, little research has focused on the economic losses due to sheep mortalities caused by these ailments in tropical areas. Fasciolosis is categorized in the list of the most neglected tropical zoonotic diseases to sensitize research, education and awareness creation on the problem (WHO 2009). The incidence of sheep mortality due to haemonchosis and fasciolosis infections also has been reported by Wilson et al (2017). The losses caused by C. tenuicollis might be due to high infestation, which caused severe liver damage or secondary bacterial infection resulting in the death (FAO 1994).

Plant toxin accounted for 3.43% of the demises. The incidence of sheep mortalities due to plant toxin in this study might occur as a result of poor forage conditions, which forced deprived animals to graze large extents of noxious plants (Merrill and Schuster 1978). Plant poisoned sheep displayed the signs of weakness, mucous expulsions from oral and nasal cavities, mydriasis, hypothermia, muscular tremor, lying down, leg splashing and finally death. Postmortem examinations showed petechial hemorrhages in the lungs, heart, kidneys and diffused areas of hemorrhages in intestines. The deaths might occur due to ingestion of excessive amount of toxic plants making plentiful poison being absorbed into the individual cells and inhibited the cellular respiration (Tegzes et al 2003). A diverse of toxic plants has caused significant negative impact on the management of small ruminants in many parts of the word due to loss of productivity and death (Desta 2019). Onyeyili et al (1994) reported outbreak of mass death in sheep after grazing a poisonous plant containing complex alkaloids. The poisonous plant species present at West Kilimanjaro location that can impact adversely on the health and productivity of domestic ruminants include Solanum incanum, Datura stramonium, Rhaponticum repens and Cynodon dactylon. The poisonous effect of C. dactylon results from ergot-induced alkaloid toxin produced by strains of Claviceps spp. invading the grass (Diaz 2011).

Regarding bloat, it was incriminated in 3.16% of the death cases. Seventeen (2.34%) frothy and 6(0.82%) abomasal bloat cases were observed at postmortem. Frothy bloat was observed in sheep with mature rumen and abomasal bloat was obtained in lambs below four weeks of age. Frothy bloat might occur because of too much consumption of wet grasses and succulent pastures. Sheep with frothy bloat showed the signs of distended abdomen, discomfort, considerable distress, kicking at the belly, difficulty in breathing, recumbent and sudden death. Postmortem results showed anterior congestion, generalized reddening of the tracheal mucosa, compression of the lungs, bloody fluid in the pericardium, marked distension of the rumen and frothy ruminal contents. On the other hand, abomasal bloat might be due to bacterial infection, reduced immunity from low colostrum intake, vitamin or mineral deficiencies, pica, trichobezoars and bacterial infection (Van Metre 2018). The clinical signs diagnosed were weakness, dullness, swollen abdomen with tympany, diarrhea and eventually death. At postmortem, inflammation of the abomasum and a gas-fill were observed, the abomasal contents turned into dark colored and ulceration of the abomasal wall was seen. Bloat is a life threatening condition, which causes great losses in the livestock industry (Brewer et al 2019). It accounted for 6% of mortality cases in domestic sheep in North America (Wilson et al 2017), which is higher than the value obtained in the current study.

Traumatic injuries and snake bite were other important causes of deaths in this study. Traumatic accidents might occur due to trampling and spiking of legs at wooden flooring spaces, serious fighting and violent contact with blunt substances. Injuries attributed to death losses were subdural hematoma, fractures, wounds and ventral abdominal hernia caused by horn thrust. The incidences of injury in sheep might be generated by poor management practices such as dehorning, hoof trimming and poor housing maintenance. Besides, snake bite accidents might be occurred at grazing time. Snake bitten sheep presented the signs of respiratory distress and restlessness. At physical examination, bites were noted on lower jaw in 7 sheep, on the fetlock in 4 sheep and on the muzzle in 2 sheep. The fang marks showed profuse bleeding in 9 sheep indicating viper bite and were localized in 4 sheep indicating elapid bite. Puff adder (Viperidae) and Cobra (Elapidae) are common venomous snakes found at the farm. Postmortem results showed oedema and hemorrhage of tissues of muscles at the site of fang marks. Many carcasses had foamy releases in the tracheal lumen and obvious hemorrhages in the lungs and liver, the lungs were edematous and petechial hemorrhages were diagnosed in the peritoneum of heart. Snake bite manifests nonspecific signs and it can be diagnosed as plant poisoning, allergy or other poisoning cases when the history is absent particularly in areas where the problem is not common (Farooq et al 2014; Bolon et al 2019). But, in this study, respiratory distress, restlessness, sudden death and fang marks revealed poisonous snake bite (Farooq et al 2014). The deaths caused by snake bite envenomation in this study marks it the problem of convincing importance in terms of the disease burden, mortality in animals and economic losses. The disease is estimated to cause more than more than one hundred thousand deaths of animals in the World yearly (Banga et al 2009). However, in many countries where snake bites are common the arrangements are insufficient and the data are fragmentary, which makes the actual incidence of the disease burden to the livestock to be underestimated. As a result, snake bite was added to the list of neglected tropical diseases to enhance studies on the problem for a full understanding of its impact and awareness creation (Bolon et al 2019). Mendez and Riet-Correa (1995) reported mortality cases in sheep due to snake bite caused by Bothrops neuwiedi in Brazil.

Concerning mortality pattern between breeds, the higher mortality observed in RM indicates that the breed is less resilient to the East African tropical climate and disease conditions compared to BHP. In extreme situations such as extended drought condition and high disease incidences, RM is not naturally much equipped to tolerate against these problems as BHP manages. The low mortality observed in males might be because the males are frequently culled to keep limited number of males for breeding purpose for limited periods of time (Soundararajan 2014).

The causes of mortality pattern basing on age group are presented in Table 2. Age group mortality was higher in adults (46.0%) followed by young (31.3%) and lambs (22.7%). The deaths caused by pneumonia, starvation, abscess, plant toxin and snake bite were higher in adults than young and lambs. Hairball and helminthosis caused higher mortality in young than adults and lambs, but it is not sure if the difference between young and adults is significant since the values were not subjected to statistical test. The death losses from bloat were higher in adults, median in lambs and low in kids. In general, the results indicate that adult sheep were more vulnerable to diseases and death losses compared to young and lambs.

The causes of sheep mortality in different seasons are presented in Table 3. The death of sheep was highest during Cooler dry season (52.1%) followed by short rainy season (19.2%), hot season (18.4%) and low during long rainy season (10.3%). Also, most of the fatality cases of pneumonia, starvation, abscess, Hair ball, helminthosis, plant toxin, bloat, traumatic injuries and snake bite occurred during Cooler dry season. This might be because during this coldest and dry period of the year, which covers five months, animals undergo cold stress and deficiency of green feeds and suffer from low immunity and diseases. Mendez and Riet-Correa (1995) reported the occurrence of fatality cases of snake bite during pasture shortage season due to serious drought in Brazil, which is similar to the results of the present study. Many death losses in sheep in these seasons are accredited to variation of incidence of diseases and availability of pasture between seasons (Girma et al 2011; Tifashe et al 2017).

Conclusion and recommendations

The causes of mortality in sheep in this study were in declining order. Pneumonia, starvation and abscess were the uppermost causes of deaths between breeds and among age groups and seasons. RM was more vulnerable to diseases and mortality risks than BHP. The death losses were highest during cooler dry season. Pneumonia can be reduced by avoiding damp and poorly ventilated houses, abolishing overcrowding and delivery of nutritious feed and optimal herd health management practices. Starvation can be reduced by improving the rangeland resource through the irrigation practices especially during winter season, over-sowing the natural grassland with legumes and conservation by deferred grazing. Strategic feed supplementation in severe feed scarcity is another alternative to minimize diseases and mortalities. Caseous lymphadenitis must be controlled through serological detection and removal culling of diseased sheep. Consistent control of diseases through vaccination, vector control, prophylactic treatment and practices such as hoof trimming, dehorning and housing maintenance are important.

Limitation of the study

This observational study is a diagnostic report of mortality in sheep breeds kept at West Kilimanjaro Livestock Research Centre for conservation, research and improvement purpose. A prospective limitation of this article is that when the study was undertaken little capacity of the available resources did not permit complete diagnostic procedures such as imaging, serologic tests, histopathology, toxicological tests, etc. Nevertheless, the autopsy remains an important diagnostic procedure in establishing complete certification of the main causes of death based on advanced clinical signs.

Conflicts of interest

The authors declare that there is no conflict of interest regarding submission of this article.

Acknowledgement

The authors express heartfelt appreciation to supporting staff of the West Kilimanjaro Livestock Research Centre for their tireless participation in herd management.

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