Mortality rates and survival analysis of a cohort of owned adult dogs and puppies in Hluvukani Bushbuckridge South Africa

Abstract

Rabies is an acute, fatal, progressive, incurable viral encephalitis affecting all warm-blooded animals, including humans. Dogs are the primary reservoir of rabies virus (RABV) in Africa for both humans and animals. Although rabies can be successfully controlled through vaccination, high rates of dog population turnover through births and deaths make the maintenance of herd immunity through vaccination challenging in populations of free-roaming dogs in low-resource settings. Understanding these demographic processes may help find solutions to create stable, vaccinated populations. The primary aim of this study was to determine the rates and causes of mortality in owned, mostly free-roaming dogs in Hluvukani village of the Mnisi community, in Bushbuckridge Municipality, Ehlanzeni District, Mpumalanga Province. The study enrolled a cohort of adult dogs (one year and older) in May and June, 2014, and followed them for 12 months. Litters of puppies were also enrolled at birth and followed for 120 days each. Outcomes were recorded during frequent follow-up visits, and causes of mortality were determined through owner interview ( verbal autopsy?) and post-mortem examination. A cohort of 367 adult dogs was enrolled in the study (203 males and164 females). Of these, 27 (7.4%) died during the follow-up period, and seven (1.9%) were lost to follow-up. We observed a mortality rate of 77.6 per 1,000 dog-years in the cohort (50.8 per 1,000 dog-years in males and 112.7 per 1,000 dog-years in females). There was very high mortality in female dogs, especially from the age of 5 years and above (305.8 per 1,000 dog-years). Female dogs had a shorter survival time (mean = 341.7 days) compared to the male dogs (mean = 355.8 days); this difference was significant with the log-rank test (p = 0.04) and by Cox regression (p = 0.05). Adult dogs of age 5 years and above had shorter survival time (mean = 338.5) than dogs of ages 3-4 years, which in turn had higher survival times (mean = 358.4) than dogs of ages between 1-2 years old (mean = 348.5). The chi-square test gave a p-value of p = 0.06 and the Cox proportional hazard a p-value of p=0.3. Thus, there is a marginally significant difference in survival between the age categories of adult dogs. Twenty-seven verbal autopsy? results were collated including 15 natural deaths, 7 deliberate deaths, 3 accidental deaths and 2 euthanasia?s. Of the 15 deaths classified as natural?, two were considered to be due to snake envenomation. The remaining 13 were considered to be due to infectious and/or parasitic causes. Of the seven deaths classified as deliberate?, four were considered to have been caused by poisoning and three by trauma. The two accidental, non-anthropogenic deaths were due to animal bite wounds, and the single accidental, anthropogenic death was caused by a motor vehicle accident. Two dogs were euthanized because they were unwanted by the owner. Of the 164 females enrolled in the study, 57(34.8%) had at least one litter of puppies during the study period. The total number of puppies enrolled at birth for the study was 329, comprising 152 (46.8%) males, 148 (44.4%) females, and 29(8.8%) puppies of unknown sex. Of the 329 puppies enrolled, 135 (36.6%) died during the follow-up period, and 126 were lost to follow-up. The mortality rate for puppies was 2,389.3 deaths per 1,000 dog-years. Sex-specific mortality rates were 1,811.5 deaths per 1,000 dog-years for male puppies and 2,172.2 deaths per 1,000 dog-years for female puppies. The survival rates were not significantly different between male and female puppies (p=0.3). One hundred and thirty five verbal autopsy results were collated and analyzed from all dead puppies. Of the 54 puppy deaths classified as natural?, 53 were considered to be due to infectious and/or parasitic causes. Of the 23 deaths classified as deliberate?, six were considered to have been caused by poisoning and 17 by trauma. The 47 accidental, non-anthropogenic deaths were considered to be due to the following causes: eaten by mother (n = 18), starvation (n = 8), laid on by mother (n = 7), animal bite (n = 5), drowning (n = 4), suffocation (n = 3) and crushed by a brick (n = 2). The single accidental, anthropogenic death was caused by a motor vehicle accident. Five puppies were euthanized because they were unwanted. In conclusion, free-roaming dogs in this study were owned, and provided for by their owners without reliance on environmental resources. Mortality rate was low in the adult dog cohort. Birth rate in the female cohort was high and the mortality rate was also high in the puppies before they reached 120 days of life. Natural causes were the highest causes of death in puppies, but accidental and deliberate causes were also frequent. This study recommends future validation studies of methods of verbal autopsy in determining causes of death of dogs in underserved, resource-constrained communities like the Hluvukani village in the Mnisi community area of Mpumalanga Province, South Africa.