Descriptive Statistics and Non-Parametric Survival Analysis of Mortality and Morbidity of Goats
The results and discussion may be presented separately, or in one combined section, and may optionally be divided into headed subsections. From 671 kids born during the follow-up period, 469 (469/671 = 70%, 95% CI: 66.27–73.35%) mortality prevalence was recorded. From 347 adult goats that was joined the farm and followed during the follow-up period, 252 (252/347 = 73%, 95% CI: 67.61–77.25%) mortality prevalence was recorded. The mortality incidence rate of kids and adult goats were 0.638 and 0.302 per animal year. The 25th, 50th and 75th percentile of survival time of kids were 5, 157 and 1,274 days respectively and of adult (yearling) goats were 280, 828 and 1,557 days respectively. Incidence rate report is better than prevalence report for accurate comparison of epidemiological reports; however, incidence reports are very few in animal health studies. The present mortality rate is relatively larger than reports of boer cross breeding and evaluation research sites in Ethiopia as well as goat mortality prevalence abroad. The incidence rate is higher in new born kids than adult (yearling) age goats (0.638 vs 0.302). Incidence rate was also higher in boer goat breeds in both age age groups. In-line with our result, 22.3% of kid loss with in the 48 hrs age was reported in South Africa. Survival of boer goat kid is lower than Nguni goat kids (Lehloenya et al., 2005). The 25th and 50th percentile survival time of 5 and 157 days (~ 5 months) in the present study is shorter than 22.2%, 33.8% and 42.1% of failure rate at 3, 6, 12 months of kid age, and 6.73% and 16.6% mortality prevalence at pre-weaning and post-weaning age. In Jinka Agricultural Research Station, 45% of pre-weaning mortality prevalence were reported which is closer to our report (Molla, 2016). The probability of mortality failure rate for kids is rapid in the first few weeks of kid age and similarly adult goats are at higher risk of mortality in the first few months (~ 9 months) after joining the farm (5 days and 280 days 25th percentile survival time of kids and adult goats respectively). A similar higher mortality of kids and lambs in their early age trend were observed. Higher mortality rate of adult goats during their early time after joining the farm might be adaptation failure of the environment and the management system of goats in the farm (Table 1).
Table 1
Descriptive statistics and incidence rate of goat mortality (N = 1,031)
| | | | | | Percentile survival time (days) |
Age | Breed | Time at risk (animal years) | IR | All goats | Died | 25% | 50% | 75% |
Newborn (kids) | Boer | 129.15 | 0.836 | 151 | 108 | 8 | 199 | 841 |
CHG cross Boer | 593.83 | 0.584 | 521 | 347 | 4 | 153 | 1,955 |
CHG | 4.65 | 1.936 | 12 | 9 | 47 | 80 | 363 |
| Sub-total | 727.63 | 0.638 | 684 | 464 | 5 | 157 | 1,274 |
Adult (yearling) | Boer | 398.39 | 0.306 | 142 | 122 | 513 | 796 | 1,653 |
CHG | 434.88 | 0.299 | 205 | 130 | 64 | 980 | 2,044 |
| Sub-total | 833.27 | 0.302 | 347 | 252 | 280 | 828 | 1,735 |
Total | | 1560.90 | 0.459 | 1,031 | 716 | 25 | 467 | 1,557 |
Note: IR – Incidence rate, CHG – Central highland goat |
The distribution of causes of goat mortality and their relative contribution is presented in Table 2. During the study period, a many disease syndromes were diagnosed throgh ante-mortem and post-mortem clinical diagnosis methods. Most of the cause of death were unknown (no clear ante-mortem and/or post-mortem lesion) (44.07%). Gastro-intestinal related (diarrea, internal-parasite and others) diseases, pneumonia, weak kid, agalactia, mismothering (also called starvation-mismothering- exposure complex), hear water (cowdriosis) and others were the most diseases syndromes diagnosed as causes of goat mortality in the farm. Gastero-intestinal related diseases (internal parasite, diarrhea ) and pneumonia are most important cause of kid mortality. (Table 2).
Table 2
cause of goat mortality and their relative contribution
Cause of death | Yearling | Adult (yearling) | Total | Relative percent |
Unknown causes | 221 | 95 | 316 | 44.07 |
GIT problems | 25 | 48 | 73 | 10.18 |
Pneumonia | 40 | 24 | 64 | 8.93 |
Miss-mothering and agalagcia | 59 | 0 | 59 | 8.23 |
Weak kid | 37 | 0 | 37 | 5.16 |
Heart water (Cowdriosis) | 19 | 15 | 34 | 4.74 |
Systemic infection | 8 | 14 | 22 | 3.07 |
Internal parasite | 15 | 6 | 21 | 2.93 |
External wound | 13 | 6 | 19 | 2.65 |
Dystocia | 4 | 12 | 16 | 2.23 |
Unthriftness | 16 | 0 | 16 | 2.23 |
Liver fluke | 2 | 12 | 14 | 1.95 |
Caseous lymphadenitis | 3 | 8 | 11 | 1.53 |
Sudden death | 2 | 10 | 12 | 1.67 |
Aging | 0 | 2 | 2 | 0.28 |
Predator | 0 | 1 | 1 | 0.14 |
Total | 464 | 253 | 717 | 100.00 |
During the follow-up period, most of the clinical diseases were diagnosed related to integumentary system (skin abscess, caseous lymphadenitis), respiratory system diseases (pneumonia), gasterointestinal related disorders (diarrhea) and others (Table 3). In line with our result, respiratory problem, gastero-intestinal parasite, skin local abscess were reported in recently imported boer goats breeding and evaluation research centers (Hunduma et al., 2010; Asres et al., 2014; Molla, 2016).
Table 3
Most frequently diagnosed goat diseases category and their relative contribution in the farm
Disease category | Frequency | Relative percent |
Integumentary | 509 | 25 |
Respiratory | 429 | 21 |
Gastrointestinal | 333 | 16 |
Reproductive | 238 | 12 |
Nerveous | 217 | 11 |
Metabolic | 126 | 6 |
Other infectious | 124 | 6 |
Musculoskeletal | 52 | 3 |
Total disease cases | 2,028 | 100 |
The unadjusted Kaplan – Maier survival function curve from birth to 365 days of follow-up period of kid mortality stratified based on breed of goat, season and birth type at kid birth indicates that the failure rate is steady and the overall median survival time is around 130 days (Fig. 2D). The failure rate of pure boer breed kid is lower in their early age than the CHG cross boer kids, however, the failure rate of mortality in boer goat breed is higher after around 190 days of kid age (Fig. 2A). The hazard of mortality is higher for twin birth kids as compared to single birth kids (Fig. 2B).
The unadjusted Kaplan – Maier survival function curve from entrance to the farm (at their yearling age) to 36 months of follow-up period of mortality stratified based on breed and sex of goat, year of entrance to the farm indicates that the failure rate is steady and the overall median survival time is around 27.2 months (Fig. 3D). The failure rate of CHG breed is higher than the pure boer (Fig. 3A) and goats that joined on year 2017 was at higher failure rate (Fig. 3C).
Piecewise Exponential Model (PEM)
Constant piecewise exponential regression analysis of risk factors indicates that breed, kid BWT, doe PPWT, birth type, birth year and precipitation variables were associated with (p-value < 0.05) kid mortality rate and kid sex and doe parity number were not associated (p-value > 0.05) kid mortality rate. Pure boer kids compared with CHG cross boer goat, are 2.505 times at higher probability of mortality (p-value ≤ 0.001). A 1 kilo gram increase of kid birth weight and dam post-partum weight reduces mortality probability by 32.5% (p-value ≤ 0.001) and 6.4% (p-value ≤ 0.001) respectively. Twin birth kids are1.512 times higher rate of mortality (p-value = 0.001) compared with single born kids. Kids born during the years 2012/13, 2015/16, 2016/17 and 2017/18 were at higher risk of mortality (p < 0.05) as compared with the base 2011/12 birth year. A one-millilitre increment of 15 days average precipitation significantly reduces kid mortality by 7.8% (1-0.922, p-value ≤ 0.001). In this research area, natural feed resources for the goats like shrubs, trees and grass availability is highly dependent on availability of precipitation. Thus, kid mortality will reduced when the area gets higher precipitation. Mortality variation across year is due to the fluctuation of flock management, climatic variables, disease incidence and parasite infestation across the year. Kids born during the long rainy season were also at higher risk of mortality (P-value = 0.008) as compared with the dry season. The better survival rate of kids born in the dry season contradicts with other season effect on lamb survival in Ethiopia. Kids born at the dry season in this lowland rift valley area may be due to better access of natural feed sources like grass, shrubs and tree plants in this season. In contrast to our study, season has no significant effect on dorper cross local lambs (Tesema et al., 2020), lambs born during dry season are at higher risk than other seasons (Getachew et al., 2015). This is most probably due to breed and climate zone difference of studies. Kids in their first week of age were the most at risk and risk of mortality is lowest in their 180–2431 days of age period which agreed most studies (Table 4). Kid birth weight, goat breed, birth type and year are important risk factors for kid survival rate .
Table 4
Piecewise exponential model analysis results of explanatory variables effect on kid mortality
Risk factors | | HR | HR 95% CI | p-value |
Breed | CHG cross Boer | 1 | | | |
| Pure boer | 2.505 | 1.707 | 3.675 | ≤ 0.001 |
Sex | Female | 1 (base) | | | |
| Male | 1.184 | 0.977 | 1.435 | 0.086 |
Kid BWT | | 0.675 | 0.571 | 0.797 | ≤ 0.001 |
Doe PPWT | | 0.956 | 0.936 | 0.976 | ≤ 0.001 |
Parity | 1st | 1.074 | 0.753 | 1.533 | 0.693 |
| 2nd | 1.02 | 0.718 | 1.449 | 0.913 |
| 3rd | 1.265 | 0.877 | 1.826 | 0.209 |
| 4th | 1 (base) | | | |
| 5th | 1.203 | 0.778 | 1.859 | 0.407 |
| 6th | 0.961 | 0.467 | 1.976 | 0.913 |
Birth type | Single | 1 (base) | | | |
| Twin | 1.512 | 1.192 | 1.917 | 0.001 |
Birth year | 2011/12 | 1 (base) | | | |
| 2012/13 | 2.541 | 1.512 | 4.269 | ≤ 0.001 |
| 2013/14 | 1.511 | 0.955 | 2.392 | 0.078 |
| 2014/15 | 1.371 | 0.865 | 2.174 | 0.179 |
| 2015/16 | 2.209 | 1.371 | 3.558 | 0.001 |
| 2016/17 | 2.868 | 1.7 | 4.838 | ≤ 0.001 |
| 2017/18 | 4.124 | 2.398 | 7.093 | ≤ 0.001 |
| 2018/19 | 1.284 | 0.658 | 2.502 | 0.463 |
Kidding season | Dry | 1 (base) | | | |
| Short rain | 1.288 | 0.973 | 1.705 | 0.077 |
| Long rain | 1.863 | 1.172 | 2.962 | 0.008 |
Precipitation | | 0.922 | 0.882 | 0.964 | ≤ 0.001 |
Time interval (days) | 0–7 | 68.717 | 54 | 87.444 | ≤ 0.001 |
| 7–90 | 5.485 | 4.201 | 7.161 | ≤ 0.001 |
| 90–180 | 2.162 | 1.491 | 3.136 | ≤ 0.001 |
| 180–2431 | 1 (base) | | | |
|
Constant piecewise exponential regression analysis of risk factors indicates that breed, kid BWT, doe PPWT, birth type, birth year and precipitation variables were associated with (p-value < 0.05) kid mortality rate and kid sex and doe parity number were not associated (p-value > 0.05) kid mortality rate. Pure boer kids compared with CHG cross boer goat, are 2.505 times at higher probability of mortality (p-value ≤ 0.001). A 1 kilo gram increase of kid birth weight and dam post-partum weight reduces mortality probability by 32.5% (p-value ≤ 0.001) and 6.4% (p-value ≤ 0.001) respectively. Twin birth kids are1.512 times higher rate of mortality (p-value = 0.001) compared with single born kids. Kids born during the years 2012/13, 2015/16, 2016/17 and 2017/18 were at higher risk of mortality (p < 0.05) as compared with the base 2011/12 birth year. A one-millilitre increment of 15 days average precipitation significantly reduces kid mortality by 7.8% (1-0.922, p-value ≤ 0.001). In this research area, natural feed resources for the goats like shrubs, trees and grass availability is highly dependent on availability of precipitation. Thus, kid mortality will reduced when the area gets higher precipitation. Mortality variation across year is due to the fluctuation of flock management, climatic variables, disease incidence and parasite infestation across the year. Kids born during the long rainy season were also at higher risk of mortality (P-value = 0.008) as compared with the dry season. The better survival rate of kids born in the dry season contradicts with other season effect on lamb survival in Ethiopia. Kids born at the dry season in this lowland rift valley area may be due to better access of natural feed sources like grass, shrubs and tree plants in this season. In contrast to our study, season has no significant effect on dorper cross local lambs (Tesema et al., 2020), lambs born during dry season are at higher risk than other seasons (Getachew et al., 2015). This is most probably due to breed and climate zone difference of studies. Kids in their first week of age were the most at risk and risk of mortality is lowest in their 180–2431 days of age period which agreed most studies (Table 4). Kid birth weight, goat breed, birth type and year are important risk factors for kid survival rate .
Table 5
Constant piecewise exponential proportional hazard regression analysis results of explanatory variables effect on adult goat mortality
Variables | Class | HR | 95% CI | P-value |
Breed | CHG | Base | | |
| Boer | 1.503 | 1.067–2.118 | 0.02 |
Sex | Male | 1 | | |
| Female | 1.434 | 0.84–2.449 | 0.186 |
Year | 2011 | 1 | | |
| 2013 | 1.307 | 0.877–1.946 | 0.188 |
| 2017 | 7.083 | 4.661–10.763 | ≤ 0.001 |
Time interval | 0–6 | 1.352 | 0.982–1.861 | 0.065 |
| 6–9 | 0.219 | 0.089–0.541 | 0.001 |
| 9–18 | 0.518 | 0.346–0.775 | 0.001 |
| 18–24 | 1.29 | 0.88–1.889 | 0.192 |
| 24 - | 1 | | |