This study has investigated mortality in indigenous zebu cattle during their first year of life, and identified the main aetiological causes of death and the risk factors associated with infectious disease mortality. The all-cause mortality rate was estimated at 16.1 per 100 animal risk years, while mortality related to infectious diseases (ID-mortality) was estimated at 13.3 per 100 animal risk years. Although zebu cattle are considered well adapted to survive in environments of high disease pressure , mortality rates as observed in this study would suggest significant losses. These mortality rates are more than 3 times what is observed in most well managed dairy systems in developed countries where the all-cause mortality rates are frequently reported to be below 5% [23–25]. Unlike dairy systems which have intensive management and veterinary input, the traditional production systems under which zebu cattle are raised are largely non-interventional with little or no disease control at the farm level.
Similar mortality rates to those observed in this study have been reported among zebu calves in Tanzania [6, 26]. The review by Otte and Chilonda focussing on the production parameters among cattle raised under different agro-ecological zones and production systems in Sub-Saharan Africa reported an overall calf mortality risk of 21.7 percent in traditional smallholder mixed production systems . Among exotic and cross-bred animals in Sub-Sahara Africa, the overall mortality rates are usually higher than those observed in zebus, with some studies reporting rates as high as 35% [1, 4, 28]. From these studies it is evident that although zebu cattle have relatively lower mortality compared to exotics and cross-bred animals, calf mortality is still high and a possible significant impediment to improved livestock production.
In order to establish if there were specific periods when calves were at relatively higher risk of death, this study used the instantaneous hazard estimates plotted with kernel-smoothing to aid visualisation (Figure 2). The diagram suggests three periods of relatively higher risk for calf ID-mortality: neonatal period, age between 5 and 6 months (corresponding to period of waning maternal antibodies), and age approaching one year (corresponding to weaning time). Most studies on calf mortality identify the neonatal period (first four weeks after birth) as the period with highest risk for calf mortality [23, 24, 29, 30]. Diarrhoea and pneumonia are frequently reported to be the main causes of death during this period, with inadequate or delayed ingestion of colostrum after birth, unhygienic manual feeding of milk to the calves, poor calf feeding and poor housing being the main risk factors associated with these deaths. In this study, diarrhoea and pneumonia were uncommon and were not identified as causes of death for any study animals. Zebu calves are allowed to suckle directly from the dams, which reduces the risk of hygiene related illnesses associated with manual feeding of milk to the calves.
Initially the study sought to establish the predictors of ID-mortality at birth. High antibody titres against T.parva and B.bigemina in the dam were associated with increased risk for death. One possibility may be that the antibody titres may be a form of measure of infection pressure in the location the calf is born, with higher titres indicating higher infection pressure. Their effect was lost in the subsequent models that included calf infection data.
The husbandry practice of providing drinking water to the animals within the homestead was found to have a protective effect against ID-mortality in the recruitment model and the model with infectious and non-infectious factors observed over the one year. It would be expected that animals that do not require traveling to common watering points for groups of animals from different farms have lower exposure levels to pathogens. This may be one reason why this factor was identified associated with a protective effect against ID-mortality. It would be expected that including pathogen data would reduce the importance of this variable, and it is curious its effect remains with pathogen data included in the model. It is unclear what other variation other than that explained by pathogen data, the variable captures.
Before incorporating infection data into the models, analysis of the non-infectious factors associated with ID-mortality revealed heart girth size, the mean Normalised Difference Vegetation Index (NDVI), watering at homestead, controlling for ticks and dam antibody titres against B.bigemina as the significant non-infectious predictors of ID-mortality. The effects of mean NDVI, heart girth size (both protective effects) and antibody titre against B.bigemina (increased risk of death) were however lost when infection data was included, indicating these factors may be related to either susceptibility to infections or infection pressure. NDVI measures the health and density of vegetation with high NDVI values indicating healthy vegetation a proxy measure of environmental variables as rainfall and temperature. Since it measures the vegetation health, it may be related with the quality and quantity of feed availability for the animals or with high tick abundance and incidence of tick-borne diseases. High NDVI values may be suggestive of good feed availability to the dam and consequently to a suckling calf, and which would relate to the observed protective effect against calf ID-mortality. The heart girth size in the dam may relate to the condition of the dam and possibly the quality of care extended to the calf mainly through feeding. Absence of tick control and watering at homestead remained significant predictors of calf ID-mortality in the subsequent analysis including infection factors.
Although the study calves were not themselves sprayed with acaricides to control for ticks, tick control in the rest of the herd was associated with a 49% lower risk of death when compared to animals in farms where tick control was absent. Tick control in the herd may reduce the level of exposure to infected ticks that calves experience, thereby improving their survival probabilities. The frequency with which tick control was done within farms controlled for ticks was itself very low, with only a fraction of farms doing tick control more than two times in the year. This raises a question whether occasional tick control, even though it may not keep the cattle completely tick free, still carries some benefits especially in relation to survival of calves in the herd.
The final Cox survival model identified high intensity infection with Theileria spp. observed at microscopy, infection with Trypanosoma spp. and high strongyle faecal egg counts to significantly increase the risk of ID-mortality by a factor of 39, 8, and 1.5 (per 1000 epg increase) respectively. The model used data obtained through microscopy and did not include information such as clinical history (signs before death), gross or histo-pathology findings following post-mortem analysis.
When compared to the results obtained from the independent systematic review of laboratory, gross-pathology and histo-pathology data of all post-mortem cases done by the 7 veterinarians to establish the definitive aetiological causes of death for each case, the findings have good agreement. The review of the post-mortem examination results revealed, in order of importance, the main causes of calf mortality to be East Coast Fever, haemonchosis and heartwater disease. These three infections directly accounted for 60% of the disease-induced mortality. Larval cultures routinely carried out to identify the species of worms infecting the calves revealed that Haemonchus placei accounted for > 80% of all larvae hatched from strongyle eggs. The presence and abundance of H.placei worms was confirmed at post-mortem.
Although heartwater disease was identified as a main cause of death through the review of post-mortem examination results, it would not be possible to predict this since E.ruminantium, the causative agent for heartwater disease, is not easily detected in blood. Diagnosis of heartwater is mainly through clinical signs, although deaths may be per-acute, and confirmation by demonstration of E.ruminatum bodies in brain smears prepared during post-mortem examination. The model identifies infection with Trypanosoma spp. as significantly associated with death. From the PMs, trypanosomiasis was identified as the main cause of death for one calf and as a contributing cause of death to other cases.
The diagnosis of the three main infections (Theileria spp., Trypanosoma spp., and strongyle epg) identified important by the model is done on microscopy which is easily applicable in the field. It is also not labour intensive and requires little time to complete pointing to opportunities of applying simple diagnostic techniques whose results would help significantly reduce calf mortality.
The causes of calf mortality vary between geographical regions and production systems. Within smallholder production systems, some studies report pneumonias, digestive tract disorders including non-parasitic diarrhoeas, bloat [1, 5, 29, 30], and tick-borne diseases (TBD) [4, 6, 31] as the major causes of mortality. Although the current study covered a region within a 45 km radius semicircle from Kenya-Uganda border, differences in mortality rates and patterns between study sub-locations were evident. Higher mortality estimates were observed in sub-locations in the southern region of the study area and occurred in relatively younger animals compared to those in the northern region. These differences corresponded to the aetiological causes of death, with ECF being the main cause of death in the South and haemonchosis in the North. Such spatial heterogeneity within relatively small regions demonstrates the need for evidence based design for the control of disease and reduction of calf mortality.
The importance of the transfer of maternal antibodies into neonate calves via colostrum is known to be associated with survival chances of neonates . This is especially important in ruminants where very little transfer of such antibody occurs in-utero and where the ability of the newborn calf to absorb colostral antibodies is limited to the first few hours of life. It is important to note colostrum uptake was not directly measured in the study, as the calves were recruited 3 to 7 days after birth. Although the quality and amount of colostrum ingested was not established, data on whether the calf suckled immediately after birth was included in the analysis. In addition, antibody titres against the four main tick-bornes (T.parva, T.mutans, A.marginale and B.bigemina) in the dam were included in the analysis.