Association of Primate Veterinarians Guidelines for Weight Management in Research Nonhuman Primates

Purpose

The Association of Primate Veterinarians (APV) recognizes that primates housed in a research setting may be susceptible to weight fluctuations that can affect their welfare and research outcomes. Obesity and poor body condition can occur spontaneously, from mismatches between food provided and caloric requirements, and/or secondary to research aims. These guidelines provide recommendations for managing research NHPs that have a non-ideal body condition to maximize their health and welfare.

Background

NHPs in research settings require an appropriate diet to support healthy development, consistent maintenance, and optimal welfare. Veterinary and research staff involved in weight management of NHPs can use regular assessments of body weight, body condition score (BCS), blood parameters, and other biomarkers to identify and evaluate changes and trends and to initiate interventions as needed.¹ Obesity of research NHPs occurs spontaneously in multiple species, including macaques, baboons, marmosets, squirrel monkeys, and vervet monkeys.^2–7 Numerous intrinsic and extrinsic factors influence diet, body condition, and weight of NHPs. As the prevalence of obesity in humans has increased, research suggests that the body weights of NHPs living at primate centers and in human-influenced environments have increased concurrently.⁸

Macaques, specifically rhesus macaques, have been the dominant primate model for obesity research. There is minimal variability in body condition between captive male and female rhesus during the first 2 years of life. At puberty, males undergo a testosterone-dependent peripubertal growth spurt that is greater in magnitude and longer in duration than females’ growth.⁹ In adult macaques of many species, as with humans, rates of obesity increase with age.

Maintaining NHPs in healthy body condition is important for providing ideal animal models and minimizing research variability. For maintenance of research NHP colonies, the overarching goal of nutritional management is to provide a relatively standardized, nutritionally complete diet in appropriate quantities to prevent obesity and malnourishment. A one-size-fits-all approach to nutritional intake is unlikely to support a healthy body condition for all animals. These guidelines offer general strategies and discuss challenges associated with specific NHP populations in research settings.

Guidelines

Old World primates.

Dietary components used for captive Old World NHPs.

Commercially available NHP diets are typically differentiated into Old World and New World diets.¹⁰ Diets with adjusted protein and fiber content to target different life stages or health statuses are available for Old World monkeys. Diet components for research NHPs generally include (1) base diet (for example, commercial chow usually fed 1 to 3 times a day; although animals may effectively “graze” in some situations); (2) enrichment diet (produce, nuts, grains, forage, etc); (3) training diet (for example, high value foods that are provided as a reward during positive reinforcement training); and (4) medication diet (for example, oral medical treatments mixed with food vehicles for veterinary or research purposes).¹⁰ Treats given by animal care staff, veterinary staff, and research staff should in addition be factored into daily caloric intake, as items given during cage-side assessments are often highly palatable. All components must be considered in creating a balanced diet and avoiding overfeeding, high-calorie enrichment, and training diets.

Assessment of body condition.

Body condition scoring (on a scale of 1 to 5 or 1 to 9) is a common method to identify and monitor dietary interventions for underconditioned and overconditioned animals.¹¹ Underconditioned animals are categorized with a BCS less than 2.5 on a 5-point scale or less than 4 on a 9-point scale. Overconditioned animals are categorized with a BCS of greater than 3.5 on a 5-point scale or greater than 6 on a 9-point scale. In addition to BCS and body weight, other metrics to evaluate an animal’s condition include body weight adjusted for body length (for example, body mass index or BMI), circumferences of trunk and limbs, skinfold thicknesses, tritium or deuterium labeled water dilution,¹² and, more recently, dual-energy X-ray absorptiometry (DEXA). DEXA provides precise estimates of total body and regional body composition that correlate well with traditional somatometric measures.^13,14 Excess body fat is predominantly found in the abdominal region for both sexes. Eighteen percent body fat is considered ideal for optimally conditioned animals.¹⁵

Impact of obesity on health and welfare.

Spontaneously obese rhesus and cynomolgus macaques are excellent models of human obesity, demonstrating similar physical and biochemical alterations and development of obesity-induced metabolic diseases, such as type 2 diabetes mellitus. Obese animals may also show respiratory effects; functional residual capacity and percentage body fat showed a strong and significant negative correlation in cynomolgus macaques, and increases in abdominal fat resulted in a reduced chest volume. Obese patients have potentially altered carbohydrate metabolism, neuroendocrine signaling, proinflammatory modulation, coronary heart disease, and decreased reproductive performance.¹⁶

Research projects and IACUC considerations.

Several research-related factors may impact decisions about animal diet choices. Research aims may require dietary alterations or have body condition requirements for enrolled animals. Non-ideal body conditions may preclude animal assignment to projects, in addition to increasing risks of weight-related sequelae. For example, lean animals may be poor candidates for projects requiring frequent fasting, food or water restriction, or anesthesia, all of which typically reduce total caloric intake. Lean animals in preclinical safety studies may be less resilient to drug-induced weight loss. Conversely, obese animals may be less ideal for laparoscopic sample collection or other surgical procedures because increased abdominal fat can impair visualization of sample targets and abdominal organs. Overweight animals may be poor candidates for projects with inflammatory data points because of the proinflammatory state obesity induces. Obese animals may introduce study variability in preclinical safety studies that base drug dosing on body weight, because their increased fat stores may alter drug metabolism, blood drug levels, and dose-range responses when compared with leaner subjects. Finally, obesity may cause housing or equipment structural conflicts if animals no longer fit into the equipment (for example, MRI, single photon emission computed tomography [SPECT]-CT, and PET) or into the housing required for a particular project. For studies requiring dietary alteration, consultation between veterinary and research staff is recommended before enrolling animals or initiating changes in diet.

Strategies for nutritional management of NHPs (overweight and underweight).

The goal of nutritional management is to provide each animal with a well-balanced and nutritionally complete diet resulting in a body condition score between 2.5 and 3.5 or 4 and 6 depending on the scale used. Animals with a BCS outside of this range should be evaluated for potential clinical abnormalities and may need additional management to access appropriate food volumes and achieve optimal health status. All dietary changes should be documented in the medical record.

Indoor housed.

In an indoor housing facility, most NHPs are fed a biscuit-based diet that is predetermined in type and quantity. The quantity of biscuits offered may be determined by body weight and/or sex of the animals. Numerous factors, including genetic heterogeneity, maturity, overall health and reproductive status, and social housing, may result in a non-ideal body condition. The weight per biscuit can vary greatly within a bag of commercial feed, leading to inconsistencies in calories provided within the same number of biscuits offered. When feasible, feeding a set weight of biscuits is one option to reduce variation in caloric rations. Monitoring indoor housed animals in small groups, pairs, or individually allows for better measurement of feed intake and assessment of body condition, both visually and at physical exam. In pair-housed animals, the dominant animal may hoard food, leading to obesity, while the subordinate animal becomes underconditioned. While brief separation may be necessary for more equal distribution of food, individually housing NHPs for long periods of time is not endorsed unless required and justified for the research goal or significant veterinary reasons.¹⁷ Antinausea medications and appetite stimulants can be considered for underweight animals with temporary anorexia from illness or study sequelae (for example, ondansetron, maropitant, mirtazapine, famotidine, capromorelin, etc).

Considerations for group-housed animals.

Altering and monitoring dietary intake can be particularly complex when animals are group housed. The easiest mechanism for managing weight concerns is subdividing small groups for short periods to allow the provision of rations to individual animals. It is recommended to separate the animal receiving more food when trying to address an underweight condition or to separate all animals if trying to address an overweight animal. Many underweight animals readily accept short, 30- to 60-min separations from the group once or twice daily to receive extra provisions.

Small groups that cannot be subdivided present an additional challenge but may respond to positive reinforcement training that provides a small hole feeder box for each animal. By placing treats into the box only when a designated animal puts their hand inside, animals are trained that each individual “owns” a particular location or a particular box. The boxes may reduce food aggression and hoarding from dominant animals and can allow staff to feed grouped animals differing amounts or diets without physical separation.

Large groups may not realistically support caloric decreases for overweight animals, but in some animals, the increase in space and subsequent increase in activity level may offset the need for caloric reduction. While animals often benefit from a transition to an environment that encourages additional physical activity, interanimal variability and personalities will factor into activity levels. People-oriented or dominant animals may readily receive dietary supplementation within a larger group. Separation of socially housed animals for weight management may be recommended for very thin animals or overweight animals with mobility issues. Otherwise, separation of some socially housed overweight animals may be contraindicated since changes in housing configuration may result in acute weight loss or social group destabilization.

Special considerations for specific NHP populations.

Quarantine.

Newly arrived animals in quarantine undergo multiple tuberculosis tests that require sedation, allowing veterinary staff to make close observations in a condensed timeframe. This is an ideal opportunity to identify weight management concerns and to monitor the initial introduction to a new environment. While close observation and frequent sedations allow for more complete assessments, sedation can lead to decreased caloric intake and postsedation complications, such as nausea and general malaise. A period of stress associated with acclimation to the new environment is to be expected in recently transported NHPs. Shipment and acclimation to new environments and new foods can lead to full or partial anorexia in otherwise healthy, normal NHPs. Animals are not only acclimating to a new facility or building but also to different caging systems (outdoor compared with indoor housing, quads compared with hanging cages compared with group housing), staff, routines, and social dynamics within the housing area. Identifying and effectively treating the cause of poor appetite will increase animal welfare, even if at times, the underlying cause of stress cannot be addressed.

Pregnant and lactating animals.

Spontaneous food intake varies inversely with increasing estrogen levels in female rhesus monkeys.¹⁸ It decreases during preovulatory days and around week 5 of pregnancy, corresponding to “morning sickness,” and it does not increase appreciably during pregnancy except for younger and leaner dams.^19,20 The lack of a significant increase in food intake during pregnancy may be due to the small mass of the fetus and placenta relative to the dam’s size and the long duration of gestation compared with smaller species bearing litters after short gestations. Exogenous estradiol inhibits food intake in a dose-dependent manner.^12,21 Interestingly, there is no clear evidence of weight gain following natural or surgical menopause in rhesus monkeys.

Pregnant or lactating animals are typically given additional rations to ensure adequate caloric access during this critical period; commercially formulated high-protein biscuits may be beneficial for certain animals. However, obese animals may not require the same increase as animals of normal body condition during this period. During gestation and early infancy, obese dams can typically receive a slight reduction in calories without adverse impacts on the infant, although monitoring of fetal and infant growth is recommended. Once the infant begins to routinely consume chow, reduction of dam rations may be more complicated, but it can still be achieved if the housing environment allows the infant to access areas the dam cannot reach. Some facilities use porches with a partially closed door to allow the infant access to a food cup at the far end of the porch or use a double cage with a “doggy door” slide (one that allows the infant to access one portion of the caging that is restricted to the dam) with additional rations of food on the infant’s side of the slide. Facilities employing these tactics should provide oversight to ensure that the infant has access to the additional rations and grows as expected. Although most infants can be trained to use these systems with positive reinforcement, some may be too timid to do so or may bring the rations back to the dam. Remote monitoring can be helpful in such situations, as many shyer infants will not leave the dam with an observer immediately present. These tactics can also be used to support underweight infants (for example, with chow soaked in infant formula) or to provide oral medications to infants.

Underweight pregnant animals may be at higher risk for adverse consequences due to their body condition. It is recommended to monitor appetites closely. Pairing or grouping pregnant animals can prevent weight loss during early pregnancy. Many animals will not overtly vomit, but those exhibiting significantly reduced intake may benefit from short-term use of antinausea medications, such as ondansetron, and/or supplemental diets during this stage.

Geriatric populations.

Both sexes of rhesus monkeys gradually lose weight after approximately 25 years of age. This is mostly due to loss of lean tissue mass as fat mass does not change significantly.¹⁴ Spontaneous food and water intake are lower in older animals. These intakes are weakly correlated with body weight but significantly correlated with each other throughout adulthood.²²

Geriatric animals presenting with nonnormal body conditions should receive a full physical evaluation. Common problems associated with aging, such as degenerative joint disease and type 2 diabetes, can contribute to both under- and overweight presentations. In the case of obesity, housing and grouping adjustments to encourage increased animal locomotion can increase energy expenditure. Combining this with appropriate dietary changes, such as a decrease in ration or a decrease in caloric density of enrichment and food treats, can lead to weight loss and overall improvements in animal welfare. Pharmaceutical treatment for animals with degenerative joint disease may also be considered to decrease chronic discomfort and encourage locomotion.

Geriatric animals presenting underweight should be robustly evaluated for the presence of underlying illness, especially if they previously had a normal or obese body condition or have a sudden appetite change. Dentition can be a problem in aged animals, and correction of dental abnormalities may resolve low appetites. With or without dental problems, underweight geriatric animals may benefit from a softer or moistened chow. Adding a multivitamin or food-based enrichment, especially high-protein items, to standard chow rations may benefit aging animals that are chronically underweight without evidence of illness.

Diabetic animals.

Type 2 diabetes mellitus occurs in many Old World NHP, including macaques, chimpanzees, and sooty mangabeys.^23–25 Diabetes mellitus most commonly occurs in older, obese primates and shares many key features with human disease.^7,26–29 Blood glucose, triglycerides, body weight, and BCS, fructosamine, and hemoglobin A1C (HbA1C) are useful parameters for monitoring the diabetic state.^1,29–31 As in humans, low-carbohydrate diets have a positive effect on HbA1C, triglycerides, and HDL concentrations in captive rhesus macaques.³² Monitoring HbA1C, fructosamine, and glucose and maintaining appropriate weight through dietary adjustments are important factors for the monitoring and control of diabetic animals.^32–34

Neuroscience research.

Primate models in neuroscience research can have specific challenges for managing body condition. Animals may be overweight or obese due to receiving calorically dense rewards during training procedures. These can include both solid treats (for example, candy, dried fruit) during tasks like chair acclimation and high-calorie fluid rewards during recording procedures (for example, juice, Ensure, chocolate milk). To formulate an effective weight loss plan for animals that are obese, it is essential to establish what calories are being provided by research staff in addition to the regular rations.

Animals on water regulation may be underconditioned because they do not eat their allotted biscuits while on fluid regulation. Many animals will not eat biscuits unless given concurrent fluids. Providing biscuits when fluids are offered may encourage consumption. Body weight and BCS thresholds should be considered as checkpoints for animals that lose weight during the study. Regular “days off” with free access to fluids should be considered to help support dietary needs for underweight animals that have no other underlying clinical abnormalities.

New World primates.

The common marmoset.

The common marmoset is currently the most frequently used New World primate (NWP) species in research. Marmosets are omnivores, with fruit, leaves, tree sap, and insects comprising much of their diet in their natural habitats. In captivity, the animals are usually fed commercially available diets in the form of pellets, extruded biscuits, canned food, or gels. These commercially developed diets are intended to be nutritionally balanced. However, nutritional requirements vary according to species and life stage, and there are still many unknowns regarding specific requirements.³⁵ Commercial diets in captivity are often supplemented with a variety of edible enrichment, such as fruits, vegetables, yogurts, insects, nuts, and seeds. Described as a “cafeteria-style” diet, this approach is common for feeding colonies at many institutions, even though there is little support for the method. The result of this feeding practice is that supplements make up almost a quarter of the animals’ daily calories.³⁶ This can lead to wide variation in caloric and nutritional intake and may contribute to weight fluctuations. It is recommended that supplemental food make up no more than 10% of an animal’s daily caloric intake.³⁶ The primary source of calories should come from the commercial diet. Edible enrichment for all life stages should have a low caloric density, as in fruits and vegetables, rather than high-glucose or high-fat foods, such as marshmallows, seeds, or nuts. Some studies recommend providing supplemental food directly to individual marmosets rather than giving all animals in a cage access to the entire cage amount,³⁶ but this may not be feasible in all colonies.

Prevalence and complications of obesity in marmosets.

Obesity was reported to affect almost 50% of juvenile marmosets in one colony,⁴ and a recent survey of institutions with marmoset colonies showed wide fluctuation of adult animal weights, from 250 to over 500 g.⁴ Adult female marmosets, on average, weighed the most, although the variation in their reported weights was also the greatest. While related, studies suggest that caloric intake is not directly correlated with body mass.³⁷ Liquid-diet feeding studies found that weight gain was associated with the consumption of larger volumes per bite rather than the number of meals when total calories/volume was controlled for.⁴ This suggests that reducing the caloric density of diets may be a factor in promoting weight loss. The lack of husbandry and diet standardization may play a role since nutrients and calories can vary widely depending on the components of the cafeteria-style diet a colony is receiving.

Dental disease is a significant and commonly reported pathology in obese marmosets. Gingivitis, tooth laxity, and tooth root abscesses may be related to commercial diets where glucose is often listed as the first ingredient. The link between dietary sugars and dental caries is well established in humans, and reducing dietary sugars may decrease the incidence of dental disease in marmosets. Provision of enrichment that encourages chewing behavior combined with prophylactic dental cleaning may help reduce plaque and calculus accumulation.

Obesity in pregnant marmosets.

Energy intake in marmosets increases during lactation but not gestation, so pregnant marmosets should receive additional calories in the form of increases in their commercial diet close to parturition. Obese female marmosets are more likely to produce triplet litters or higher, which have lower prognoses for survival than the standard twin or singleton litters.³⁸

Weight loss in marmosets.

Marmosets housed in captive research environments are also susceptible to chronic wasting, usually associated with intestinal inflammation. The etiology of this disease is not well understood, but risk factors for developing chronic wasting and GI disease include a high-carbohydrate, low-fiber diet and environmental stress.³⁹ Malabsorption of vitamin D and calcium is hypothesized to result in metabolic bone disease, and dietary supplementation of both calcium and vitamin D3 is recommended as part of management. Further studies should investigate the underlying cause of chronic marmoset gastrointestinal disease so that therapies can better target the cause of the disease rather than simply address the consequences.

Other NWP species.

While the general recommendations for managing obesity in marmosets can be extrapolated across NWP species, there are some differences and species-specific attributes to consider. Squirrel monkeys undergo a natural change in body composition during their breeding season. In a process called “fattening,” males will gain up to 15% of their body weight, mostly in the upper torso, in preparation for breeding.⁴⁰ This weight gain should not be mistaken for pathology, and calories should not be restricted or diet changed as a result.

Titi and owl monkeys are more neophobic than other NWP species and are hesitant to accept food when faced with minor environmental changes.⁴¹ Consequently, care is required when introducing new diets or changing diet or enrichment composition. New feed should be mixed with the current diet and the amount slowly increased over 3 to 4 weeks.⁴² Single housing for caloric restriction may be a less viable option for these animals to prevent stress and potential anorexia. Animals should be monitored closely if any changes are introduced to their diet or environment to ensure that they continue to eat and are not showing overt signs of stress.

Weight management in NWPs is an important component of animal welfare in a captive colony. More research is needed to determine optimal nutritional guidelines for all NWP species. A better understanding of wild species’ diets and nutrient composition will allow the research community to standardize diets and enrichment plans, more accurately reflecting an animal’s physiologic needs. Refining weight management programs for NWPs will ensure better animal welfare and more valid, reproducible research.

Disclaimer

The position statements and/or guidelines produced by the Association of Primate Veterinarians (APV) are intended to be recommendations and guidance and are not a regulatory requirement. The Scientific Advisory Committee (SAC) within APV is tasked with the generation and revision of guidance documents for use by the membership and primate specialists worldwide. A subcommittee of current APV members and subject matter experts that have expertise in the area of interest are recruited to draft a document that is then sent out for comment and input from the SAC committee, the APV Board of Directors, and the APV membership. The final version was approved by the Board of Directors before being published on the APV website. We extend special thanks to the committee members that worked on and contributed to this document: Carmen Arsuaga, DVM, MS, DACLAM (Abigail Wexner Research Institute, Nationwide Children’s Hospital), Matthew Breed, PhD, BVSc, DACLAM (Clemson University), Mallory G. Brown, DVM, DACLAM (University of Texas MD Anderson Cancer Center), Joseph W. Kemnitz, PhD (Wisconsin National Primate Research Center), Kristin E. Killoran, PhD, DVM, DACLAM (Frederick National Laboratory for Cancer Research, NIH), Mia Lieberman, DVM, PhD, DACLAM (Harvard Medical School), Julie Mattison, PhD (National Institute on Aging, NIH), and Heather Sidener, DVM, DACLAM (Oregon National Primate Research Center).